NullPointerException尝试访问@Inject bean在构造函数 [英] NullPointerException while trying to access @Inject bean in constructor

问题描述

我有一个会话作用域bean：

I've a session scoped bean:

@Named
@SessionScoped
public class SessionBean implements Serializable {

    private String someProperty;

    public String getSomeProperty() {
        return someProperty;
    }

}

在请求作用域bean中并用它初始化：

I'd like to inject this in a request scoped bean and initialize with it:

@Named
@RequestScoped
public class RequestBean {

    @Inject
    private SessionBean sessionBean;  

    public RequestBean() {
        System.out.println(sessionBean.getProperty());
    }

}

但是，它会抛出以下异常：

However, it throws the following exception:

java.lang.NullPointerException
    at com.example.RequestBean.<init>(RequestBean.java:42)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:525)
    at org.jboss.weld.introspector.jlr.WeldConstructorImpl.newInstance(WeldConstructorImpl.java:206)
    at org.jboss.weld.injection.ConstructorInjectionPoint.newInstance(ConstructorInjectionPoint.java:117)
    at org.jboss.weld.bean.ManagedBean.createInstance(ManagedBean.java:336)
    at org.jboss.weld.bean.ManagedBean$ManagedBeanInjectionTarget.produce(ManagedBean.java:200)
    at org.jboss.weld.bean.ManagedBean.create(ManagedBean.java:292)
    ...

这是如何引起的，我该如何解决？

How is this caused and how can I solve it?

推荐答案

您期望在构造bean之前注入的依赖关系可用。您期望它的工作方式如下：

You're expecting that the injected dependency is available before the bean is constructed. You're expecting that it works like this:

RequestBean requestBean;
requestBean.sessionBean = sessionBean; // Injection.
requestBean = new RequestBean(); // Constructor invoked.

然而这不是真的，在技术上是不可能的。依赖关系在之后注入。

This is however not true and technically impossible. The dependencies are injected after construction.

RequestBean requestBean;
requestBean = new RequestBean(); // Constructor invoked.
requestBean.sessionBean = sessionBean; // Injection.

您应该使用 @PostConstruct 方法，而不是直接在bean构造之后基于注入的依赖性执行业务逻辑

You should be using a @PostConstruct method instead if you intend to perform business logic based on injected dependencies directly after bean's construction.

删除构造函数并添加此方法：

Remove the constructor and add this method:

@PostConstruct
public void init() {
    System.out.println(sessionBean.getSomeProperty());
}

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