org.aspectj/docs/devGuideDB/ltw.xml

<chapter id="ltw" xreflabel="Load-Time Weaving">
    <title>Load-Time Weaving</title>
    
    <sect1 id="ltw-introduction">
        <title>Introduction</title>
        
        <para> The AspectJ 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,
                and may themselves be woven by aspects. </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> As of AspectJ 5 aspects (code style or annotation style) and woven classes are
            	reweavable by default. If you are developing AspectJ applications that are to be used 
            	in a load-time weaving environment with an older version of the compiler 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> <para>All aspects to be used for weaving must be defined to the weaver before any
                types to be woven are loaded.  This avoids types being "missed" by aspects added
                later, with the result that invariants across types fail.</para></listitem>
            <listitem> <para>All aspects visible to the weaver are usable.
                A visible aspect is one defined by the
                weaving class loader or one of its parent class loaders.
                All concrete visible aspects are woven and all abstract visible aspects
                may be extended.
            </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>New in AspectJ 5 are a number of mechanisms to make load-time weaving
        easy to use. 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 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>
                <varlistentry>
                    <term>Command-line wrapper scripts <literal>aj</literal></term>
                    <listitem>
                        <para>The <command>aj</command> command runs Java programs in Java 1.4 or
                        later by setting up <literal>WeavingURLClassLoader</literal> as the 
                            system class loader.  
                            For more information, see <xref linkend="aj"/>.
                        </para>
                        <para>The <command>aj5</command> command runs Java programs in Java 5
                        by using the <literal>-javaagent:pathto/aspectjweaver.jar</literal> option 
                            described above.
                            For more information, see <xref linkend="aj"/>.
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>Custom class loader</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 explicitly restrict by class loader which classes can be woven.
                            For more information, see <xref linkend="aj"/> and the
                            API documentation and source for
                            <literal>WeavingURLClassLoader</literal> and
                            <literal>WeavingAdapter</literal>.
                        </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
                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"
                               precedence="com.xyz.first, *">
                <pointcut name="tracingScope" expression="within(org.maw.*)"/>
              </concrete-aspect>

              <!-- Of the set of aspects declared to the weaver
                   use aspects matching the type pattern "com..*" for weaving. -->
              <include within="com..*"/>

              <!-- Of the set of aspects declared to the weaver
                   do not use any aspects with the @CoolAspect annotation for weaving -->
              <exclude within="@CoolAspect *"/>

            </aspects>

            <weaver options="-verbose">
              <!-- 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.*"/>

              <!-- Do not weave types within the "bar" pakage -->
              <exclude within="bar.*"/>

              <!-- Dump all types within the "somepack" package,
                   both before are after they are woven,
                   to the "./_ajdump" folder on disk (for diagnostic purposes) -->
              <dump within="somepack.*" />
            </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.
                Refer to the next section for more details.
            </para>

            <para>
                The <literal>aspects</literal> element may optionally contain one or more <literal>include</literal> and
                <literal>exclude</literal> 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 <literal>within</literal> 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>
                Note that <literal>include</literal> and <literal>exclude</literal> elements affect all aspects 
                declared to the weaver including those in other aop.xml files. To help avoid unexpected 
                behaviour a lint warning is issued
                if an aspect is not declared as a result of of applying these filters.
                Also note <literal>aspect</literal> and <literal>concrete-aspect</literal> elements
                must be used to delare aspects to the weaver i.e. <literal>include</literal> and <literal>exclude</literal>
                elements cannot be used find aspects on the class loader search path.
            </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 visible to the weaver will be woven.
            </para>
          
          
            <para> When several configuration files are visible from a given weaving class loader
                their contents are conceptually merged. 
                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 concrete aspect
                is defined 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> can be generated by
                using either the <literal>-outxml</literal> or <literal>-outxmlfile</literal> options of the AspectJ compiler. 
                It will simply contain a (possibly empty) set of aspect elements; one for
                each abstract or concrete aspect defined. 
                When used in conjuction with the <literal>-outjar</literal> option
                a JAR is produced that can be used
                with the <command>aj5</command> command or a load-time weaving environment.</para>
        </sect2>

        <sect2 id="concrete-aspect" xreflabel="concrete-aspect">
            <title>Using Concrete Aspects</title>
            <para>
                It is possible to make an abstract aspect concrete by means of the <literal>META-INF/aop.xml</literal>
                file. This is useful way to implement abstract pointcuts at deployment time, and also gives control
                over precedence through the <literal>precedence</literal> attribute of the
                <literal>concrete-aspect</literal> XML element.
                Consider the following:
            </para>
            <programlisting><![CDATA[
            package mypack;

            @Aspect
            public abstract class AbstractAspect {

                // abstract pointcut: no expression is defined
                @Pointcut
                abstract void scope();

                @Before("scope() && execution(* *..doSome(..))")
                public void before(JoinPoint jp) {
                   ....
                }
            }
            ]]></programlisting>
            <para>
                This aspect is equivalent to the following in code style:
            </para>
            <programlisting><![CDATA[
            package mypack;

            public abstract aspect AbstractAspect {

                // abstract pointcut: no expression is defined
                abstract pointcut scope();

                before() : scope() && execution(* *..doSome(..)) {
                   ....
                }
            }
            ]]></programlisting>
            <para>
                This aspect (in either style) can be made concrete using <literal>META-INF/aop.xml</literal>.
                It defines the abstract pointcut <literal>within()</literal>. When using this mechanism the 
                following rules apply:
                <itemizedlist>
                    <listitem><para>The parent aspect must be abstract. It can be an @AspectJ or a
                            regular code style aspect.</para></listitem>
                    <listitem><para>Only a simple abstract pointcut can be implemented i.e. a pointcut that doesn't expose
                            state (through <literal>args(), this(), target(), if()</literal>). In @AspectJ syntax
                            as illustrated in this sample, this means the method that hosts the pointcut must be abstract,
                            have no arguments, and return void.</para></listitem>
                    <listitem><para>The concrete aspect must implement all inherited abstract pointcuts.</para></listitem>
                    <listitem><para>The concrete aspect may not implement methods so the abstract aspect it 
                            extends may not contain any abstract methods.</para></listitem>
                </itemizedlist>
                If more complex aspect inheritance is required use regular aspect
                inheritance instead of XML.
                The following XML definition shows a valid concrete sub-aspect for the abstract aspects above:
            </para>
            <programlisting><![CDATA[
            <aspectj>
                <conrete-aspect name="mypack.__My__AbstractAspect" extends="mypack.AbstractAspect">
                    <pointcut name="scope" expression="within(yourpackage..*)"/>
                </concrete-aspect>
            </aspectj>
            ]]></programlisting>
            <para>
                It is important to remember that the <literal>name</literal> attribute in the
                <literal>concrete-aspect</literal> directive defines the fully qualified name that will be given to the
                concrete aspect. It must a valid class name because the aspect will be generated on the fly by the weaver.
                You must
                also ensure that there are no name collisions. Note that the concrete aspect will be
                defined at the classloader level for which the aop.xml is visible. This implies that if you need
                to use the <literal>aspectof</literal> methods to access the aspect instance(s) (depending on the perclause
                of the aspect it extends) you have to use the helper API <literal>org.aspectj.lang.Aspects.aspectOf(..)</literal>
                as in:
            </para>
            <programlisting><![CDATA[
                // exception handling omitted
                Class myConcreteAspectClass = Class.forName("mypack.__My__AbstractAspect");

                // here we are using a singleton aspect
                AbstractAspect concreteInstance = Aspects.aspectOf(myConcreteAspectClass);
            ]]></programlisting>
       </sect2>

        <sect2 id="concrete-aspect-precedence" xreflabel="concrete-aspect-precedence">
            <title>Using Concrete Aspects to define precedence</title>
            <para>
                As described in the previous section, the <literal>concrete-aspect</literal> element in
                <literal>META-INF/aop.xml</literal> gives the option to declare the precedence, just as
                <literal>@DeclarePrecedence</literal> or <literal>declare precedence</literal> do in
                aspect source code.
            </para>
            <para>
                Sometimes it is necessary to declare precedence without extending any abstract aspect.
                It is therefore possible to use the <literal>concrete-aspect</literal>
                element without the <literal>extends</literal> attribute and without any
                <literal>pointcut</literal> nested elements, just a <literal>precedence</literal>
                attribute.
                Consider the following:
            </para>
            <programlisting><![CDATA[
                <aspectj>
                    <concrete-aspect name="mypack.__MyDeclarePrecedence"
                                     precedence="*..*Security*, Logging+, *"/>
                </aspectj>
            ]]></programlisting>
            <para>
                This deployment time definitions is only declaring a precedence rule. You have to remember
                that the <literal>name</literal> attribute must be a valid fully qualified class name
                that will be then reserved for this concrete-aspect and must not conflict with other classes
                you deploy.
            </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
          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. Messages issued while the weaver is being 
                                   bootstrapped are accumulated until all options are parsed. If the messages are required to be output
                                   immediately you can use the option <literal>-Daj.weaving.verbose=true</literal> on the JVM startup command line.
                            </entry>
                        </row>
        				<!-- TODO option parsed but not used -->
        				<!--
                        <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>-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>
        				<!-- TODO option parsed but not used -->
        				<!--
                        <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 the
                            <literal>org.aspectj.bridge.IMessageHandler</literal> interface
                            and is visible to the classloader with which the weaver being configured is associated.
                            Exercise caution when packaging a custom message handler with an application that is to 
                            be woven. The handler (as well as classes on which it depends) cannot itself be woven
                            by the aspects that are declared to the same weaver.
                            </entry>
                        </row>
                    </tbody>
                </tgroup>
            </informaltable>
        </sect2>
    </sect1>

    <sect1 id="ltw-specialcases">
        <title>Special cases</title>
        <para>
            The following classes are not exposed to the LTW infrastructure regardless of 
            the <literal>aop.xml</literal> file(s) used:
            <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>

        <para>
            Some lint options behave differently when used under load-time weaving. The <literal>adviceDidNotMatch</literal>
            won't be handled as a warn (as during compile time) but as an info message.
        </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>