java - hibernate jpa java.lang.ClassCastException : model. Dog 无法转换为 model.Dog

Question

我有一个奇怪的异常，我不知道为什么。我在内存中有数据库中的列表，但是当我尝试访问 getter 或将实体保存在变量中时，它会引发异常。

这是实体:

package model;

import java.io.Serializable;
import javax.persistence.*;

@Entity
@NamedQuery(name="Dog.findAll", query="SELECT d FROM Dog d")
public class Dog implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

    private String name;

    public Dog() {
    }

    public int getId() {
        return this.id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

这是我的 session bean 实现:

@Stateless(name = "mytable")
public class TableBean implements TableLocal {

@PersistenceContext(unitName = "sample")
private EntityManager manager;

@Override
public Dog getAll() {
    Dog h = manager.find(Dog.class, 1); 
    Hibernate.initialize(h);
    return h;
}

@Override
@SuppressWarnings("unchecked")
public List<Dog> findAll() {
    Query query = manager.createQuery("SELECT h FROM Dog h",Dog.class);
    List<Dog> col = query.getResultList();
    Hibernate.initialize(col);
    return col;
}

@Override
public void addDog() {
    Dog d = new Dog();
    //d.setId(2);
    d.setName("chappy");
    manager.persist(d);
}
}

我也有一个服务bean，但是没有意义

@Stateless(name = "service")
public class ServiceBean implements ServiceLocal {

    @EJB(beanName = "mytable")
    TableLocal table;

    @Override
    public Dog getAll() {
        return table.getAll();
    }

    @Override
    public List<Dog> findAll() {
        return table.findAll();
    }

    @Override
    public void addDog() {
        table.addDog();

    }
}

和我的 servlet:

public class HelloServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    @EJB(beanName = "service")
    ServiceLocal table;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        List<Dog> tables = table.findAll();
        Iterator<Dog> it = tables.iterator();
        while (it.hasNext()) {
            Dog t = it.next();
            resp.getWriter().println(t.getName());
        }
        /*Dog h = table.getAll();
        resp.getWriter().println(h.getId() + " " + h.getName());*/
        /*table.addDog();
        resp.getWriter().println("Added!");*/
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }

}

错误是:

2015-07-28T13:38:17.385+0300|Warning: StandardWrapperValve[HelloServlet]: Servlet.service() for servlet HelloServlet threw exception
java.lang.ClassCastException: model.Dog cannot be cast to model.Dog
    at HelloServlet.doGet(HelloServlet.java:26)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
    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: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)

和我的 persistence.xml:

<?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="sample">
        <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
        <jta-data-source>MySQL</jta-data-source>
        <class>model.Dog</class>
        <properties>
            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect" />
            <property name="hibernate.hbm2ddl" value="update" />
            <property name="hibernate.transaction.jta.platform"
                value="org.hibernate.service.jta.platform.internal.SunOneJtaPlatform" />
            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.format_sql" value="true"/>
        </properties>
    </persistence-unit>
</persistence>

Answer 1

在你的findAll()方法，使用 TypedQuery 而不是 Query。原因是，TypedQuery 的 getResultList()正在返回类型列表的列表。即在你的情况下，List<Dog>但 Query 的 getResultList 返回原始 List

您可以尝试使用以下代码:-

@Override
@SuppressWarnings("unchecked")
public List<Dog> findAll() {
    TypedQuery<Dog> query = manager.createQuery("SELECT h FROM Dog h",Dog.class);
    List<Dog> col = query.getResultList();
    Hibernate.initialize(col);
    return col;
}