java.lang.UnsupportedClassVersionError Unsupported major.minor version 51.0

I am trying to build an application, but it gives some error. My JDK version is given below:

java version "1.6.0_30"
Java(TM) SE Runtime Environment (build 1.6.0_30-b12)
Java HotSpot(TM) Client VM (build 20.5-b03, mixed mode, sharing)

Here is my Error Log:

Buildfile: build.xml

compile:
     [echo] Compiling the service....
     [echo]   ...mkdir for classes first....
     [echo]   ...java classes next....
     [echo]   ...mkdir for generated code next....
     [echo]  ...generate stubs/skeletons next....
[wscompile] command line: wscompile -d /home/vivekray/program/MTrans/WEB-INF/classes -features:wsi,documentliteral -gen:server -keep -model /home/vivekray/program/MTrans/WEB-INF/model.gz -verbose -Xprintstacktrace /home/vivekray/program/MTrans/src/config.xml -classpath /home/vivekray/program/MTrans/WEB-INF:/home/vivekray/program/MTrans/WEB-INF/classes:/home/vivekray/program/MTrans/lib/log4j-1.2.16.jar:/home/vivekray/jwsdp-2.0/jwsdp-shared/lib/mail.jar:/home/vivekray/jwsdp-2.0/jwsdp-shared/lib/activation.jar:/home/vivekray/jwsdp-2.0/jaxp/lib/jaxp-api.jar:/home/vivekray/jwsdp-2.0/jaxp/lib/endorsed/dom.jar:/home/vivekray/jwsdp-2.0/jaxp/lib/endorsed/sax.jar:/home/vivekray/jwsdp-2.0/jaxp/lib/endorsed/xalan.jar:/home/vivekray/jwsdp-2.0/jaxp/lib/endorsed/xercesImpl.jar:/home/vivekray/jwsdp-2.0/jaxrpc/lib/jaxrpc-api.jar:/home/vivekray/jwsdp-2.0/jaxrpc/lib/jaxrpc-spi.jar:/home/vivekray/jwsdp-2.0/jaxrpc/lib/jaxrpc-impl.jar:/home/vivekray/jwsdp-2.0/saaj/lib/saaj-api.jar:/home/vivekray/jwsdp-2.0/saaj/lib/saaj-impl.jar:/home/vivekray/jwsdp-2.0/jwsdp-shared/lib/relaxngDatatype.jar:/home/vivekray/jwsdp-2.0/jwsdp-shared/lib/xsdlib.jar:/home/vivekray/jwsdp-2.0/jwsdp-shared/lib/jax-qname.jar:/home/vivekray/jwsdp-2.0/apache-ant/lib/ant.jar:/home/vivekray/jwsdp-2.0/fastinfoset/lib/FastInfoset.jar:/home/vivekray/Softwares/apache-tomcat-7.0.27/lib/servlet-api.jar
[wscompile] [creating model: mtransservice]
[wscompile] [creating service: Mtransservice]

BUILD FAILED
java.lang.UnsupportedClassVersionError: in/cdac/mobile/computing/mtrans/MtransService : 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 org.apache.tools.ant.AntClassLoader.defineClassFromData(AntClassLoader.java:1146)
	at org.apache.tools.ant.AntClassLoader.getClassFromStream(AntClassLoader.java:1324)
	at org.apache.tools.ant.AntClassLoader.findClassInComponents(AntClassLoader.java:1388)
	at org.apache.tools.ant.AntClassLoader.findClass(AntClassLoader.java:1341)
	at org.apache.tools.ant.AntClassLoader.loadClass(AntClassLoader.java:1088)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:295)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:247)
	at com.sun.xml.rpc.processor.modeler.rmi.RmiUtils.getLoadableClassName(RmiUtils.java:78)
	at com.sun.xml.rpc.processor.modeler.rmi.RmiUtils.getRealName(RmiUtils.java:54)
	at com.sun.xml.rpc.processor.modeler.rmi.RmiModeler.modelPort(RmiModeler.java:676)
	at com.sun.xml.rpc.processor.modeler.rmi.RmiModeler.buildLiteralModel(RmiModeler.java:603)
	at com.sun.xml.rpc.processor.modeler.rmi.RmiModeler.buildModel(RmiModeler.java:457)
	at com.sun.xml.rpc.processor.config.ModelInfo.buildModel(ModelInfo.java:88)
	at com.sun.xml.rpc.processor.Processor.runModeler(Processor.java:63)
	at com.sun.xml.rpc.tools.wscompile.CompileTool.run(CompileTool.java:714)
	at com.sun.xml.rpc.util.ToolBase.run(ToolBase.java:43)
	at com.sun.xml.rpc.tools.ant.Wscompile.execute(Wscompile.java:746)
	at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:288)
	at sun.reflect.GeneratedMethodAccessor3.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
	at org.apache.tools.ant.Task.perform(Task.java:348)
	at org.apache.tools.ant.Target.execute(Target.java:357)
	at org.apache.tools.ant.Target.performTasks(Target.java:385)
	at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1337)
	at org.apache.tools.ant.Project.executeTarget(Project.java:1306)
	at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:41)
	at org.apache.tools.ant.Project.executeTargets(Project.java:1189)
	at org.apache.tools.ant.Main.runBuild(Main.java:758)
	at org.apache.tools.ant.Main.startAnt(Main.java:217)
	at org.apache.tools.ant.launch.Launcher.run(Launcher.java:257)
	at org.apache.tools.ant.launch.Launcher.main(Launcher.java:104)

Total time: 1 second

These guys gave you the reason why is failing but not how to solve it. This problem may appear even if you have a jdk which matches JVM which you are trying it into.

Project -> Properties -> Java Compiler

Enable project specific settings.

Then select Compiler Compliance Level to 1.6 or 1.5, build and test your app.

Now, it should be fine.

java.lang.UnsupportedClassVersionError happens because of a higher JDK during compile time and lower JDK during runtime.

Here's the list of versions:

Java SE 9 = 53,
Java SE 8 = 52,
Java SE 7 = 51,
Java SE 6.0 = 50,
Java SE 5.0 = 49,
JDK 1.4 = 48,
JDK 1.3 = 47,
JDK 1.2 = 46,
JDK 1.1 = 45

Version 51 is Java 7, you probably use the wrong JDK. Check JAVA_HOME.

As mentioned here, under Binary Compatibility

It means that you compiled your classes under a specific JDK, but then try to run them under older version of JDK.

I encountered the same issue, when jdk 1.7 was used to compile then jre 1.4 was used for execution.

My solution was to set environment variable PATH by adding pathname C:\glassfish3\jdk7\bin in front of the existing PATH setting. The updated value is "C:\glassfish3\jdk7\bin;C:\Sun\SDK\bin". After the update, the problem was gone.

Make sure you're using the correct SDK when compiling/running and also, make sure you use source/target 1.7.

Use Maven and use the maven-compiler-plugin to explicitly call the actual correct version JDK javac.exe command, because Maven could be running any version; this also catches the really stupid long standing bug in javac that does not spot runtime breaking class version jars and missing classes/methods/properties when compiling for earlier java versions! This later part could have easily been fixed in Java 1.5+ by adding versioning attributes to new classes, methods, and properties, or separate compiler versioning data, so is a quite stupid oversight by Sun and Oracle.