COMP 405/505 Spring 2016	Google Web Toolkit (GWT)

Having problems with GWT?  See the Known Issues and Problems below and/or the GAE How-To's page.

Google Web Toolkit (GWT) is a development toolkit used to build browser-based web applications that use GAE-based server-side processing.   The toolkit enables client-side applications to be written in Java but deployed as Javascript.   The toolkit also includes a UI designer, ala WindowBuilder, for drag-and-drop browser-based UI creation.   The toolkit also includes debugging and performance monitoring capabilities for GWT applications.

Note: GWT is included in the Google Plugin for Eclipse.   No additional installations are necessary.   Be sure to chose the GWT and GAE options when installing the Google Plugin.

GWT web pages act differently than regular web pages.   Instead of progressing to a new page in response to a user action, a GWT page acts more like a desktop application which mutates its view in response to user actions.  

Key Parts of a GWT Project

All files and folders mentioned here, unless explicitly identified otherwise, are specified relative to the root of the GWT code's package

[GWT module name].gwt.xml

• Redefines the  GWT module name
  • Normally, the default module name is module's root package name
  • The <module rename-to='short_name'> tag renames the module to be the 'short_name'.
  • The compiled Javascript code for the client browser will be put in a folder with the 'short_name' off the root of the WAR folder.
  • Be careful about name collisions with other entities in the WAR folder, e.g. regular HTML or JSP files.   Eclipse does NOT check and will simply overwrite folders with a name collision.   Suggestion:  use  a clear naming convention for all modules, e.g. their names all end in '_serv'.
• Defines the entry point class  (fully qualtfied pathname) which is typically [root package].client.[GWT module name]
• Defines the folders of the client and shared code (typically, path='client' and path='shared')

client/[GWT module name].java

• The entry point class, extends com.google.gwt.core.client.EntryPoint
• Has onModuleLoad() method that instantiates and adds dynamically created HTML components to the GUI and connects those components to their handlers.
• Can be opened with the GWT Designer in Eclipse

shared/[RPC service name].java, preferably, though in the sample code it is client/[RPC service name].java

• This interface is the same as the one used by the server-side RPC service.
• The Eclipse sample code has this file in the client folder, but the server code must also implement this interface so one gets better decoupling if it is in the shared folder instead.
• Normal method with input parameter(s) and rturn value plus throws IllegalArgumentException
• Extends com.google.gwt.user.client.rpc.RemoteService
• Contains the annotation @RemoteServiceRelativePath(relative_path) where the relative path is with respect to the GWT module's package root.   The net path should match the URL route in app.yaml.

shared/[RPC service name]Async.java, preferably, though in the sample code it is client/[RPC service name]ASync.java

• Must be in the same folder as the RPC service interface.   Recommended to be in the shared folder for better client/server decoupling.
• The interface can be auto-generated by Eclipse by first writing the RPC service interface and then selecting the "Quick fix" when Eclipse marks the RPC service interface in error because the Async interface is missing.
• Instance of this interface is dynamically generated using com.google.gwt.core.client.GWT.create([RPC service name].class) in client/[GWT module name].java
• Method is the same as client/[RPC service name].java except that type T return is now void and AsyncCallback<T> parameter added to method.
• Client uses this asynchronous proxy to make call to server side services.

server/[RPC service name]Impl.java

• Server-side implementation of RPC service
• Implements client/[RPC service name].java interface
• Extends RemoteServiceServlet

/war/[path]/[GWT Module name].html

• The stastic HTML web page framework into which the dynamic GWT content is injected.  Is the web page the browser actually looks at.
• In WAR folder
• May have associated CSS and other support files
• Loads client code that has been compiled into Javascript.
  • <script type="text/javascript" language="javascript" src="/[Module short name]/[Module short name].nocache.js"></script>
    • Module short name = the redefine short name for the module specified in the .gwt.xml file.
• Has tag ID's used by client code to identify dynamic HTML code injection points.  
  • The dynamically injected code essentially replaces the tag with the matching ID.  
  • <div> tags are a typical tag to set the ID on.

/war/WEB-INF/app.yaml

• In WAR folder
• May contain URL route to  /war/[path]/[GWT Module name].html -- where the module name is
• Contains URL route to server-side Remote Procedure Call (RPC) service:  
  • - url:  /[Module short name]/[relative_path]
      servlet = [server-side implementation class] - the fully qualified classname.
    • Module short name = the short name defined in the the .gwt.xml file or the full module name if no rename defined.
    • relative_path = the relative path defined by the @RemoteServiceRelativePath([relative_path]) annotation in the remote service interface definition.

References:

• Google Web Toolkit
  • Latest GWT documentation
• Showcase of GWT Features  (includes CSS and source code)
• GWT Widget Gallery
• Organizing Projects
• Different techniques for communicating with the server
• GWT and Eclipse
• Testing GWT
• Client and Server-side Logging
• Tutorials
  • GWT Examples
  • Build a Sample GWT Application (StockWatcher)
  • GWT Tutorial (vogella.com) -- for JDK 1.6, GWT 2.3 and Eclipse 3.7

Making a new GWT web page

See the "GWT Wizards" topic in the Google Plugin for Eclipse documentation.

1. Make a new GWT Module:  File/New/Other.../Google Web Toolkit/Module
   • Only need this step if not adding an entry point to an existing module
   • Creates the .gwt.xml file in the root of the specified package 
     •  The <source path="client"> entry is automatically created, but one must manually add another entry for the shared folder.
     • Add the following line to enable default access to the CSS definitions corresponding to the "Clean" visual style.    Some components will not display properly without this.  These CSS files can be found under WAR/[module short name]gwt/clean.
       <inherits name='com.google.gwt.user.theme.clean.Clean'/>
     • Add the lines needed to perform logging in the GWT browser client. See the notes on Logging.
2. Make a new GWT Entry Point: File/New/Other.../Google Web Toolkit/Entry Point Class
   • The package should be the "client" package below your module package root.
   • Add some startup code to the onModuleLoad() method of the newly created EntryPoint class so that the GWT Designer will work properly (see below).  This code may be changed later; this is just to get the GWT Designer started.  In onModuleLoad():
     • Create a private field and instantiate a com.google.gwt.user.client.ui.DockPanel  -- this type of panel uses a BorderLayout-like layout which is convenient for top-level containers.
     • Make onModuleLoad() delegate to a private void initGUI() method.
     • In initGUI(), add the panel to the RootPanel, using a reasonable name for the location of the top-level container, e.g. "body_div" here which refers to the ID of a div tag in the body of the HTML web page.
       RootPanel.get("body_div").add(myDockPanel);
   • Run a GWT Compile to create the folders in the WAR folder that hold the compiled Javascript code.   
     1. Highlight the Eclipse project -- GWT compile will NOT work correctly if anything but the entire project is selected.
     2.  Either pull down the Google toolbar icon or right click the project and open up "Google".
     3. Select "GWT Compile" and click "Compile".   Be sure that a "GWT Compile" dialog box comes up!    If not, you are probably not selecting the entire project.
        • If you have created a new GWT module, you must manually add it to the list of "Entry Point Modules" in the GWT Compile dialog window.
3. Make a new GWT HTML page: File/New/Other.../Google Web Toolkit/HTML Page
   • May not properly connect to the generated Javascript.   There should be a script tag that references a file called something like this::  /[pathname]/[module name].nocache.js   If not, locate this file in the WAR folder heirarchey and set the script tag to reference it.
   • Add a place to put the dynamically generated widgets.    As per the previous example, in the <body> of the web page, an empty div tag with the id set to the name used above is convenient:    <div id="body_div"></div>
   • Add a URL route to the new HTML page in app.yaml, if necessary.   This is only necessary if you want to page to be accessed as something other than it's full pathname from the root of the WAR folder.   Note that if the filename, e.g. index.html is included in the welcome_files directive in app.yaml, then the page can be accessed by only using its folder name.

GWT Designer

Be sure that you have the GWT Designer configured properly before you start!

The GWT Designer can be used to edit EntryPoint classes in much the same way as WindowBuilder.  There are a few caveats to remember however:

• Make sure you are using widgets from com.google.gwt.user.client.ui, not the usual javax.swing, java.awt, etc.!
• There must be at least one container widget, e.g. a DropPanel, instantiated in onModuleLoad() or the Designer will not be able to add more widgets. 
• Only widgets that have either themselves directly or indirectly via their containing widgets,  been added to the RootPanel will show up in the Designer. 
• The code for adding a widget to the RootPanel must be added manually, not through the Designer:   RootPanel.get("anID").add(aWidget);
•  It is possible to add widgets to different locations in the RootPanel's HTML page;  simply add the widget using a different ID (make sure the ID exists in the HTML page).
• Changes made in the Designer will NOT show up in the running application until a GWT Compile has been performed!
  • The Development Mode Server does not generally need to be restarted when a GWT Compile is performed.

Server-side processing (Remote Procedure Calls)

The Java code in a GWT module is cross-compiled into client-side Javascript.   This makes it easy to off-load a fair amount of GUI processing onto the client.   But to make a server-sdie call, one can use GWT's Remote Procedure Call (RPC) mechanism to make what is really a servlet call into what seems to be a simple method call on an client-side object.   This is conceptually the same as what Java RMI does, though a little different because a normally blocking, synchronous call must be translated into a non-blocking, asynchronous call.

To create an RPC call from a GWT client to a servlet backend, several pieces are needed:

1. In the module's "shared" folder (be sure the shared folder is listed as a "source" in the module's .gwt.xml file!):
   1. The interface that defines the server to the client. i.e. defines what RPC calls can be made.
      • See the requirements for shared/[RPC service name].java above.
   2. An "Async" proxy of the above interface.   This can be auto-generated from the above interface.
      • See the requirements for shared/[RPC service name]Async.java above.
2. In the module's "server" folder (the shared folder is NOT listed in the module's .gwt.xml file because it's contents are not cross-compiled into Javascript):
   • See the requirements for server/[RPC service name]Impl.java above.
   • This implements the RPC service interface above (the synchronous original, not the asynchronous proxy).
3. In the module's EntryPoint class
   • See the requirements for client/[GWT module name].java above
   • Contains the field or variable of the sort MyServiceAsync myService = GWT.create(MyService.class)
   • All calls are made through the asynchronous proxy, myService using AsyncCallback<ReturnType> lambdas (success/fail visitors actually).
4. In the app.yaml configuration
   • See the requirements for the app.yaml file above.
   • The URL pattern to the server implementation is the combination of the specifications in the module "rename-to" in the module's .gwt.xml file (the "short name") and the relative_path defined in the RPC service interface.

Be careful when refactoring that you don't break the relationships between the above 5 entities!

Returning Objects from the Server to the Client

It is critical to always remember that a GWT server is a Java program but a GWT client is a Javascript program.   Thus, any interaction between them is fundamentally a cross-language process as well as a cross-machine one.    This has several specific consequences:

1. No returning of anonymous inner classes -- the GWT compiler will complain about not finding a specific class to serialize.  Plus, the closure issues are just going to cause trouble.
2. Any returned objects must implement com.google.gwt.user.client.rpc.IsSerializable  -- this is the GWT equivalent of java.io.Serializable.
3. Any returned object type must have a public no-parameter constructor -- This is because the deserializer will first call the no-parameter constructor to create an instance of the object and then proceed to set the field values.   Having an additional, parameterized constructor is fine for use by the server to easily instantiate the object.
4. Access to contained data should be via getters and setters -- encapsulate the data.   Read-only data simply means a getter but no setter for the affected value(s).

The above restrictions are essentially the description of a Java Bean..   Because any returned data types are shared between the client and server, the code should be in the "shared" folder of the GWT module.

References:

• Communicate with a Server
• RPC Interfaces

System Design with GWT

Reusing common code

If multiple modules use the same operations or data or GUI elements, then one should refactor that code out into a separate module.   Note that modules are not required to have an entry point, which is just the UI portion of the module.   This is particularly useful for defining common data classes that are used for information transport between the front and backends. 

To use the common code in another modules, do the following:

1. "Inherit" the common module by adding the following line to the .gwt.xml file of the module using it:   <inherits name="[common module root package].[common module name]">,  e.g. <inherits name="edu.rice.comp405.common.Common"/>
2. If the utilizing module is using common RPC server functionalities, in the utilizing module's EntryPoint class, create an Async proxy for the common RPC server implementation.
   • This additional proxy can be used as normal.
3.  Create a hybrid URL route in the app.yaml since the common code is actually imbedded in the untilizing module's Javascript.   The URL route should look like:

  - url: utilizing_module_short_name/common_module_relative_path   # e.g. user_serv/common_service
    servlet: common_server_classname   # e.g. edu.rice.comp405.common.server.CommonServerImpl
    name:   unique_name     # e.g. userCommonServerImpl

All 3 steps above must be repeated every time the common module is used in another utilizing module.

References:

• Large scale application development and Model-View-Presenter (MVP)  (2-part article)
• Structuring GWT modules for large applications
• Security for GWT Applications

Custom GWT Widgets

When creating a complex GUI with multiple, non-trivial panels of controls or when you want the same generic set of controls to be used in multiple places in your system, you should avoid duplicating code by creating a custom GWT widget.

To make a custom GWT widget, you need to use the rather non-obviously named GWT "UiBinder" wizard  under "New/Other.../Google Web Toolkit/UiBinder".   Give the new widget class an appropiate name and make sure that for "Create UI based on:" is set to "GWT widgets".    Deselect the "Generate sample content" because the sample code is more confusing than useful.

Custom GWT widgets are slightly different than GWT module EntryPoint classes.   Two files will be generated:

• [Widget Classname].java -- this is where the Java code to run the widget is located, e.g. event handlers, constructor, etc.   GWT Designer will add some annotations that are not present in a regular EntryPoint class.
• [Widget Classname].ui.xml -- this is the GUI controls layout file and thus, what the GWT Designer can open to do graphical editing of the Widget.    The GWT Designer will automatically put any initialization and event code into the Java file.  

The Java code that you normally see in the "Source" tab in the GWT Designer is in the associated Java file, not the renamed "XML Source" tab.   The graphical layout editor is still on the "Design" tab.

You can add public or package private methods to the widget's Java code to enable the user of the widget to set values and otherwise interact with it.    Adding events can be a bit tricky however.

Attaching events to custom GWT widgets

When you use the GWT Designer to create an event handler for a widget, it will create the event handler in the Java code as a method rather than as an event handler installed into a widget.    The problem is that the custom widget is an encapsulated entity with regards to theparts of the system that use the widget.   You need event handlers that can communicate both with the rest of your system and the internal pieces of the widget.   There are two approaches to this problem:

1. Make a non-private method of your widget that will install an externally supplied event  handler into the desired GUI control.
   • Big drawback:  Because the event handler is outside of the widget's encapsulation it cannot interact with any of the widget's other GUI controls.   For instance, suppose your custom widget has a text field and a button.  You cannot install an event handler for the button that will retrieve the text from the text field because the event handler cannot access the private text field.     You could create non-private methods that break the widget's encapsulation and expose the desired controls, but that solution is problematic.    This technique is thus only really useful if the event handler does not need to interact with the rest of the custom widget.
2. Delegate to an overridden method
   • In the event handler auto-generated by the GWT Designer, delegate to a non-private, abstract method of the widget's class.
     • The input parameters of this new method should be the control(s) and/or data that the the event needs to access.   The event handler code should supply the references to the control(s) and/or data when it makes the delegating call.   In general, passing data rather than control references will protect the custom widget's encapsulation better.   For instance, pass a text field's text rather than a reference to the text field itself if all you want to do is to read the text field.   If the event code needs to mutate the control you can eitehr pass the reference to the control or add protected methods to the widget to mutate the control.
   • In the code that uses the custom widget, simply use an anonymous inner class to override the abstract method and supply the usage-specific code that is needed.  

Known Issues and Problems

GWT Designer very slow to load

The first time it is invoked, the GWT Designer can take over a minute to load.    It will also eat up a large fraction of your system resources.   Subsequent loads shold be faster.   Increasing the amount of memory available to Eclipse may help, see below.

 "This is not a GUI class and can't be edited graphically" Error

This error will arise if you try to edit an EntryPoint class with a defined but empty onModuleLoad() method, e.g. a newly created file.   For some reason, the GWT Designer wants at least one GUI widget to be defined in the onModuleLoad() method before it can perform graphical editing.   Follow the directions above for creating some startup code for the GWT Designer to work with.

References:

• https://groups.google.com/forum/#!msg/google-plugin-eclipse/pjZMk1rOFRk/6WcMMjIep1YJ

Out of Memory Error

If Eclpse crashes with an "out of memory error", try the following configuration changes:

1. Start your favorite text editor (Windows:  start "As Administrator")
2. In the root of the Eclipse EE installation folder, open eclipse.ini
3.  Find the -Xms and -Xmx parameters, probably at the end of the file (the exact values may be different):
   

   -Xms40m
   -Xmx512m

4. Try doubling these values and see if GWT still crashes.
5. If it still crashes, add the -XX:PermSize and -XMMaxPermSize values right after the  -Xms and -Xmx declarations:
   

   -Xms80m
   -Xmx1024m
   -XX:PErsmSize=512m
   -XX:MaxPermSize=1024m

6. Adjust the parameter values until the "Out of memory" error goes away.

Google recommends the following settings:

-vmargs 
-XX:MaxPermSize=128m 
-Xms256m 
-Xmx512m

References:

• What should I do if I encounter an OutOfMemoryError using GWT Designer?
• http://code.google.com/p/gwt-examples/wiki/gwtEclipseFaqs#Make_Eclipse_Faster
• http://blog.ankursharma.org/2009/08/vm-arguments-in-eclipse_29.html
• http://www.eclipsezone.com/eclipse/forums/t61618.html

SVN Errors

The Development Mode Server must be stopped when doing a commit to SVN because the development server has a hold of some temporary files that SVN will fail to commit.

One can set svn:ignore for the gwt-unitCache folder since it is just files to help the GWT compiler remember what has been compiled.

Another option is to mark the gwt-unitCache folder as "Derived" in the folder's Properties.

Removing gwt-unitCache from SVN

The best thing seems to be to set the svn:ignore before the gwt-unitCache is ever committed for the first time to SVN.   But, if you have already committed the cache to the repository at least once already, follow these steps to get SVN to ignore it:

1. Commit everything as usual.
2. In Eclipse, delete the entire gwt-unitCache directory.
3. Commit the entire project again.
4. Right-click the project root and select Team/Set Property
   • Property name = svn:ignore
   • Text property = gwt-unitCache
5. Save everything and commit again.  

If you now do a GWT compile, the gwt-unitCache folder will automatically be recreated. but it should now show up in Eclipse's  Project Explorer with no adornments on its icon indicating any SVN status.  The cached files will not be committed to the repository anymore.

References:

• GWT Home page:  http://www.gwtproject.org
  • Developer's Guide Docs
    • GWT "Super Dev Mode" docs
• http://stackoverflow.com/questions/5663465/setup-a-gwt-project-correctly-with-svn-and-eclipse
• http://kolovos.wordpress.com/2012/11/15/sharing-gwt-projects-to-svn-with-subclipse/

© 2015 by Stephen Wong