创建可执行的jar包

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

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 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

Setting an Entry Point with the JAR Tool

The 'e' flag (for 'entrypoint'), introduced in JDK 6, creates or overrides the manifest's Main-Class attribute. It can be used while creating or updating a jar file. Use it to specify the application entry point without editing or creating the manifest file.
For example, this command creates app.jar where the Main-Class attribute value in the manifest is set to MyApp:

jar cfe app.jar MyApp MyApp.class

You can directly invoke this application by running the following command:

java -jar app.jar

If the entrypoint class name is in a package it may use a '.' (dot) character as the delimiter. For example, if Main.class is in a package called foo the entry point can be specified in the following ways:

jar cfe Main.jar foo.Main foo/Main.class

Adding Classes to the JAR File's Classpath

You may need to reference classes in other JAR files from within a JAR file.

For example, in a typical situation an applet is bundled in a JAR file whose manifest references a different JAR file (or several different JAR files) that serves as utilities for the purposes of that applet.

You specify classes to include in the Class-Path header field in the manifest file of an applet or application. The Class-Path header takes the following form:

Class-Path: jar1-name jar2-name directory-name/jar3-name

By using the Class-Path header in the manifest, you can avoid having to specify a long -classpath flag when invoking Java to run the your application.


Note: 

The Class-Path header points to classes or JAR files on the local network, not JAR files within the JAR file or classes accessible over internet protocols. To load classes in JAR files within a JAR file into the class path, you must write custom code to load those classes. For example, if MyJar.jar contains another JAR file called MyUtils.jar, you cannot use the Class-Path header in MyJar.jar's manifest to load classes in MyUtils.jar into the class path.


An Example

We want to load classes in MyUtils.jar into the class path for use in MyJar.jar. These two JAR files are in the same directory.

We first create a text file named Manifest.txt with the following contents:

Class-Path: MyUtils.jar

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 cfm MyJar.jar Manifest.txt MyPackage/*.class

This creates the JAR file with a manifest with the following contents:

Manifest-Version: 1.0
Class-Path: MyUtils.jar
Created-By: 1.6.0 (Sun Microsystems Inc.)

The classes in MyUtils.jar are now loaded into the class path when you run MyJar.jar.

posted on 2011-09-29 15:13 大浩子 阅读(...) 评论(...) 编辑 收藏

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