classpath is a parameter—set either on the command-line, or through an environment variable—that tells the Java Virtual Machine or the Java compiler where to look for user-defined classes and packages.

Overview and architecture

Similar to the classic dynamic loading behavior, when executing Java programs, the Java Virtual Machine finds and loads classes lazily (it loads the bytecode of a class only when this class is first used). The classpath tells Javawhere to look in the filesystem for files defining these classes.

The virtual machine searches for and loads classes in this order:

bootstrap classes: the classes that are fundamental to the Java Platform (comprising the public classes of the Java Class Library, and the private classes that are necessary for this library to be functional).

extension classes: packages that are in the extension directory of the JRE or JDK, jre/lib/ext/

user-defined packages and libraries

By default only the packages of the JDK standard API and extension packages are accessible without needing to set where to find them. The path for all user-defined packages and libraries must be set in the command-line (or in the Manifest associated with the Jar file containing the classes).

Setting the path to execute Java programs

Supplying as application argument

Suppose we have a package called org.mypackage containing the classes:

HelloWorld (main class)

SupportClass

UtilClass

and the files defining this package are stored physically under the directory D:\myprogram (on Windows) or /home/user/myprogram (on Linux).

The file structure will look like this:

D:\myprogram\

|

---> org\

|

---> mypackage\

|

---> HelloWorld.class

---> SupportClass.class

---> UtilClass.class

/home/user/myprogram/

|

---> org/

|

---> mypackage/

|

---> HelloWorld.class

---> SupportClass.class

---> UtilClass.class

When we invoke Java, we specify the name of the application to run: org.mypackage.HelloWorld. However we must also tell Java where to look for the files and directories defining our package. So to launch the program, we use the following command:

java -classpath D:\myprogram org.mypackage.HelloWorld

java -classpath /home/user/myprogram org.mypackage.HelloWorld

where:

java is a java application launcher, a type of sdkTool(A command-line tool, such as java, javac, javadoc, or apt)

-classpath D:\myprogram sets the path to the packages used in the program (on Linux, -classpath /home/user/myprogram) and

org.mypackage.HelloWorld is the name of the main class

Setting the path through an environment variable

The environment variable named CLASSPATH may be alternatively used to set the classpath. For the above example, we could also use on Windows:

set CLASSPATH=D:\myprogram

java org.mypackage.HelloWorld

The rule is that -classpath option, when used to start the java application, overrides the CLASSPATH environment variable. If none are specified, the current working directory is used as classpath. This means that when our working directory is D:\myprogram\ (on Linux,/home/user/myprogram/), we would not need to specify the classpath explicitly. When overriding however, it is advised to include current folder "." into the classpath in the case when loading classes from current folder is desired.

The same applies not only to java launcher but also to javac, the java compiler.

Setting the path of a Jar file

If a program uses a supporting library enclosed in a Jar file called supportLib.jar, physically in the directory D:\myprogram\lib\ and the corresponding physical file structure is:

D:\myprogram\

|

---> lib\

|

---> supportLib.jar

|

---> org\

|

--> mypackage\

|

---> HelloWorld.class

---> SupportClass.class

---> UtilClass.class

the following command-line option is needed:

java -classpath D:\myprogram;D:\myprogram\lib\supportLib.jar org.mypackage.HelloWorld

or alternatively:

set CLASSPATH=D:\myprogram;D:\myprogram\lib\supportLib.jar

java org.mypackage.HelloWorld

Adding all JAR files in a directory

In Java 6 and higher, one can add all jar-files in a specific directory to the classpath using wildcard notation.

Windows example:

java -classpath ".;c:\mylib\*" MyApp

Linux example:

java -classpath '.:/mylib/*' MyApp

This works for both -classpath options and environment classpaths.

Setting the path in a Manifest file

Suppose that a program has been enclosed in a Jar file called helloWorld.jar, put directly in the D:\myprogram directory. We have the following file structure:

D:\myprogram\

|

---> helloWorld.jar

|

---> lib\

|

---> supportLib.jar

The manifest file defined in this Jar file has this definition:

Main-Class: org.mypackage.HelloWorld

Class-Path: lib/supportLib.jar

It's important that the manifest file ends with either a new line or carriage return.

To launch the program, we can use the following command:

java -jar D:\myprogram\helloWorld.jar [app arguments]

This will automatically start the org.mypackage.HelloWorld specified in the Main-Class with the arguments and user cannot replace this class name using java -jar options. The Class-Path meantime describes the location of the supportLib.jar file relative to the location of thehelloWorld.jar. Neither absolute file path (which is permitted in -classpath parameter on the command line) nor jar-internal paths are supported. This particularly means that if main class file is contained in a jar, org/mypackage/HelloWorld.class must be a valid path on the root within the jar.

Multiple classpath entries are separated with spaces:

Class-Path: lib/supportLib.jar lib/supportLib2.jar

OS specific notes

Being closely associated with the file system, the command-line Classpath syntax depends on the operating system. For example:

on all Unix-like operating systems (such as Linux and Mac OS X), the directory structure has a Unix syntax, with separate file paths separated by a colon (":").

on Windows, the directory structure has a Windows syntax, and each file path must be separated by a semicolon (";").

This does not apply when the Classpath is defined in manifest files, where each file path must be separated by a space (" "), regardless of the operating system.

Diagnose

Application programmers may want to find out/debug the current settings under which the application is running:

System.getProperty("java.class.path")

linux 分隔符是冒号:    windows分隔符是分号;

将所以jar包的分号换成冒号即可

Java classpath 和 classpath引入和不引入星号(*) 区别：

classpath 不引入星号：如:D:\abc\ 只会到你的class路径中查找找文件;

classpath 引入星号*： 如:D:\abc\* 不仅包含class路径，还包括jar文件中(class路径)进行查找.