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+<chapter id="ltw" xreflabel="Load-Time Weaving">
+    <title>Load-Time Weaving</title>
+    
+    <sect1 id="ltw-introduction">
+        <title>Introduction</title>
+        
+        <para> The AspectJ 5 weaver takes class files as input and produces class files as output.
+            The weaving process itself can take place at one of three different times: compile-time,
+            post-compile time, and load-time. The class files produced by the weaving process (and
+            hence the run-time behaviour of an application) are the same regardless of the approach
+            chosen. </para>
+            
+        <itemizedlist>
+            <listitem> Compile-time weaving is the simplest approach. When you have the source code
+                for an application, ajc will compile from source and produce woven class files as
+                output. The invocation of the weaver is integral to the ajc compilation process. The
+                aspects themselves may be in source or binary form. </listitem>
+            <listitem> Post-compile weaving (also sometimes called binary weaving) is used to weave
+                existing class files and JAR files. As with compile-time weaving,
+                the aspects used for weaving may be in source or binary form. </listitem>
+            <listitem> Load-time weaving (LTW) is simply binary weaving defered until the point that
+                a class loader loads a class file and defines the class to the JVM. To support this,
+                one or more "weaving class loaders", either provided explicitly by the run-time
+                environment or enabled through a "weaving agent" are required. </listitem>
+        </itemizedlist>
+        
+        <para> You may also hear the term "run-time weaving". We define this as the weaving of
+            classes that have already been defined to the JVM (without reloading those
+            classes). AspectJ 5 does not provide explicit support for run-time weaving although
+            simple coding patterns can support dynamically enabling and disabling advice in aspects. </para>
+        
+        <sect2>
+            <title>Weaving class files more than once</title>
+        
+            <para> By default a class file that has been woven by the AspectJ compiler cannot
+                subsequently be rewoven (passed as input to the weaver). If you are developing
+                AspectJ applications that are to be used in a load-time weaving environment, you
+                need to specify the <literal>-Xreweavable</literal> compiler option when building
+                them. This causes AspectJ to save additional state in the class files that is used
+                to support subsequent reweaving. </para>
+        </sect2>
+    </sect1>
+    
+    <sect1 id="ltw-rules">
+        <title>Load-time Weaving Requirements</title>
+    
+        <para> All load-time weaving is done in the context of a class loader, and hence the set of
+            aspects used for weaving and the types that can be woven are affected by the class
+            loader delegation model. This ensures that LTW complies with the Java 2 security model.
+            The following rules govern the interaction of load-time weaving with class loading: </para>
+    
+        <orderedlist>
+            <listitem> All aspects to be used for weaving must be defined to the weaver before any
+                types to be woven are loaded.</listitem>
+            <listitem> All abstract and concrete aspects visible to the weaver
+                are available for extending (abstract aspects) and using for weaving. 
+                A visible aspect is one defined by the
+                weaving class loader or one of its parent class loaders.</listitem>
+            <listitem>A class loader may only weave classes that it defines. It may not weave
+                classes loaded by a delegate or parent class loader.</listitem>    
+        </orderedlist>
+        
+    </sect1>
+    
+    <sect1 id="ltw-configuration">
+        <title>Configuration</title>
+        <para>AspectJ 5 supports a number of mechanisms designed to make load-time weaving as
+        easy to use as possibe. The load-time weaving mechanism is chosen through JVM startup options. 
+        Configuration files determine the set of aspects to be used for weaving and which 
+        types will be woven. Additional diagnostic options allow the user to debug the configuration and 
+        weaving process. </para>
+        
+        <sect2>
+            <title>Enabling Load-time Weaving</title>
+            <para> AspectJ 5 supports three different ways of enabling load-time weaving for
+            an application: agents, a command-line launch script, and a set of interfaces for
+            integration of AspectJ load-time weaving in custom environments. </para>
+            <variablelist>
+                <varlistentry>
+                    <term>Agents</term>
+                    <listitem>
+                        <para>AspectJ 5 ships with a number of load-time weaving agents that
+                            enable load-time weaving. These agents and their configuration
+                            are execution environment dependent.
+                            Using Java 5 for example, you can specify the "-javaagent" option
+                            to the JVM. Configuration for the supported environments is discussed
+                            later in this chapter. AspectJ 5
+                            has several agents including those that use JVMTI, and the 
+                            JRockit MAPI. </para>
+                    </listitem>
+                </varlistentry>
+                <varlistentry>
+                    <term>Command line</term>
+                    <listitem>
+                        <para> AspectJ includes a script "aj" that allows programs executed at
+                            the command line to take advantage of load-time weaving. 
+                            The script is customized when AspectJ is installed depending on the chosen 
+                            JDK. For example, for JDK 1.4 the script uses the
+                            <literal>-Djava.system.class.loader</literal> system property to replace
+                            the system class loader with a weaving class loader allowing classes 
+                            loaded from the CLASSPATH to be woven. 
+                            For JDK 1.5 the JVMTI weaving agent is used allowing classes loaded by all
+                            class loaders to be woven. Versions of the JDK prior to 1.3 are not
+                            supported by the "aj" mechanism. </para>
+                    </listitem>
+                </varlistentry>
+                <varlistentry>
+                    <term>Custom Integration</term>
+                    <listitem>
+                        <para> A public interface is provided to allow a user written class loader
+                            to instantiate a weaver and weave classes after loading and before
+                            defining them in the JVM. This enables load-time weaving to be supported in
+                            environments where no weaving agent is available. It also allows the
+                            user to explicity restrict by class loader which classes can be woven. </para>
+                    </listitem>
+                </varlistentry>
+            </variablelist>
+        </sect2>
+        
+        <sect2>
+            <title>Configuring Load-time Weaving with aop.xml files</title>
+            
+            <para>The weaver is configured using one or more <literal>META-INF/aop.xml</literal>
+                files located on the class loader search path. Each file may define a list of
+                concrete aspects to be used for weaving, type patterns describing which types
+                should woven, and a set of options to be passed to the weaver. In addition AspectJ 5
+                supports the definition of concrete aspects in XML. Aspects defined in this way
+                must extend an abstract aspect visible to the weaver. The abstract aspect
+                may define abstract pointcuts (but not abstract
+                methods). The following example shows a simple aop.xml file: </para>
+            <programlisting><![CDATA[
+          <aspectj>
+          
+            <aspects>
+              <!-- declare two existing aspects to the weaver -->
+              <aspect name="com.MyAspect"/>
+              <aspect name="com.MyAspect.Inner"/>
+
+              <!-- define a concrete aspect inline -->
+              <concrete-aspect name="com.xyz.tracing.MyTracing" extends="tracing.AbstractTracing">
+                <pointcut name="tracingScope" expression="within(org.maw.*)"/>
+              </concrete-aspect>
+		  	  
+              <!-- Of the set of aspects known to the weaver, use aspects matching
+                   the type pattern "com..*" for weaving. -->
+              <include within="com..*"/>
+			  
+              <!-- Do not use any aspects with the @CoolAspect annotation for weaving -->
+              <exclude within="@CoolAspect *"/>    	
+			  
+            </aspects>
+
+            <weaver options="-verbose -XlazyTjp">
+              <!-- Weave types that are within the javax.* or org.aspectj.*
+                   packages. Also weave all types in the foo package that do
+                   not have the @NoWeave annotation. -->
+              <include within="javax.*"/>
+              <include within="org.aspectj.*"/>
+              <include within="(!@NoWeave foo.*) AND foo.*"/>
+            </weaver>
+          
+          </aspectj>
+          
+		  ]]></programlisting>
+          
+            <para>
+                An aop.xml file contains two key sections: "aspects" defines one
+                or more aspects to the weaver and controls which aspects are to be
+                used in the weaving process; "weaver" defines weaver options and which
+                types should be woven.
+            </para>
+            
+            <para>
+                The simplest way to define an aspect to the weaver is to 
+                specify the fully-qualified name of the aspect type in an aspect element. 
+                You can also
+                declare (and define to the weaver) aspects inline in the aop.xml file.
+                This is done using the "concrete-aspect" element. A concrete-aspect
+                declaration must provide a pointcut definition for every abstract 
+                pointcut in the abstract aspect it extends. This mechanism is a 
+                useful way of externalizing configuration for infrastructure and
+                auxiliary aspects where the pointcut definitions themselves can be
+                considered part of the configuration of the service.
+            </para>
+            
+            <para>
+                The aspects element may optionally contain one or more include and
+                exclude elements (by default, all defined aspects are used for weaving).
+                Specifying include or exclude elements restricts the set of defined
+                aspects to be used for weaving to those that are matched by an include
+                pattern, but not by an exclude pattern. The 'within' attribute accepts
+                a type pattern of the same form as a within pcd, except that &amp;&amp;
+                and || are replaced by 'AND' and 'OR'.
+            </para>
+            
+            <para>
+                The weaver element is used to pass options to the weaver and to specify
+                the set of types that should be woven. If no include elements are specified
+                then all types seen by the weaver will be woven.
+            </para>
+          
+          
+            <para> When several configuration files are visible from a given weaving class loader
+                their contents are conceptually merged (this applies to both aop.xml files
+                and to aop.properties files as described in the next section). 
+                The files are merged in the order they are
+                found on the search path (regular <literal>getResourceAsStream</literal> lookup)
+                according to the following rules: </para>
+            <itemizedlist>
+                <listitem> The set of available aspects is the set of all
+                    declared and defined aspects (<literal>aspect</literal> and
+                    <literal>concrete-aspect</literal> elements of the <literal>aspects</literal>
+                    section).</listitem>
+                <listitem>The set of aspects used for weaving is the subset of the available
+                    aspects that are matched by at least one include statement and are not matched
+                    by any exclude statements. If there are no include statements then all non-excluded
+                    aspects are included.</listitem>
+                <listitem> The set of types to be woven are those types matched by at
+                    least one weaver <literal>include</literal> element and not matched by any 
+                    weaver <literal>exclude</literal> element. If there are no weaver include
+                    statements then all non-excluded types are included.</listitem>
+                <listitem> The weaver options are derived by taking the union of the
+                    options specified in each of the weaver options attribute specifications. Where an
+                    option takes a value e.g. <literal>-warn:none</literal> the most recently defined value
+                    will be used.</listitem>
+            </itemizedlist>
+
+            <para>It is not an error for the same aspect to be defined to the weaver in 
+                more than one visible <literal>META-INF/aop.xml</literal> file. 
+                However, if a declarative concrete aspect
+                is declared in more than aop.xml file then an error will be issued.
+                 A concrete aspect 
+                defined in this way will be used to weave types loaded by the
+                class loader that loaded the aop.xml file in which it was defined. 
+                </para>
+            
+            <para> A <literal>META-INF/aop.xml</literal> file will automatically be generated when
+                using the <literal>-outjar</literal> option of the AspectJ compiler. 
+                It will simply contain a (possibly empty) set of aspect elements, one for
+                each concrete aspect included in the JAR. </para>
+        </sect2>
+        
+        <sect2>
+            <title>Configuring Load-time Weaving with Properties Files</title>
+            <para> For memory constrained environments or those without support for XML a simple
+                Java Properties file can be used to configure LTW. Just like XML files,
+                <literal>META-INF/aop.properties</literal> files are loaded from the class loader
+                search path. Everything that can be configured through XML can be configured using a
+                Properties file, with the exception of declarative concrete aspects. For example: </para>
+            <programlisting><![CDATA[
+          aspects.names=com.MyAspect,com.MyAspect.Inner
+          aspects.include=com..*
+          aspects.exclude=@CoolAspect
+
+          weaver.options=-verbose -XlazyTjp
+          weaver.include=javax.* OR org.aspectj.*
+		  ]]></programlisting>
+        </sect2>
+        
+        <sect2>
+            <title>Weaver Options</title>
+            <para> The table below lists the AspectJ options supported by LTW. All other options
+                will be ignored and a warning issued. </para>
+            <informaltable>
+                <tgroup cols="2">
+                    <thead>
+                        <row>
+                            <entry>Option</entry>
+                            <entry>Purpose</entry>
+                        </row>
+                    </thead>
+                    <tbody>
+                        <row>
+                            <entry>
+                                <literal>-1.5</literal>
+                            </entry>
+                            <entry>Run the weaver in 1.5 mode (supports autoboxing in
+                            join point matching)</entry>
+                        </row>
+                        <row>
+                            <entry>
+                                <literal>-XlazyTjp</literal>
+                            </entry>
+                            <entry>Performance optimization for aspects making use
+                            of thisJoinPoint (non-static parts)</entry>
+                        </row>
+                        <row>
+                            <entry>
+                                <literal>-nowarn, -warn:none</literal>
+                            </entry>
+                            <entry>Suppress warning messages</entry>
+                        </row>
+                        <row>
+                            <entry>
+                                <literal>-proceedOnError</literal>
+                            </entry>
+                            <entry>Continue weaving even if errors occur (for example,
+                             "... already woven" errors)</entry>
+                        </row>
+                        <row>
+                            <entry>
+                                <literal>-verbose</literal>
+                            </entry>
+                            <entry>Issue informational messages about the weaving process</entry>
+                        </row>
+                        <row>
+                            <entry>
+                                <literal>-Xreweavable</literal>
+                            </entry>
+                            <entry>Produce class files that can subsequently be rewoven</entry>
+                        </row>
+                        <row>
+                            <entry>
+                                <literal>-Xnoinline</literal>
+                            </entry>
+                            <entry>Don't inline around advice.</entry>
+                        </row>
+                        <row>
+                            <entry>
+                                <literal>-showWeaveInfo</literal>
+                            </entry>
+                            <entry>Issue informational messages whenever the weaver touches a class file</entry>
+                        </row>
+                        <row>
+                            <entry>
+                                <literal>-XmessageHolderClass</literal>
+                            </entry>
+                            <entry>Provide alternative output destination to stderr for all weaver messages</entry>
+                        </row>
+                    </tbody>
+                </tgroup>
+            </informaltable>
+        </sect2>
+    </sect1>
+    
+    <sect1 id="ltw-packaging">
+        <title>Runtime Requirements for Load-time Weaving</title>
+        <para> To use LTW the <literal>aspectjweaver.jar</literal> library must be added to the
+            classpath. This contains the AspectJ 5 runtime, weaver, weaving class loader and
+            weaving agents. It also contains the DTD for parsing XML weaving configuration files. </para>
+    </sect1>
+    
+    <sect1 id="ltw-agents">
+        <title>Supported Agents</title>
+        <sect2>
+            <title>JVMTI</title>
+            <para> When using JDK 1.5 the JVMTI agent can be used by starting the JVM with the
+                following option: </para>
+            <programlisting><![CDATA[
+			-javaagent=aspectjweaver.jar          
+		  ]]></programlisting>
+        </sect2>
+        <sect2>
+            <title>JRockit</title>
+            <para> The JRockit agent is configured with the following JVM option: </para>
+            <programlisting><![CDATA[
+			-Xmanagement:class=org.aspectj.weaver.tools.JRockitWeavingAgent
+		  ]]></programlisting>
+        </sect2>
+    </sect1>
+</chapter>