mismatch errors when deploying session bean

i have written a session bean xyzBean,jar and deployed it using weblogic700 server.The first time it deployed successfully.But  i made some code shanges in my bean and recompiled an made a new jar file xyzBean.jar and replaced the old jar file with the new one.Now when i restarted the weblogic server i got error :

Unable to deploy EJB: xyzBean from xyzBean.jar:

There is a mismatch between the bean code and generated  code.xyzBean(Application: xyzBean, EJBComponent: MT
alkBean) .       Please rerun ejbc on the bean code.The error was java.lang.NoSuchFieldException
       at java.lang.Class.getField0(Native Method)
       at java.lang.Class.getField(Class.java:821)
       at weblogic.ejb20.deployer.ClientDrivenBeanInfoImpl.setMDField(ClientDrivenBeanInfoImpl.java:688)
       at weblogic.ejb20.deployer.ClientDrivenBeanInfoImpl.setMethodDescriptor(ClientDrivenBeanInfoImpl.java:8
24)
       at weblogic.ejb20.deployer.ClientDrivenBeanInfoImpl.setMethodDescriptors(ClientDrivenBeanInfoImpl.java:
755)
       at weblogic.ejb20.deployer.ClientDrivenBeanInfoImpl.prepare(ClientDrivenBeanInfoImpl.java:907)
       at weblogic.ejb20.deployer.EJBDeployer.setupBeanInfo(EJBDeployer.java:1067)
       at weblogic.ejb20.deployer.EJBDeployer.prepare(EJBDeployer.java:1273)
       at weblogic.ejb20.deployer.EJBModule.prepare(EJBModule.java:232)
       at weblogic.j2ee.J2EEApplicationContainer.prepareModule(J2EEApplicationContainer.java:1570)
       at weblogic.j2ee.J2EEApplicationContainer.prepare(J2EEApplicationContainer.java:737)
       at weblogic.j2ee.J2EEApplicationContainer.prepare(J2EEApplicationContainer.java:555)
       at weblogic.j2ee.J2EEApplicationContainer.prepare(J2EEApplicationContainer.java:458)
       at weblogic.management.deploy.slave.SlaveDeployer.prepareAllStagedApplications(SlaveDeployer.java:490)
       at weblogic.management.deploy.slave.SlaveDeployer.initialize(SlaveDeployer.java:253)
       at weblogic.management.deploy.DeploymentManagerServerLifeCycleImpl.initialize(DeploymentManagerServerLi
feCycleImpl.java:150)