在托管 bean 构造函数中访问注入的依赖项会导致 NullPointerException [英] Accessing injected dependency in managed bean constructor causes NullPointerException

问题描述

我正在尝试将 DAO 作为托管属性注入.

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bean创建后注入DAO对象，但它在构造函数中为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

注入只能发生在构建之后，因为构建之前没有符合条件的注入目标.想象一下下面这个虚构的例子:

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.实际情况如下:

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.

您应该使用带有 @PostConstruct 注释的方法 在构建后直接执行操作和依赖注入(例如通过 Spring bean、@ManagedProperty、@EJB、@注入等).

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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