参考资料：

高并发Java（7）：并发设计模式 

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
    
     (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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