Sealing Packages in Extensions

You can optionally seal packages in extension JAR files as an additional security measure. If a package is sealed, 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.
Sealing packages in extensions is no different than sealing any JAR-packaged classes. 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 packages:
com/myCompany/package_1/
com/myCompany/package_2/
com/myCompany/package_3/
com/myCompany/package_4/
Suppose that 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 that 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 of 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, and that header overrides the archive-level sealing for that package. Therefore this manifest will cause all packages to be sealed except for com.myCompany.package_2.

Making Extensions Secure