Setting the class path

Synopsis

C:> sdkTool -classpath classpath1 ; classpath2.

C:> set CLASSPATH= classpath1 ; classpath2.

sdkTool A command-line tool, such as java , javac , javadoc , or apt . For a listing, see JDK Tools. classpath1;classpath2 Class paths to the .jar, .zip or .class files. Each classpath should end with a filename or directory depending on what you are setting the class path to:

• For a .jar or .zip file that contains .class files, the class path ends with the name of the .zip or .jar file.
• For .class files in an unnamed package, the class path ends with the directory that contains the .class files.
• For .class files in a named package, the class path ends with the directory that contains the «root» package (the first package in the full package name).

Multiple path entries are separated by semi-colons. With the set command, it’s important to omit spaces from around the equals sign (=).

The default class path is the current directory. Setting the CLASSPATH variable or using the -classpath command-line option overrides that default, so if you want to include the current directory in the search path, you must include «.» in the new settings.

Classpath entries that are neither directories nor archives (.zip or .jar files) nor * are ignored.

Description

The class path tells JDK tools and applications where to find third-party and user-defined classes — that is, classes that are not Java extensions or part of the Java platform. The class path needs to find any classes you’ve compiled with the javac compiler — its default is the current directory to conveniently enable those classes to be found.

The JDK, the JVM and other JDK tools find classes by searching the Java platform (bootstrap) classes, any extension classes, and the class path, in that order. (For details on the search strategy, see How Classes Are Found.) Class libraries for most applications will want to take advantage of the extensions mechanism. You only need to set the class path when you want to load a class that’s (a) not in the current directory or in any of its subdirectories, and (b) not in a location specified by the extensions mechanism.

If you are upgrading from an older version of the JDK, your startup settings may include CLASSPATH settings that are no longer needed. You should remove any settings that are not application-specific, such as classes.zip . Some third-party applications that use the Java Virtual Machine may modify your CLASSPATH environment variable to include the libaries they use. Such settings can remain.

You can change the class path by using the JDK tools’ -classpath option when you invoke the JVM or other JDK tools or by using the CLASSPATH environment variable. Using the -classpath option is preferred over setting CLASSPATH environment variable because you can set it individually for each application without affecting other applications and without other applications modifying its value.

Classes can be stored either in directories (folders) or in archive files. The Java platform classes are stored in rt.jar . For more details on archives and information on how the class path works, see Understanding the class path and package names near the end of this document.

Important Note: Some older versions of the JDK sofware included a jdk-dir >/classes entry in the default class path. That directory exists for use by the JDK software, and should not be used for application classes. Application classes should be placed in a directory outside of the JDK directory hierarcy. That way, installing a new JDK does not force you to reinstall application classes. For compatibility with older versions, applications that use the jdk-dir >/classes directory as a class library will run in the current version, but there is no guarantee that they will run in future versions.

Using the JDK tools’ -classpath option

The JDK tools java, jdb, javac, and javah have a -classpath option which replaces the path or paths specified by the CLASSPATH environment variable while the tool runs. This is the recommended option for changing class path settings, because each application can have the class path it needs without interfering with any other application.

The runtime tool java has a -cp option, as well. This option is an abbreviation for -classpath .

For very special cases, both java and javac have options that let you change the path they use to find their own class libraries. The vast majority of users will never to need to use those options, however.

Using the CLASSPATH environment variable

In general, you will want to use the -classpath command-line option, as explained in the previous section. This section shows you how to set the CLASSPATH environment variable if you want to do that, or clear settings left over from a previous installation.

Setting CLASSPATH

The CLASSPATH environment variable is modified with the set command. The format is:

The paths should begin with the letter specifying the drive, for example, C:\ . That way, the classes will still be found if you happen to switch to a different drive. (If the path entries start with backslash ( \ ) and you are on drive D: , for example, then the classes will be expected on D: , rather than C: .)

Clearing CLASSPATH

If your CLASSPATH environment variable has been set to a value that is not correct, or if your startup file or script is setting an incorrect path, you can unset CLASSPATH by using:

This command unsets CLASSPATH for the current command prompt window only. You should also delete or modify your startup settings to ensure that you have the right CLASSPATH settings in future sessions.

Changing Startup Settings

If the CLASSPATH variable is set at system startup, the place to look for it depends on your operating system:

Operating System	Method
Windows 95 and 98	Examine autoexec.bat for the set command.
Other (Windows NT, Windows 2000, . )	The CLASSPATH environment variable can be set using the System utility in the Control Panel.

Understanding class path wildcards

Class path entries can contain the basename wildcard character *, which is considered equivalent to specifying a list of all the files in the directory with the extension .jar or .JAR . For example, the class path entry foo/* specifies all JAR files in the directory named foo . A classpath entry consisting simply of * expands to a list of all the jar files in the current directory.

A class path entry that contains * will not match class files. To match both classes and JAR files in a single directory foo , use either foo;foo/* or foo/*;foo . The order chosen determines whether the classes and resources in foo are loaded before JAR files in foo , or vice versa.

Subdirectories are not searched recursively. For example, foo/* looks for JAR files only in foo , not in foo/bar , foo/baz , etc.

The order in which the JAR files in a directory are enumerated in the expanded class path is not specified and may vary from platform to platform and even from moment to moment on the same machine. A well-constructed application should not depend upon any particular order. If a specific order is required then the JAR files can be enumerated explicitly in the class path.

Expansion of wildcards is done early, prior to the invocation of a program’s main method, rather than late, during the class-loading process itself. Each element of the input class path containing a wildcard is replaced by the (possibly empty) sequence of elements generated by enumerating the JAR files in the named directory. For example, if the directory foo contains a.jar , b.jar , and c.jar , then the class path foo/* is expanded into foo/a.jar;foo/b.jar;foo/c.jar , and that string would be the value of the system property java.class.path .

The CLASSPATH environment variable is not treated any differently from the -classpath (or -cp ) command-line option. That is, wildcards are honored in all these cases. However, class path wildcards are not honored in the Class-Path jar-manifest header.

Understanding the class path and package names

Java classes are organized into packages which are mapped to directories in the file system. But, unlike the file system, whenever you specify a package name, you specify the whole package name — never part of it. For example, the package name for java.awt.Button is always specified as java.awt .

For example, suppose you want the Java runtime to find a class named Cool.class in the package utility.myapp . If the path to that directory is C:\java\MyClasses\utility\myapp , you would set the class path so that it contains C:\java\MyClasses .

To run that app, you could use the following JVM command:

When the app runs, the JVM uses the class path settings to find any other classes defined in the utility.myapp package that are used by the Cool class.

Note that the entire package name is specified in the command. It is not possible, for example, to set the class path so it contains C:\java\MyClasses\utility and use the command java myapp.Cool . The class would not be found.

(You may be wondering what defines the package name for a class. The answer is that the package name is part of the class and cannot be modified, except by recompiling the class.)

Note: An interesting consequence of the package specification mechanism is that files which are part of the same package may actually exist in different directories. The package name will be the same for each class, but the path to each file may start from a different directory in the class path.

Folders and archive files

When classes are stored in a directory (folder), like c:\java\MyClasses\utility\myapp , then the class path entry points to the directory that contains the first element of the package name. (in this case, C:\java\MyClasses , since the package name is utility.myapp .)

But when classes are stored in an archive file (a .zip or .jar file) the class path entry is the path to and including the .zip or .jar file. For example, to use a class library that is in a .jar file, the command would look something like this:

Multiple specifications

To find class files in the directory C:\java\MyClasses as well as classes in C:\java\OtherClasses , you would set the class path to:

Note that the two paths are separated by a semicolon.

Specification order

The order in which you specify multiple class path entries is important. The Java interpreter will look for classes in the directories in the order they appear in the class path variable. In the example above, the Java interpreter will first look for a needed class in the directory C:\java\MyClasses . Only if it doesn’t find a class with the proper name in that directory will the interpreter look in the C:\java\OtherClasses directory.

PATH and CLASSPATH

This section explains how to use the PATH and CLASSPATH environment variables on Microsoft Windows, Solaris, and Linux. Consult the installation instructions included with your installation of the Java Development Kit (JDK) software bundle for current information.

After installing the software, the JDK directory will have the structure shown below.

The bin directory contains both the compiler and the launcher.

Update the PATH Environment Variable (Microsoft Windows)

You can run Java applications just fine without setting the PATH environment variable. Or, you can optionally set it as a convenience.

Set the PATH environment variable if you want to be able to conveniently run the executables ( javac.exe , java.exe , javadoc.exe , and so on) from any directory without having to type the full path of the command. If you do not set the PATH variable, you need to specify the full path to the executable every time you run it, such as:

The PATH environment variable is a series of directories separated by semicolons ( ; ). Microsoft Windows looks for programs in the PATH directories in order, from left to right. You should have only one bin directory for the JDK in the path at a time (those following the first are ignored), so if one is already present, you can update that particular entry.

The following is an example of a PATH environment variable:

It is useful to set the PATH environment variable permanently so it will persist after rebooting. To make a permanent change to the PATH variable, use the System icon in the Control Panel. The precise procedure varies depending on the version of Windows:

Windows XP

1. Select Start, select Control Panel. double click System, and select the Advanced tab.
2. Click Environment Variables. In the section System Variables, find the PATH environment variable and select it. Click Edit. If the PATH environment variable does not exist, click New .
3. In the Edit System Variable (or New System Variable) window, specify the value of the PATH environment variable. Click OK. Close all remaining windows by clicking OK.

Windows Vista:

1. From the desktop, right click the My Computer icon.
2. Choose Properties from the context menu.
3. Click the Advanced tab (Advanced system settings link in Vista).
4. Click Environment Variables. In the section System Variables, find the PATH environment variable and select it. Click Edit. If the PATH environment variable does not exist, click New .
5. In the Edit System Variable (or New System Variable) window, specify the value of the PATH environment variable. Click OK. Close all remaining windows by clicking OK.

Windows 7:

1. From the desktop, right click the Computer icon.
2. Choose Properties from the context menu.
3. Click the Advanced system settings link.
4. Click Environment Variables. In the section System Variables, find the PATH environment variable and select it. Click Edit. If the PATH environment variable does not exist, click New .
5. In the Edit System Variable (or New System Variable) window, specify the value of the PATH environment variable. Click OK. Close all remaining windows by clicking OK.

Update the PATH Variable (Solaris and Linux)

You can run the JDK just fine without setting the PATH variable, or you can optionally set it as a convenience. However, you should set the path variable if you want to be able to run the executables ( javac , java , javadoc , and so on) from any directory without having to type the full path of the command. If you do not set the PATH variable, you need to specify the full path to the executable every time you run it, such as:

To find out if the path is properly set, execute:

This will print the version of the java tool, if it can find it. If the version is old or you get the error java: Command not found, then the path is not properly set.

To set the path permanently, set the path in your startup file.

For C shell ( csh ), edit the startup file (

For bash , edit the startup file (

For ksh , the startup file is named by the environment variable, ENV . To set the path:

For sh , edit the profile file (

Then load the startup file and verify that the path is set by repeating the java command:

For C shell ( csh ):

For ksh , bash , or sh :

Checking the CLASSPATH variable (All platforms)

The CLASSPATH variable is one way to tell applications, including the JDK tools, where to look for user classes. (Classes that are part of the JRE, JDK platform, and extensions should be defined through other means, such as the bootstrap class path or the extensions directory.)

The preferred way to specify the class path is by using the -cp command line switch. This allows the CLASSPATH to be set individually for each application without affecting other applications. Setting the CLASSPATH can be tricky and should be performed with care.

The default value of the class path is «.», meaning that only the current directory is searched. Specifying either the CLASSPATH variable or the -cp command line switch overrides this value.

To check whether CLASSPATH is set on Microsoft Windows NT/2000/XP, execute the following:

On Solaris or Linux, execute the following:

If CLASSPATH is not set you will get a CLASSPATH: Undefined variable error (Solaris or Linux) or simply %CLASSPATH% (Microsoft Windows NT/2000/XP).

To modify the CLASSPATH , use the same procedure you used for the PATH variable.

Class path wildcards allow you to include an entire directory of .jar files in the class path without explicitly naming them individually. For more information, including an explanation of class path wildcards, and a detailed description on how to clean up the CLASSPATH environment variable, see the Setting the Class Path technical note.