EntityManager 注入 - NullPointerException

发布于 2024-10-01 11:39:14 字数 764 浏览 4 评论 0原文

在我的 Spring+JPA/Hibernate+Wicket 应用程序中，我有一个 QueryBuilder bean，我想在我的 DAO 之一中使用它，它在 Criteria API 的帮助下生成类型化查询：

@Service(value="inboxQueryBuilder")
public class InboxQueryBuilder {

    @PersistenceContext
    EntityManager em;

    CriteriaBuilder cb;

    public InboxQueryBuilder() {
        cb = em.getCriteriaBuilder();
    }

    public TypedQuery<App> getQueryForApps(AppSearchObject aso) {
           ...
    }

    ...
}

但是，当我运行该应用程序时，我得到一个 null行指针异常：

cb = em.getCriteriaBuilder();

即 EntityManager 没有被注入。你知道为什么吗？

另外，这种使用是否正确且线程安全，或者我应该为每个查询实例化我的 InboxQueryBuilder 吗？在这种情况下，我还应该注入 EntityManager 还是应该将其作为构造函数参数传递（InboxQueryBuilder 将为 DAO 中具有注入的 EntityManager 实例的每个查询进行实例化）？

原文

In my Spring+JPA/Hibernate+Wicket app, I have a QueryBuilder bean that I want to use in one of my DAOs which generates a typed query with the help of Criteria API:

@Service(value="inboxQueryBuilder")
public class InboxQueryBuilder {

    @PersistenceContext
    EntityManager em;

    CriteriaBuilder cb;

    public InboxQueryBuilder() {
        cb = em.getCriteriaBuilder();
    }

    public TypedQuery<App> getQueryForApps(AppSearchObject aso) {
           ...
    }

    ...
}

However, when I run the app, I get a null pointer exception for line:

cb = em.getCriteriaBuilder();

i.e. the EntityManager doesn't get injected. Do you know why?

Also, is this use correct and thread-safe or should I instantiate my InboxQueryBuilder for each query? In that case, should I also inject the EntityManager or should I just pass it as a constructor parameter (the InboxQueryBuilder would get instantiated for each query in the DAO which has an injected instance of EntityManager)?

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枕梦 2024-10-08 11:39:14

您无法在构造函数中访问 EntityManager。看看@PostConstruct-Annotation

@Service(value="inboxQueryBuilder")
public class InboxQueryBuilder {

@PersistenceContext
EntityManager em;

CriteriaBuilder cb;

public InboxQueryBuilder() {
    // em= null
}

@PostConstruct
public void toSomething(){
    // em set by Container  
    cb = em.getCriteriaBuilder();
}


public TypedQuery<App> getQueryForApps(AppSearchObject aso) {
       ...
}

...
}

编辑：
再次阅读您的文章后，我开始不确定我是否正确。我知道 JBoss 中的 Java EE 依赖注入按照我所描述的方式工作，但我不确定 spring-IOC。

You can't access the EntityManager within the constructor. Take a look at the @PostConstruct-Annotation

@Service(value="inboxQueryBuilder")
public class InboxQueryBuilder {

@PersistenceContext
EntityManager em;

CriteriaBuilder cb;

public InboxQueryBuilder() {
    // em= null
}

@PostConstruct
public void toSomething(){
    // em set by Container  
    cb = em.getCriteriaBuilder();
}


public TypedQuery<App> getQueryForApps(AppSearchObject aso) {
       ...
}

...
}

EDIT:
After reading your post again, I start to became unsure, if I'm right. I know the Java EE-Dependency-Injection within a JBoss works as I described, but I'm not sure about spring-IOC.

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追星践月 2024-10-08 11:39:14

您的应用程序上下文中是否有此 bean？

<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalEntityManagerFactoryBean">
  <property name="persistenceUnitName" value="myPersistenceUnit"/>
</bean>

Do you have this bean somewhere in your application context?

<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalEntityManagerFactoryBean">
  <property name="persistenceUnitName" value="myPersistenceUnit"/>
</bean>

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秋千易 2024-10-08 11:39:14

Spring 使用 Java Beans 机制，所以我很确定这是不够的：

@PersistenceContext
EntityManager em;

这是标准方法：

private EntityManager entityManager;

@PersistenceContext
public void setEntityManager(final EntityManager entityManager){
    this.entityManager = entityManager;
}

Spring uses the Java Beans mechanism, so I am pretty sure this is insufficient:

@PersistenceContext
EntityManager em;

Here's the standard way:

private EntityManager entityManager;

@PersistenceContext
public void setEntityManager(final EntityManager entityManager){
    this.entityManager = entityManager;
}

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