Sealing Packages within a JAR File

Packages within JAR files can be optionally sealed, which means that all classes defined in that package must be archived in the same JAR file. You might want to seal a package, for example, to ensure version consistency among the classes in your software.

You seal a package in a JAR file by adding the Sealed header in the manifest, which has the general form:

Name: myCompany/myPackage/
Sealed: true

The value myCompany/myPackage/ is the name of the package to seal.

Note that the package name must end with a "/".

An Example

We want to seal two packages firstPackage and secondPackage in the JAR file MyJar.jar.

We first create a text file named Manifest.txt with the following contents:

Name: myCompany/firstPackage/
Sealed: true

Name: myCompany/secondPackage/
Sealed: true

We then create a JAR file named MyJar.jar by entering the following command:

jar cmf MyJar.jar Manifest.txt MyPackage/*.class

This creates the JAR file with a manifest with the following contents:

Manifest-Version: 1.0
Created-By: 1.6.0 (Sun Microsystems Inc.)
Name: myCompany/firstPackage/
Sealed: true
Name: myCompany/secondPackage/
Sealed: true

Sealing JAR Files

If you want to guarantee that all classes in a package come from the same code source, use JAR sealing. A sealed JAR specifies that all packages defined by that JAR are sealed unless overridden on a per-package basis.

To seal a jar file, use the Sealed manifest header with the value true. For example,

Sealed: true

specifies that all packages in this archive are sealed unless explicitly overridden for particular packages with the Sealed attribute in a manifest entry.