I recently uninstalled Java 8, to use Java 6 as I want my code/creations to be usable by more people than just those on Java 8. When I do mvn - version it returns:
Exception in thread "main" java.lang.UnsupportedClassVersionError: org/apache/maven/cli/MavenCli : Unsupported major.minor version 51.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClassFromSelf(ClassRealm.java:401)
at org.codehaus.plexus.classworlds.strategy.SelfFirstStrategy.loadClass(SelfFirstStrategy.java:42)
at org.codehaus.plexus.classworlds.realm.ClassRealm.unsynchronizedLoadClass(ClassRealm.java:271)
at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClass(ClassRealm.java:254)
at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClass(ClassRealm.java:239)
at org.codehaus.plexus.classworlds.launcher.Launcher.getMainClass(Launcher.java:144)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:266)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
When I do java -version:
java version "1.6.0_45"
Java(TM) SE Runtime Environment (build 1.6.0_45-b06)
Java HotSpot(TM) 64-Bit Server VM (build 20.45-b01, mixed mode)
Everything seems fine with Java, but this also happens when I try to run executable JARs. I got around it by manually doing java -jar (jar name)
According to maven website, the last version to support Java 6 is 3.2.5, and 3.3 and up use Java 7. My hunch is that you're using Maven 3.3 or higher, and should either upgrade to Java 7 (and set proper source/target attributes in your pom) or downgrade maven.
That version number (51.0) indicates that you are trying to run classes compiled for Java 7. You will need to recompile them for Java 6.
Note, however, that some features may no longer be compatible with Java 6, which is very old, and no longer (publicly) supported by Oracle.
i also faced similar issue. I could able to solve this by setting JAVA_HOME in Environment variable in windows. Setting JAVA_HOME in batch file is not working in this case.
I ran into the same problem. I use jdk 1.8 and maven 3.3.9 Once I export JAVA_HOME, I did not see this error. export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/
I face the same problem and solved by adding the JAVA_HOME variable with updated version of java in my Ubuntu Machine(16.04). if you are using "Apache Maven 3.3.9" You need to upgrade your JAVA_HOME with java7 or more
Step to Do this
1-sudo vim /etc/environment
2-JAVA_HOME=JAVA Installation Directory (MyCase-/opt/dev/jdk1.7.0_45/)
3-Run echo $JAVA_HOME will give the JAVA_HOME set value
4-Now mvn -version will give the desired output
Apache Maven 3.3.9
Maven home: /usr/share/maven
Java version: 1.7.0_45, vendor: Oracle Corporation
Java home: /opt/dev/jdk1.7.0_45/jre
Default locale: en_US, platform encoding: UTF-8
OS name: "linux", version: "4.4.0-36-generic", arch: "amd64", family: "unix"
The problem is because you haven't set JDK version properly.You should use jdk 7 for major number 51. Like this:
JAVA_HOME=/usr/java/jdk1.7.0_79
来源：https://stackoverflow.com/questions/31394456/java-6-unsupported-major-minor-version-51-0