class file has wrong file version 51.0, should be 49.0

my environment is :

<pre>

windows 7, 32 bit, two jdk, one is 1.5, another is 1.7, google gwt, jetty, ant, two eclipse, one is kepler, another is luna.

</pre>

for kepler, I set its jdk is 1.5, and use ant to compile and jar, this works, and eclipse can open the gwt window when debugging.

then i use luna, check out project(gwt project) from cvs, there is error said the jdk is 1.5 when compiling with ant, therefore I changed the jdk to 1.7 since this project requires this version, eventually compile is ok with ant.  However, when I debug it in eclipse, it opens a gwt window said unable to compile to jsp, and the error in console is : 

<pre>

10:20:48.193 [btpool0-8] ERROR org.apache.jasper.compiler.Compiler - Javac exception

Compile failed; see the compiler error output for details.

    at org.apache.tools.ant.taskdefs.Javac.compile(Javac.java:933)

    at org.apache.tools.ant.taskdefs.Javac.execute(Javac.java:757)

    at org.apache.jasper.compiler.Compiler.generateClass(Compiler.java:382)

    at org.apache.jasper.compiler.Compiler.compile(Compiler.java:472)

    at org.apache.jasper.compiler.Compiler.compile(Compiler.java:451)

    at org.apache.jasper.compiler.Compiler.compile(Compiler.java:439)  

   

.....

10:20:48.194 [btpool0-8] ERROR org.apache.jasper.compiler.Compiler - Env: Compile:

javaFileName=/C:/Users/FRANKH~1/AppData/Local/Temp/Jetty_localhost_8001_www__Login__-wcglsc/jsp//org/apache/jsp\login_jsp.java

   

cannot access cn.com.yict.easyport.web.service.CommonUtil

wrong class file： D:\cvs\eInvoice\common-web\target\classes\cn\com\yict\easyport\web\service\CommonUtil.class

class file has wrong version 51.0, should be 49.0

please delete this file or make sure ...

import cn.com.yict.easyport.web.service.CommonUtil;

</pre>

what happened?

Thanks in advance!

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