尝试在构造函数中访问 @Inject bean 时出现 NullPointerException [英] NullPointerException while trying to access @Inject bean in constructor

问题描述

我有一个会话范围的 bean:

I've a session scoped bean:

@Named
@SessionScoped
public class SessionBean implements Serializable {

    private String someProperty;

    public String getSomeProperty() {
        return someProperty;
    }

}

我想将它注入到请求范围的 bean 中并用它进行初始化:

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());
    }

}

然而，它抛出了以下异常:

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)
    ...

这是怎么引起的，我该如何解决?

How is this caused and how can I solve it?

推荐答案

您期望注入的依赖项在构建 bean 之前可用.你期望它是这样工作的:

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，而且在技术上是不可能的.依赖在构造之后注入.

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 如果您打算在 bean 构建后直接执行基于注入的依赖项的业务逻辑，请使用此方法.

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

移除构造函数并添加这个方法:

Remove the constructor and add this method:

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

这篇关于尝试在构造函数中访问 @Inject bean 时出现 NullPointerException的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！