How to create jar file for the java app you make.

Modifying a Manifest File

You use the m command-line option to add custom information to the manifest during creation of a JAR file. This section describes the m option. The Jar tool automatically puts a default manifest with the 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, modifying the default manifest involves adding special-purpose headers to the manifest that allow the JAR file to perform a particular desired function.
To modify the manifest, you must first prepare a text file containing the information you wish to add to the manifest. You then use the Jar tool's m option to add the information in your file to the manifest.

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Warning:  The text file from which you are creating the manifest 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.

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The basic command has this format:

jar cfm jar-file manifest-addition 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 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 standard output.
• manifest-addition is the name (or path and name) of the existing text file whose contents you want to add to the contents of 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 m and f options must be in the same order as the corresponding arguments.

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Note: The contents of the manifest must be encoded in UTF8.

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The remaining sections of this lesson demonstrate specific modifications you may want to make to the manifest file.

Setting an Application's Entry Point

If you have an application bundled in a JAR file, you need some way to indicate which class within the JAR file is your application's entry point. You provide this information with the Main-Class header in the manifest, which has the general form:

Main-Class: classname

The value classname is the name of the class that is your application's entry point. Recall that the entry point is a class having a method with signature public static void main(String[] args).
After you have set the Main-Class header in the manifest, you then run the JAR file using the following form of the java command:

java -jar JAR-name

The main method of the class specified in the Main-Class header is executed.

An Example

We want to execute the main method in the class MyClass in the package MyPackage when we run the JAR file.
We first create a text file named Manifest.txt with the following contents:

Main-Class: MyPackage.MyClass

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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.

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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: 1.6.0 (Sun Microsystems Inc.)
Main-Class: MyPackage.MyClass

When you run the JAR file with the following command, the main method of MyClass executes:

java -jar MyJar.jar

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