Maven中的Java_home

Question

When I ran mvn -version , I noticed the java_home points to ...jdk\\jre (as shown below). Is that wrong? Isn't it supposed to point to ...\\jdk.xyz (without the \\jre )? If so, how do I reset it? (In global %java_home% points to the jdk directory)

C:\Users\Owner>mvn -version
Apache Maven 2.2.1 (r801777; 2009-08-06 15:16:01-0400)
Java version: 1.7.0_17
Java home: C:\Program Files\Java\jdk1.7.0_17\jre
Default locale: en_US, platform encoding: Cp1252
OS name: "windows 7" version: "6.1" arch: "amd64" Family: "windows"

Answer 1

No, it's not wrong. It is pointing to the JRE used by your JDK, which is what it's supposed to. If you print out JAVA_HOME outside maven, it should print correctly:

C:\>echo %JAVA_HOME%
C:\Program Files\Java\jdk1.7.0_07

C:\>mvn -version
Apache Maven 3.0.4 (r1232337; 2012-01-17 10:44:56+0200)
Maven home: C:\APPS\apache-maven-3.0.4\bin\..
Java version: 1.7.0_07, vendor: Oracle Corporation
Java home: C:\Program Files\Java\jdk1.7.0_07\jre
Default locale: en_US, platform encoding: Cp1252
OS name: "windows 7", version: "6.1", arch: "amd64", family: "windows"
C:\>

So basically JAVA_HOME needs to point to a JDK installation (maven needs the tools.jar) but maven actually uses the jre within the JDK to run itself.

When using mvn -version , maven uses java internal java.home property, as can be seen from source code :

version.append( "Java home: " + System.getProperty( "java.home", "<unknown java home>" ) ).append( LS );

This property is not the same thing as JAVA_HOME environment setting, so it might fool you. It is actually dynamic property showing you which JRE is running your code. If you compile and execute a Test.java test class printing the same, you can see that if your JAVA_HOME points to a JDK, the value of java.home is not equal to your JAVA_HOME. This is expected.

Quoting this :

What's the difference between JAVA_HOME and java.home?

JAVA_HOME is the JDK install directory, eg, C:\\jdk5. It's meant to be set as an environment variable and referenced in Windows batch files or Unix scripts. I always have it in my Windows Control Panel and .tcsh files,along with other common environment variables. Some Java applications use the name jdk.home for this purpose, which I think is a better name. But JAVA_HOME has been used since the beginning and is now a convention.

java.home is the JRE install directory, eg, C:\\jdk5\\jre, or C:\\Program Files\\Java\\jre1.5.0_06. Unlike JAVA_HOME, I never seen java.home as an environment variable. java.home is a build-in Java system property, whose value is the JRE install directory. Since all Java system properties are also exposed as Ant build properties, you can also use ${java.home} in build files.

Would jre.home be a better name? Maybe, but I don't think Sun will change it.

You can see that maven uses JAVA_HOME on mvn.bat:

:endInit
SET MAVEN_JAVA_EXE="%JAVA_HOME%\bin\java.exe"
..
%MAVEN_JAVA_EXE% %MAVEN_OPTS% -classpath %CLASSWORLDS_JAR% ..

And if you want to make sure, you can comment out "@echo off" statement in mvn.bat, so you can see that it is being used.

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TL;DR: Based on the information you've given, your configuration is correct, no need to change anything.

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Edit: thanks to this thread, there was also an issue about this being confusing , which resulted in change of output for Maven version 3.5.4.

Answer 2

What helped me debugging mvn/jdk issues is to set this before running mvn: set MAVEN_BATCH_ECHO=on

You get verbose output (on windows), including info how Maven picks java.