The TwoPhaseCoreAnnotationProcessor class is an abstract class that implements the APT CoreAnnotationProcessor interface. It breaks the work of the process() method of the CoreAnnotationProcessor down into two distinct phases, represented as abstract method of TwoPhaseCoreAnnotationProcessor that are to be implemented by concrete subclasses.

The two phases of processing are: The check phase is used to validate input Declarations that have been annotated with annotations claimed by the processor to ensure that it is semantically valid. If the presence of the input Declaration implies the need to add new files, and those files need to be visible during the check phase for other Declarations, then the CoreAnnotationProcessorEnv's Filer API should be used to add those files in this phase. The adding of such files at this point should typically not result in their emission to persistent storage (i.e. disk), but rather be kept in memory to be referenced by the check phase of other Declarations. The generate phase will actually emit any source, binary, or class files that are derived from the input Declaration, including files added via the Filer API during the check phase. The Filer API may also be used in this phase to add new files, however, such additions will not be visible during the check phase of any Declarations.

The benefits of breaking process() down into check() and generate() phases are: Makes it possible to perform the semantic validation of Declarations without necessarily resulting in code generation. Provides a clearer association between input Declarations and generator output. TwoPhaseCoreAnnotationProcessor is intended provide a uniform mechanism for writing CoreAnnotationProcessor implementations that can be used in tooling environments more sophisticated than command-line tools (that may not do all their work on source in a single pass). Such environments will typically also provide implementations of the CoreAnnotationProcessorEnv and associated interfaces (Messager, Filer etc).