【GOF23设计模式】单例模式

来源：http://www.bjsxt.com/ 

 1 package com.test.singleton; 2 /** 3 * 测试饿汉式单例模式 4 */ 5 public class SingletonDemo01 { 6 7   //类初始化时，立即加载这个对象(没有延时加载的优势)。加载类时，天然的是get='_blank'>线程安全的！ 8   private static SingletonDemo01 instance = new SingletonDemo01(); 9 10   private SingletonDemo01(){//私有化构造器11   }12 13   //方法不用同步，调用效率高！14   public static SingletonDemo01 getInstance(){15     return instance;16   }17 }

4、懒汉式 

 1 package com.test.singleton; 2 /** 3 * 测试懒汉式单例模式 4 */ 5 public class SingletonDemo02 { 6 7   //类初始化时，不初始化这个对象(延时加载，真正用的时候再创建) 8   private static SingletonDemo02 instance; 9 10   private SingletonDemo02(){//私有化构造器11   }12 13   //方法同步，调用效率低！14   public static synchronized SingletonDemo02 getInstance(){15     if(null==instance){16       instance = new SingletonDemo02();17     }18     return instance;19   }20 }

二、【GOF23设计模式】_单例模式、双重检查锁式、静态内部类式、枚举式、UML类图

双重检测锁实现 

 1 package com.test.singleton; 2 /** 3 * 测试双重检测锁式单例模式 4 */ 5 public class SingletonDemo03 { 6 7   //类初始化时，不初始化这个对象(延时加载，真正用的时候再创建) 8   private static SingletonDemo03 instance; 9 10   private SingletonDemo03(){//私有化构造器11   }12 13   //调用效率低！14   public static SingletonDemo03 getInstance(){15     if(null==instance){16       SingletonDemo03 sc;17       synchronized(SingletonDemo03.class){18         sc = instance;19         if(null==sc){20           synchronized (SingletonDemo03.class) {21             if(null==sc){22               sc = new SingletonDemo03();23             }24           }25           instance = sc;26         }27       }28     }29     return instance;30   }31 }

静态内部类实现 

 1 package com.test.singleton; 2 /** 3 * 测试静态内部类实现单例模式 4 * 这种方式：线程安全，调用效率高，并且实现了延时加载！ 5 */ 6 public class SingletonDemo04 { 7 8   private static class SingletonClassInstance { 9     private static final SingletonDemo04 instance = new SingletonDemo04();10   }11 12   private SingletonDemo04(){//私有化构造器13   }14 15   //方法没有同步，调用效率高！16   public static SingletonDemo04 getInstance(){17     return SingletonClassInstance.instance;18   }19 }

枚举实现 

 1 package com.test.singleton; 2 /** 3 * 测试枚举式实现单例模式(没有延时加载) 4 */ 5 public enum SingletonDemo05 { 6 7   //这个枚举元素，本身就是单例对象！ 8   INSTANCE; 9 10   //添加自己需要的操作！11   public void singletonOperation(){12   }13 }

 1 package com.test.singleton; 2 3 import java.io.FileInputStream; 4 import java.io.FileOutputStream; 5 import java.io.ObjectInputStream; 6 import java.io.ObjectOutputStream; 7 import java.lang.reflect.Constructor; 8 9 /**10 * 测试反射和反序列化破解单例模式11 */12 public class Client2 {13   public static void main(String[] args) throws Exception {14     SingletonDemo06 s1 = SingletonDemo06.getInstance();15     SingletonDemo06 s2 = SingletonDemo06.getInstance();16     System.out.println(s1);17     System.out.println(s2);18 19     //通过反射的方式直接调用私有构造器20 /*   Class<SingletonDemo06> clazz = (Class<SingletonDemo06>) Class.forName("com.test.singleton.SingletonDemo06");21     Constructor<SingletonDemo06> c = clazz.getDeclaredConstructor(null);22     c.setAccessible(true);//访问私有，破解单例23     SingletonDemo06 s3 = c.newInstance();24     SingletonDemo06 s4 = c.newInstance();25     System.out.println(s3);//SingletonDemo06的私有构造器中加了代码防止破解26     System.out.println(s4);*/27 28     //通过反序列化的方式构造多个对象29     FileOutputStream fos = new FileOutputStream("g:/java/test/a.txt");30     ObjectOutputStream oos = new ObjectOutputStream(fos);31     oos.writeObject(s1);32     oos.close();33     fos.close();34 35     ObjectInputStream ois = new ObjectInputStream(new FileInputStream("g:/java/test/a.txt"));36     SingletonDemo06 s3 = (SingletonDemo06) ois.readObject();37     System.out.println(s3);//SingletonDemo06定义了readResolve()防止破解38   }39 }

 1 package com.test.singleton; 2 3 import java.util.concurrent.CountDownLatch; 4 5 /** 6 * 测试多线程环境下五种创建单例模式的效率 7 */ 8 public class Client3 { 9   public static void main(String[] args) throws Exception{10 11     int threadNum = 10;12     final CountDownLatch countDownLatch = new CountDownLatch(threadNum); 13 14     long start = System.currentTimeMillis();15     for(int i=0;i<10;i++){16       new Thread(new Runnable(){17         @Override18         public void run() {19           for (int i = 0; i < 1000000; i++) {20             Object o = SingletonDemo01.getInstance();//分别测试几种单例模式的效率21             //Object o = SingletonDemo05.INSTANCE;22           }23 24           countDownLatch.countDown();//计数器减125         }26       }).start();27     }28 29     countDownLatch.await();//main线程阻塞，直到计数器变为0，才会继续往下执行！30 31     long end = System.currentTimeMillis();32     System.out.println("总耗时：" + (end-start));33   }34 }

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原标题：【GOF23设计模式】单例模式

关键词：设计模式