java “@inject”-ed 属性保持为空
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"@inject"-ed attribute remains null
提问by martijnve
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.
我正在尝试将服务注入到我的 bean 中，但它始终是null. 我收到以下错误： WELD-001000 将属性 userBean 解析为空值时出错。
Some code snippets:
一些代码片段：
index.xhtml
索引.xhtml
<h:body>
Hello from Facelets
#{userBean.name}
</h:body>
userbean.java
用户bean.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
用户服务.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();
}
}
采纳答案by Andreas Dolk
The error message couldbe 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:
错误消息可能是一个提示，即 JVM 无法创建UserBean. 以下是一些猜测，必须证明：
Dependecy Injection requires a dependency injector, a tool that injectsan instance of UserServiceinto 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.
依赖注入需要一个依赖注入器，一种将的实例注入UserService到UserBean. 在您的代码中，您已经在 bean 的实例化过程中使用了这个注入的实例：您在构造函数中调用了注入的服务。
Ifthe dependency injector starts it's work afterthe bean is created, thenthe call to the service inside the constructor will raise a NullPointerException(because serviceis still nullat that time). It's worth checking that by trying to catch NPEs in the UserBeanconstructor 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)
如果依赖注入器在创建 bean后启动它的工作，那么对构造函数内的服务的调用将引发一个NullPointerException（因为service仍然null在那个时候）。值得通过尝试在UserBean构造函数中捕获 NPE 来检查一下。如果你发现一个 - 瞧 - 依赖注入器在 bean 被创建后开始运行，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）
Workaround idea: implement a small service provider helper class - inner class could work:
解决方法的想法：实现一个小的服务提供者助手类 - 内部类可以工作：
public class UserBean implements Serializable {
static class UserServiceProvider {
@Inject static UserService service;
}
// ...
public UserBean() {
this.user = UserServiceProvider.service.findUser("kaas");
}
// ...
}
Untested but could work - the service should be injected in the provider class beforeyou use it in the beans constructor.
未经测试但可以工作 -在您在 beans 构造函数中使用它之前，应该将服务注入到提供者类中。
回答by Gere
Another alternative is use @PostConstruct method annotation.
另一种选择是使用@PostConstruct 方法注释。
@SessionScoped
public class UserBean implements Serializable {
@Inject UserService service;
private User user;
public UserBean() {
}
@PostConstruct
void init(){
this.user = service.findUser("kaas");
}
}
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