Using CDI in a singleton pattern

Question

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

The code almost looks like this:

Logger class:

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

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;
    }

}

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.

Answer 1

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

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!");
}

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

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!");
    }

}

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

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

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!");
    }
}

Answer 2

This will not work, because injection, as already mentioned, will be performed after the constructor is called.

Methods annotated with @PostConstruct are called after injection has been finished and before the object itself will be supplied somewhere else.

However, injection only works if the instance of your class is provided by injection itself. This is due to the injection depending on proxying.

Therefore you will need to inject your MySingleton wherever you need it. To be sure it is a singleton, annotate it @Singleton and the container will work that out for you.

Addiotnally beware, that singleton in terms of CDI spec does not mean only one instantiation, but rather only one initialiation of @PostConstruct.