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JEP 217: Annotations Pipeline 2.0

Summary​

Redesign the javac annotations pipeline to better address the requirements of annotations and tools that process annotations.

Goals​

  • The compiler should behave correctly with regard to annotations and type annotations: the emitted class files should have correctly-formatted attributes for all kinds of annotations.

  • Compile-time reflection (javax.lang.model and javax.annotation.processing) should properly handle all annotations in signature-visible positions.

  • Run-time reflection (Core Reflection) should work properly with annotations in class files.

  • The Checkers Framework must continue to work.

  • The Java Compiler API should continue to work as designed.

Non-Goals​

No new language features or APIs will be added. It is not a goal to update support for annotations in related tools such as javadoc and javap.

Motivation​

Java SE 8 introduced two new annotation features, Repeating Annotations (JEP 120) and Annotations on Java Types (JSR 308 / JEP 104). In addition Lambda Expressions (JSR 335 / JEP 126) added new syntactic position for annotations. These features, none of which existed when the javac annotation pipeline was initially conceived, can be combined together, leading to patterns such as:

Function<String, String> fss = (@Anno @Anno String s) -> s;

The existing annotation pipeline could not handle such cases out of the box; as a result, the original design has been stretched in order to accommodate the new use cases, leading to a brittle and hard-to-maintain implementation. The goal of this work is to replace this aging architecture with a new one which supports the new use cases in a more straightforward fashion, leading to more-correct and maintainable code.

Description​

Refactor the javac annotation pipeline. This should not be externally noticeable except where we fix bugs and improve correctness. The first step is to improve testing coverage so we can measure and evaluate our exit criteria. After that follows a series of incremental refactorings. This work will be done in the OpenJDK Annotations Pipeline 2.0 Project.

The javadoc tool has some related issues with regards to type annotations. javadoc is, however, undergoing significant work as part of the Javadoc.Next Project. Part of that work includes converting javadoc to use the javax.lang.model API instead of the older com.sun.javadoc API. It is, therefore, not a goal of this project to work on javadoc to ensure that annotations, including type annotations, are presented correctly. It is expected that, as part of the JavaDoc.Next Project, javadoc will be enhanced to leverage the updates to the javax.lang.model API that are a goal of this project.

Testing​

There is already good coverage of a substantial set of end-to-end use cases of annotations. This includes JCK and langtools regression tests. A big part of this work is to develop further tests to ensure the measurability of the success metrics.

We will create tests that exercise the intersection of the new features in Java SE 8, as mentioned above.