jar 是java的一个压缩文件名 (java archive)，但是格式仍是zip的，所以你可以用winzip等支持zip格式的软件打开，如果你是想自己生成一个jar文件，你可以用 jar工具来做。
官方文档： http://docs.oracle.com/javase/tutorial/deployment/jar/appman.html
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: 1.7.0_06 (Oracle Corporation) 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') 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
----------------------------------------------------------------------------------------------
在命令行打 jar
即可看到它的帮助：
用法：jar {ctxu}[vfm0M] [jar-文件] [manifest-文件] [-C 目录] 文件名 ...
选项：
-c 创建新的归档
-t 列出归档内容的列表
-x 展开归档中的命名的（或所有的〕文件
-u 更新已存在的归档
-v 生成详细输出到标准输出上
-f 指定归档文件名
-m 包含来自指定的清单（manifest〕文件的清单（manifest〕信息
-0 只存储方式；未用ZIP压缩格式
-M 不产生所有项的清单（manifest〕文件
-i 为指定的jar文件产生索引信息
-C 改变到指定的目录，并且包含下列文件：
如果一个文件名是一个目录，它将被递归处理。
清单（manifest〕文件名和归档文件名都需要被指定，按'm' 和 'f'标志指定的相同顺序。
示例1：将两个class文件归档到一个名为 'classes.jar' 的归档文件中：
jar cvf classes.jar Foo.class Bar.class
示例2：用一个存在的清单（manifest〕文件 'mymanifest' 将 foo/ 目录下的所有
文件归档到一个名为 'classes.jar' 的归档文件中：
jar cvfm classes.jar mymanifest -C foo/ .