尝试在托管 bean 构造函数中访问 @EJB bean 时出现 NullPointerException

我有一个 EJB 服务。

@Stateless
public class SomeService {}

我想将其注入到 viewscoped bean 中并用它进行初始化：

@ManagedBean
@ViewScoped
public class ViewBean implements Serializable {

    @EJB
    private SomeService someService;

    public ViewBean() {
        System.out.println(someService.getEntity());
    }

}

但是，它会引发以下异常：

com.sun.faces.mgbean.ManagedBeanCreationException: Cant instantiate class: com.example.ViewBean.
    at com.sun.faces.mgbean.BeanBuilder.newBeanInstance(BeanBuilder.java:193)
    at com.sun.faces.mgbean.BeanBuilder.build(BeanBuilder.java:102)
    at com.sun.faces.mgbean.BeanManager.createAndPush(BeanManager.java:409)
    at com.sun.faces.mgbean.BeanManager.create(BeanManager.java:269)
    at com.sun.faces.el.ManagedBeanELResolver.resolveBean(ManagedBeanELResolver.java:244)
    at com.sun.faces.el.ManagedBeanELResolver.getValue(ManagedBeanELResolver.java:116)
    at com.sun.faces.el.DemuxCompositeELResolver._getValue(DemuxCompositeELResolver.java:176)
    at com.sun.faces.el.DemuxCompositeELResolver.getValue(DemuxCompositeELResolver.java:203)
    [snip]
Caused by: java.lang.NullPointerException
    at com.example.ViewBean.<init>(ViewBean.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 java.lang.Class.newInstance0(Class.java:374)
    at java.lang.Class.newInstance(Class.java:327)
    at com.sun.faces.mgbean.BeanBuilder.newBeanInstance(BeanBuilder.java:188)
    ... 62 more

这是如何引起的以及如何解决？

换句话说，您期望 EJB 注入在幕后工作，如下所示：

ViewBean viewBean;
viewBean.someService = new SomeService(); // EJB injected, so that constructor can access it.
viewBean = new ViewBean(); // ViewBean constructed.

然而，这在技术上是不可能的。当实例根本没有被构造时，不可能分配实例变量。

在构造后直接执行基于注入依赖项的任务的规范方法是使用@PostConstruct带注释的方法。

因此，要解决您的具体问题，只需更换

public ViewBean() {

by

@PostConstruct
public void init() { // Note: Method name is fully free to your choice.

这样，该过程将大致如下：

ViewBean viewBean;
viewBean = new ViewBean(); // ShiftBean constructed.
viewBean.someService = new SomeService(); // EJB injected.
viewBean.init(); // PostConstruct invoked.

请注意，具体问题与视图范围完全无关。当使用请求、会话或应用程序范围的 bean 时，您会遇到完全相同的问题。因此，这是另一个证据，表明您从未通过使用不同的范围进行测试来真正排除它的原因。