EJB,struts问题

各位大侠帮忙看看这个错误：我在一个ejb中有这样一个方法：
  public ArrayList UserList() {
    ArrayList userlist = new ArrayList();
    UserVO obj1 = new UserVO();    obj1.setUserID("item1");
    obj1.setUserName("item2");
    
    userlist.add(obj1);
    return userlist;
  }UserVO.java：
package testproject;import testproject.ValueObject;public class UserVO {
  private String UserID;
  private String UserName;  public UserVO() {
  }  public void setUserID(String pUserID) {
    UserID = pUserID;
  }
  public String getUserID(){
    return UserID;
  }
  
  public void setUserName(String pUserName) {
    UserName = pUserName;
  }
  public String getUserName(){
    return UserName;
  }}
在action中通过EJB组件接口调用Bean方法的时候，报了下面的错误：
Error 500--Internal Server Error 
java.io.NotSerializableException: testproject.UserVO
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1054)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:278)
at java.util.ArrayList.writeObject(ArrayList.java:530)
at sun.reflect.GeneratedMethodAccessor28.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:795)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1294)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1245)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1052)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:278)
at weblogic.rmi.internal.CBVOutputStream.writeObject(CBVOutputStream.java:78)
at weblogic.rmi.internal.CBVWrapper.copy(CBVWrapper.java:47)
at testproject.UserInfo_1571he_EOImpl_CBV.UserList(Unknown Source)
at testproject.myAction.execute(myAction.java:39)
at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:484)
at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:274)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)
at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:525)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:760)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at weblogic.servlet.internal.ServletStubImpl$ServletInvocationAction.run(ServletStubImpl.java:971)
at weblogic.servlet.internal.ServletStubImpl.invokeServlet(ServletStubImpl.java:402)
at weblogic.servlet.internal.ServletStubImpl.invokeServlet(ServletStubImpl.java:305)
at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.run(WebAppServletContext.java:6354)
at weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.java:317)
at weblogic.security.service.SecurityManager.runAs(SecurityManager.java:118)
at weblogic.servlet.internal.WebAppServletContext.invokeServlet(WebAppServletContext.java:3635)
at weblogic.servlet.internal.ServletRequestImpl.execute(ServletRequestImpl.java:2585)
at weblogic.kernel.ExecuteThread.execute(ExecuteThread.java:197)
at weblogic.kernel.ExecuteThread.run(ExecuteThread.java:170)
--------------- nested within: ------------------
weblogic.utils.AssertionError: ***** ASSERTION FAILED ***** - with nested exception:
[java.io.NotSerializableException: testproject.UserVO]
at weblogic.rmi.internal.CBVWrapper.copy(CBVWrapper.java:55)
at testproject.UserInfo_1571he_EOImpl_CBV.UserList(Unknown Source)
at testproject.myAction.execute(myAction.java:39)
at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:484)
at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:274)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)
at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:525)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:760)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at weblogic.servlet.internal.ServletStubImpl$ServletInvocationAction.run(ServletStubImpl.java:971)
at weblogic.servlet.internal.ServletStubImpl.invokeServlet(ServletStubImpl.java:402)
at weblogic.servlet.internal.ServletStubImpl.invokeServlet(ServletStubImpl.java:305)
at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.run(WebAppServletContext.java:6354)
at weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.java:317)
at weblogic.security.service.SecurityManager.runAs(SecurityManager.java:118)
at weblogic.servlet.internal.WebAppServletContext.invokeServlet(WebAppServletContext.java:3635)
at weblogic.servlet.internal.ServletRequestImpl.execute(ServletRequestImpl.java:2585)
at weblogic.kernel.ExecuteThread.execute(ExecuteThread.java:197)
at weblogic.kernel.ExecuteThread.run(ExecuteThread.java:170)