The Linker subsystem takes care of writing the GWT compiler’s output. It’s responsible for each output file’s name, location, and content.

Inside the compiler, each output file is represented by an Artifact.

You usually do not need to care about them, unless you have specific requirements.

Linkers create several kinds of files (see EmittedArtifact.Visibility):

  • Public files are those intended to be downloaded by web browsers. This includes JavaScript and other resource files.
  • Source files are only served to browsers during development. This includes Java source code and sourcemaps that browser debuggers use.
  • Deployed files may be needed by application servers but shouldn’t be served to browsers.
  • Private files include various optional reports about what happened during the compile. Soyc reports are one example.
  • There are also non-emitted artifacts which are used to pass data from one linker to another, but aren’t written as output.

Linkers are divided into three categories:

  • pre-linkers run before the primary linker, in the lexical order of their name.
  • primary linker is the main linker. Only one primary linker is run for a compilation.
  • post-linkers run after the primary linker, in reverse lexical order of their name.

To use a Linker, simply add <add-linker name="linker_name" /> to your Module.gwt.xml file:

  <inherits name="" />
  <add-linker name="sso" />

Or you can also inherit another module which adds the linker for you.

This tag is additive for pre- and post-linkers; only the last primary linker will be run. The xsiframe linker is the default if nothing is specified.

Available linkers

Please refer to the documentation of each linker to check how to use it.

The following primary linkers are defined:

  • xsiframe (class CrossSiteIframeLinker): This linker uses an iframe to hold the code and a script tag to download the code. This is the default linker if no linker is specified.
  • direct_install (class DirectInstallLinker): A linker that adds a script tag directly to the iframe.
  • sso (class SingleScriptLinker): A Linker for producing a single JavaScript file from a GWT module.

The following linkers are deprecated:

  • xs: use xsiframe instead.
  • std: use xsiframe instead.

Custom linkers

In case you have specific needs not covered by the existing linkers, you can create and use your custom linkers.

First, implement your linker. It must be a class extending the Linker abstract class (check its documentation), or AbstractLinker.

Using the @LinkerOrder annotation, you can specify whether it is a pre-, post- or primary linker. You must also annotate it with the @Shardable annotation.

Second, register your linker by putting <define-linker name="short_name" class="fully_qualified_class_name" /> in your Module.gwt.xml file. The name attribute must be a valid Java identifier and is used to identify the Linker in <add-linker> tags.

It is permissible to redefine an already-defined Linker by declaring a new <define-linker> tag with the same name.

You can then use your linker with <add-linker> like any other linker.

Internal linkers

For reference, here are some other linkers that GWT uses. They are not part of GWT’s public API and you shouldn’t refer to them directly.

The following pre-linkers are defined:

  • rpcPolicyManifest (class RpcPolicyManifestLinker): Emit a file containing a map of RPC proxy classes to the partial path of the RPC policy file. Automatically turned on for applications that use GWT-RPC.

The following post-linkers are defined:

  • soycReport (class SoycReportLinker): Converts SOYC report files into emitted private artifacts. This linker is enabled by default, but the SOYC reports themselves are off by default and enabled by the -soyc compiler flag. If the compiler doesn’t generate the reports then this linker doesn’t do anything.
  • symbolMaps (class SymbolMapsLinker): This Linker exports the symbol maps associated with each compilation result as a private file. This linker is enabled by default.
  • rpcLog (class RpcLogLinker): This linker emits RpcLogArtifact` as output files. Automatically turned on for applications that use GWT-RPC.
  • precompress (class PrecompressLinker): A linker that precompresses the public artifacts that it sees. Used through inheriting