Jetty running on 1.6 JVM is unable to run webapp compiled on Java 1.6

Question

I am trying to start very simple Java webapp on Jetty (v9.0) webserver.

Using maven I built and created the war and as can be see in the manifest file, I used Java 1.6u43 for compilation on OS X

Contents of META-INF/MANIFEST.MF

Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Created-By: Apache Maven
Built-By: irraju
Build-Jdk: 1.6.0_43

I am running Jetty on Ubuntu and as can be seen, I have slightly higher version of the java 1.6.0_45. If java is backward compatible, my java application should run on jetty but it is failing with unsupported minor version

root# java -version
java version "1.6.0_45"
Java(TM) SE Runtime Environment (build 1.6.0_45-b06)
Java HotSpot(TM) Client VM (build 20.45-b01, mixed mode, sharing)

root# java -DDEBUG -jar start.jar 
Exception in thread "main" java.lang.UnsupportedClassVersionError: org/eclipse/jetty/start/Main : 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$1.run(URLClassLoader.java:197)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
Could not find the main class: org.eclipse.jetty.start.Main. Program will exit.

Other people on the internet are facing this issue when they are trying to run Java 1.7 compiled applications on 1.6* JVMs. I am unable to figure out why this is happening in my case where both the ENVs are 1.6* and deployment ENV has slightly higher version of JVM.

If I deploy the application on Tomcat that is using java 1.6.0_45, it doesn't complain though and works fine. Am I missing something very trivial here?

Answer 1

Jetty 9 requires to be ran with Java 7, according to this list of Jetty versions.

You can try to compile Jetty 9 yourself using Java 6, but that will most likely not work (otherwise, the Jetty devs might've done that themselves as well).