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Trail: Deployment
Lesson: Packaging Programs in JAR Files
Section: Working with Manifest Files: The Basics
Packaging Programs in JAR Files
Using JAR Files: The Basics
Creating a JAR File
Viewing the Contents of a JAR File
Extracting the Contents of a JAR File
Updating a JAR File
Running JAR-Packaged Software
Working with Manifest Files: The Basics
Understanding the Default Manifest
Modifying a Manifest File
Setting an Application's Entry Point
Adding Classes to the JAR File's Classpath
Setting Package Version Information
Sealing Packages within a JAR File
Signing and Verifying JAR Files
Understanding Signing and Verification
Signing JAR Files
Verifying Signed JAR Files
Using JAR-related APIs
The JarClassLoader Class
The JarRunner Class
Home Page > Deployment > Packaging Programs in JAR Files
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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: 1.6.0 (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.

> For further information on how to seal jar files, see JDC TechTips

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