How do you use JPA objects with GWT in a different project? - gwt

Question

I'm new to GWT, and I'm setting up a project to hold the UI portion of a complicated application. The JPA data objects are defined in a separate project using MyBatis generated POJOs. Once I manually modify the JPA objects to include 'implements Serializable' (this is a separate problem I need to solve), the gwtc step in the build process can't continue because it needs the source code for the JPA object, but can't find them.
I've created the 'core' project's jar files so that it includes the source .java files, but this didn't work. I've tried adding a classpath element to the gwtc ant java call.
How can I make my UI project compile while keeping the JPA objects defined in a separate project?
Errors:
Buildfile: /Users/user/source/user_interface/build.xml
libs:
javac:
gwtc:
[java] Compiling module org.mysite.ui
[java] Validating newly compiled units
[java] [ERROR] Errors in 'file:/Users/user/source/user_interface/src/org/mysite/ui/client/MyService.java'
[java] [ERROR] Line 21: No source code is available for type org.mysite.core.model.Person; did you forget to inherit a required module?
[java] [ERROR] Errors in 'file:/Users/user/source/user_interface/src/org/mysite/ui/client/MyServiceAsync.java'
[java] [ERROR] Line 19: No source code is available for type org.mysite.core.model.Person; did you forget to inherit a required module?
[java] [ERROR] Errors in 'file:/Users/user/source/user_interface/src/org/mysite/ui/client/presenter/MainTabPresenter.java'
[java] [ERROR] Line 75: No source code is available for type org.mysite.core.model.Person; did you forget to inherit a required module?
[java] Scanning for additional dependencies: file:/Users/user/source/user_interface/src/org/mysite/ui/client/MyApp.java
[java] Computing all possible rebind results for 'org.mysite.ui.client.MyService'
[java] Rebinding org.mysite.ui.client.MyService
[java] Checking rule <generate-with class='com.google.gwt.user.rebind.ui.ImageBundleGenerator'/>
[java] [ERROR] Unable to find type 'org.mysite.ui.client.MyService'
[java] [ERROR] Hint: Previous compiler errors may have made this type unavailable
[java] [ERROR] Hint: Check the inheritance chain from your module; it may not be inheriting a required module or a module may not be adding its source path entries properly
[java] [ERROR] Errors in 'file:/Users/user/source/user_interface/src/org/mysite/ui/client/MyApp.java'
[java] [ERROR] Line 23: Failed to resolve 'org.mysite.ui.client.MyService' via deferred binding
[java] [ERROR] Cannot proceed due to previous errors
BUILD FAILED
/Users/user/source/user_interface/build.xml:39: Java returned: 1
Total time: 26 seconds
Ant gwtc target:
<target name="gwtc" depends="javac" description="GWT compile to JavaScript (production mode)">
<java failonerror="true" fork="true" classname="com.google.gwt.dev.Compiler">
<classpath>
<pathelement location="src"/>
<pathelement location="../core/src"/>
<path refid="project.class.path"/>
</classpath>
<!-- add jvmarg -Xss16M or similar if you see a StackOverflowError -->
<jvmarg value="-Xmx256M"/>
<arg line="-war"/>
<arg value="web"/>
<!-- Additional arguments like -style PRETTY or -logLevel DEBUG -->
<arg line="${gwt.args}"/>
<arg value="org.mysite.ui.MyApp"/>
</java>
</target>

Answer 1

GWT has always had this problem.
The approach I have taken in the past is to make a .gwt.xml module in the other JAR including the JPA POJOs and putting the JPA annotation classes in an "emul" javax.persistence folder. This won't work w/ many providers that use lazy fetching of course, so Gilead (as mentioned in another answer here) will be needed.
Another way is to make copies of the JPA objects and just Dozer them before sending across RPC
Luckily, GWT 2.1 has solved this problem. Use Entity proxies and request factories. It works well.

Answer 2

Either use the new GWT 2.1 RequestFactory / EntityProxy approach, or
Better, ditch the JPA and use Objectify or Twig.
We use Objectify, because JPA is not a good fit for Datastore. JPA was created with SQL-based relational databases in mind, while GAE Datastore is an entirely different thing.
Google ported JPA layer to Datastore to lure in developers, pretending that Datastore is just a plain old SQL relational database. This later blows up in their faces once projects grow big.
You really need to understand the Datastore tradeoffs if you want to use AppEngine. JPA is not making you a favor here.

Answer 3

Your best bet if you want to serialize POJOs to GWT is to use Gilead http://noon.gilead.free.fr/gilead/
All you have to do on your POJO is extend LightEntity and implement Serializable
The tutorial is pretty straight forward in how to set up the RPC.