## Incremental project builders

An incremental project builder is an object that manipulates the resources in a project in a particular way. Incremental project builders are often used to apply a transformation on a resource to produce a resource or artifact of another kind.

Plug-ins contribute incremental project builders to the platform in order to implement specialized resource transformations. For example, the Java development tools (JDT)define an incremental project builder that compiles a Java source file into a class file any time a file is added or modified in a Java project. It also keeps track of dependent files and recompiles them when necessary.

From an API point of view, the platform defines two basic types of builds:

- A full build performs a build from scratch. It treats all resources in a project as if they have never been seen by the builder.
- An incremental build uses a "last build state," maintained internally by the builder, to do an optimized build based on the changes in the project since the last build.

Incremental builds are seeded with a resource change delta. The delta reflects the net effect of all resource changes since the builder last built the project. This delta is similar to the one used inside resource change events.

Projects can be periodically cleaned by the user in order to force a rebuild of a complete project the next time an incremental build is performed on that project. Cleaning a project removes build information such as problem markers and class files.

Builders are best understood by example. The JDT Java compiler is driven by a Java incremental project builder which recompiles the files in a project that are affected by changes. When a full build is triggered, (or an incremental build after a clean), all of the .java files in the project are compiled. Any compile problems encountered are added as problem markers on the affected .java files. When an incremental build is triggered, the builder selectively recompiles the added, changed, or otherwise affected .java files that are described in the resource delta and updates the problem markers as necessary. Any .class files or markers that are no longer appropriate are removed.

Incremental building has obvious performance benefits for projects with hundreds or thousands of resources, most of which are unchanging at any given point in time.

The technical challenge for incremental building is to determine exactly what needs to be rebuilt. For example, the internal state maintained by the Java builder includes things like a dependency graph and a list of compilation problems reported. This information is used during an incremental build to identify which classes need to be recompiled in response to a change in a Java resource.

Although the basic structure for building is defined in the platform, the real work is done in the builder code. Patterns for implementing complex incremental builders are beyond the scope of this discussion, since the implementation is dependent on the specific builder design.

### Invoking a build

A builder can be invoked explicitly in one of the following ways:

- IProject.build() runs the build processing on the receiving project according to the build method's arguments.
- IWorkspace.build() runs the build processing on all open projects in the workspace.

In practice, the workbench user triggers a build by selecting corresponding commands in the resource navigator menu.

Incremental project builders are also invoked implicitly by the platform during an auto-build. If enabled, auto-builds run whenever the workspace is changed.

### Defining an incremental project builder

The org.eclipse.core.resources.builders extension point is used to contribute an incremental project builder to the platform. The following markup shows how the hypothetical plug-in com.example.builders could contribute an incremental project builder.

       <extension
          id="mybuilder" name="My Sample Builder" point="org.eclipse.core.resources.builders">
          <builder
             <run
                class="com.example.builders.BuilderExample">
                <parameter name="optimize" value="true" />
                <parameter name="comment" value="Builder comment" />
             </run>
          </builder>
       </extension>

The class identified in the extension point must extend the platform class IncrementalProjectBuilder.

       public class BuilderExample extends IncrementalProjectBuilder {
          IProject[] build(int kind, Map args, IProgressMonitor monitor)
                throws CoreException {
             // add your build logic here
             return null;
          }
          protected void startupOnInitialize() {
             // add builder init logic here
          }
          protected void clean(IProgressMonitor monitor) {
             // add builder clean logic here
          }
       }

Build processing begins with the build() method, which includes information about the kind of build that has been requested. The build is one of the following values:

- FULL_BUILD indicates that all resources in the project should be built.
- INCREMENTAL_BUILD indicates that the build is incremental.
- AUTO_BUILD indicates that an incremental build is being triggered automatically because a resource has changed and the autobuild feature is on.

If an incremental build has been requested, a resource delta is provided to describe the changes in the resources since the last build. The following snippet further refines the build() method.

       protected IProject[] build(int kind, Map args, IProgressMonitor monitor
             throws CoreException {
          if (kind == IncrementalProjectBuilder.FULL_BUILD) {
             fullBuild(monitor);
          } else {
             IResourceDelta delta = getDelta(getProject());
             if (delta == null) {
                fullBuild(monitor);
             } else {
                incrementalBuild(delta, monitor);
             }
          }
          return null;
       }

It sometimes happens that when building project "X," a builder needs information about changes in some other project "Y." (For example, if a Java class in X implements an interface provided in Y.) While building X, a delta for Y is available by calling getDelta(Y). To ensure that the platform can provide such deltas, X's builder must have declared the dependency between X and Y by returning an array containing Y from a previous build() call. If a builder has no dependencies, it can simply return null. See IncrementalProjectBuilder for further information.

#### Full build

The logic required to process a full build request is specific to the plug-in. It may involve visiting every resource in the project or even examining other projects if there are dependencies between projects. The following snippet suggests how a full build might be implemented.

       protected void fullBuild(final IProgressMonitor monitor) throws CoreException {
          try {
             getProject().accept(new MyBuildVisitor());
          } catch (CoreException e) { }
       }

The build visitor would perform the build for the specific resource (and answer true to continue visiting all child resources).

       class MyBuildVisitor implements IResourceVisitor {
          public boolean visit(IResource res) {
             //build the specified resource.
             //return true to continue visiting children.
             return true;
          }
       }

The visit process continues until the full resource tree has been traveled.

#### Incremental build

When performing an incremental build, the builder works with a resource change delta instead of a complete resource tree.

       protected void incrementalBuild(IResourceDelta delta,
             IProgressMonitor monitor) throws CoreException {
          // the visitor does the work.
          delta.accept(new MyBuildDeltaVisitor());
       }

The visit process continues until the complete resource delta tree has been traveled. The specific nature of changes is similar to that described in Implementing a resource change listener. One important difference is that with incremental project builders, you are working with a resource delta based on a particular project, not the entire workspace.

#### Cleaning before a build

The workbench allows users to clean a project or set of projects before initiating a build. This feature allows the user to force a rebuild from scratch on only certain projects. Builders should implement this method to clean up any problem markers and derived resources in the project.

### Associating an incremental project builder with a project

To make a builder available for a given project, it must be included in the build spec for the project. A project's build spec is a list of commands to run, in sequence, when the project is built. Each command names a single incremental project builder.

NOTE: The builder name in a build command is the fully qualified id of the builder extension. The fully qualified id of an extension is created by combining the plug-in id with the simple extension id in the plugin.xml file. For example, a builder with simple extension id "mybuilder" in the plug-in "com.example.builders" would have the name "com.example.builders.mybuilder"

The following snippet adds a new builder as the first builder in the existing list of builders.

       final String BUILDER_ID = "com.example.builders.mybuilder";
       IProjectDescription desc = project.getDescription();
       ICommand[] commands = desc.getBuildSpec();
       boolean found = false;

       for (int i = 0; i < commands.length; ++i) {
          if (commands[i].getBuilderName().equals(BUILDER_ID)) {
             found = true;
             break;
          }
       }
       if (!found) {
          //add builder to project
          ICommand command = desc.newCommand();
          command.setBuilderName(BUILDER_ID);
          ICommand[] newCommands = new ICommand[commands.length + 1];

          // Add it before other builders.
          System.arraycopy(commands, 0, newCommands, 1, commands.length);
          newCommands[0] = command;
          desc.setBuildSpec(newCommands);
          project.setDescription(desc, null);
       }

Configuring a project's builder is done just once, usually as the project is being created.