[Java High Concurrency Best Practices] Thread-safe Single Instance Mode

References:

Thread-safe singleton mode under high concurrency (the most complete and classic)

High Concurrency Java (7): Concurrency Design Patterns  http://www.importnew.com/21312.html

 

1. Two best practices for thread safety in single instance mode:

public class SQLFactory
{	
    private Logger logger = LoggerFactory.getLogger(getClass());//Log.getLog(getClass());
   
    /*//old method not thread-safe
    private static SQLFactory instance = null;	
	public static SQLFactory getInstance() {
		if (instance == null) {
			instance = new SQLFactory();
		}
		return instance;
	}
    */

    //thread-safe best practice	
	public static SQLFactory getInstance() {
		return SQLHolder.instance;
	}
    private static class SQLHolder {
		private static SQLFactory instance = new SQLFactory();
	}
    
    /*//also good thread-safe
    volatile private static SQLFactory instance = null;
	public static SQLFactory getInstance() {
		synchronized (SQLFactory.class) {
			if (instance == null) {
				instance = new SQLFactory();
			}
		}
		return instance;
    }*/
}

 

2. Test method:

import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

import com.highgo.admin.migrator.controller.SQLFactory;

public class TestThreadSafe {
	public void doing() {
		System.out.println(SQLFactory.getInstance().hashCode());
	}

	public static void main(String[] args) {
		ThreadPoolExecutor threadPool = new ThreadPoolExecutor(4, 8, 101, TimeUnit.SECONDS,
				new LinkedBlockingQueue<Runnable>(2000));

		final TestThreadSafe tf = new TestThreadSafe();
		for (int i = 0; i < 10; i++) {
			threadPool.submit(() -> {
				tf.doing();
			});
		}
	} 
}

3. Test results: The same instance hash value means that it is always the same instance.

1090303302
1090303302
1090303302
1090303302
1090303302
1090303302
1090303302
1090303302
1090303302
1090303302