Question

我是JavaEE的新手。我在这个网站和其他网站上用EJB注入查找了很多关于NullPinterException的问题，但还没有成功。我无法找到解决问题的方法。

我使用JPA，JAX-RS，Glassfish Web Extensions方面在Eclipse中创建了DynamicWebProject。这是我的项目的结构（web.xml文件位于glassfish-web.xml文件下）：

Project Structure

我尝试调用部署到localhost Glassfish服务器的getemployees webservice，并尝试从我的数据库（也在localhost）获取有关所有员工的信息。这是我的GetEmployees类的代码。

package bean;

import javax.faces.bean.ApplicationScoped;
import javax.faces.bean.ManagedBean;
import javax.inject.Inject;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.Response;

import com.google.gson.Gson;

import ejb.EmployeeEJB;

@ApplicationScoped
@ManagedBean
@Path("/getemployees")
public class GetEmployees {

    @Inject
    EmployeeEJB employeeEJB;

    @GET
    @Produces("application/json; charset=utf-8")
    public Response getAllEmployees() throws Exception {
        Gson gson = new Gson();
        String jsonString = null;
        try {
            jsonString = gson.toJson(employeeEJB.getAll());
        } catch (Exception e) {
            e.printStackTrace();
        }
        return Response.status(200).entity(jsonString).build();
    }
}

我创建了托管和会话bean。我从我的GetEmployees RESTful webservice类中创建了Managed bean。这是对的吗？

EmployeeEJB是会话bean：

package ejb;

import java.util.List;

import javax.ejb.LocalBean;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;

import entity.Employee;

@LocalBean
@Stateless
public class EmployeeEJB {

    @PersistenceContext
    EntityManager em;

    public void saveEmployee(Employee employee) {
        em.merge(employee);
    }

    public void deleteEmployee(Employee employee) {
        em.remove(employee);
    }

    public List<Employee> getAll() {
        Query query = em.createQuery("SELECT emp FROM Employee emp", Employee.class);
        List<Employee> emp = (List<Employee>) query.getResultList();
        return emp;
    }

}

这是我的persistence.xml文件。在Glassfish管理控制台中创建的JDBC连接池和JDBC资源。与数据库的连接是好的。

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence" 
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
            xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
    <persistence-unit name="FirstDynamicWebProject">
    <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
    <jta-data-source>jdbc/second_attempt_hibernate</jta-data-source>
    <class>entity.Docstatus</class>
    <class>entity.Doctype</class>
    <class>entity.Document</class>
    <class>entity.Employee</class>
    <class>entity.Mailorder</class>
    <class>entity.Mailorderstatus</class>
    </persistence-unit>
</persistence>

当我尝试在GetEmployees类中调用employeeEJB.getAll（）方法时发生NullPointerException：

2017-04-10T00:59:07.063+0500|Severe: java.lang.NullPointerException
    at bean.GetEmployees.getAllEmployees(GetEmployees.java:28)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.sun.jersey.spi.container.JavaMethodInvokerFactory$1.invoke(JavaMethodInvokerFactory.java:60)
    at com.sun.jersey.server.impl.model.method.dispatch.AbstractResourceMethodDispatchProvider$TypeOutInvoker._dispatch(AbstractResourceMethodDispatchProvider.java:185)
    at com.sun.jersey.server.impl.model.method.dispatch.ResourceJavaMethodDispatcher.dispatch(ResourceJavaMethodDispatcher.java:75)
    at com.sun.jersey.server.impl.uri.rules.HttpMethodRule.accept(HttpMethodRule.java:302)
    at com.sun.jersey.server.impl.uri.rules.ResourceClassRule.accept(ResourceClassRule.java:108)
    at com.sun.jersey.server.impl.uri.rules.RightHandPathRule.accept(RightHandPathRule.java:147)
    at com.sun.jersey.server.impl.uri.rules.RootResourceClassesRule.accept(RootResourceClassesRule.java:84)
    at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1542)
    at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1473)
    at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1419)
    at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1409)
    at com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:409)
    at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:558)
    at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:733)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
    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:416)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:283)
    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:206)
    at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:180)
    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:283)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:200)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:132)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:111)
    at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
    at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:536)
    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$Worker.doWork(AbstractThreadPool.java:591)
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:571)
    at java.lang.Thread.run(Thread.java:745)

我认为注入EmployeeEJB是不正确的，但不知道如何赢得它。

Answer 1

我想问题是你使用了@ApplicationScoped的错误包。而且我认为你不需要@ManagedBean。

您正在使用：

import javax.faces.bean.ApplicationScoped;

尝试以下方法：

import javax.enterprise.context.ApplicationScoped;

另见：

Why are there different bean management annotations

调用EJB方法时出现NullPointerException

1 个答案: