java常用設計模式一：單例模式

?1、餓漢式

package singleton.demo;/*** @author Administrator* @date 2019/01/07*/
public class Singleton {//在調用getInstance方法前，實例已經創建好private static Singleton instance = new Singleton();//私有構造，防止被實例化private Singleton() {}public static Singleton getInstance() {if (instance == null) {instance = new Singleton();}return instance;}
}

2、懶漢式

package singleton.demo;/*** @author Administrator* @date 2019/01/07*/
public class Singleton {//初始設置為null，需要時才被加載，延遲加載private static Singleton instance = null;//私有構造，防止被實例化private Singleton() {}public static Singleton getInstance() {if (instance == null) {instance = new Singleton();}return instance;}
}

?

以上兩種模式：只適合在單線程環境使用，多線程環境容易出現不同步的情況。在多線程模式下，可能產生問題。比如下面的實例

package singleton.demo;/*** @author Administrator* @date 2019/01/07*/
public class Singleton {//初始設置為null，需要時才被加載，延遲加載private static Singleton instance = null;//私有構造，防止被實例化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();}
}

執行結果：

1369340138
1555013046
285003072
270952636
84064941

?之所以出現不同步，是因為，當多個線程執行?getInstance() 的時候，都發現instance是null，所以每個線程各自創建了一個instance

?3、使用static塊實現線程安全的單例模式

注意：以下try-catch不是模式需要的代碼，這里只是為了測試多線程環境才加上的

package singleton.demo;/*** @author Administrator* @date 2019/01/07*/
public class Singleton {//初始設置為null，需要時才被加載，延遲加載private static Singleton instance = null;//私有構造，防止被實例化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

?