【Java高并发最佳实践】线程安全的单实例模式

参考资料：

高并发下线程安全的单例模式（最全最经典）

高并发Java（7）：并发设计模式 http://www.importnew.com/21312.html

1.单实例模式的线程安全的两种上佳实践：

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.测试方法：

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.测试结果：相同的实例哈希值表示始终都是同一个实例。

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