在使用JPA的remove（T entity）方法时，我得到了一个java.lang.NullPointerException

请帮助我解决这个NullPointerException错误，我在尝试使用JPA中的remove（T entity）方法时遇到了这个错误

这是我的密码

ControllerServlet

}else if (userPath.equals("/confirm_delete")){

       String userid = request.getParameter("userToDelete");

       int usrId = Integer.valueOf(userid);

        SystemusersFacade usertoDelete = new SystemusersFacade();

       Systemusers user = new Systemusers();
            user =  systemusersFacade.find(usrId);

       usertoDelete.remove(user);

        userPath ="/view_users";

SystemusersFacade扩展了包含remove（T实体）方法的AbstractFacade）

下面是我得到的NullPointerException

Info:   theBankingGuide was successfully deployed in 13,022 milliseconds.
Info:   WebModule[null] ServletContext.log():ControllerServlet:
Info:   entity.Systemusers[ userId=1 ]
Warning:   StandardWrapperValve[ControllerServlet]: Servlet.service() for servlet ControllerServlet threw exception
java.lang.NullPointerException
    at session.AbstractFacade.remove(AbstractFacade.java:35)
    at controller.ControllerServlet.doPost(ControllerServlet.java:212)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:344)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:214)
    at org.glassfish.tyrus.servlet.TyrusServletFilter.doFilter(TyrusServletFilter.java:295)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:256)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:214)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:316)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
    at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
    at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
    at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
    at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:415)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:282)
    at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java:459)
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:167)
    at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:201)
    at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:175)
    at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:235)
    at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:284)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:201)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:133)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:112)
    at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
    at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:561)
    at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:112)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:117)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:56)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:137)
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:565)
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:545)
    at java.lang.Thread.run(Thread.java:745)

我确信该实体没有作为null传递，因为当我使用System.out.Print时，它会给我一个值，如您在上面的日志中所看到的 Info:entity.Systemusers[userId=1]

我还使用以下参数检查了调试器和实体变量exit

entity  Systemusers #1232   
Static          
_persistence_cacheKey   HardCacheWeakIdentityMap$ReferenceCacheKey  #1280   
_persistence_fetchGroup     null    
_persistence_href       null    
_persistence_listener   AttributeChangeListener #1281   
_persistence_primaryKey Integer ObjectFieldVariable _persistence_primaryKey 
_persistence_relationshipInfo       null
_persistence_session        null    
_persistence_shouldRefreshFetchGroup    boolean false   
adsCollection   IndirectList    size = 0
mmtransinfoCollection   IndirectList    size = 0
securitiesinfoCollection    IndirectList    size = 0
systemUsersTime  Date  02-Jan-2015 20:09:52
userEmail   String  jonahkayizzi@gmail.com
userId  Integer 1
userName    String  "Jonah"
userPassword    String  "0xgl#kylZ"
userType    String  "Manager"

我还尝试在SystemusersFacade中创建我的custome delete函数，但得到了相同的错误

请帮忙

编辑

package session; 

import entity.Systemusers; 
import javax.ejb.Stateless; 
import javax.persistence.EntityManager; 
import javax.persistence.PersistenceContext; 

/** 
* 
* @author Jonah 
*/ 
@Stateless 
public class SystemusersFacade extends AbstractFacade<Systemusers> { 

@PersistenceContext(unitName = "theBankingGuidePU") 
    private EntityManager em; 

    @Override 
    protected EntityManager getEntityManager() { 
        return em; 
    } 

    public SystemusersFacade() { 
        super(Systemusers.class); 
    } 
}

package session; 

import java.util.List; 
import javax.persistence.EntityManager; 
import javax.persistence.Query; 

public abstract class AbstractFacade<T> { 

    private Class<T> entityClass; 

    public AbstractFacade(Class<T> entityClass) { 
        this.entityClass = entityClass; 
    } 

    protected abstract EntityManager getEntityManager(); 

    public void remove(T entity) { 
        System.out.println(entity);
        getEntityManager().remove(getEntityManager().merge(entity)); 
    } 

}

包会话；
导入实体.Systemusers；
导入javax.ejb.Stateless；
导入javax.persistence.EntityManager；
导入javax.persistence.PersistenceContext；
/** 
* 
*@作者约拿
*/ 
@无国籍
公共类SystemusersFacade扩展了AbstractFacade{
@持久性上下文（unitName=“theBankingGuidePU”）
私人实体管理者；
@凌驾
受保护的EntityManager getEntityManager（）{
返回em；
} 
public SystemusersFacade（）{
super（Systemusers.class）；
} 
}
一揽子会议；
导入java.util.List；
导入javax.persistence.EntityManager；
导入javax.persistence.Query；
公共抽象类AbstractFacade{
私有类实体类；
公共抽象外观（类entityClass）{
this.entityClass=entityClass；
} 
受保护的抽象EntityManager getEntityManager（）；
公共无效删除（T实体）{
系统输出打印项次（实体）；
getEntityManager（）.remove（getEntityManager（）.merge（entity））；
} 
}

SystemusersFacade usertoDelete=new SystemusersFacade（）

是您的问题（忽略奇怪的名称，可能是复制/粘贴错误）。由于您使用

new

实例化它，因此您绕过了容器管理，因此不会处理注释。您需要将bean注入到使用它的类中。用这样的东西

@EJB
private SystemusersFacade systemusersFacade;

...

String userid = request.getParameter("userToDelete");
int usrId = Integer.valueOf(userid);
Systemusers user = systemusersFacade.find(usrId);
systemusersFacade.remove(user);

---

getEntityManager（）

返回null？调试器中的

getEntityManager（）.merge（entity）

返回什么？在Servlet中执行SystemusersFacade usertoDelete=new SystemusersFacade（）时，如何假设getEntityManager（）返回的SystemusersFacade中的EntityManager被实例化？它可以是null以外的任何内容吗？@Predrag，getEntityManager（）。合并（实体）似乎在调试器中返回null，尽管实体不是null。名称类型值实体Systemusers#1858 emnull@AlanHay，在这种情况下，如何使EntityManager不为null。我需要什么来稳定？下面是SystemusersFacade

@无状态公共类SystemusersFacade扩展AbstractFacade{@PersistenceContext（unitName=“theBankingGuidePU”）私有EntityManager em；@Override protected EntityManager getEntityManager（）{return em；}公共SystemusersFacade（）{super]的代码（Systemusers.class）；}}

对不起，我对JPAYes有些陌生，你是对的@Predrag，'SystemusersFacade usertoDelete=new SystemusersFacade（）'是不必要的，实际上是导致NullPointerException的。我刚刚摆脱了它，一切都正常工作。谢谢：）

@EJB
private SystemusersFacade systemusersFacade;

...

String userid = request.getParameter("userToDelete");
int usrId = Integer.valueOf(userid);
Systemusers user = systemusersFacade.find(usrId);
systemusersFacade.remove(user);