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Annotation processors

Annotation processors are basically plugins for the Java compilation workflow. They are presented the opportunity to analyze the compiled sources and generate other sources, classes, and resources based on them. They are a very versatile and useful tool for compile-time code generation.

Processors are supported by the task and allows them running in an incremental manner. Processors can be specified for the task using the AnnotationProcessors parameter:
	SourceDirectories: src,
		ClassPath: lib/my_processor.jar,
		Class: example.MyProcessor

The above is one of the simplest example for adding an annotation processor for the compilation task. The processor with the class name example.MyProcessor is loaded from the lib/my_processor.jar archive, and is called during the compilation.

Processor restrictions

Due to the nature of the system environment and the implementation of the incremental annotation processing, the following restrictions are placed on the processor implementations:

  • They must only use APIs to access environmental resources that is available to them through the object that they were initialized with.
    • This means that they can only use the ProcessingEnvironment and related classes to read files and other properties of the current compilation environment.
    • They can only use the Filer API to read and write files.
    • They must not use the, java.nio.file.Files or related APIs.
  • Non-aggregating must report the elements based on which that they generate the output resources.

Processor classification

In order to reduce the work that needs to be done for incremental compilation, we separate the annotation processor type into two category: Aggregating, and non-aggregating.

The definition for an aggregating processor is as follows: A processor is considered to be Aggregating if there exists an addition-wise modification to the compiled Java classes that causes it to generate different classes or resources.

What does it mean in practice? Generally, if an annotation processor generates resources by taking information from multiple unrelated parts of the codebase, then it is most likely an aggregating processor. One example for one is the following.

Aggregating processor example

Let's imagine a processor, that takes the list of compiled class names, and writes them to a custom output file. If we compile the following classes:

package example;
public class Bar { }
package example;
public class Foo { }

It will generate a text file with the following contents:

// output.txt

Why is it aggregating? Because if we add a new class Baz, it will generate an output with different contents:

package example;
public class Baz { }

Will result in:

// output.txt

We can see that from an incremental perspective, an unrelated Java codebase modification that is not part of the inputs of the output.txt generated file may cause it to have different contents.

The aggregating nature of a processor will result in that the whole compilation set will be passed to it in order for it to do its work. It signals the compiler task that it may not perform additional optimizations as that could lead to incorrect output.

All annotation processors are considered to be aggregating unless specified otherwise.

Non-aggregating processors

If a processor is classified as non-aggregating, then the compiler task is allowed to perform optimizations in order to do less work. Non-aggregating processors generally don't need the whole compilation set as their input, and they can work on a subset of the input.

If a non-aggregating processor creates an output resource, then the contents of that output resource only depends on the specified input elements, and there exists no Java codebase related modification that affects a non-input element, and causes different output resource to be created.

Non-aggregating processor example

An example for a non-aggregating processor is one that simply generates a factory class for annotated classes. See the following inputs and outputs:

package example;
public class Foo { }

The processor will generate the following class for it:

// (output by processor)
package example;
public class FooFactory {
	public static Foo create() {
		return new Foo();

We can see that in this case the input and output have a one-to-one relation. Adding a class to the Java codebase will not cause the processor to generate different FooFactory. If we add a new class, the compiler task is allowed to not pass Foo as its input class element, since it was not modified. This results in the processor doing less work in case of incremental scenarios. However, if Foo is modified, the processor will be re-run accordingly.

Non-aggregating restrictions

In order to work properly, non-aggregating processor implementations need to adhere to the following:

  • They must report the originating elements for the generated sources, classes, and resources. They must use the Filer API correctly when generating resources.
    • The originating elements should contain all Java elements that is an input information used to derive the generated resources.
    • If they don't report the originating elements, that can cause incremental errors. These error are most likely silently ignored by the compiler task, as it has incorrect information to work with.
  • Processors can't generate resources based on the absence of a Java source element. That behaviour would make them aggregating.
  • Non-aggregating processors aren't limited to generating resources using a one-to-one relation. They may take Java elements from multiple locations into account.