How to set Java environment variables

Download the jdk required for java environment variable setting and install it. Download address: http://download.oracle.com/otn-pub/java/jdk/7u2-b13/jdk-7u2-windows-i586.exe

1, java environment variable setting-lite version

1. Right-click to open My Computer->Properties->Advanced->Environment Variables

2. Create a new system Variables JAVA_HOME and CLASSPATH

Variable name (N): JAVA_HOME

Variable value (V): C:\Program Files\Java\jdk1.7.0

Variable name (N ): CLASSPATH

Variable value (V): .;%JAVA_HOME%\lib\dt.jar;%JAVA_HOME%\lib\tools.jar;

3. Select "System Variables" In the environment variable named "Path", double-click the variable, add the absolute path of the bin directory in the jdk installation path to the value of the Path variable, and use a half-width English semicolon to separate it from the existing path.

Variable name (N): Path

Variable value (V): %JAVA_HOME%\bin;%JAVA_HOME%\jre\bin;

2, java environment variables Settings - Detailed version

After installing jdk to the computer (here jdk1.7 is used to install it, the same is true for other versions), and then setting it up, the java environment can take effect.

First, right-click My Computer. Open properties. Then select "Environment Variables" in "Advanced". In the newly opened interface, you need to set three system variable attributes "JAVA_HOME", "Path", and "CLASSPATH". Before jdk was installed, the path attribute originally existed, but the two system variables

JAVA_HOME and CLASSPATH did not exist and needed to be created.

Click "New", and then write JAVA_HOME in "Variable Name (N)". The JAVA_HOME variable represents the installation path of java. Then write the path just installed "C:\jdk1.7" in the change value (N).

(Note: If the installation path is not disk C or is not in the jdk1.7 folder, it can be modified accordingly. The following text is assumed to be installed in C:\jdk1.7.)

Second, find Path in the system variable, and then click "Edit". The Path variable indicates that the system can recognize the java command in any path. Then write ".;%JAVA_HOME%\bin" in the change value (N) (where "%JAVA_HOME%" means the value of JAVA_HOME just set), or directly write "C:\jdk1 .7\bin", it is recommended to use the former because of its high reusability.

Third, click "New" again, and then write CLASSPATH on the variable name. The CLASSPATH variable represents the java loading class (class or lib) path. Only the class is in CLASSPATH , the java command can recognize it. Then write ".;%JAVA_HOME%\lib\dt.jar;%JAVA_HOME%\lib\toos.jar" in the change value (N) (add . to indicate the current path)

Finally, after the java environment variable is set, press "OK" until the properties window disappears, that is, the save is successful.

Verify whether the java environment variable is set successfully. First open "Start"->"Run" and type "cmd" to enter the DOS system interface. Then enter "java -version" if the installation is successful. The system will display java version jdk "1.7.0". This way the java environment variable setting is OK!

Summary, the purpose of setting java environment variables:

1,JAVA_HOME

(1) For the convenience of reference, for example, your jdk is installed in C:\ In the Program Files\Java\jdk1.7.0 directory, set JAVA_HOME as the directory path. Then when you want to use this path in the future, you only need to enter %JAVA_HOME% to avoid entering a long path string for each reference;

(2) Normalization principle, when your jdk path needs to be changed, you only need to change the variable value of JAVA_HOME, otherwise you will have to change any document that uses an absolute path to reference the jdk directory. If you fail to change everything and a certain program cannot find jdk, the consequence may be that the program crashes;

(3) The third-party software will reference the agreed JAVA_HOME variable so that the software can be used normally. After using JAVA for a long time, you will know that if a certain software cannot be used normally, it is probably caused by the incorrect JAVA_HOME setting;

2, CLASSPATH

The purpose of setting CLASSPATH is In order for the program to find the corresponding ".class" file, for example: Compile a JAVA program ---A.java, and you will get an A.class class file. Execute java A in the current directory, and you will get the corresponding Result (provided that

CLASSPATH has been set to "."). Now, move A.class to another directory (for example: "e:\"), execute java A, and java will be thrown .lang.NoClassDefFoundError exception, the reason is that the .class file cannot be found. Now you increase CLASSPATH to: ".;e:\" and then run java A. The result is that everything is normal. The java command found the .class file through CLASSPATH!

3,PATH

Do you want to use %JAVA_HOME%\bin\java to execute java commands at any time? Of course not, so you can choose to add %JAVA_HOME%\bin to the PATH path, so that we can You can just use java to execute commands under the path. (When you enter your code in the command prompt window, the operating system will search for the corresponding application in the current directory and the PATH variable directory, and execute it.

Three, java environment variable setting - error case

Error: java.lang.NoClassDefFoundError

Description: When the Java virtual machine or ClassLoader instance attempts to define a class This exception is thrown when the class is loaded (as part of a normal method call or as part of a new instance created using a new expression), but the definition of the class cannot be found.

Cause: The java environment variable is set incorrectly.

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