“@ inject”-ed属性保持为null [英] "@inject"-ed attribute remains null

问题描述

我正在尝试将服务注入到我的bean中，但它总是 null 。
我收到以下错误：WELD-001000错误解析属性userBean对基础null。

I am trying to inject a service into my bean but it is always null. I get the following error: WELD-001000 Error resolving property userBean against base null.

一些代码片段：

index.xhtml

index.xhtml

<h:body>
    Hello from Facelets
    #{userBean.name}
</h:body>

userbean.java

userbean.java

package beans;

import Domain.User;
import java.io.Serializable;
import javax.enterprise.context.SessionScoped;
import javax.inject.Inject;
import javax.inject.Named;
import service.UserService;

@Named
@SessionScoped
public class UserBean implements Serializable{
    @Inject UserService service;
    private User user;

    public UserBean(){
        this.user = service.find_user("foo");
    }

    public String getName(){
        return "bar";
    }
}

UserService.java

UserService.java

package service;

import Domain.User;
import javax.ejb.Stateless;
import javax.inject.Named;

@Named
@Stateless
public class UserService {
    public UserService(){}

    public User find_user(String name){
        return new User();
    }
}

推荐答案

错误消息可能是一个提示，JVM无法创建 UserBean 的实例。以下是一些猜测，必须证明：

The error message could be a hint, that the JVM wasn't able to create an instance of UserBean. The following is some guessing and would have to be proven:

Dependecy Injection需要一个依赖注入器，一个注入 <$ em>实例的工具c $ c> UserService 进入 UserBean 。在你的代码中，你已经在bean的实例化过程中使用了这个注入的实例：你在构造函数中调用注入的服务。

Dependecy Injection requires a dependency injector, a tool that injects an instance of UserService into a UserBean. In your code you're already using this injected instance during instantiation of the bean: you call the injected service in the constructor.

如果依赖注入器启动它的工作在创建bean之后，然后对构造函数内部服务的调用将引发 NullPointerException （因为 service 当时仍为 null ）。通过尝试在 UserBean 构造函数中捕获NPE一会儿值得检查。如果你抓住一个 - il - 依赖注入器在创建bean之后开始运行，因此，我们不能在类实例化期间使用注入的服务（=在构造函数中）

If the dependency injector starts it's work after the bean is created, then the call to the service inside the constructor will raise a NullPointerException (because service is still null at that time). It's worth checking that by trying to catch NPEs in the UserBean constructor for a moment. If you catch one - voilà - the dependency injector starts runnning after the bean has been created and, as a consequence, we can't use injected services during class instantiation (= in the constructor)

解决方法想法：实现一个小型服务提供商助手类 - 内部类可以工作：

Workaround idea: implement a small service provider helper class - inner class could work:

public class UserBean implements Serializable {
    static class UserServiceProvider {
      @Inject static UserService service;
    }

    // ... 

    public UserBean() {
      this.user = UserServiceProvider.service.findUser("kaas"); 
    }

    // ...
}

未经测试但可以正常工作 - 在中使用bean构造函数之前，应该在提供程序类中注入服务。

Untested but could work - the service should be injected in the provider class before you use it in the beans constructor.

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