我调用userDao.findById（整数I）并给出java.lang.NullPointerException我给出的是db中的确切id

这是我的密码：

public class Test_JPA {
    public static void main(String[] args) {

        UserDao uDao = new UserDaoImplement();
        User u = uDao.findById(4);
        System.out.println(u.getName());
    }

}

在my

UserDaoImplement（）上

这是我的persistence.xml

<persistence-unit name="WebApp" 
transaction-type="RESOURCE_LOCAL"
>
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <class>com.model.Profile</class>
        <class>com.model.User</class>
<properties>
        <!-- <property name="hibernate.dialect" value="org.hibernate.dialect.DB2Dialect" />-->
        <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
        <property name="hibernate.hbm2ddl.auto" value="update" />
        <property name="hibernate.show_sql" value="true" />
        <property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/test"/>
        <property name="hibernate.connection.username" value="root"/>
        <property name="hibernate.connection.password" value=""/>
        <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
        <!--  <property name="hibernate.current_session_context_class" value="thread"/>
        -->
        <!-- per hibernate 4.3 -->
    <!--    <property name="hibernate.currrent_session_context_class" value="org.hibernate.context.internal.ThreadLocal‌​SessionContext"/>
    -->
</properties>
</persistence-unit>


org.hibernate.ejb.HibernatePersistence
com.model.Profile
com.model.User

对不起，忘记了用户实体

@实体
@桌子
公共类用户
{

}
UserDaoImplement.java中第85行的内容。我愿意打赌它是整个管理者。您似乎已经完成了
UserDao uDao=newuserdaoomplement（）这是您的测试用例。您对私有EntityManager EntityManager的期望是什么
要在测试用例中填充
uDao
实例？
请访问
User
实体。另外，请添加完整的堆栈跟踪。2015年2月15日下午6:28:34 com.dao.impl.userdaoomplement findById严重：在com.dao.impl.userdaoomplement.findById（userdaoomplement.java:85）获取失败的java.lang.NullPointerException在com.test_jpa.test_jpa.main（test_jpa.java:14）线程“main”中的异常java.lang.NullPointerException在com.dao.impl.UserDaoImplement.findById（UserDaoImplement.java:85）在com.test_jpa.test_jpa.main（test_jpa.java:14）中的异常，那么您得到了一个NPE，那么什么是null呢？是实体经理吗？那么为什么它是空的呢？实际上，我认为@PersistanceContext不需要spring模块，因为我是JPA和hibernate的新手；这是第85行。我只是给出了您正在使用entitymanager的注释
@PersistenceContext
。必须有人读取该注释并将PersistenceContext注入entitymanager变量中。通常，Spring或依赖项注入框架会这样做。但是当您执行
newuserdaoomplement（）
时，谁负责注入PersistenceContext？答案是没有人。这就是为什么会出现空指针异常。非常感谢：）如果没有spring，我只需要创建带有持久性单元的EntityManagerFactory？不。请阅读有关spring的“集成测试”的内容。您可以将spring上下文注入测试用例中，然后按原样使用Springbean，而无需任何
new
调用。
<persistence-unit name="WebApp" 
transaction-type="RESOURCE_LOCAL"
>
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <class>com.model.Profile</class>
        <class>com.model.User</class>
<properties>
        <!-- <property name="hibernate.dialect" value="org.hibernate.dialect.DB2Dialect" />-->
        <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
        <property name="hibernate.hbm2ddl.auto" value="update" />
        <property name="hibernate.show_sql" value="true" />
        <property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/test"/>
        <property name="hibernate.connection.username" value="root"/>
        <property name="hibernate.connection.password" value=""/>
        <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
        <!--  <property name="hibernate.current_session_context_class" value="thread"/>
        -->
        <!-- per hibernate 4.3 -->
    <!--    <property name="hibernate.currrent_session_context_class" value="org.hibernate.context.internal.ThreadLocal‌​SessionContext"/>
    -->
</properties>
</persistence-unit>

private Integer id;
private Profile profile;
private String name;
private String email;
private String username;
private String password;

public User() {
}

public User(Profile profile, String name, String email, String username,
        String password) {
    this.profile = profile;
    this.name = name;
    this.email = email;
    this.username = username;
    this.password = password;
}

@Id
@GeneratedValue(strategy = IDENTITY)
@Column(name = "id", unique = true, nullable = false)
public Integer getId() {
    return this.id;
}

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

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "profile_id")
public Profile getProfile() {
    return this.profile;
}

public void setProfile(Profile profile) {
    this.profile = profile;
}

@Column(name = "name", length = 45)
public String getName() {
    return this.name;
}

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

@Column(name = "email", length = 45)
public String getEmail() {
    return this.email;
}

public void setEmail(String email) {
    this.email = email;
}

@Column(name = "username", length = 20)
public String getUsername() {
    return this.username;
}

public void setUsername(String username) {
    this.username = username;
}

@Column(name = "password", length = 20)
public String getPassword() {
    return this.password;
}

public void setPassword(String password) {
    this.password = password;
}