Java不支持major.minor版本51.0运行jar文件

Question

运行我编译的jar文件时出现此错误：

java.lang.UnsupportedClassVersionError: eventtrigger/test/FulfillPurchaseOrder : Unsupported major.minor version 51.0

我跟踪了与此错误相关的stackoverflow中的大部分答案：

>java -version
java version "1.6.0_41"
OpenJDK Runtime Environment (IcedTea6 1.13.13) (6b41-1.13.13-0ubuntu0.14.04.1)
OpenJDK 64-Bit Server VM (build 23.41-b41, mixed mode)

>javac -version
java version "1.6.0_41"

> sudo update-alternatives --config java
* 2            /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java   1061      manual mode

> sudo update-alternatives --config javac
* 1            /usr/lib/jvm/java-6-openjdk-amd64/bin/javac   1061      manual mode

我的eclipse编译器也设置为1.6，尽管我使用ant来构建jar文件：

build.xml

<?xml version="1.0"?>

<project name="EventTriggerTestFulfillPurchaseOrder" default="main" basedir=".">

    <!-- set global properties for this build -->
    <property name="jar.name" value="plugin-event-trigger-test-fulfill-purchase-order.jar"/>
    <property name="src.dir" value="src"/>
    <property name="build.dir" value="build"/>
    <property name="build.deploy.dir" value="${build.dir}/deploy"/>
    <property name="build.classes.dir" value="${build.dir}/classes"/>
    <property name="emp-jar.dir" value="/usr/java/jboss/server/default/deploy"/>
    <property name="lib.dir" value="/usr/java/jboss/server/default/lib"/>

    <!-- define classpath -->   
    <path id="base.path">
        <pathelement location="${build.classes.dir}" />
        <fileset dir="${lib.dir}">
            <include name="*.jar"/>
        </fileset>      
        <fileset dir="${emp-jar.dir}">
            <include name="emp-*.jar"/>
        </fileset>
    </path>

    <target name="init">
        <mkdir dir="${build.classes.dir}"/>
        <mkdir dir="${build.deploy.dir}"/>
    </target>     

    <!-- Compile the java code from ${src.dir} into ${build.classes.dir} -->
    <target name="compile" depends="init">
        <javac
            srcdir="${src.dir}"
            destdir="${build.classes.dir}"
            debug="on"
            deprecation="off"
            optimize="on"
            includes="*.java"
            classpathref="base.path"
        >       
        </javac>
    </target>   

    <!-- Put everything in ${build.classes.dir} into the ${build.deploy.dir}/${jar}.jar file -->
    <target name="jar" depends="compile">
        <echo message="Compiling jar files"/>
        <delete file="${build.deploy.dir}/${jar.name}"/>
        <jar jarfile="${build.deploy.dir}/${jar.name}" 
            basedir="${build.classes.dir}">
            <manifest>
                <attribute name="Main-Class" value="eventtrigger.test.FulfillPurchaseOrder" />                
            </manifest>              
            <!-- <zipgroupfileset dir="${lib.dir}" includes="gson-2.2.2.jar" />-->                       
        </jar>
    </target>

    <target name="main" depends="jar">
    </target>     

   <!-- Cleans up the current ${build.dir} -->
   <target name="clean">
      <delete dir="${build.dir}}"/>
   </target>

</project>

它正确编译而没有问题：

init:

compile:
    [javac] /home/test/test-optional/plugins/eventtrigger/test/FulfillPurchaseOrder/build.xml:40: warning: 'includeantruntime' was not set, defaulting to build.sysclasspath=last; set to false for repeatable builds
    [javac] Compiling 1 source file to /home/test/test-optional/plugins/eventtrigger/test/FulfillPurchaseOrder/build/classes
    [javac] Note: /home/test/test-optional/plugins/eventtrigger/test/FulfillPurchaseOrder/src/FulfillPurchaseOrder.java uses    unchecked or unsafe operations.
    [javac] Note: Recompile with -Xlint:unchecked for details.

jar:
     [echo] Compiling jar files
    [delete] Deleting: /home/test/test-optional/plugins/eventtrigger/test/FulfillPurchaseOrder/build/deploy/plugin-event-trigger-test-fulfill-purchase-order.jar
      [jar] Building jar: /home/test/test-optional/plugins/eventtrigger/test/FulfillPurchaseOrder/build/deploy/plugin-event-trigger-test-fulfill-purchase-order.jar

main:

BUILD SUCCESSFUL
Total time: 1 second

我正在使用ubuntu。

有任何建议吗？