In some recent classes, my students have encountered a package-info.java file tucked inside of some of the sample code we get from courseware providers.  “What’s that thing?”, they usually ask.

package-info.java’s purpose

The package-info.java is a Java file that can be added to any Java source package.  Its purpose is to provide a home for package level documentation and package level annotations. Simply create the package-info.java file and add the package declaration that it relates to in the file.  In fact, the only thing the package-info.java file must contain is the package declaration.

The package-info.java file above must sit in the com.intertech.services package.

Package Documentation

Prior to Java 5, package level documentation (the documentation shown in Javadocs for a package) was placed in package.html.  Today, the description and other related documentation for a package can be written up in the package-info.java file and it gets used in the production of the Javadocs.  As a demonstration, the example package-info.java…

… results in the following Javadocs.

Package Annotations

Perhaps more importantly to today’s annotation driven programmer, the package-info.java file contains package level annotations. An annotation with ElementType.PACKAGE as one of its targets is a package-level annotation and there are many of them.  Using your favorite IDE’s code assistant (shown in Eclipse below) in a package-info.java file and you will find a number package annotation options.

For example, perhaps you want to deprecate all the types in a package. You could annotate each individual type (the classes, interfaces, enums, etc. defined in their .java files) with @Deprecated (as shown below).

Or, you could use the @Deprecated on the package declaration in package-info.java.  This has the effect of deprecating everything in the package in one fell swoop.

Help adding package-info.java to your packages

While you can add the package-info.java file to your packages by hand (just as you can create Java classes by hand), IDE’s often offer you the option to include a package-info.java file each time you create a new package.  Eclipse, shown below, offers a simple (and often overlooked) checkbox in the New Java Package creation wizard.

Wrap Up
So now you know what that package-info.java file is all about and you know how to use it.

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