Editing JAR (file format) (section)

====Package Sealing====
Optionally, a package within a JAR file can be sealed, which means that all classes defined in that package are archived in the same JAR file. A package might be sealed to ensure version consistency among the classes in the software or as a security measure.

To seal a package, a Name entry needs to appear, followed by a Sealed header, such as:
<syntaxhighlight lang="properties">
Name: myCompany/myPackage/
Sealed: true
</syntaxhighlight>
The Name header's value is the package's relative pathname. Note that it ends with a '/' to distinguish it from a filename. Any headers following a Name header, without any intervening blank lines, apply to the file or package specified in the Name header. In the above example, because the <code>Sealed</code> header occurs after the <code>Name: myCompany/myPackage</code> header with no intervening blank lines, the <code>Sealed</code> header applies (only) to the package <code>myCompany/myPackage</code>.

The feature of sealed packages is outmoded by the Java Platform Module System introduced in Java 9, in which modules cannot split packages.<ref name="JEP 261: Module System">{{cite web | url=https://openjdk.java.net/jeps/261 | title=JEP 261: Module System | access-date=2021-02-06}}</ref>