How to synchronize static method in java

问题:

I come up with this question when implementing singleton pattern in Java. Even though the example listed below is not my real code, yet very similar to the original one.

public class ConnectionFactory{
    private static ConnectionFactory instance;

    public static synchronized ConnectionFactory getInstance(){
        if( instance == null ){
            instance = new ConnectionFactory();
        }

        return instance;
    }

    private ConnectionFactory(){
        // private constructor implementation
    }
}

Because I'm not quite sure about the behavior of a static synchronized method, I get some suggestion from google -- do not have (or as less as possible) multiple static synchronized methods in the same class. I guess when implementing static synchronized method, a lock belongs to Class object is used so that multiple static synchronized methods may degrade performance of the system.

Am I right? or JVM use other mechanism to implement static synchronized method? What's the best practice if I have to implement multiple static synchronized methods in a class?

Thank you all!

Kind regards!

回答1:

The best approach (which makes as few changes in your code as possible) is to do like this:

public class ConnectionFactory{
    private static ConnectionFactory instance = new ConnectionFactory();

    public static ConnectionFactory getInstance(){
        return instance;
    }

    private ConnectionFactory(){
    }
}

As you can see, there is no real need in getInstance method now, so you can simplify the code to:

public class ConnectionFactory{
    public static final ConnectionFactory INSTANCE = new ConnectionFactory();

    private ConnectionFactory(){
    }
}

UPD about synchronization: the best way is synchronizing on a lock which is not visible to outer classes, i.e.:

public class ConnectionFactory{
    private static final Object lock = new Object();

    public static void doSmth() {
        synchronized (lock) {

           ...
        }
    }

    public static void doSmthElse() {
        synchronized (lock) {

           ...
        }
    }
}

There are many discussions about "why synchronizing on this is a bad idea" (like this one), I think that the same is actual for synchronizing on class.

回答2:

There are a couple of ways you can create a singleton.

One recommended way is to use an enum (guaranteed to only create one instance):

public enum ConnectionFactory {

  INSTANCE;

}

Or you can create it statically when the class loads:

public class ConnectionFactory {

  private static ConnectionFactory INSTANCE = new ConnectionFactory();

  private ConnectionFactory() {}

  public static ConnectionFactory getInstance() {
    return INSTANCE;
  }    

}

If you need to lazily load it you can use this idiom (rather than the double checked locking anti-pattern )

public class ConnectionFactory {

  private static class ConnectionFactoryHolder {
    private static ConnectionFactory INSTANCE = new ConnectionFactory();
  }

  public static ConnectionFactory getInstance() {
    return ConnectionFactoryHolder.INSTANCE;
  }

}

回答3:

Yes, static methods are synchronized on their class object. I wouldn't worry about performance here, as probably this will not be your performance hot spot. Do it simple, optimize when and where you need to.

回答4:

Static synchronized methods use the lock on the class. In the case of your example it would be accessing the lock on the ConnectionFactory class object. Best practice is not to hold onto locks for longer than you have to. Whether you have multiple synchronized methods is not a problem in itself.

回答5:

Effective Java recommends using Enums to create singleton. So you code would look something like this:

public enum ConnectionFactory{
INSTANCE;

// Other factory methods go here.

}

}