GWT Recipes

0. Resources

• Search the GWT google group. This is a treasure trove of good material. Alas, the information is sometimes a bit scattered (which is inherent in the nature of the source).
• Roughian Examples: Very helpful if you need small examples for GWT's classes.
  

1. Client

1.1. Using existing Java code

• You can only use Java classes whose source code is available. The source code is needed to generate JavaScript.
  
• To integrate existing libraries, create a module XML file and inherit the module whenever you want to use the library.
• You can use <super-source path=""> to provide a source code that is adapted to the GWT needs
• The source code is nested inside the the package like my/package/other/package, but the package declaration must still say other.package.
• The path for both super-source and source can be empty. Usually, I like nested packages better, though.
• You can take liberties with the source code, especially with super-source. This comes in handy when methods introduce unwanted dependencies on other classes: Just make that other class empty and the dependency is not as much of a problem, any more.

Problems

• A method depends on a class that is too complicated for the client side or not needed.
• Use super-source (see above). Wish: An annotation for marking methods whose JavaScript code should not be generated.
• How do I find out if my client-side code is executed as Java bytecode or as JavaScript code?
• GWT.isScript()
  

1.2. Graphical user interfaces

• Example of manual layout: Mail.java (in samples).
  

Problems

• DockPanel does not work like BorderLayout when it comes to its CENTER position!
• You have to set the cell size to "100%" for it to work like that.
• Debug: setBorder() to 1.
• My widgets don't fill out the complete available area.
• Make sure that the body of your HTML page is sized to fill out 100%. For example, like this:
  

  <style>
      html, body { 
          width: 100%;    /* make the body expand to fill the visible window */
          height: 100%;
          overflow: hidden;;      /* erase window level scrollbars */
          padding: 0 0 0 0;
          margin: 0 0 0 0;
      }
  </style>
  

1.3. GWT and JavaScript

Problems

• I need to invoke GWT from JavaScript.
• Normally, method names are compacted by the GWT compiler. But mentioning them in JSNI code prevents that scrambling. This leads to the following trick (which also works with static methods). Invoke that method in onModuleLoad().
  

      private native void initJavaScriptApi() /*-{
          var that = this;
          $wnd.hyenaAdmin = {
              addIndexContainer: function(s) {
                  that.@de.hypergraphs.hyena.gwt.admin.client.AdminGui::addIndexContainer(Ljava/lang/String;)(s);
              }
          };
      }-*/;

• Signatures are specified here. Use the following invocation to show signatures (-s prints out signatures, -p shows private members). JSNI omits the type after the parentheses.
  javap -p -s my/qualified/Class

2. Server and client-server communication

2.1. Client-server communication

Problems

• Class literals: How can the server tell the client what instances to create?
• The problem is that GWT.create() only works with class literals. This is practically the same as calling a constructor. The reason is that this method encapsulates a whole (pluggable) suite of functions, among other starting RPC on the client side. Solution: Send factories (that must then be serializable). Wish: An annotation for classes whose class literals are to be used as factories. Then either GWT.create() or another client-side method would be used to create instances.
  

2.2. Serialization

 public final class Foo_CustomFieldSerializer {

  public static void serialize(SerializationStreamWriter streamWriter,
      Foo instance) throws SerializationException {
      streamWriter.writeObject(instance.getA());
      streamWriter.writeFloat(instance.getB());
  }

  public static void deserialize(SerializationStreamReader streamReader,
      Foo instance) throws SerializationException {
		instance.setA(streamRead.readObject());
		instance.setB(streamRead.readFloat());
  }
 }

Problems

• An interface cannot be serialized: You need at least one implementing class that is serializable.
• Cannot change an external class to be serializable: Create a serializable subclass

3. Development and tools

3.1. Simulating a server environment in hosted mode

• The tomcat/ directory is your friend
• library jars: tomcat/webapps/ROOT/WEB-INF/lib/
• Application-global (non-service) servlets via tomcat/webapps/ROOT/WEB-INF/web.xml
• Especially if you need to auto-load a servlet.
• Scripts projectCreator, applicationCreator
• Option -ant: generates an ant build file
• Option -out: generates the source code so that it can live somewhere else (than the current directory)

Problems

• I want hosted mode to start with a different URL.
• Change: *.launch, *-shell
• The paths of my local GWT installation are hard-coded into the scripts, making it impossible to share them between different computers. How can I change that?
• Step 1: Define variables (different with each installation, operating system and/or computer):
  
• Create two user libraries (Preferences -> Java -> Build Path -> User Libraries)
• gwt-user: gwt-user.jar
• gwt-dev: gwt-dev-{mac,windows,linux}.jar
• Create one runtime variable (Preferences -> Run/Debug -> String Substitution)
• extra-jvm-argument: "-XstartOnFirstThread" on Mac OS X (otherwise empty)
• Step 2: Modify the project settings and standard scripts so that they become installation-independent (and can thus be checked into version control etc.)
  
• Use the run dialog to edit the *.launch configuration
• Classpath -> User entries: remove the gwt-dev-{mac,windows,linux}.jar, add gwt-dev (add as a library)
• Arguments -> VM arguments: -Xmx256M ${extra-jvm-argument}
• Change the build path to include gwt-dev-{mac,windows,linux}.jar and gwt-user as a library (not as a jar).
• Edit *-compile: Remove gwt-dev-{mac,windows,linux}.jar, gwt-user.jar from its classpath. [Actually, it might be more complicated than that, this still has to be investigated.]
  
• Edit *-shell: Same as above