ids for this class must be manually assigned before calling save(): po.Project
org.hibernate.id.IdentifierGenerationException: ids for this class must be manually assigned before calling save(): po.Project
at org.hibernate.id.Assigned.generate(Assigned.java:33)
at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:99)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:187)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:172)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:94)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
at org.hibernate.impl.SessionImpl.fireSaveOrUpdate(SessionImpl.java:508)
at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:500)
at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:496)
at dao.ProjectDao.addProject(ProjectDao.java:25)
at action.AddProjectAction.execute(AddProjectAction.java:68)
at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431)
at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)


------解决方案--------------------
hibernate出现这个错误的原因有可能因为，你的表中有个主键。 但是你插入的（调用save)时那个值是null

你要操作的数据表中的id（即主键）的类型设置成了“自动增长类型”，而在你的

hibernate.cfg.xml中，id的生成方式是assigned，即

XML code

    <id name="id" type="integer">  
        <column name="id" />  
        <generator class="assigned" />  
    </id>

------解决方案--------------------

你看一下传过来的。String project_id=request.getParameter("project_id");是不是null.如果是的话和没设是一样的。