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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

---

Warning: The text file must end with a new line or carriage return. The last line will not be parsed properly if it does not end with a new line or carriage return.

---

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: 1.5.0_01 (Sun Microsystems Inc.)
Name: myCompany/firstPackage/
Sealed: true
Name: myCompany/secondPackage/
Sealed: true

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