在受管Bean构造函数中访问注入的依赖会导致NullPointerException [英] Accessing injected dependency in managed bean constructor causes NullPointerException

问题描述

我试图注入一个DAO作为托管属性。

I'm trying to inject a DAO as a managed property.

public class UserInfoBean {

    private User user;

    @ManagedProperty("#{userDAO}")
    private UserDAO dao;

    public UserInfoBean() {
        this.user = dao.getUserByEmail("[e-mail]");
    }

    // Getters and setters.
}

在创建bean之后注入DAO对象， c $ c> null ，因此导致 NullPointerException 。如何使用注入的托管属性初始化托管bean？

The DAO object is injected after the bean is created, but it is null in the constructor and therefore causing NullPointerException. How can I initialize the managed bean using the injected managed property?

推荐答案

注入只能在< >建设只是因为在建设之前没有合格的注射目标。想象以下虚拟示例：

Injection can only take place after construction simply because before construction there's no eligible injection target. Imagine the following fictive example:

UserInfoBean userInfoBean;
UserDao userDao = new UserDao();
userInfoBean.setDao(userDao); // Injection takes place.
userInfoBean = new UserInfoBean(); // Constructor invoked.

这在技术上根本不可能。实际上，发生了什么：

This is technically simply not possible. In reality the following is what is happening:

UserInfoBean userInfoBean;
UserDao userDao = new UserDao();
userInfoBean = new UserInfoBean(); // Constructor invoked.
userInfoBean.setDao(userDao); // Injection takes place.

您应该使用 @PostConstruct 直接在构建之后执行操作依赖注入（通过例如Spring bean， @ManagedProperty ， @EJB ， @Inject ，etc）。

You should be using a method annotated with @PostConstruct to perform actions directly after construction and dependency injection (by e.g. Spring beans, @ManagedProperty, @EJB, @Inject, etc).

@PostConstruct
public void init() {
    this.user = dao.getUserByEmail("[e-mail]");
}

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