java常用设计模式一：单例模式

 1、饿汉式

package singleton.demo;/*** @author Administrator* @date 2019/01/07*/
public class Singleton {//在调用getInstance方法前，实例已经创建好private static Singleton instance = new Singleton();//私有构造，防止被实例化private Singleton() {}public static Singleton getInstance() {if (instance == null) {instance = new Singleton();}return instance;}
}

2、懒汉式

package singleton.demo;/*** @author Administrator* @date 2019/01/07*/
public class Singleton {//初始设置为null，需要时才被加载，延迟加载private static Singleton instance = null;//私有构造，防止被实例化private Singleton() {}public static Singleton getInstance() {if (instance == null) {instance = new Singleton();}return instance;}
}

 

以上两种模式：只适合在单线程环境使用，多线程环境容易出现不同步的情况。在多线程模式下，可能产生问题。比如下面的实例

package singleton.demo;/*** @author Administrator* @date 2019/01/07*/
public class Singleton {//初始设置为null，需要时才被加载，延迟加载private static Singleton instance = null;//私有构造，防止被实例化private Singleton() {}public static Singleton getInstance() {if (instance == null) {try {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}instance = new Singleton();}return instance;}
}

package singleton.demo;import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;/*** @author Administrator* @date 2019/01/07*/
public class Test {public static void main(String[] args){ExecutorService executorService = Executors.newCachedThreadPool();for(int i=0;i<5;i++){executorService.submit(new Runnable() {public void run() {System.out.println(Singleton.getInstance().hashCode());}});}executorService.shutdown();}
}

执行结果：

1369340138
1555013046
285003072
270952636
84064941

 之所以出现不同步，是因为，当多个线程执行 getInstance() 的时候，都发现instance是null，所以每个线程各自创建了一个instance

 3、使用static块实现线程安全的单例模式

注意：以下try-catch不是模式需要的代码，这里只是为了测试多线程环境才加上的

package singleton.demo;/*** @author Administrator* @date 2019/01/07*/
public class Singleton {//初始设置为null，需要时才被加载，延迟加载private static Singleton instance = null;//私有构造，防止被实例化private Singleton() {}static {instance = new Singleton();}public static Singleton getInstance() {if (instance == null) {try {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}instance = new Singleton();}return instance;}
}

package singleton.demo;import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;/*** @author Administrator* @date 2019/01/07*/
public class Test {public static void main(String[] args){ExecutorService executorService = Executors.newCachedThreadPool();for(int i=0;i<5;i++){executorService.submit(new Runnable() {public void run() {System.out.println(Singleton.getInstance().hashCode());}});}executorService.shutdown();}
}

2050945573
2050945573
2050945573
2050945573
2050945573