title: AnnotationProcessor using multiple source-files to create one file
body: <p>I have two classes with methods and i want to combine the methods of the two classes to one class. </p>
  
  <pre><code>@Service("ITestService")
  public interface ITest1
  {
     @Export
     void method1();
  }
  
  @Service("ITestService")
  public interface ITest2
  {
     @Export
     void method2();
  }
  </code></pre>
  
  <p>Result should be:</p>
  
  <pre><code>public interface ITestService extends Remote
  {
    void method1();
    void method2();
  }
  </code></pre>
  
  <p>The first run of my AnnotationProcessor generates the correct output (because the RoundEnvironment contains both classes).</p>
  
  <p>But if I edit one of the classes (for example adding a new method), the RoundEnviroment contains only the edited class and so the result is follwing (adding newMethod() to interface ITest1)</p>
  
  <pre><code>public interface ITestService extends Remote
  {
    void method1();
    void newMethod();
  }
  </code></pre>
  
  <p><strong>Now method2 is missing. I don't know how to fix my problem. Is there a way (Enviroment), to access all classes in the project? Or is there another way to solve this?</strong></p>
  
  <p>The code to generate the class is pretty long, so here a short description how i generate the class. I iterate through the Elements with <code>env.getElementsAnnotatedWith(Service.class)</code> and extract the methods and write them into the new file with:</p>
  
  <pre><code>FileObject file = null;
  file = filer.createSourceFile("com/test/" + serviceName);
  file.openWriter().append(serviceContent).close();
  </code></pre>