Purpose of setting the environment variable XX_HOME
When installing Hadoop tools, you need to set XX_HOME, which is sometimes confusing. Because I use the brew package installer on Mac OS x to install these tools, and then I can run commands on the terminal without setting XX_HOME.
Java-related environment variables are set:
1. Set JAVA_HOME:
1. For reference convenience, for example, if your JDK is installed in the C: \ Program Files \ Java \ jdk1.6.0 directory, set JAVA_HOME to the directory path, to use this path in the future, you only need to enter % JAVA_HOME % to avoid entering a long path string for each reference;
Ii. Normalization principle: when your JDK path is forced to change, you only need to change the variable value of JAVA_HOME. Otherwise, you need to change any document that uses the absolute path to reference the JDK directory, if you fail to complete the changes and a program cannot find the JDK, the consequence is that the system crashes!
3. The third-party software will reference the agreed JAVA_HOME variable. Otherwise, you will not be able to use the software normally. After using JAVA for a long time, you will know that if a software cannot be used normally, think about the problem.
2. Set CLASSPATH:
This is a very interesting question. Of course, it is also a question that affects beginners. The purpose of this variable setting is to allow the program to find the corresponding ". for example, compile a java program ---. java,. class file. If you execute java A in the current directory, the corresponding result will be obtained (provided that you have set CLASSPATH ". "). now. when the class is moved to another directory (for example, "e: \") and java A is executed, there will be A NoClassDefFindError exception because it cannot be found. class file, now you add CLASSPATH :".; e: \ "run java A again to see what the result will be ~~ :)~~~, Everything is normal. The java Command finds the. class file through CLASSPATH!
3. Set PATH:
The principle is very simple. Do you want to use % JAVA_HOME % \ bin \ java to execute java commands at any time? Of course not, so, you can add % JAVA_HOME % \ bin to the PATH. In this way, we can use java to execute commands in any PATH. (When you enter your code in the Command Prompt window, the operating system will find the corresponding application in the current directory and PATH variable directory and execute it .)
