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
myCompany/myPackage/ is the name of the package to seal.
Note that the package name must end with a "/".
We want to seal two packages
secondPackage in the JAR file
We first create a text file named
Manifest\.txt with the following contents:
Name: myCompany/firstPackage/ Sealed: true Name: myCompany/secondPackage/ Sealed: true
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 cfm MyJar.jar Manifest.txt MyPackage/*.class
This creates the JAR file with a manifest with the following contents:
Manifest-Version: 1.0 Created-By: 1.7.0_06 (Oracle Corporation) Name: myCompany/firstPackage/ Sealed: true Name: myCompany/secondPackage/ Sealed: true
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,
specifies that all packages in this archive are sealed unless explicitly overridden for particular packages with the
Sealed attribute in a manifest entry.