JAR File Format and 'jar' Tool

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(Continued from previous part...)

What Is "manifest"?

"manifest" in a JAR file is a file named as META-INF/MANIFEST.MF. It contains attributes about the JAR file and its contents.

Attributes in "manifest" are recorded as a list of name-value pairs in the form of "name: value\n". The list is divided into a main section for package-level attributes and multiple sub sections for entry-level attributes. Note that each attribute must be ended with a new line character.

The main section may contain only the package level attributes like:

• Manifest-Version
• Created-By
• Main-Class
• Class-Path
• Implementation-Vendor
• Implementation-URL

Sub sections are optional. They are used to provide entry-level attributes with one section per entry. A sub sections must be preceded with a blank line and started with a "Name" attribute to associate this section with a content entry in the JAR file. Example of entry-level attributes are:

• Content-Type
• Java-Bean

Adding "manifest" to JAR Files

There are two ways to add "manifest" to a JAR file.

1. Adding "manifest" through "jar" command line. Store all your manifest attributes in a file. And specify this file in the "jar" command line with the "m" option:

jar c[v0]mf manifest jarfile inputfiles

For example, I created my own manifest file called manifest.txt with one attribute in it:

Main-Class: Hello

Remember to press the <Enter> key at the end of the attribute. This will insert a new line character (\n) to terminate the attribute.

Here is how I added my manifest to a JAR file with the "m" option:

>jar cvmf manifest.txt hello.jar Hello.class
added manifest
adding: Hello.class(in = 416) (out= 285)(deflated 31%)

>jar xvf hello.jar
  created: META-INF/
extracted: META-INF/MANIFEST.MF
extracted: Hello.class

>type META-INF\MANIFEST.MF
Manifest-Version: 1.0
Created-By: 1.4.2 (Sun Microsystems Inc.)
Main-Class: Hello

As you can see that "jar" command copied the attribute from my manifest file to the end of the auto-generated MANIFEST.MF file.

2. Adding "manifest" through META-INF/MANIFEST.MF file. Create META-INF/MANIFEST.MF as a text file. Enter all your manifest attributes in this file. And include META-INF/MANIFEST.MF as an input file to JAR file.

For example, I created my own META-INF/MANIFEST.MF with a text editor as:

Manifest-Version: 3.3
Created-By: Herong Yang
Main-Class: Hello

Here is how I added my manifest to a JAR file with the "M" option:

>jar cvMf tutu.jar Hello.class META-INF
adding: Hello.class(in = 416) (out= 285)(deflated 31%)
adding: META-INF/(in = 0) (out= 0)(stored 0%)
adding: META-INF/MANIFEST.MF(in = 19) (out= 21)(deflated -10%)

C:\herong\jtool_20051119\src>jar tf tutu.jar
Hello.class
META-INF/
META-INF/MANIFEST.MF

Note that:

• The "M" option stops "jar" to auto-generate the META-INF/MANIFEST.MF file.
• When MANIFEST.MF is included as an input file, it was compressed. This is different than the auto-generated version, which was not compressed at all.

(Continued on next part...)

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