Classpath java как прописать

How to Set Classpath in Java?

CLASSPATH describes the location where all the required files are available which are used in the application. Java Compiler and JVM (Java Virtual Machine) use CLASSPATH to locate the required files. If the CLASSPATH is not set, Java Compiler will not be able to find the required files and hence will throw the following error.

The above error is resolved when CLASSPATH is set.

Set the CLASSPATH in JAVA in Windows

Command Prompt:

Note: Semi-colon (;) is used as a separator and dot (.) is the default value of CLASSPATH in the above command.

1. Select Start

2. Go to the Control Panel

Select Control Panel

3. Select System and Security

select Advanced System Settings

4. Select Advanced System settings

5. Click on Environment Variables

Click on Environment variables

6. Click on New under System Variables

Click on New under System Variables

7. Add CLASSPATH as variable name and path of files as a variable value.

Add CLASSPATH as variable name and path of files as a variable value

8. Select OK.

Set the CLASSPATH on Linux

Command Line:

Find out where you have installed Java, basically, it’s in /usr/lib/jvm path. Set the CLASSPATH in /etc/environment using

Add the following lines,

Note: Colon (:) is used as a separate directory and dot (.) is the default value of CLASSPATH in the above command.

2 Setting the Class Path

The class path is the path that the Java Runtime Environment (JRE) searches for classes and other resource files.

This chapter covers the following topics:


The class search path (class path) can be set using either the -classpath option when calling a JDK tool (the preferred method) or by setting the CLASSPATH environment variable. The -classpath option is preferred because you can set it individually for each application without affecting other applications and without other applications modifying its value.

sdkTool -classpath classpath1;classpath2.

set CLASSPATH=classpath1;classpath2.

A command-line tool, such as java , javac , javadoc , or apt . For a listing, see JDK Tools and Utilities at


Class paths to the JAR, zip or class files. Each class path should end with a file name or directory depending on what you are setting the class path to, as follows:

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, which is the first package in the full package name.

Multiple path entries are separated by semicolons with no spaces around the equals sign (=) in Windows and colons in Oracle Solaris.

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, then you must include a dot ( . ) in the new settings.

Class path entries that are neither directories nor archives (.zip or JAR files) nor the asterisk ( * ) wildcard character are ignored.


The class path tells the JDK tools and applications where to find third-party and user-defined classes that are not extensions or part of the Java platform. See The Extension Mechanism at

The class path needs to find any classes you have compiled with the javac compiler. The 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 about the search strategy, see How Classes Are Found at

Class libraries for most applications use the extensions mechanism. You only need to set the class path when you want to load a class that is (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 upgrade from an earlier release of the JDK, then your startup settings might include CLASSPATH settings that are no longer needed. You should remove any settings that are not application-specific, such as . Some third-party applications that use the Java Virtual Machine (JVM) can modify your CLASSPATH environment variable to include the libraries they use. Such settings can remain.

You can change the class path by using the -classpath or -cp option of some Java commands when you call the JVM or other JDK tools or by using the CLASSPATH environment variable. See JDK Commands Class Path Options. Using the -classpath option is preferred over setting the CLASSPATH environment variable because you can set it individually for each application without affecting other applications and without other applications modifying its value. See CLASSPATH Environment Variable.

Classes can be stored in directories (folders) or in archive files. The Java platform classes are stored in rt.jar. For more details about archives and information about how the class path works, see Class Path and Package Names.

Note: Some earlier releases of the JDK had 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 hierarchy. That way, installing a new JDK does not force you to reinstall application classes. For compatibility with earlier releases, applications that use the <jdk-dir>/classes directory as a class library run in the current release, but there is no guarantee that they will run in future releases.

JDK Commands Class Path Options

The following commands have a -classpath option that replaces the path or paths specified by the CLASSPATH environment variable while the tool runs: java , jdb , javac , javah and jdeps .

The -classpath option 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 java command also has a -cp option that is an abbreviation for -classpath .

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

CLASSPATH Environment Variable

As explained in JDK Commands Class Path Options, the -classpath command-line option is preferred over the CLASSPATH environment variable. However, if you decide to use the CLASSPATH environment variable, this section explains how to set and clear it.


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:.


If your CLASSPATH environment variable was set to a value that is not correct, or if your startup file or script is setting an incorrect path, then you can unset CLASSPATH with:

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 correct CLASSPATH settings in future sessions.

Change Startup Settings

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

Windows 95 and 98: Examine autoexec.bat for the set command.

Other (Windows NT, Windows 2000, . ): The CLASSPATH environment variable can be set with the System utility in the Control Panel.

If the CLASSPATH variable is set at system startup, then the place to look for it depends on the shell you are running:

The csh , tcsh shells : Examine your .cshrc file for the setenv command.

The sh , ksh shells : Examine your .profile file for the export command.

Class Path Wild Cards

Class path entries can contain the base name wildcard character (*), which is considered equivalent to specifying a list of all of the files in the directory with the extension .jar or .JAR . For example, the class path entry mydir/* specifies all JAR files in the directory named mydir . A class path entry consisting of * expands to a list of all the jar files in the current directory. Files are considered regardless of whether they are hidden (have names beginning with ‘.’).

A class path entry that contains an asterisk (*) does not match class files. To match both classes and JAR files in a single directory mydir , use either mydir:mydir/* or mydir/*:mydir . The order chosen determines whether the classes and resources in mydir are loaded before JAR files in mydir or vice versa.

Subdirectories are not searched recursively. For example, mydir/* searches for JAR files only in mydir , not in mydir/subdir1 , mydir/subdir2 , and so on.

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 wild cards is done early, before the invocation of a program’s main method, rather than late, during the class-loading process. Each element of the input class path that contains 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 mydir contains a.jar, b.jar, and c.jar, then the class path mydir/* is expanded into mydir/a.jar:mydir/b.jar:mydir/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 options. Wild cards are honored in all of these cases. However, class path wild cards are not honored in the Class-Path jar-manifest header.

Class Path and Package Names

Java classes are organized into packages that 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 and 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 JRE to find a class named Cool.class in the package utility.myapp. If the path to that directory is C:\java\MyClasses\utility\myapp , then you would set the class path so that it contains C:\java\MyClasses . To run that application, you could use the following java command:

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 might wonder 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.

An interesting consequence of the package specification mechanism is that files that are part of the same package can exist in different directories. The package name is the same for each class, but the path to each file might start from a different directory in the class path.

Folders and Archive Files

When classes are stored in a directory (folder), such as 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 , because the package name is utility.myapp).

When classes are stored in an archive file (a zi p or JAR file) the class path entry is the path to and including the zip or JAR file. For example, the command to use a class library that is in a JAR file as follows:

Multiple Specifications

To find class files in the directory C:\java\MyClasses and classes in C:\java\OtherClasses , you would set the class path to the following. 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 previous example, the Java interpreter will first look for a needed class in the directory C:\java\MyClasses . Only when it does not find a class with the proper name in that directory will the interpreter look in the C:\java\OtherClasses directory.

Pro Java

Теперь быстренько разберемся classpath, так как это достаточно важная тема для разработки на Java. Естественно я тут не смогу разобрать все тонкости и хитрости, но постараюсь представить самое необходимое чтобы было понимание этой темы. Для желающих разобраться глубже приведу линки для самостоятельного изучения. Ну и как всегда гугль в помощь.

Чтобы понимать что происходит под капотом у любой Java IDE когда она собирает проект и запускает его надо хоть немного попрактиковаться в использовании компилятора javac, среды исполнения java и понять classpath.

По существу classpath указывает компилятору или виртуальной машине где искать классы необходимые для сборки проекта или же его запуска. Немного об этом мы уже узнали тут. Теперь разберемся с этим получше.

Указать где компилятору или виртуальной машине искать классы можно через ключ –classpath или же системную переменную окружения CLASSPATH. Мы рассмотрим оба этих варианта.

Начнем с простого. Вернемся к нашему проекту Hello World (00004E_HelloWorld), там где мы разделили его на два файла и

Теперь попробуем создать исполняемый (jar) файл этого проекта из среды Eclipse. Так как скомпилированные, читай готовые к исполнению, файлы в Java имеют расширение class, а классов в реальных программах, могут быть сотни и тысячи, то их собирают в один или несколько jar архивов и таким образом запускают. То есть уже существует не россыпь файлов с расширением class, а один или несколько jar файлов.

И так! Понеслась! Воспользуемся Export для создания jar



После этого мы получим файл HelloWorld.jar готовый к исполнению на виртуальной машине java. Запустим его из командной строки:


Запускать jar файлы надо с ключом –jar как показано на скрине выше. Если этот ключ не использовать то вылетит ошибка:


Нам сообщили что не знают где искать главный (main) класс для HelloWorld.jar. Но запустить все же можно и без ключа –jar, но уже воспользовавшись на ключом –classpath , для которого существует сокращенный вариант –cp . Вот как это можно сделать:


Почему строчка запуска выглядит именно так? Вспоминаем что именно класс содержит у нас метод main.


Класс такого метода не имеет.


Как я уже говорил метод main – это точка входа в программу, то есть место от куда начинается ее выполнение и поэтому виртуальной машине java надо знать, от куда надо начинать выполнять программу. Если она не может найти метод main, то она начинает ругаться, как это было показано выше.

И так в нашей строчке

java -cp HelloWorld.jar Hello

мы указали что искать метод main надо в классе Hello (расширение .class опускается) по пути HelloWorld.jar. Jar и zip архивы рассматриваются виртуальной машиной как каталоги, поэтому их надо указывать по полному имени с расширением.

Теперь скомпилируем наши классы и самостоятельно из командной строки без помощи Eclipse, чтобы еще глубже понять как все работает. Для чистоты эксперимента рекомендую удалить все файлы с расширением .class и .jar из каталога bin.

Для начал просто скомпилируем исходники в class файлы без упаковки их в jar, чтобы было понятнее.

Переходим в коневой каталог 00004E_HelloWorld и от туда даем команду компиляции

javac -encoding utf8 -d bin src\ src\


Поскольку у нас программа состоит из двух классов Hello и Word, то их обоих сразу надо указать компилятору. Кроме того так же надо указать и кодировочку исходников. Так же мы указали папку bin – это то куда будут складываться откомпилированные файлы.

Теперь у нас в каталоге bin два файла Hello.class и Word.class. Перейдем в него чтобы запустить программу.


Все работает. Но файлики у нас там лежат россыпью классво .class


Теперь упакуем эти файлики .class в jar архив командой

jar cf HelloWorld.jar Hello.class Word.class

и попробуем запустить HelloWorld.jar


И вылетела ошибочка. Почему так? Ведь у нас уже есть jar файл в который упакованы оба класса.


Но все равно не работает. Это происходит потому, что внутри jar файла мы не указали какой файл у нас имеет метод main.

Запустить наш jar файл все таки можно указав дополнительно, какой класс содержит метод main.

java -cp HelloWorld.jar Hello


Теперь все работает. Но согласитесь так запускать jar файл не удобно, так как всегда надо знать какой класс содержит метод main. Если вы смотрели внимательно, то видимо заметили внутри архива HelloWorld.jar папку META-INF. В ней содержится файл MANIFEST.MF


Вот в нем и должна содержаться информация о классе содержащем метод main, но пока в нем ее нет.


Исправим эту ошибочку. Удалим файлик HelloWorld.jar и создадим его заново, но уже с добавлением информации о классе содержащим метод main. Сделаем это следующей командой

jar cfe HelloWorld.jar Hello Hello.class Word.class

И запустим файл HelloWorld.jar уже как полагается без танцев с бубном.


Как видим все работает нормально. Это произошло потому, что файл MANIFEST.MF уже содержит информацию о классе содержащем метод main.


Ну вот теперь мы имеем хоть какое-то представление о том что происходит когда какая-либо IDE создает исполняемый jar файл, а так же получили представление о classpath. В следующей статье мы немного углубим его.

P.S. Так же стоит знать что по умолчанию для виртуальной машины java доступны все классы стандартной библиотеки java, а так же все классы в текущем каталоге от куда запускается главный класс содержащий метод main.

Ну и на последок ссылка где про classpath рассказано достаточно подробно. Правда я не знаю как долго она проживет.

Java Classpath

Learn how to set classpath in Java either as an environment variable and pass as the command-line argument. During runtime of any Java application, the CLASSPATH is a parameter that tells the JVM where to look for classes and packages.

  • The default value of the classpath is “ . ” (dot) , meaning that only the current directory is searched for dependencies.
  • Specifying either the CLASSPATH environment variable or the -cp command line switch overrides this value.
  • The order in which you specify multiple classpath entries is important. The Java interpreter will look for classes in the directories in the order they appear in the classpath variable.

Java Classpath separators are OS specific.

Windows – ; [Semicolon]
Linux/Unix – : [Colon]

1. Setting CLASSPATH as Environment Variable

When you have set the location of jar files that are always required during the application runtime, then it’s probably the best to add them in the machine’s environment variable ‘CLASSPATH’ .

During application runtime, application class loader will always scan the jar files and classes at specified paths in this variable.

To set CLASSPATH environment variable, find the location of user environment variables in your machine and add all paths where Jar files are stored. Use the separator between different two folders, jar files or classes.

You can find the user environment variables window by –

  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 CLASSPATH environment variable and select it. Click Edit. If the CLASSPATH environment variable does not exist, click New .
  5. Add all folders separated with separator. Click OK. Close all remaining windows by clicking OK.

If you are creating CLASSPATH for the first time, you need to specify the name for Variable Name in the Windows 10. Use ‘.’ (dot) to denote current directory.

2. Setting CLASSPATH from Command Line

Use -classpath argument to set classpath from command prompt/console. Use below given commands to set classpath for different requirements.

Let’s say we have a folder named dependency where JAR files and other classes are placed.

2.1. Add a single jar file in classpath

Below syntax examples will add single jar file in classpath.

2.2. Add multiple jar files in classpath

Below syntax examples will add more than one jar file in classpath. To do so, simply use the delimiter for your operating system (either ; or : ) as a separator between the locations specified for the CLASSPATH.

To add all JAR files present in a directory, use wildcard character ( ‘*’ ).

2.3. Add multiple classes to classpath

Many times, you may need to add individual classes in classpath as well. To do so, simply add the folder where classfile is present. e.g. let’s say there are five .class files are present in location folder which you want to include in classpath.

As a best practice, always organize all JAR files and application classes inside one root folder. This may be the workspace for the application.

2.4. Clearing Classpath

If your CLASSPATH environment variable was set to a value that is not correct, then you can unset CLASSPATH with specifying empty value to it.

3. Executing programs with ‘-classpath’ or ‘-cp’ option in Java

Apart from setting classpath to the environment variable, you can pass additional classpath to Java runtime while launching the application using –classpath option or –cp option.

Use the . (dot) to include the current path into the classpath where the .class file has been generated.

4. How to Find and Print CLASSPATH Value

Anytime you wish to verify all path entries in CLASSPATH variable, you can verify using echo command.

If CLASSPATH is not set you will get a CLASSPATH: Undefined variable error (Solaris or Linux) console or simply %CLASSPATH% printed in windows command prompt.

Добавить комментарий

Ваш адрес email не будет опубликован. Обязательные поля помечены *