以单例模式使用CDI [英] Using CDI in a singleton pattern

问题描述

我正在尝试在一个按照单例方法实现的类中注入一个logger对象。

I'm trying to inject a logger object in a class that is implemented following a singleton approach.

代码几乎如下所示：

记录器类：

public class LoggerFactory {
    @Produces 
    public Logger getLogger(InjectionPoint caller){
        return Logger.getLogger(caller.getMember().getDeclaringClass().getName());
    }
}

然后我创建一个需要记录器的类并实现单例模式：

Then I create a class that needs logger and implements the Singleton Pattern:

public class MySingleton{
    @Inject
    private Logger logger;

    private MySingleton instance;

    /*
     * Private constructor for singleton implementation
     */
    private MySingleton(){
        logger.info("Creating one and only one instance here!");
    }

    public MySingleton getInstance(){

        if(instance == null) {
            instance = new MySingleton();
        }

        return instance;
    }

}

如果我运行代码（在Glassfish 3.1.2.2上），我会在尝试使用记录器时立即获得NPE。
我做错了什么（ beans.xml 文件到位了）？
我也尝试使用 @Inject 使用setter方法获取 Logger 对象，但没有运气。

If I run the code (on Glassfish 3.1.2.2) I get a NPE as soon as I try to use the logger. What I'm doing wrong (beans.xml file is in place)? I've also tried using @Inject with a setter method for the Logger object but with no luck.

推荐答案

在构造之后进行注射。所以你不能在构造函数中使用它。

Injections happens after construct. So you cant use it in the constructor.

一种方法是添加一个带注释的@PostConstruct方法，可以在注入后调用它。

One way is to add a method annotated @PostConstruct that can will be invoked after injections.

@PostConstruct
public void init() {
    logger.info("Creating one and only one instance here!");
}

在旁注上我认为你正以错误的方式解决问题。 CDI有一个很好的单身人士支持

On a sidenote i Think you are aprouching the problem in the wrong way. CDI has a nice singleton support

创建一个带注释的类@Singleton

create a class annotated @Singleton

@Singleton
public class MySingleton {

    @Inject
    Logger logger;

    @PostConstruct
    public void init() {
        logger.info("Creating one and only one instance here!");
    }

}

以上假设您使用的是CDI java ee（JSR-299）。

Above assumes you are using CDI for java ee (JSR-299).

如果您正在使用JSR 330依赖注入（guice等） link

If you are using JSR 330 Dependency Injection (guice etc.) link

你可以使用构造函数注入：

You could use constructor injection:

@Singleton
public class MySingleton {


    private final Logger logger;

    @Inject
    public MySingleton (Logger logger) {
        this.logger = logger;
        logger.info("Creating one and only one instance here!");
    }
}

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