= Weaving

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A responsibility of all ORMs is lazy loading of related objects (so as not to load all the data in one go), and tracking of objects as they are modified (to flush back to the database).

With JPA, the code that performs this is "weaved" into your own domain entity code.
This "weaving" can be done either statically during compile-time, or dynamically at runtime using a Java agent.



[#compiletime]
== Static (Compile-time) Weaving

To enable static weaving, set xref:refguide:config:sections/eclipselink.adoc#eclipselink.weaving[eclipselink.weaving]:

[source,properties]
.application.properties
----
eclipselink.weaving=static
----

In addition, add the link:https://github.com/ethlo/eclipselink-maven-plugin[eclipselink-maven-plugin] to the `pom.xml` of all modules that contain JPA entities, specifying the base package(s) to be weaved.:

[source,xml]
.pom.xml
----
 <build>
    <plugins>
        <plugin>
            <groupId>com.ethlo.persistence.tools</groupId>
            <artifactId>eclipselink-maven-plugin</artifactId>
            <version>2.7.9.1</version>
            <executions>
                <execution>
                    <id>weave</id>
                    <phase>process-classes</phase>
                    <goals>
                        <goal>weave</goal>
                    </goals>
                    <configuration>
                        <basePackages>
                            <basePackage>domainapp.modules.simple.dom</basePackage> <!--.-->
                        </basePackages>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
<build>
----
<.> Update as required.

And in the IDE, you may need to also configure to ensure that this weaving is performed as necessary, for example:

image::enable-static-weaving-in-IntelliJ.png[]

NOTE: Thanks for the contribution via link:https://the-asf.slack.com/archives/CFC42LWBV/p1712451654657869?thread_ts=1709070676.947439&cid=CFC42LWBV[Slack] channel.


[#runtime]
== Dynamic (Runtime) Weaving

The xref:docs:starters:simpleapp.adoc[SimpleApp] and xref:docs:starters:helloworld.adoc[HelloWorld] starter apps demonstrate the dynamic agent approach.

To enable runtime weaving, set xref:refguide:config:sections/eclipselink.adoc#eclipselink.weaving[eclipselink.weaving]:

[source,properties]
.application.properties
----
eclipselink.weaving=true
----

It's also necessary to run the application with the `spring-instrument.jar` Java agent, which actually performs the weaving at load-time.

* Download this jar file using:
+
[source,bash]
----
mvn dependency:get -DgroupId=org.springframework -DartifactId=spring-instrument -Dversion=XXX
----
=
changing "XXX" to the value that `${spring-framework.version}` resolves to, from the Causeway parent `pom.xml`.

* Move and rename this file, eg to `lib/spring-instrument.jar`.

* Run the application using:
+
[source,bash]
----
-javaagent:lib/spring-instrument.jar
----
+
as a JVM option.



== Configuration

There are a number of other EclipseLink configuration options relate to weaving:

* in addition, optionally set the following (their default values are shown):
+
[source,properties]
.application.properties
----
eclipselink.weaving.changetracking=true
eclipselink.weaving.eager=false
eclipselink.weaving.fetchgroups=true
eclipselink.weaving.internal=true
eclipselink.weaving.lazy=true
----
+
These all default to `true` except for xref:refguide:config:sections/eclipselink.adoc#eclipselink.weaving.eager[eclipselink.weaving.eager], which you should only enable if you fully understand its consequences.

The weaving process modifies the classes themselves, introducing additional `public` methods.
Depending upon the value of xref:refguide:config:sections/causeway.core.meta-model.introspector.adoc#causeway.core.meta-model.introspector.policy[causeway.core.meta-model.introspector.policy] configuration property, these could be picked up as part of the framework's metamodel, which is not what you want.

Therefore, to use JPA, you will also need to change this configuration property, either:

* to xref:refguide:applib:index/annotation/Introspection.adoc#ANNOTATION_REQUIRED[IntrospectionPolicy.ANNOTATION_REQUIRED]
+
[source,properties]
.application.properties
----
causeway.core.meta-model.introspector.policy=annotation_required
----
+
or,

* to xref:refguide:applib:index/annotation/Introspection.adoc#ENCAPSULATION_ENABLED[IntrospectionPolicy.ENCAPSULATION_ENABLED]
+
[source,properties]
.application.properties
----
causeway.core.meta-model.introspector.policy=encapsulation_enabled
----

The xref:docs:starters:simpleapp.adoc[SimpleApp] and xref:docs:starters:helloworld.adoc[HelloWorld] starter apps both use the latter option.
