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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> <para>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. </para></listitem>
+            <listitem> <para>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. </para></listitem>
+            <listitem> <para>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. </para></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 id="weaving-class-files-more-than-once" xreflabel="weaving-class-files-more-than-once">
+            <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>
+            <para><!-- FIXME AV -->As per AspectJ 1.5.0 M3 aspects (code style or annotation style) are
+            reweavable by default, and weaved classes may be as well in 1.5.0 final.</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> <para>All aspects to be used for weaving must be defined to the weaver before any
+                types to be woven are loaded.</para></listitem>
+            <listitem> <para>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.</para></listitem>
+            <listitem><para>A class loader may only weave classes that it defines. It may not weave
+                classes loaded by a delegate or parent class loader.</para></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 id="enabling-load-time-weaving" xreflabel="enabling-load-time-weaving">
+            <title>Enabling Load-time Weaving</title>
+            <para> AspectJ 5 supports several 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. Configuration for the supported environments is discussed
+                            later in this chapter.</para>
+                            <para>
+                            Using Java 5 JVMTI you can specify the <literal>-javaagent:pathto/aspectjweaver.jar</literal> option
+                            to the JVM.</para><para>
+                            Using BEA JRockit and Java 1.3/1.4, the very same behavior can be obtained using BEA JRockit JMAPI features with
+                            the <literal>-Xmanagement:class=org.aspectj.weaver.loadtime.JRockitAgent</literal>
+                        </para>
+                    </listitem>
+                </varlistentry>
+                <!-- FIXME: must be made consistent (aop.xml , CL hierarchy etc) -->
+<!--                <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 id="configuring-load-time-weaving-with-aopxml-files" xreflabel="configuring-load-time-weaving-with-aopxml-files">
+            <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.*"/>
+              <dump within="somepack.*"/><!-- will dump weaved classes to the "./_ajdump" folder on disk (for diagnostic purpose) -->
+            </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>
+                <emphasis>
+                Note: concrete-aspect is not available in AspectJ 1.5 M3.
+                </emphasis>
+            </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>
+                <!-- FIXME AV - looks like we can refine conf in a child CL - not good -->
+                <listitem> <para>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).</para></listitem>
+                <listitem> <para>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.</para></listitem>
+                <listitem> <para> 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.</para></listitem>
+                <listitem> <para> 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.</para></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>
+
+        <!-- TODO someone implement that -->
+        <!--
+        <sect2 id="configuring-load-time-weaving-with-properties-files" xreflabel="configuring-load-time-weaving-with-properties-files">
+            <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 id="weaver-options" xreflabel="weaver-options">
+            <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>-verbose</literal>
+                            </entry>
+                            <entry>Issue informational messages about the weaving process. If ever you need to have information
+                                   when the load time weaving engine is bootstrapped (hence its logger as per <literal>-XmessageHandlerClass:...</literal> not ready yet),
+                                   you can use the option <literal>-Daj.weaving.verbose=true</literal> on the JVM startup command line. Messages will then be printed
+                                   on stderr as long as the message handler class is not ready.
+                            </entry>
+                        </row>
+                        <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>-Xlintfile:pathToAResource</literal>
+                            </entry>
+                            <entry>Configure lint messages as specified in the given resource (visible from this aop.xml file' classloader)</entry>
+                        </row>
+                        <row>
+                            <entry>
+                                <literal>-Xlint:default, -Xlint:ignore, ...</literal>
+                            </entry>
+                            <entry>Configure lint messages, refer to documentation for meaningfull values</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>-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>-XmessageHandlerClass:...</literal>
+                            </entry>
+                            <entry>Provide alternative output destination to stdout/stderr for all weaver messages.
+                            The given value must be the full qualified class name of a class that implements
+                            <literal>org.aspectj.bridge.IMessageHandler</literal>
+                            and that is visible from where the <literal>aop.xml</literal> is packed.
+                            If more than one such options are used,
+                            the first occurence only is taken into account.
+                            You must also be very cautious about using a custom handler since it is likely that it will be invoked
+                            (as well as all its third parties) while the weaving is done, which means that f.e. it cannot be weaved
+                            by the aspects that are configured within the same deployment unit.
+                            </entry>
+                        </row>
+                    </tbody>
+                </tgroup>
+            </informaltable>
+        </sect2>
+    </sect1>
+
+    <sect1 id="ltw-specialcases">
+        <title>Special cases</title>
+        <para>
+            Those classes are not exposed to the LTW infrastructure, no matter
+            the configuration of the <literal>aop.xml</literal> file(s):
+            <itemizedlist>
+                <listitem> <para>All <literal>org.aspectj.*</literal> classes (and subpackages) - as those are needed by the infrastructure itself</para></listitem>
+                <listitem> <para>All <literal>java.*</literal> and <literal>javax.*</literal> classes (and subpackages)</para></listitem>
+                <listitem> <para>All <literal>sun.reflect.*</literal> classes - as those are JDK specific classes used when reflective calls occurs</para></listitem>
+            </itemizedlist>
+        </para>
+        <para>
+            Despite these restrictions, it is perfectly possible to match call join points for calls to these types providing the calling
+            class is exposed to the weaver. Subtypes of these excluded types that are exposed to the weaver may of course be woven.
+        </para>
+        <para>
+            Note that dynamic proxy representations are exposed to the LTW infrastructure and are not considered
+            a special case.
+        </para>
+    </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 id="jvmti" xreflabel="jvmti">
+            <title>JVMTI</title>
+            <para> When using Java 5 the JVMTI agent can be used by starting the JVM with the
+                following option (adapt according to the path to aspectjweaver.jar): </para>
+            <programlisting><![CDATA[
+			-javaagent:pathto/aspectjweaver.jar
+		  ]]></programlisting>
+        </sect2>
+        <sect2 id="jrockit" xreflabel="jrockit">
+            <title>JRockit with Java 1.3/1.4 (use JVMTI on Java 5)</title>
+            <para> The JRockit agent is configured with the following JVM option: </para>
+            <programlisting><![CDATA[
+			-Xmanagement:class=org.aspectj.weaver.loadtime.JRockitAgent
+		  ]]></programlisting>
+        </sect2>
+    </sect1>
+</chapter>
+