Speeding up GWT compilation with Ant’s parallel task

November 27th, 2008 Alex Moffat Posted in GWT | 6 Comments »

For Blueprint we use Ant to invoke the GWT compiler even though the rest of the build process is controlled by maven. If you’re also using Ant the technique I describe is a simple way to get a nice reduction in compilation time.

In our build.xml file we have a generic CompileGWTModule target that is invoked multiple times with the antcall task to compile the different GWT modules in the application. So we end up with something like this

  <antcall target="CompileGWTModule">
     <param name="gwt.module" value="LoginPage"/>
  </antcall>
  <antcall target="CompileGWTModule">
     <param name="gwt.module" value="ProcessPage"/>
  </antcall>
  <antcall target="CompileGWTModule">
     <param name="gwt.module" value="ProjectPage"/>
  </antcall>
  ....

On my MacBook Pro the GWT compilation process takes about eight minutes to complete when building all pages for all browsers. On my desktop it takes about seven and three quarter minutes. Ant has a parallel task that will invoke the tasks it surrounds in parallel. I simply wrapped the calls to CompileGWTModule in a parallel element like this

  <parallel threadsPerProcessor="1">
    <antcall target="CompileGWTModule">
       <param name="gwt.module" value="LoginPage"/>
    </antcall>
    <antcall target="CompileGWTModule">
       <param name="gwt.module" value="ProcessPage"/>
    </antcall>
    <antcall target="CompileGWTModule">
       <param name="gwt.module" value="ProjectPage"/>
    </antcall>
    ....
  </parallel>

The threadsPerProcessor attribute tells the task to use one thread for each processor/core in the machine. With these settings the same compilation takes about four and a quarter minutes on the dual core MacBook and two and three quarter minutes on the four core desktop. That’s significantly quicker.

Of course for maximum reduction in compilation time you should also

• Use hosted mode as much as possible so that you don’t compile when you don’t need to.
• Edit your GWT module xml files so that when you’re testing you only compile for the browser you need to test for. Using a common module xml file inherited by all your others makes this easy to do.
• Structure your build scripts so that you can choose which modules you compile.