01 August 2008

Compiling package-info.java over and over again...

Java packages may have a corresponding package-info.java source file. This file provides a central location for package-wide information. A Java compiler will compile the package-info.java source file just like any other Java source file... sort of.

The thing is, the package-info.java source file is a special sort of source file:

  • Its contents typically consist of Javadoc documentation comments that describe the package. In fact, the package-info.java feature replaces the legacy package.html file that served the same purpose.
  • Sometimes a package is annotated in its package-info.java file.
  • Usually, package-info.java is devoid of source code proper.

See The Java Language Specification, 3rd ed. sec "Package Annotations" for more information on the intention of the package-info.java feature.

No corresponding package-info.class file is produced by the compiler when a package-info.java file contains only Javadoc comments. Why is this an issue? It's an issue because most build tools interpret the presence of an up-to-date .class file as a reason not to compile a corresponding .java file. Without a package-info.class, a build system might compile package-info.java every time the build is invoked regardless of whether package-info.java has changed. Such unnecessary compiler invocations have the effect of increasing build execution time. This is not such a big deal for small projects, but the impact on build time can become annoying as the size of a codebase grows.

In my environment a corresponding package-info.class file is created only if at least one package annotation exists. Intuitively, it would seem that such annotations would need to have retention policy "stronger" than java.lang.annotation.RetentionPolicy.SOURCE. However, I observe the generation of a package-info.class file even if no annotation exists with a stronger retention policy (RetentionPolicy.CLASS (the default) or RetentionPolicy.RUNTIME).

The presence of source code proper does not cause a package-info.class file to be produced. Rather, class files with a names appropriate to the types defined in the source code are generated. For example, if I write a package private Foo class in package-info.java, then Foo.class is generated.

So then, how can a build be protected from unnecessarily invoking the compiler due to the lack of a package-info.class file? One solution to this issue is to annotate each package with an annotation that serves no purpose other than to force the generation of a corresponding package-info.class file.

The annotation I wrote for this solution is called ForcePackageByteCode. I gave it a retention policy of RetentionPolicy.CLASS which clearly states that it needs to be in a .class file. Just annotate the package statement in a package-info.java file with ForcePackageByteCode:

 * This package handles all of the foo.
package com.example.foo;

Admittedly, this solution is not ideal because it creates extraneous output for the sole purpose of optimizing future build invocations. On the plus side, the size of the extraneous files is relatively small. This minor cost in size can be well worth the build time savings over the course of a project.

It would be nice if release candidate builds could "turn-off" the extraneous file generation by simply changing the ForcePackageByteCode retention policy to RetentionPolicy.SOURCE. Instead, a global search and replace opertion that changes all occurrences of the annotation to a comment for all package-info.java files will do the trick:


There are other solutions to this problem. The Apache Ant build tool is addressing this issue with built-in tool support. The Ant team has devised some heuristics for deciding when to ignore package-info.java files during compilation. Such work is helpful, but relying on support from a particular build tool has the drawback of being a partial solution if source is built in several build environments.

Alternatively, the need for a solution like the ForcePackageByteCode annotation is reduced if there is another reason for a project to annotate packages. For example, the static byte code analysis tool FindBugs supports the edu.umd.cs.findbugs.annotations.DefaultAnnotation annotation. This annotation declares that specified annotations be applied to all classes, fields, and methods of a package. For example, annotating a package with @DefaultAnnotation(NonNull.class) tells FindBugs that the NonNull annotation should be applied to all classes, fields, and methods of a package. Applying DefaultAnnotation to a package-info.java file causes a package-info.class to be produced obviating the need for ForcePackageByteCode.

ForcePackageByteCode source code is available as part of the Virtual Team Tools project.