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Trail: JAR Files
Lesson: Using JAR Files: The Basics

## Modifying a Manifest File

> You can modify the contents of a JAR file in a couple of ways. One method, available in both versions 1.1 and 1.2 of the Jar tool, uses the `m` command-line option to add custom information to the manifest during creation of a JAR file. The `m` option is described in this section.

> Version 1.2 of the Jar tool also provides a `u` option which you can used to update the contents of an existing JAR file, including its manifest. The `u` option is covered in the section called Updating a JAR File.

> The Jar tool automatically puts a default manifest with pathname `META-INF/MANIFEST.MF` into any JAR file you create. You can enable special JAR file functionality, such as package sealing, by modifying the default manifest. Typically, this involves adding special-purpose headers to the manifest that allow the JAR file to perform a particular desired function. For examples of some special-purpose headers, and for more information about manifest files in general, see the Understanding the Manifest section of this lesson.

> The Jar tool's `m` option allows you to add information to the default manifest during creation of a JAR file. You must first prepare a text file containing the information you wish to add to the default manifest. You can then use the Jar tool's `m` option to add the information in your file to the default manifest.

> The basic command has this format:

> >     jar cmf manifest-addition jar-file input-file(s)

> Let's look at the options and arguments used in this command:
> - The `c` option indicates that you want to create a JAR file.
> - The `m` option indicates that you want to merge information from an existing manifest file into the manifest file of the JAR file you're creating.
> - The `f` option indicates that you want the output to go to a file (the JAR file you're creating) rather than to stdout.
> - `manifest-addition` is the name (or path and name) of the existing text file whose contents you want included in the JAR file's manifest.
> - `jar-file` is the name that you want the resulting JAR file to have.
> - The `input-file(s)` argument is a space-separated list of one or more files that you want to be placed in your JAR file.

> The `c`, `m`, and `f` options can appear in any order, but there must not be any whitespace between them.

### An Example

> In version 1.2 of the JavaTM platform, 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.

> A package can be sealed by adding the `Sealed` header beneath the header naming the package that's to be sealed.:

> >     Name: myCompany/myPackage/
> >     Sealed: true

> The default manifest created by the Jar tool does not contain any `Sealed` headers, of course, because packages are not sealed by default. To seal a package, you therefore have to add the `Sealed` header yourself. To insert the `Sealed` header in a JAR file's manifest, you first need to write a text file containing the appropriate headers. The file you write doesn't have to be a complete manifest file; it can contain just enough information for the Jar tool to know where and what information to merge into the default manifest.

> Let's suppose, for example, that your JAR file is to contain these four packages:

> >     myCompany/firstPackage/
> >     myCompany/secondPackage/
> >     myCompany/thirdPackage/
> >     myCompany/fourthPackage/

> To seal `firstPackage` and `thirdPackage`, you would write a text file with contents that look like this:

> >     Name: myCompany/firstPackage/
> >     Sealed: true

> >     Name: myCompany/thirdPackage/
> >     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.

> Note that the package names end with a "`/`". This file contains the information that needs to be added to the default manifest to seal the desired packages. Each `Sealed` header is immediately preceeded by a `Name` header that indicates which package is to be sealed.

> Let's suppose that:

> - you named your text file `sealInfo`
> - the JAR file you want to create will have the name `myJar.jar`
> - the current directory is the parent directory of `myCompany`

> You would create the JAR file with this command:

> >     jar cmf sealInfo myJar.jar myCompany

> The precise look of the resulting manifest file in `myJar.jar` would depend upon whether you were using version 1.1 or version 1.2 of the JavaTM Development Kit. In either case, the `Sealed` header would be included for `firstPackage` and `thirdPackage`. If you were using the version 1.2 Jar tool, the manifest would look like this:

> >     Manifest-Version: 1.0

> >     Name: myCompany/firstPackage/
> >     Sealed: true

> >     Name: myCompany/thirdPackage/
> >     Sealed: true

> Only the first line, `Manifest-Version: 1.0`, is part of the default manifest. The other lines are in the manifest because you added them when the Jar file was created by using the `m` option.

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