src/documentation/content/xdocs/poifs/html/POIFSDesignDocument.html


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<HTML>
    <HEAD>
        <TITLE>POIFS Design Document</TITLE>
    </HEAD>
    <BODY>
        <FONT SIZE="+3"><B>POIFS Design Document</B></FONT>
        <P>
            This document describes the design of the POIFS system. It is
	    organized as follows:
        </P>
        <UL>
            <LI>
                <A HREF="#Scope">Scope</A> A description of the limitations of
		this document.
            </LI>
            <LI>
                <A HREF="#Assumptions">Assumptions</A> The assumptions on
		which this design is based.
            </LI>
            <LI>
                <A HREF="#Considerations">Design Considerations</A> The
		constraints and goals applied to the design.
            </LI>
            <LI>
                <A HREF="#Design">Design</A> The design of the POIFS system.
            </LI>
        </UL>
        <P></P>
        <OL TYPE="I">
            <LI>
                <A NAME="Scope"><FONT
                SIZE="+2"><B>Scope</B></FONT></A>
                <P>
                    This document is written as part of an iterative process.
		    As that process is not yet complete, neither is this
		    document.
                </P>
            </LI>
            <LI>
                <A NAME="Assumptions"><FONT
                SIZE="+2"><B>Assumptions</B></FONT></A>
                <P>
                    The design of POIFS is not dependent on the code written
		    for the proof-of-concept prototype POIFS package.
                </P>
            </LI>
            <LI>
                <A NAME="Considerations"><FONT SIZE="+2"><B>Design
                Considerations</B></FONT></A>
                <P>
                    As usual, the primary considerations in the design of the
		    POIFS assumption involve the classic space-time tradeoff.
		    In this case, the main consideration has to involve
		    minimizing the memory footprint of POIFS. POIFS may be
		    called upon to create relatively large documents, and in
		    web application server, it may be called upon to create
		    several documents simultaneously, and it will likely
		    co-exist with other Serializer systems, competing with
		    those other systems for space on the server.
                </P>
                <P>
                    We've addressed the risk of being too slow through a
		    proof-of-concept prototype. This prototype for POIFS
		    involved reading an existing file, decomposing it into its
		    constituent documents, composing a new POIFS from the
		    constituent documents, and writing the POIFS file back to
		    disk and verifying that the output file, while not
		    necessarily a byte-for-byte image of the input file, could
		    be read by the application that generated the input file.
		    This prototype proved to be quite fast, reading,
		    decomposing, and re-generating a large (300K) file in 2 to
		    2.5 seconds.
                </P>
                <P>
                    While the POIFS format allows great flexibility in laying
		    out the documents and the other internal data structures,
		    the layout of the filesystem will be kept as simple as
		    possible.
                </P>
            </LI>
            <LI>
                <A NAME="Design"><FONT
                SIZE="+2"><B>Design</B></FONT></A>
                <P>
                    The design of the POIFS is broken down into two parts:
		    <A HREF="#Classes">discussion of the classes and
		    interfaces</A>, and <A HREF="#Scenarios">discussion of how
		    these classes and interfaces will be used to convert an
                    appropriate Java InputStream (such as an XML stream) to a
		    POIFS output stream containing an HSSF document</A>.
                </P>
                <A NAME="Classes"><FONT SIZE="+1"><B>Classes and Interfaces</B></FONT></A>
                <P>
                    The classes and interfaces used in the POIFS are broken
		    down as follows:
                </P>
                <TABLE BORDER="1">
                    <TR>
                        <TH><B>Package</B></TH>
                        <TH><B>Contents</B></TH>
                    </TR>
                    <TR>
                        <TD><A
                        HREF="#BlockClasses">net.sourceforge.poi.poifs.storage</A></TD>
                        <TD>Block classes and interfaces</TD>
                    </TR>
                    <TR>
                        <TD><A
                        HREF="#PropertyClasses">net.sourceforge.poi.poifs.property</A></TD>
                        <TD>Property classes and interfaces</TD>
                    </TR>
                    <TR>
                        <TD><A
                        HREF="#FilesystemClasses">net.sourceforge.poi.poifs.filesystem</A></TD>
                        <TD>Filesystem classes and interfaces</TD>
                    </TR>
                    <TR>
                        <TD><A
                        HREF="#UtilityClasses">net.sourceforge.poi.util</A></TD>
                        <TD>Utility classes and interfaces</TD>
                    </TR>
                </TABLE>
                <OL>
                    <LI>
                        <A NAME="BlockClasses"><B>Block Classes and
                        Interfaces</B></A>
                        <P>
                            The block classes and interfaces are shown
                            in the following class diagram.
                        </P>
                        <P>
                            <IMG SRC="BlockClassDiagram.gif">
                        </P>
                        <TABLE BORDER="1">
                            <TR>
                                <TH><B>Class/Interface</B></TH>
                                <TH><B>Description</B></TH>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="BATBlock"><B>BATBlock</B></A></TD>
                                <TD>The <B>BATBlock</B> class
                                represents a single big block
                                containing 128 <A
                                HREF="POIFSFormat.html#BAT">BAT
                                entries</A>.<BR>Its
                                <CODE><I>_fields</I></CODE> array is
                                used to read and write the BAT entries
                                into the <CODE><I>_data</I></CODE>
                                array.<BR>Its
                                <CODE><I>createBATBlocks</I></CODE>
                                method is used to create an array of
                                BATBlock instances from an array of
                                int BAT entries.<BR>Its
                                <CODE><I>calculateStorageRequirements</I></CODE>
                                method calculates the number of BAT
                                blocks necessary to hold the specified
                                number of BAT entries.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="BigBlock"><B>BigBlock</B></A></TD>
                                <TD>The <B>BigBlock</B> class is an
                                abstract class representing the common
                                big block of 512 bytes. It implements
                                <A
                                HREF="#BlockWritable">BlockWritable</A>,
                                trivially delegating the
                                <CODE><I>writeBlocks</I></CODE> method
                                of BlockWritable to its own abstract
                                <CODE><I>writeData</I></CODE>
                                method.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="BlockWritable"><B>BlockWritable</B></A></TD>
                                <TD>The <B>BlockWritable</B> interface
                                defines a single method,
                                <CODE><I>writeBlocks</I></CODE>, that
                                is used to write an implementation's
                                block data to an
                                <CODE>OutputStream</CODE>.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="DocumentBlock"><B>DocumentBlock</B></A></TD>
                                <TD>The <B>DocumentBlock</B> class is
                                used by a <A
                                HREF="#Document">Document</A> to holds
                                its raw data. It also retains the
                                number of bytes read, as this is used
                                by the Document class to determine the
                                total size of the data, and is also
                                used internally to determine whether
                                the block was filled by the
                                <CODE>InputStream</CODE> or
                                not.<BR>The
                                <CODE><I>DocumentBlock</I></CODE>
                                constructor is passed an
                                <CODE>InputStream</CODE> from which to
                                fill its <CODE><I>_data</I></CODE>
                                array.<BR>The <CODE><I>size</I></CODE>
                                method returns the number of bytes
                                read (<CODE><I>_bytes_read</I></CODE>
                                when the instance was
                                constructed.<BR>The
                                <CODE><I>partiallyRead</I></CODE>
                                method returns true if the
                                <CODE><I>_data</I></CODE> array was
                                not completely filled, which may be
                                interpreted by the Document as having
                                reached the end of file
                                point.<BR>Typical use of the
                                DocumentBlock class is like
                                this:<BR><CODE>while
                                (true)<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;DocumentBlock
                                block = new
                                DocumentBlock(stream);<BR>&nbsp;&nbsp;&nbsp;&nbsp;blocks.add(block);<BR>&nbsp;&nbsp;&nbsp;&nbsp;size
                                +=
                                block.size();<BR>&nbsp;&nbsp;&nbsp;&nbsp;if
                                (block.partiallyRead())<BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;break;<BR>&nbsp;&nbsp;&nbsp;&nbsp;}<BR>}</CODE></TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="HeaderBlock"><B>HeaderBlock</B></A></TD>
                                <TD>The <B>HeaderBlock</B> class is
                                used to contain the data found in a
                                POIFS header.<BR>Its <A
                                HREF="#IntegerField">IntegerField</A>
                                members are used to read and write the
                                appropriate entries into the
                                <CODE><I>_data</I></CODE>
                                array.<BR>Its
                                <CODE><I>setBATBlocks</I></CODE>,
                                <CODE><I>setPropertyStart</I></CODE>,
                                and <CODE><I>setXBATStart</I></CODE>
                                methods are used to set the
                                appropriate fields in the
                                <CODE><I>_data</I></CODE>
                                array.<BR>The
                                <CODE><I>calculateXBATStorageRequirements</I></CODE>
                                method is used to determine how many
                                XBAT blocks are necessary to
                                accommodate the specified number of
                                BAT blocks.
                                </TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="PropertyBlock"><B>PropertyBlock</B></A></TD>
                                <TD>The <B>PropertyBlock</B> class is
                                used to contain <A
                                HREF="#Property">Property</A>
                                instances for the <A
                                HREF="#PropertyTable">PropertyTable</A>
                                class.<BR>It contains an array,
                                <CODE><I>_properties</I></CODE> of 4
                                Property instances, which together
                                comprise the 512 bytes of a <A
                                HREF="#BigBlock">BigBlock</A>.<BR>The
                                <CODE><I>createPropertyBlockArray</I></CODE>
                                method is used to convert a
                                <CODE>List</CODE> of Property
                                instances into an array of
                                PropertyBlock instances. The number of
                                Property instances is rounded up to a
                                multiple of 4 by creating empty
                                anonymous inner class extensions of
                                Property.</TD>
                            </TR>
                        </TABLE>
                    </LI>
                    <LI>
                        <A NAME="PropertyClasses"><B>Property Classes
                        and Interfaces</B></A>
                        <P>
                            The property classes and interfaces are
                            shown in the following class diagram.
                        </P>
                        <P>
                            <IMG SRC="PropertyTableClassDiagram.gif">
                        </P>
                        <TABLE BORDER="1">
                            <TR>
                                <TH><B>Class/Interface</B></TH>
                                <TH><B>Description</B></TH>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="Directory"><B>Directory</B></A></TD>
                                <TD>The <B>Directory</B> interface is
                                implemented by the <A
                                HREF="#RootProperty">RootProperty</A>
                                class. It is not strictly necessary
                                for the initial POIFS implementation,
                                but when the POIFS supports <A
                                HREF="POIFSFormat.html#directoryEntry">directory
                                elements</A>, this interface will be
                                more widely implemented, and so is
                                included in the design at this point
                                to ease the eventual support of
                                directory elements.<BR>Its methods are
                                a getter/setter pair,
                                <CODE><I>getChildren</I></CODE>,
                                returning an <CODE>Iterator</CODE> of
                                <A HREF="#Property">Property</A>
                                instances; and
                                <CODE><I>addChild</I></CODE>, which
                                will allow the caller to add another
                                Property instance to the Directory's
                                children.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="DocumentProperty"><B>DocumentProperty</B></A></TD>
                                <TD>The <B>DocumentProperty</B> class
                                is a trivial extension of <A
                                HREF="#Property">Property</A> and is
                                used by <A
                                HREF="#Document">Document</A> to keep
                                track of its associated entry in the
                                <A
                                HREF="#PropertyTable">PropertyTable</A>.<BR>Its
                                constructor takes a name and the
                                document size, on the assumption that
                                the Document will not create a
                                DocumentProperty until after it has
                                created the storage for the document
                                data and therefore knows how much data
                                there is.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="File"><B>File</B></A></TD>
                                <TD>The <B>File</B> interface
                                specifies the behavior of reading and
                                writing the next and previous child
                                fields of a <A
                                HREF="#Property">Property</A>.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="Property"><B>Property</B></A></TD>
                                <TD>The <B>Property</B> class is an
                                abstract class that defines the basic
                                data structure of an element of the <A
                                HREF="POIFSFormat.html#PropertyTable">Property
                                Table</A>.<BR>Its <A
                                HREF="#ByteField">ByteField</A>, <A
                                HREF="#ShortField">ShortField</A>, and
                                <A
                                HREF="#IntegerField">IntegerField</A>
                                members are used to read and write
                                data into the appropriate locations in
                                the <CODE><I>_raw_data</I></CODE>
                                array.<BR>The
                                <CODE><I>_index</I></CODE> member is
                                used to hold a Propery instance's
                                index in the <CODE>List</CODE> of
                                Property instances maintained by <A
                                HREF="#PropertyTable">PropertyTable</A>,
                                which is used to populate the child
                                property of parent <A
                                HREF="#Directory">Directory</A>
                                properties and the next property and
                                previous property of sibling <A
                                HREF="#File">File</A>
                                properties.<BR>The
                                <CODE><I>_name</I></CODE>,
                                <CODE><I>_next_file</I></CODE>, and
                                <CODE><I>_previous_file</I></CODE>
                                members are used to help fill the
                                appropriate fields of the _raw_data
                                array.<BR>Setters are provided for
                                some of the fields (name, property
                                type, node color, child property,
                                size, index, start block), as well as
                                a few getters (index, child
                                property).<BR>The
                                <CODE><I>preWrite</I></CODE> method is
                                abstract and is used by the owning
                                PropertyTable to iterate through its
                                Property instances and prepare each
                                for writing.<BR>The
                                <CODE><I>shouldUseSmallBlocks</I></CODE>
                                method returns true if the Property's
                                size is sufficiently small - how small
                                is none of the caller's business.
                                </TD>
                            </TR>
                            <TR>
                                <TD><B>PropertyBlock</B></TD>
                                <TD>See the description in <A
                                HREF="#PropertyBlock">PropertyBlock</A>.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="PropertyTable"><B>PropertyTable</B></A></TD>
                                <TD>The <B>PropertyTable</B> class
                                holds all of the <A
                                HREF="#DocumentProperty">DocumentProperty</A>
                                instances and the <A
                                HREF="#RootProperty">RootProperty</A>
                                instance for a <A
                                HREF="#Filesystem">Filesystem</A>
                                instance.<BR>It maintains a
                                <CODE>List</CODE> of its <A
                                HREF="#Property">Property</A>
                                instances
                                (<CODE><I>_properties</I></CODE>), and
                                when prepared to write its data by a
                                call to <CODE><I>preWrite</I></CODE>,
                                it gets and holds an array of <A
                                HREF="#PropertyBlock">PropertyBlock</A>
                                instances
                                (<CODE><I>_blocks</I></CODE>.<BR>It
                                also maintains its start block in its
                                <CODE><I>_start_block</I></CODE>
                                member.<BR>It has a method,
                                <CODE><I>getRoot</I></CODE>, to get
                                the RootProperty, returning it as an
                                implementation of <A
                                HREF="#Directory">Directory</A>, and a
                                method to add a Property,
                                <CODE><I>addProperty</I></CODE>, and a
                                method to get its start block,
                                <CODE><I>getStartBlock</I></CODE>.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="RootProperty"><B>RootProperty</B></A></TD>
                                <TD>The <B>RootProperty</B> class acts
                                as the <A
                                HREF="#Directory">Directory</A> for
                                all of the <A
                                HREF="#DocumentProperty">DocumentProperty</A>
                                instance. As such, it is more of a
                                pure <A
                                HREF="POIFSFormat.html#directoryEntry">directory
                                entry</A> than a proper <A
                                HREF="POIFSFormat.html#RootEntry">root
                                entry</A> in the <A
                                HREF="POIFSFormat.html#PropertyTable">Property
                                Table</A>, but the initial POIFS
                                implementation does not warrant the
                                additional complexity of a full-blown
                                root entry, and so it is not modeled
                                in this design.<BR>It maintains a
                                <CODE>List</CODE> of its children,
                                <CODE><I>_children</I></CODE>, in
                                order to perform its
                                directory-oriented duties.</TD>
                            </TR>
                        </TABLE>
                    </LI>
                    <LI>
                        <A NAME="FilesystemClasses"><B>Filesystem
                        Classes and Interfaces</B></A>
                        <P>
                            The property classes and interfaces are
                            shown in the following class diagram.
                        </P>
                        <P>
                            <IMG SRC="POIFSClassDiagram.gif">
                        </P>
                        <TABLE BORDER="1">
                            <TR>
                                <TH><B>Class/Interface</B></TH>
                                <TH><B>Description</B></TH>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="Filesystem"><B>Filesystem</B></A></TD>
                                <TD>The <B>Filesystem</B> class is the
                                top-level class that manages the
                                creation of a POIFS document.<BR>It
                                maintains a <A
                                HREF="#PropertyTable">PropertyTable</A>
                                instance in its
                                <CODE><I>_property_table</I></CODE>
                                member, a <A
                                HREF="#HeaderBlock">HeaderBlock</A>
                                instance in its
                                <CODE><I>_header_block</I></CODE>
                                member, and a <CODE>List</CODE> of its
                                <A HREF="#Document">Document</A>
                                instances in its
                                <CODE><I>_documents</I></CODE>
                                member.<BR>It provides methods for a
                                client to create a document
                                (<CODE><I>createDocument</I></CODE>),
                                and a method to write the Filesystem
                                to an <CODE>OutputStream</CODE>
                                (<CODE><I>writeFilesystem</I></CODE>).</TD>
                            </TR>
                            <TR>
                                <TD><B>BATBlock</B></TD>
                                <TD>See the description in <A
                                HREF="#BATBlock">BATBlock</A></TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="BATManaged"><B>BATManaged</B></A></TD>
                                <TD>The <B>BATManaged</B> interface
                                defines common behavior for objects
                                whose location in the written file is
                                managed by the <A
                                HREF="POIFSFormat.html#BAT">Block
                                Allocation Table</A>.<BR>It defines
                                methods to get a count of the
                                implementation's <A
                                HREF="#BigBlock">BigBlock</A>
                                instances
                                (<CODE><I>countBlocks</I></CODE>), and
                                to set an implementation's start block
                                (<CODE><I>setStartBlock</I></CODE>).</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="BlockAllocationTable"><B>BlockAllocationTable</B></A></TD>
                                <TD>The <B>BlockAllocationTable</B> is
                                an implementation of the POIFS <A
                                HREF="POIFSFormat.html#BAT">Block
                                Allocation Table</A>. It is only
                                created when the <A
                                HREF="#Filesystem">Filesystem</A> is
                                about to be written to an
                                <CODE>OutputStream</CODE>.<BR>It
                                contains an <A
                                HREF="#IntList">IntList</A> of block
                                numbers for all of the <A
                                HREF="#BATManaged">BATManaged</A>
                                implementations owned by the
                                Filesystem,
                                <CODE><I>_entries</I></CODE>, which is
                                filled by calls to
                                <CODE><I>allocateSpace</I></CODE>.<BR>It
                                fills its array,
                                <CODE><I>_blocks</I></CODE>, of <A
                                HREF="#BATBlock">BATBlock</A>
                                instances when its
                                <CODE><I>createBATBlocks</I></CODE>
                                method is called. This method has to
                                take into account its own storage
                                requirements, as well as those of the
                                XBAT blocks, and so calls
                                <CODE><I>BATBlock.calculateStorageRequirements</I></CODE>
                                and
                                <CODE><I>HeaderBlock.calculateXBATStorageRequirements</I></CODE>
                                repeatedly until the counts returned
                                by those methods stabilize.<BR>The
                                <CODE><I>countBlocks</I></CODE> method
                                returns the number of BATBlock
                                instances created by the preceding
                                call to createBlocks.</TD>
                            </TR>
                            <TR>
                                <TD><B>BlockWritable</B></TD>
                                <TD>See the description in <A
                                HREF="#BlockWritable">BlockWritable</A></TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="Document"><B>Document</B></A></TD>
                                <TD>The <B>Document</B> class is used
                                to contain a document, such as an HSSF
                                workbook.<BR>It has its own <A
                                HREF="#DocumentProperty">DocumentProperty</A>
                                (<CODE><I>_property</I></CODE>) and
                                stores its data in a collection of <A
                                HREF="#DocumentBlock">DocumentBlock</A>
                                instances
                                (<CODE><I>_blocks</I></CODE>).<BR>It
                                has a method,
                                <CODE><I>getDocumentProperty</I></CODE>,
                                to get its DocumentProperty.</TD>
                            </TR>
                            <TR>
                                <TD><B>DocumentBlock</B></TD>
                                <TD>See the description in <A
                                HREF="#DocumentBlock">DocumentBlock</A></TD>
                            </TR>
                            <TR>
                                <TD><B>DocumentProperty</B></TD>
                                <TD>See the description in <A
                                HREF="#DocumentProperty">DocumentProperty</A></TD>
                            </TR>
                            <TR>
                                <TD><B>HeaderBlock</B></TD>
                                <TD>See the description in <A
                                HREF="#HeaderBlock">HeaderBlock</A></TD>
                            </TR>
                            <TR>
                                <TD><B>PropertyTable</B></TD>
                                <TD>See the description in <A
                                HREF="#PropertyTable">PropertyTable</A></TD>
                            </TR>
                        </TABLE>
                    </LI>
                    <LI>
                        <A NAME="UtilityClasses"><B>Utility Classes
                        and Interfaces</B></A>
                        <P>
                            The utility classes and interfaces are
                            shown in the following class diagram.
                        </P>
                        <P>
                            <IMG SRC="utilClasses.gif">
                        </P>
                        <TABLE BORDER="1">
                            <TR>
                                <TH><B>Class/Interface</B></TH>
                                <TH><B>Description</B></TH>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="BitField"><B>BitField</B></A></TD>
                                <TD>The <B>BitField</B> class is used
                                primarily by HSSF code to manage
                                bit-mapped fields of HSSF records. It
                                is not likely to be used in the POIFS
                                code itself and is only included here
                                for the sake of complete documentation
                                of the POI utility classes.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="ByteField"><B>ByteField</B></A></TD>
                                <TD>The <B>ByteField</B> class is an
                                implementation of <A
                                HREF="#FixedField">FixedField</A> for
                                the purpose of managing reading and
                                writing to a byte-wide field in an
                                array of <CODE>bytes</CODE>.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="FixedField"><B>FixedField</B></A></TD>
                                <TD>The <B>FixedField</B> interface
                                defines a set of methods for reading a
                                field from an array of
                                <CODE>bytes</CODE> or from an
                                <CODE>InputStream</CODE>, and for
                                writing a field to an array of
                                <CODE>bytes</CODE>. Implementations
                                typically require an offset in their
                                constructors that, for the purposes of
                                reading and writing to an array of
                                <CODE>bytes</CODE>, makes sure that
                                the correct <CODE>bytes</CODE> in the
                                array are read or written.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="HexDump"><B>HexDump</B></A></TD>
                                <TD>The <B>HexDump</B> class is a
                                debugging class that can be used to
                                dump an array of <CODE>bytes</CODE> to
                                an <CODE>OutputStream</CODE>. The
                                static method <CODE><I>dump</I></CODE>
                                takes an array of <CODE>bytes</CODE>,
                                a <CODE>long</CODE> offset that is
                                used to label the output, an open
                                <CODE>OutputStream</CODE>, and an
                                <CODE>int</CODE> index that specifies
                                the starting index within the array of
                                <CODE>bytes</CODE>.<BR>The data is
                                displayed 16 bytes per line, with each
                                byte displayed in hexadecimal format
                                and again in printable form, if
                                possible (a byte is considered
                                printable if its value is in the range
                                of 32 ... 126).<BR>Here is an example
                                of a small array of <CODE>bytes</CODE>
                                with an offset of
                                0x110:<BR><CODE>00000110&nbsp;C8&nbsp;00&nbsp;00&nbsp;00&nbsp;FF&nbsp;7F&nbsp;90&nbsp;01&nbsp;00&nbsp;00&nbsp;00&nbsp;00&nbsp;00&nbsp;00&nbsp;05&nbsp;01&nbsp;................<BR>00000120&nbsp;41&nbsp;00&nbsp;72&nbsp;00&nbsp;69&nbsp;00&nbsp;61&nbsp;00&nbsp;6C&nbsp;00&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A.r.i.a.l.</CODE></TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="IntegerField"><B>IntegerField</B></A></TD>
                                <TD>The <B>IntegerField</B> class is
                                an implementation of <A
                                HREF="#FixedField">FixedField</A> for
                                the purpose of managing reading and
                                writing to an integer-wide field in an
                                array of <CODE>bytes</CODE>.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="IntList"><B>IntList</B></A></TD>
                                <TD>The <B>IntList</B> class is a
                                work-around for functionality missing
                                in Java (see <A
                                HREF="http://developer.java.sun.com/developer/bugParade/bugs/4487555.html">http://developer.java.sun.com/developer/bugParade/bugs/4487555.html</A>
                                for details); it is a simple growable
                                array of <CODE>ints</CODE> that gets
                                around the requirement of wrapping and
                                unwrapping <CODE>ints</CODE> in
                                <CODE>Integer</CODE> instances in
                                order to use the
                                <CODE>java.util.List</CODE>
                                interface.<BR><B>IntList</B> mimics
                                the functionality of the
                                <CODE>java.util.List</CODE> interface
                                as much as possible.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="LittleEndian"><B>LittleEndian</B></A></TD>
                                <TD>The <B>LittleEndian</B> class
                                provides a set of static methods for
                                reading and writing
                                <CODE>shorts</CODE>,
                                <CODE>ints</CODE>, <CODE>longs</CODE>,
                                and <CODE>doubles</CODE> in and out of
                                <CODE>byte</CODE> arrays, and out of
                                <CODE>InputStreams</CODE>, preserving
                                the Intel byte ordering and encoding
                                of these values.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="LittleEndianConsts"><B>LittleEndianConsts</B></A></TD>
                                <TD>The <B>LittleEndianConsts</B>
                                interface defines the width of a
                                <CODE>short</CODE>, <CODE>int</CODE>,
                                <CODE>long</CODE>, and
                                <CODE>double</CODE> as stored by Intel
                                processors.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="LongField"><B>LongField</B></A></TD>
                                <TD>The <B>LongField</B> class is an
                                implementation of <A
                                HREF="#FixedField">FixedField</A> for
                                the purpose of managing reading and
                                writing to a long-wide field in an
                                array of <CODE>bytes</CODE>.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="ShortField"><B>ShortField</B></A></TD>
                                <TD>The <B>ShortField</B> class is an
                                implementation of <A
                                HREF="#FixedField">FixedField</A> for
                                the purpose of managing reading and
                                writing to a short-wide field in an
                                array of <CODE>bytes</CODE>.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="ShortList"><B>ShortList</B></A></TD>
                                <TD>The <B>ShortList</B> class is a
                                work-around for functionality missing
                                in Java (see <A
                                HREF="http://developer.java.sun.com/developer/bugParade/bugs/4487555.html">http://developer.java.sun.com/developer/bugParade/bugs/4487555.html</A>
                                for details); it is a simple growable
                                array of <CODE>shorts</CODE> that gets
                                around the requirement of wrapping and
                                unwrapping <CODE>shorts</CODE> in
                                <CODE>Short</CODE> instances in order
                                to use the <CODE>java.util.List</CODE>
                                interface.<BR> <B>ShortList</B> mimics
                                the functionality of the
                                <CODE>java.util.List</CODE> interface
                                as much as possible.</TD>
                            </TR>
                            <TR>
                                <TD><A
                                NAME="StringUtil"><B>StringUtil</B></A></TD>
                                <TD>The <B>StringUtil</B> class
                                manages the processing of Unicode
                                strings.</TD>
                            </TR>
                        </TABLE>
                    </LI>
                </OL>
                <A NAME="Scenarios"><FONT
                SIZE="+1"><B>Scenarios</B></FONT></A>
                <P>
                    This section describes the scenarios of how the
                    POIFS classes and interfaces will be used to
                    convert an appropriate XML stream to a POIFS
                    output stream containing an HSSF document.
                </P>
		<P>
		    It is broken down as suggested by the following
		    scenario diagram:
		</P>
		<P>
		    <IMG SRC="POIFSLifeCycle.gif">
		</P>
		<TABLE BORDER="1">
		    <TR>
		        <TH><B>Step</B></TH>
			<TH><B>Description</B></TH>
		    </TR>
		    <TR>
		        <TD><B>1</B></TD>
			<TD><A HREF="Initialization">The Filesystem is
			created by the client application.</A></TD>
		    </TR>
		    <TR>
		        <TD><B>2</B></TD>
			<TD><A HREF="CreateDocument">The client
			application tells the Filesystem to create a
			document</A>, providing an
			<CODE>InputStream</CODE> and the name of the
			document. This may be repeated several
			times.</TD>
		    </TR>
		    <TR>
		        <TD><B>3</B></TD>
			<TD><A HREF="Initialization">The client
			application asks the Filesystem to write its
			data to an <CODE>OutputStream</CODE>.</A></TD>
		    </TR>
		</TABLE>
                <OL>
                    <LI>
                        <P>
                            <A
                            NAME="Initialization">Initialization</A>
                        </P>
                        <P>
                            Initialization of the POIFS system is
                            shown in the following scenario diagram:
                        </P>
                        <P>
                            <IMG SRC="POIFSInitialization.gif">
                        </P>
                        <TABLE BORDER="1">
                            <TR>
                                <TH><B>Step</B></TH>
                                <TH><B>Description</B></TH>
                            </TR>
                            <TR>
                                <TD><B>1</B></TD>
                                <TD>The <A
                                HREF="#Filesystem">Filesystem</A>
                                object, which is created for each
                                request to convert an appropriate XML
                                stream to a POIFS output stream
                                containing an HSSF document, creates
                                its <A
                                HREF="#PropertyTable">PropertyTable</A>.</TD>
                            </TR>
                            <TR>
                                <TD><B>2</B></TD>
                                <TD>The <A
                                HREF="#PropertyTable">PropertyTable</A>
                                creates its <A
                                HREF="#RootProperty">RootProperty</A>
                                instance, making the RootProperty the
                                first <A HREF="#Property">Property</A>
                                in its <CODE>List</CODE> of Property
                                instances.</TD>
                            </TR>
                            <TR>
                                <TD><B>3</B></TD>
                                <TD>The <A
                                HREF="#Filesystem">Filesystem</A>
                                creates its <A
                                HREF="#HeaderBlock">HeaderBlock</A>
                                instance. It should be noted that the
                                decision to create the HeaderBlock at
                                Filesystem initialization is
                                arbitrary; creation of the HeaderBlock
                                could easily and harmlessly be
                                postponed to the appropriate moment in
                                <A HREF="#WriteFilesystem">writing the
                                filesystem</A>.</TD>
                            </TR>
                        </TABLE>
                    </LI>
                    <LI>
                        <P>
                            <A NAME="CreateDocument">Creating a
                            Document</A>
                        </P>
                        <P>
                            Creating and adding a document to a POIFS
                            system is shown in the following scenario
                            diagram:
                        </P>
                        <P>
                            <IMG SRC="POIFSAddDocument.gif">
                        </P>
                        <TABLE BORDER="1">
                            <TR>
                                <TH><B>Step</B></TH>
                                <TH><B>Description</B></TH>
                            </TR>
                            <TR>
                                <TD><B>1</B></TD>
                                <TD>The <A
                                HREF="#Filesystem">Filesystem</A>
                                instance creates a new <A
                                HREF="#Document">Document</A>
                                instance. It will store the newly
                                created Document in a
                                <CODE>List</CODE> of <A
                                HREF="#BATManaged">BATManaged</A>
                                instances.</TD>
                            </TR>
                            <TR>
                                <TD><B>2</B></TD>
                                <TD>The <A
                                HREF="#Document">Document</A> reads
                                data from the provided
                                <CODE>InputStream</CODE>, storing the
                                data in <A
                                HREF="#DocumentBlock">DocumentBlock</A>
                                instances. It keeps track of the byte
                                count as it reads the data.</TD>
                            </TR>
                            <TR>
                                <TD><B>3</B></TD>
                                <TD>The <A
                                HREF="#Document">Document</A> creates
                                a <A
                                HREF="#DocumentProperty">DocumentProperty</A>
                                to keep track of its property
                                data. The byte count is stored in the
                                newly created DocumentProperty
                                instance.</TD>
                            </TR>
                            <TR>
                                <TD><B>4</B></TD>
                                <TD>The <A
                                HREF="#Filesystem">Filesystem</A>
                                requests the newly created <A
                                HREF="#DocumentProperty">DocumentProperty</A>
                                from the newly created <A
                                HREF="#Document">Document</A>
                                instance.</TD>
                            </TR>
                            <TR>
                                <TD><B>5</B></TD>
                                <TD>The <A
                                HREF="#Filesystem">Filesystem</A>
                                sends the newly created <A
                                HREF="#DocumentProperty">DocumentProperty</A>
                                to the Filesystem's <A
                                HREF="#PropertyTable">PropertyTable</A>
                                so that the PropertyTable can add the
                                DocumentProperty to its
                                <CODE>List</CODE> of <A
                                HREF="#Property">Property</A>
                                instances.</TD>
                            </TR>
                            <TR>
                                <TD><B>6</B></TD>
                                <TD>The <A
                                HREF="#Filesystem">Filesystem</A> gets
                                the <A
                                HREF="#RootProperty">RootProperty</A>
                                from its <A
                                HREF="#PropertyTable">PropertyTable</A>.</TD>
                            </TR>
                            <TR>
                                <TD><B>7</B></TD>
                                <TD>The <A
                                HREF="#Filesystem">Filesystem</A> adds
                                the newly created <A
                                HREF="#DocumentProperty">DocumentProperty</A>
                                to the <A
                                HREF="#RootProperty">RootProperty</A>.</TD>
                            </TR>
                        </TABLE>
                        <P>
                            Although typical deployment of the POIFS
                            system will only entail adding a single <A
                            HREF="#Document">Document</A> (the
                            workbook) to the <A
                            HREF="#Filesystem">Filesystem</A>, there
                            is nothing in the design to prevent
                            multiple Documents from being added to the
                            Filesystem. This flexibility can be
                            employed to write summary information
                            document(s) in addition to the workbook.
                        </P>
                    </LI>
                    <LI>
                        <P>
                            <A NAME="WriteFilesystem">Writing the
                            Filesystem</A>
                        </P>
                        <P>
                            Writing the filesystem is shown in the
                            following scenario diagram:
                        </P>
                        <P>
                            <IMG SRC="POIFSWriteFilesystem.gif">
                        </P>
                        <TABLE BORDER="1">
                            <TR>
                                <TH><B>Step</B></TH>
                                <TH COLSPAN="2"><B>Description</B></TH>
                            </TR>
                            <TR>
                                <TD><B>1</B></TD>
                                <TD COLSPAN="2">The <A
                                HREF="#Filesystem">Filesystem</A> adds
                                the <A
                                HREF="#PropertyTable">PropertyTable</A>
                                to its <CODE>List</CODE> of <A
                                HREF="#BATManaged">BATManaged</A>
                                instances and calls the
                                PropertyTable's
                                <CODE><I>preWrite</I></CODE>
                                method. The action taken by the
                                PropertyTable is shown in the <A
                                HREF="#PropertyTablePreWrite">PropertyTable
                                preWrite scenario diagram</A>.</TD>
                            </TR>
                            <TR>
                                <TD><B>2</B></TD>
                                <TD COLSPAN="2">The <A
                                HREF="#Filesystem">Filesystem</A>
                                creates the <A
                                HREF="#BlockAllocationTable">BlockAllocationTable</A>.</TD>
                            </TR>
                            <TR>
                                <TD><B>3</B></TD>
                                <TD>The <A
                                HREF="#Filesystem">Filesystem</A> gets
                                the block count from the <A
                                HREF="#BATManaged">BATManaged</A>
                                instance.</TD> <TD
                                ROWSPAN="3"><B>These three steps are
                                repeated for each <A
                                HREF="#BATManaged">BATManaged</A>
                                instance in the <A
                                HREF="#Filesystem">Filesystem</A>'s
                                <CODE>List</CODE> of BATManaged
                                instances (i.e., the <A
                                HREF="#Document">Documents</A>, in
                                order of their addition to the
                                Filesystem, followed by the <A
                                HREF="#PropertyTable">PropertyTable</A>).</B></TD>
                            </TR>
                            <TR>
                                <TD><B>4</B></TD>
                                <TD>The <A
                                HREF="#Filesystem">Filesystem</A>
                                sends the block count to the <A
                                HREF="#BlockAllocationTable">BlockAllocationTable</A>,
                                which adds the appropriate entries to
                                is <A HREF="#IntList">IntList</A> of
                                entries, returning the starting block
                                for the newly added entries.</TD>
                            </TR>
                            <TR>
                                <TD><B>5</B></TD>
                                <TD>The <A
                                HREF="#Filesystem">Filesystem</A>
                                gives the start block number to the <A
                                HREF="#BATManaged">BATManaged</A>
                                instance. If the BATManaged instance
                                is a <A HREF="#Document">Document</A>,
                                it sets the start block field in its
                                <A
                                HREF="#DocumentProperty">DocumentProperty</A>.</TD>
                            </TR>
                            <TR>
                                <TD><B>6</B></TD>
                                <TD COLSPAN="2">The <A
                                HREF="#Filesystem">Filesystem</A>
                                tells the <A
                                HREF="#BlockAllocationTable">BlockAllocationTable</A>
                                to create its <A
                                HREF="#BATBlock">BatBlocks</A>.</TD>
                            </TR>
                            <TR>
                                <TD><B>7</B></TD>
                                <TD COLSPAN="2">The <A
                                HREF="#Filesystem">Filesystem</A>
                                gives the BAT information to the <A
                                HREF="#HeaderBlock">HeaderBlock</A> so
                                that it can set its BAT fields and, if
                                necessary, create XBAT blocks.</TD>
                            </TR>
                            <TR>
                                <TD><B>8</B></TD>
                                <TD COLSPAN="2">If the filesystem is
                                unusually large (over <B>7MB</B>), the
                                <A HREF="#HeaderBlock">HeaderBlock</A>
                                will create XBAT blocks to contain the
                                BAT data that it cannot hold
                                directly. In this case, the <A
                                HREF="#Filesystem">Filesystem</A>
                                tells the HeaderBlock where those
                                additional blocks will be stored.</TD>
                            </TR>
                            <TR>
                                <TD><B>9</B></TD>
                                <TD COLSPAN="2">The <A
                                HREF="#Filesystem">Filesystem</A>
                                gives the <A
                                HREF="#PropertyTable">PropertyTable</A>
                                start block to the <A
                                HREF="#HeaderBlock">HeaderBlock</A>.</TD>
                            </TR>
                            <TR>
                                <TD><B>10</B></TD>
                                <TD COLSPAN="2">The <A
                                HREF="#Filesystem">Filesystem</A>
                                tells the <A
                                HREF="#BlockWritable">BlockWritable</A>
                                instance to write its blocks to the
                                provided
                                <CODE>OutputStream</CODE>.<BR>This
                                step is repeated for each
                                BlockWritable instance, in this
                                order:<BR>
                                <OL>
                                    <LI>
                                        The <A
                                        HREF="#HeaderBlock">HeaderBlock</A>.
                                    </LI>
                                    <LI>
                                        Each <A
                                        HREF="#Document">Document</A>,
                                        in the order in which it was
                                        added to the <A
                                        HREF="#Filesystem">Filesystem</A>.
                                    </LI>
                                    <LI>
                                        The <A
                                        HREF="#PropertyTable">PropertyTable</A>.
                                    </LI>
                                    <LI>
                                        The <A
                                        HREF="#BlockAllocationTable">BlockAllocationTable</A>
                                    </LI>
                                    <LI>
                                        The XBAT blocks created by the
                                        <A
                                        HREF="#HeaderBlock">HeaderBlock</A>,
                                        if any.
                                    </LI>
                                </OL></TD>
                            </TR>
                        </TABLE>
                        <P>
                            <A
                            NAME="PropertyTablePreWrite"><B>PropertyTable
                            preWrite scenario diagram</B></A>
                        </P>
                        <P>
                            <IMG SRC="POIFSPropertyTablePreWrite.gif">
                        </P>
                        <TABLE BORDER="1">
                            <TR>
                                <TH><B>Step</B></TH>
                                <TH><B>Description</B></TH>
                            </TR>
                            <TR>
                                <TD><B>1</B></TD>
                                <TD>The <A
                                HREF="#PropertyTable">PropertyTable</A>
                                calls <CODE><I>setIndex</I></CODE> for
                                each of its <A
                                HREF="#Property">Property</A>
                                instances, so that each Property now
                                knows its index within the
                                PropertyTable's <CODE>List</CODE> of
                                Property instances.</TD>
                            </TR>
                            <TR>
                                <TD><B>2</B></TD> <TD>The <A
                                HREF="#PropertyTable">PropertyTable</A>
                                requests the <A
                                HREF="#PropertyBlock">PropertyBlock</A>
                                class to create an array of <A
                                HREF="#PropertyBlock">PropertyBlock</A>
                                instances.</TD>
                            </TR>
                            <TR>
                                <TD><B>3</B></TD>

                                <TD>The <A
                                HREF="#PropertyBlock">PropertyBlock</A>
                                calculates the number of empty <A
                                HREF="#Property">Property</A>
                                instances it needs to create and
                                creates them. The algorithm for the
                                number to create is:<BR>
                                <CODE>block_count = (properties.size()
                                + 3) / 4;<BR> emptyPropertiesNeeded =
                                (block_count * 4) -
                                properties.size();</CODE></TD>
                            </TR>
                            <TR>
                                <TD><B>4</B></TD> <TD>The <A
                                HREF="#PropertyBlock">PropertyBlock</A>
                                creates the required number of <A
                                HREF="#PropertyBlock">PropertyBlock</A>
                                instances from the <CODE>List</CODE>
                                of <A HREF="#Property">Property</A>
                                instances, including the newly created
                                empty <A HREF="#Property">Property</A>
                                instances.</TD>
                            </TR>
                            <TR>
                                <TD><B>5</B></TD>
                                <TD>The <A
                                HREF="#PropertyTable">PropertyTable</A>
                                calls <CODE><I>preWrite</I></CODE> on
                                each of its <A
                                HREF="#Property">Property</A>
                                instances. For <A
                                HREF="#DocumentProperty">DocumentProperty</A>
                                instances, this call is a no-op. For
                                the <A
                                HREF="#RootProperty">RootProperty</A>,
                                the action taken is shown in the <A
                                HREF="#RootPropertyPreWrite">RootProperty
                                preWrite scenario diagram</A>.</TD>
                            </TR>
                        </TABLE>
                        <P>
                            <A
                            NAME="RootPropertyPreWrite"><B>RootProperty
                            preWrite scenario diagram</B></A>
                        </P>
                        <P>
                            <IMG SRC="POIFSRootPropertyPreWrite.gif">
                        </P>
                        <TABLE BORDER="1">
                            <TR>
                                <TH><B>Step</B></TH>
                                <TH COLSPAN="2"><B>Description</B></TH>
                            </TR>
                            <TR>
                                <TD><B>1</B></TD>
                                <TD COLSPAN="2">The <A
                                HREF="#RootProperty">RootProperty</A>
                                sets its child property with the index
                                of the child <A
                                HREF="#Property">Property</A> that is
                                first in its <CODE>List</CODE> of
                                children.</TD>
                            </TR>
                            <TR>
                                <TD><B>2</B></TD>
                                <TD>The <A
                                HREF="#RootProperty">RootProperty</A>
                                sets its child's next property field
                                with the index of the child's next
                                sibling in the RootProperty's
                                <CODE>List</CODE> of children. If the
                                child is the last in the
                                <CODE>List</CODE>, its next property
                                field is set to <CODE>-1</CODE>.</TD>
                                <TD ROWSPAN="2"><B>These two steps are
                                repeated for each <A
                                HREF="#File">File</A> in the <A
                                HREF="#RootProperty">RootProperty</A>'s
                                <CODE>List</CODE> of
                                children.</B></TD>
                            </TR>
                            <TR>
                                <TD><B>3</B></TD>
                                <TD>The <A
                                HREF="#RootProperty">RootProperty</A>
                                sets its child's previous property
                                field with a value of
                                <CODE>-1</CODE>.</TD>
                            </TR>
                        </TABLE>
                    </LI>
                </OL>
            </LI>
        </OL>
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