Documentation TaskDoc JavaDoc Packages
  2. Extending
  3. Task development
  4. Hello world

Hello world

Let's start with the simplest task possible that only writes a simple string to the output:

public class HelloWorldTaskFactory 
		implements TaskFactory<Void>, Externalizable {
	public HelloWorldTaskFactory() {

	public Task<? extends Void> createTask(ExecutionContext executioncontext) {
		return new Task<Void>() {
			public Void run(TaskContext taskcontext) throws Exception {
				taskcontext.println("Hello world!");
				return null;

	public void writeExternal(ObjectOutput out) 
			throws IOException {

	public void readExternal(ObjectInput in) 
			throws IOException, ClassNotFoundException {

	public int hashCode() {
		return getClass().getName().hashCode();

	public boolean equals(Object obj) {
		return ObjectUtils.isSameClass(this, obj);

The actual printing to the output happens in the run(TaskContext) method. It calls the println(String) method on the task context of the task, which will result in the "Hello world!" string to be displayed in the output.

All the other methods are seemingly extraneous, but they all have a specific functionality in order to work with the system properly. Let's view them one by one.


The overriden createTask() method of TaskFactory is called when the build system starts to invoke the task. A Task instance is created, that is a stateful object, and it will have its run(TaskContext) method called by the build system.

The ExecutionContext is the context object for accessing features of the build system that is associated with the current build execution. The TaskContext is the context object for accessing features of the build system that is strictly related to the task, but not the whole execution. It can be used to report dependencies, get the deltas from previous run, start new tasks, and others.

In this createTask() method we will return a new anonymous class for our task, only for simplification reasons. In normal scenarios it's strongly recommened to export the task to a static inner class, or a new top level class.


This method is defined by the Task interface, and it is called when the build system invokes the task to execute its operations. This is the method where the computations are being done.

taskcontext.println("Hello world!");

In the above implementation we just print out a simple message which will be displayed in the standard output of the build. When we execute the build the next time for our task, it won't be run, as it has not changed, but the build system will display the printed message again.

The run method declares itself to be able to throw any kind of Exception if the operations of it fails.


We defined the HelloWorldTaskFactory to implement Externalizable. This is in order to have a more fine grained control over the serialization of the task factory. Using Externalizable instead of Serializable is likely to be more performant as well. Another note is that the system implements its own serialization methods for persisting the build database, in which Externalizable is strongly preferred.


In the above example, the task factory has no fields. It will always do the same thing, therefore any other task factories which have the same class as HelloWorldTaskFactory is considered to be equal. In order to implement this, we only check the class identity of the argument in equals, using the ObjectUtils utility class.

ObjectUtils.isSameClass(this, obj);

The hashCode computes its result based on the current class' name instead of calling hashCode() directly on the class. This is in order to have a stable hash code between executions. This is not required, but can be beneficial in some cases. (See Task caching.)



The class needs a public no-arg constructor for two reasons. First is for it to be instantiatable via reflection when it is loaded. Second is because the Externalizable interface requires it in order to properly deserialize an instance.

Some repository implementations may not require a public no-arg constructor, but if you decide not to include one in your TaskFactory implementation, make sure you test it beforehand for proper operation.

Further examples

In further examples, where the other methods are semantically the same as in the above example, we're not going to include them in the article example codes, and will only focus on the run(TaskContext) function or the Task implementation in whole.