Setting Extension Security Privileges

Sealing Packages in Extensions

You can optionally seal packages in extension JAR files as an additional security measure. If a package is sealed, it means that all classes defined in that package must originate from a single JAR file.

Without sealing, a "hostile" program could create a class and define it to be a member of one of your extension packages. The hostile software would then have free access to package-protected members of your extension package.

To seal your extension packages, you must add the Sealed header to the manifest of the JAR file containing your extension. You can seal individual packages by associating a Sealed header with the packages' Name headers. A Sealed header not associated with an individual package in the archive signals that all packages are sealed. Such a "global" Sealed header is overridden by any Sealed headers associated with individual packages.

The value associated with the Sealed header is either true or false.

Examples

Let's look at a few sample manifest files. For these examples, suppose that the JAR file contains these four packages:

com/myCompany/package_1/
com/myCompany/package_2/
com/myCompany/package_3/
com/myCompany/package_4/

Suppose you want to seal all the packages. You could do so by simply adding an archive-level Sealed header to the manifest like this:

Manifest-Version: 1.0
Sealed: true

All packages in any JAR file having this manifest will be sealed.

If you wanted to seal only com.myCompany.package_3, you could do so with this manifest:

Manifest-Version: 1.0

Name: com/myCompany/package_3/
Sealed: true

In this example, the only Sealed header is that associated with the Name header of package com.myCompany.package_3, so only that package is sealed. (The Sealed header is associated with the Name header because there are no blank lines between them.)

For a final example, suppose you wanted to seal all packages except for com.myCompany.package_2. You could accomplish that with a manifest like this:

Manifest-Version: 1.0
Sealed: true

Name: com/myCompany/package_2/
Sealed: false

In this example, the archive-level Sealed: true header indicates that all the packages in the JAR file are to be sealed. However, the manifest also has a Sealed: false header associated with package com.myCompany.package_2 which overrides the archive-level sealing for that package. Therefore, this manifest will cause all packages except for com.myCompany.package_2 to be sealed.

Setting Extension Security Privileges