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Trail: The Extension Mechanism
Lesson: Making Extensions Secure

## 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`.

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