Java : Unsupported major.minor version 51.0

1. Problem

Run a “jar” on Debian 7.5, and hits the following error messages :

$ java -jar ripecrawler.jar
Exception in thread "main" java.lang.UnsupportedClassVersionError: 
	com/mkyong/whois/job/RipeCrawlJob : Unsupported major.minor version 51.0
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:643)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
	at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:323)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:268)

A quick check on the Java version, shows the Debian is using 1.6

$ java -version
java version "1.6.0_31"
OpenJDK Runtime Environment (IcedTea6 1.13.3) (6b31-1.13.3-1~deb7u1)
OpenJDK 64-Bit Server VM (build 23.25-b01, mixed mode)

2. Reason

This is because the “jar” is compiled with JDK 1.7, but you try to run in a JDK 1.6 environment. Refer to the following Wikipedia Java class reference.

J2SE 8 = 52 (0x34 hex)
J2SE 7 = 51 (0x33 hex)
J2SE 6.0 = 50 (0x32 hex)
J2SE 5.0 = 49 (0x31 hex)
JDK 1.4 = 48 (0x30 hex)
JDK 1.3 = 47 (0x2F hex)
JDK 1.2 = 46 (0x2E hex)
JDK 1.1 = 45 (0x2D hex)

P.S major.minor version 51.0 = JDK 1.7 / 7

For example, if you compiled your “jar” with JDK 1.6, and try to run in a JDK 1.5 environment, the error message will change to this :

Unsupported major.minor version 50.0

3. Solution

To fix it, download JDK 7, and change the default JDK from 1.6 to 1.7

3.1 Download and install openjdk-7-jdk

$ sudo apt-get install openjdk-7-jdk openjdk-7-jre

3.2 Change the default Java to 1.7 (Working on Debian like system only).

$ sudo update-alternatives --config java
There are 2 choices for the alternative java (providing /usr/bin/java).
  Selection    Path                                            Priority   Status
------------------------------------------------------------
* 0            /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java   1061      auto mode
  1            /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java   1061      manual mode
  2            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1051      manual mode
Press enter to keep the current choice[*], or type selection number: 2
update-alternatives: using /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java 
to provide /usr/bin/java (java) in manual mode

$ java -version
java version "1.7.0_55"
OpenJDK Runtime Environment (IcedTea 2.4.7) (7u55-2.4.7-1~deb7u1)
OpenJDK 64-Bit Server VM (build 24.51-b03, mixed mode)

Done.