GWT projects can be organized in a variety of ways. However, particular conventions are encouraged to make it easy to identify which code is intended to run on the client browser, the server, or both.
This section describes the fundamentals of project organization with GWT as well as the recommended conventions.
SCRIPT tag. This HTML page is referred to as a host page from the GWT application's point of view. A typical HTML host page for an application written with GWT from scratch might not include any visible HTML body content at all. The example below shows how to embed a GWT application that will use the entire browser window.
Note that the body of the page contains only a
SCRIPT tag and an
IFRAME tag. It is left to the GWT application to then fill in all the visual content.
But GWT was designed to make it easy to add GWT functionality to existing web applications with only minor changes. It is possible to allow the GWT module to selectively insert widgets into specific places in an HTML page. To accomplish this, use the
id attribute in your HTML tags to specify a unique identifier that your GWT code will use to attach widgets to that HTML element. For example:
<body> <!-- ... other sample HTML omitted --> <table align=center> <tr> <td id="slot1"></td> <td id="slot2"></td> </tr> </table> </body>
Notice that the
td tags include an
id attribute associated with them. This attribute is accessible through the
DOM class. You can easily attach widgets using the method
RootPanel.get(). For example:
final Button button = new Button("Click me"); final Label label = new Label(); ... RootPanel.get("slot1").add(button); RootPanel.get("slot2").add(label);
In this manner, GWT functionality can be added as just a part of an existing page, and changing the application layout can be done in plain HTML. The
I18N sample uses this technique heavily.
A host HTML page does not have to be static content. It could also be generated by a servlet, or by a JSP page.
GWT projects are overlaid onto Java packages such that most of the configuration can be inferred from the classpath and the module definitions.
If you are not using the Command-line tools to generate your project files and directories, here are some guidelines to keep in mind when organizing your code and creating Java packages.
For example, all the files for the “DynaTable” sample are organized in a main project directory also called “DynaTable”.
The src directory contains an application's Java source files, the module definition, and external resource files.
The war directory is the deployment image of your web application. It is in the standard expanded war format recognized by a variety of Java web servers, including Tomcat, Jetty, and other J2EE servlet containers. It contains a variety of resources:
A detailed description of the war format is beyond the scope of this document, but here are the basic pieces you will want to know about:
The test directory contains the source files for any JUnit tests.
Individual units of GWT configuration are called modules. A module bundles together all the configuration settings that your GWT project needs:
Modules are defined in XML and placed into your project's package hierarchy. Modules may appear in any package in your classpath, although it is strongly recommended that they appear in the root package of a standard project layout.
A module entry-point is any class that is assignable to EntryPoint and that can be constructed without parameters. When a module is loaded, every entry point class is instantiated and its EntryPoint.onModuleLoad() method gets called.
The default source path is the client subpackage underneath where the Module XML File is stored.
Image widget, you should construct the URL like this:
GWT.getModuleBaseForStaticFiles() + "resourceName.png". When referencing public resources from a Module XML File, just use the relative path within the public folder, the module’s base URL will be prepended automatically. When a module inherits other modules, their public paths are combined so that each module will have access to the static resources it expects.
The default public path is the public subdirectory underneath where the Module XML File is stored.
Modules are defined in XML files with a file extension of .gwt.xml. Module XML files should reside in your project's root package.
If you are using the standard project structure, your module XML can be as simple as the following example:
<module rename-to="dynatable"> <inherits name="com.google.gwt.user.User" /> <entry-point class="com.google.gwt.sample.dynatable.client.DynaTable" /> </module>
Module XML files are found on the Java classpath. Modules are always referred to by their logical names. The logical name of a module is of the form pkg1.pkg2.ModuleName (although any number of packages may be present). The logical name includes neither the actual file system path nor the file extension.
For example, if the module XML file has a file name of...
...then the logical name of the module is:
<module> element supports an optional attribute
rename-to that causes the compiler to behave as though the module had a different name than the long, fully-qualified name. Renaming a module has two primary use cases:
<module rename-to="com.foo.MyModule"> <inherits name="com.foo.MyModule" /> <set-property name="user.agent" value="ie6" /> <set-property name="locale" value="default" /> </module>
WorkingModule.gwt.xml is compiled, the compiler will produce only an
ie6 variant using the default locale; this will speed up development compilations. The output from the
WorkingModule.gwt.xml will be a drop-in replacement for
MyModule.gwt.xml because the compiler will generate the output using the alternate name. (Of course, if
com.foo.MyModule was itself renamed, you would just copy its rename-to attribute.)
Creating a second module doesn‘t necessarily mean that that module must define an entry point. Typically, you create a new module when you want to package up a library of GWT code that you want to reuse in other GWT projects. An example of this is the Google API Library for GWT (GALGWT), specifically the Gears for GWT API binding. If you download the library and take a look at the
gwt-google-apis/com/google/gwt/gears you’ll find the
Gears.gwt.xml file for the module which doesn't define an entry point. However, any GWT project that would like to use Gears for GWT will have to inherit the Gears.gwt.xml module. For example, a module named “Foo” might want to use GALGWT, so in
<inherits> entry would be needed:
<module> ... <inherits name='com.google.gwt.gears.Gears' />
If you have multiple GWT modules in your application, there are two ways to approach loading them.
<script>tag in your HTML host page.
The first approach may seem the easiest and most obvious. However, the second approach will lead to much better end-user performance. The problem with loading multiple modules is that each module has to be downloaded separately by the end-user's browser. In addition, each module will contain redundant copies of GWT library code and could possibly conflict with each other during event handling. The second approach is strongly recommended.
For more information, see Linkers
<super-source> tag instructs the compiler to “re-root” a source path. This is useful for cases where you want to be re-use an existing Java API for a GWT project, but the original source is not available or not translatable. A common reason for this is to emulate part of the JRE not implemented by GWT.
For example, suppose you want implement the UUID class provided by the JRE under
java.util. Assume your project's module file is
com/example/myproject/MyProject.gwt.xml. Place the source for the UUID class into
com/example/myproject/jre/java/util/UUID.java. Then add a line to
<super-source path="jre" />
This tells the compiler to add all subfolders of
com/example/myproject/jre/ to the source path, but to strip off the path prefix up to and including
jre. As a result,
com/google/myproject/gwt/jre/java/util/UUID.java will be visible to the compiler as
java/util/UUID.java, which is the intended result.
The GWT project uses this technique internally for the JRE emulation classes provided with GWT. One caveat specific to overriding JRE classes in this way is that they will never actually be used in development mode. In development mode, the native JRE classes always supercede classes compiled from source.
<super-source> element supports pattern-based filtering to allow fine-grained control over which resources get copied into the output directory during a GWT compile.
This section documents the most commonly used elements in the module XML file.
<inherits name="_logical-module-name_" /> : Inherits all the settings from the specified module as if the contents of the inherited module's XML were copied verbatim. Any number of modules can be inherited in this manner. See also this advice about deciding which modules to inherit.
<entry-point class="_classname_" /> : Specifies an entry point class. Any number of entry-point classes can be added, including those from inherited modules. Entry points are all compiled into a single codebase. They are called sequentially in the order in which they appear in the module file. So when the
onModuleLoad() of your first entry point finishes, the next entry point is called immediately.
<source path="_path_" /> : Each occurrence of the
<source> tag adds a package to the source path by combining the package in which the module XML is found with the specified path to a subpackage. Any Java source file appearing in this subpackage or any of its subpackages is assumed to be translatable. The
<source> element supports pattern-based filtering to allow fine-grained control over which resources get copied into the output directory during a GWT compile.
<source>element is defined in a module XML file, the client subpackage is implicitly added to the source path as if
<source path="client" />had been found in the XML. This default helps keep module XML compact for standard project layouts.
<public path="_path_" />: Each occurrence of the
<public>tag adds a package to the public path by combining the package in which the module XML is found with the specified path to identify the root of a public path entry. Any file appearing in this package or any of its subpackages will be treated as a publicly-accessible resource. The
<public>element supports pattern-based filtering to allow fine-grained control over which resources get copied into the output directory during a GWT compile.
<public>element is defined in a module XML file, the public subpackage is implicitly added to the public path as if
<public path="public">had been found in the XML. This default helps keep module XML compact for standard project layouts.
<servlet path="_url-path_" class="_classname_" />: For RPC, this element loads a servlet class mounted at the specified URL path. The URL path should be absolute and have the form of a directory (for example,
/calendar). Your client code then specifies this URL mapping by annotating the service interface with the @RemoteServiceRelativePath attribute. Any number of servlets may be loaded in this manner, including those from inherited modules.
<servlet>element applies only to GWT's embedded server server-side debugging feature.
NOTE: as of GWT 1.6, this tag does no longer loads servlets in development mode, instead you must configure a
WEB-INF/web.xmlin your war directory to load any servlets needed.
<stylesheet src="_css-url_" /> : Automatically injects the external CSS file located at the location specified by src. See automatic resource inclusion for details. If the specified URL is not absolute, the resource will be loaded from the module's base URL (in other words, it would most likely be a public resource).
<extend-property name="_client-property-name_" values="_comma-separated-values_" /> : Extends the set of values for an existing client property. Any number of values may be added in this manner, and client property values accumulate through inherited modules. You will likely only find this useful for specifying locales in internationalization.
The following elements are used for defining deferred binding rules. Deferred binding is not commonly used in user projects.
<replace-with class="_replacement_class_name_"> : A directive to use deferred binding with replacement.
<generate-with class="_generator_class_name_"> : A directive to use deferred binding using a Generator
<define-property name="_property_name_" values="_property_values_"> : Define a property and allowable values (comma-separated identifiers). This element is typically used to generate a value that will be evaluated by a rule using a
<set-property name="_property_name_" value="_property_value_"> : Set the value of a previously-defined property (see
<define property> above). This element is typically used to generate a value that will be evaluated by a rule using a
<when...> element. Note that
property-provider on the same value will overwrite each other. The last definition encountered is the one that is used.
<when...> element. To see examples of
<property-provider> definitions in action, see the files
UserAgent.gwt.xml in the GWT source code. Note that
property-provider on the same value will overwrite each other. The last definition encountered is the one that is used.
<generate-with-class> elements can take a
<when...> child element that defines when this rule should be used, much like the
WHERE predicate of an SQL query. The three different types of predicates are:
<when-property-is name="_property_name_" value="_value_" /> : Deferred binding predicate that is true when a named property has a given value.
<when-type-assignable class="_class_name_" /> : Deferred binding predicate that is true for types in the type system that are assignable to the specified type.
<when-type-is class="_class_name_" /> : Deferred binding predicate that is true for exactly one type in the type system.
Several different predicates can be combined into an expression. Surround your
<when...> elements using the following nesting elements begin/end tags:
</all> : Predicate that ANDs all child conditions.
</any> : Predicate that ORs all child conditions.
</none> : Predicate that NANDs all child conditions.
As an example module XML file that makes use of deferred binding rules, here is a module XML file from the GWT source code, Focus.gwt.xml:
<module> <inherits name="com.google.gwt.core.Core" /> <inherits name="com.google.gwt.user.UserAgent" /> <!-- old Mozilla, and Opera need a different implementation --> <replace-with class="com.google.gwt.user.client.ui.impl.FocusImplOld"> <when-type-is class="com.google.gwt.user.client.ui.impl.FocusImpl" /> <any> <when-property-is name="user.agent" value="gecko" /> <when-property-is name="user.agent" value="opera" /> </any> </replace-with> <!-- Safari needs a different hidden input --> <replace-with class="com.google.gwt.user.client.ui.impl.FocusImplSafari"> <when-type-is class="com.google.gwt.user.client.ui.impl.FocusImpl" /> <when-property-is name="user.agent" value="safari" /> </replace-with> <!-- IE's implementation traps exceptions on invalid setFocus() --> <replace-with class="com.google.gwt.user.client.ui.impl.FocusImplIE6"> <when-type-is class="com.google.gwt.user.client.ui.impl.FocusImpl" /> <any> <when-property-is name="user.agent" value="ie6" /> </any> </replace-with> </module>
GWT libraries are organized into modules. The standard modules contain big pieces of functionality designed to work independently of each other. By selecting only the modules you need for your project (for example the JSON module rather than the XML module), you minimize complexity and reduce compilation time.
Generally, you want to inherit at least the User module. The User module contains all the core GWT functionality, including the EntryPoint class. The User module also contains reusable UI components (widgets and panels) and support for the History feature, Internationalization, DOM programming, and more.
GWT 1.5 also provides several theme modules which contain default styles for widgets and panels. You can specify one theme in your project's module XML file to use as a starting point for styling your application, but you are not required to use any of them.
To inherit a module, edit your project's module XML file and specify the logical name of the module you want to inherit in the
Note: Modules are always referred to by their logical names. The logical name of a module is of the form pkg1.pkg2.ModuleName (although any number of packages may be present). The logical name includes neither the actual file system path nor the file extension.
Versions of GWT prior to 1.4 required a script-ready function to determine when an included script was loaded. This is no longer required; all included scripts will be loaded when your application starts, in the order in which they are declared.
Stylesheet inclusion is a convenient way to automatically associate external CSS files with your module. Use the following syntax to cause a CSS file to be automatically attached to the host page.
You can add any number of stylesheets this way, and the order of inclusion into the page reflects the order in which the elements appear in your module XML.
If an absolute URL is specified in the
src attribute, that URL will be used verbatim. However, if a non-absolute URL is used (for example, “foo.css”), the module‘s base URL is prepended to the resource name. This is identical to constructing an absolute URL using
GWT.getModuleBaseForStaticFiles() + "foo.css" in client code. This is useful when the target resource is from the module’s public path.
The module XML format's
<super-source> elements supports certain attributes and nested elements to allow pattern-based inclusion and exclusion in the public path. These elements follow the same rules as Ant's
FileSet element. Please see the documentation for
FileSet for a general overview. These elements do not support the full
FileSet semantics. Only the following attributes and nested elements are currently supported:
Other attributes and nested elements are not supported.
The default value of
true. By default, the patterns listed here are excluded.
Consider the following HTML page that loads a GWT module:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html> <body onload='alert("w00t!")'> <img src='bigImageZero.jpg'></img> <script source='externalScriptZero.js'></script> <img src='bigImageOne.jpg'></img> <img src='reallyBigImageTwo.jpg'></img> <script src='myApp/myApp.nocache.js'></script> <script src='externalScriptOne.js'></script> </body> </html>
The following principles are needed to understand the sequence of operations that will occur in this page:
<script>tags always block evaluation of the page until the script is fetched and evaluated.
<img>tags do not block page evaluation.
body.onload()event will only fire once all external resources are fetched, including images and frames.
myApp/myApp.nocache.js) will be fetched and evaluated like a normal script tag, but the compiled script will be fetched asynchronously.
onModuleLoad()can be called at any point after the outer document has been parsed.
Applying these principles to the above example, we obtain the following sequence:
bigImageZero.jpgcompletes (let's assume). Parsing is blocked until
externalScriptZero.jsis done fetching and evaluating.
bigImageOne.jpgcompletes (let's assume again).
myApp/myApp.nocache.jsbegins fetching and evaluating.
myApp/myApp.nocache.jscompletes, and the compiled script (
<hashname>.cache.html) begins fetching in a hidden
IFRAME(this is non-blocking).
onModuleLoad()is not called yet, as we're still waiting on
externalScriptOne.jsto complete before the document is considered ‘ready’.
externalScriptOne.jscompletes. The document is ready, so onModuleLoad() fires.
body.onload()fires, in this case showing an alert() box.
This is a bit complex, but the point is to show exactly when various resources are fetched, and when
onModuleLoad() will be called. The most important things to remember are that
<img>tags are not guaranteed to be done loading when
<script>tags are guaranteed to be done loading when
If you have multiple EntryPoints (the interface that defines
onModuleLoad()) within a module, they will all be called in sequence as soon as that module (and the outer document) is ready.
If you are loading multiple GWT modules within the same page, each module‘s EntryPoint will be called as soon as both that module and the outer document is ready. Two modules’ EntryPoints are not guaranteed to fire at the same time, or in the same order in which their selection scripts were specified in the host page.