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• 在多線程下,單例是否仍能實現(xiàn)?

單例從概念上講: 一個JVM里,只能有一個實例
多線程,仍舊是基于同一進程下,自然也是基于同一JVM下,因此符合單例的概念.

• 我們可以按兩種多線程方式分別進行測試分析:

1.多實例多線程

單例類: public class MySingleton1 {     private static final MySingleton1 instance =new MySingleton1();          private MySingleton1(){}          public static MySingleton1 getInstance() {         return instance;     }  }

線程類:run()方法里面調(diào)用單例     public void run(){         MySingleton1 mySingleton1=null;         mySingleton1 = MySingleton1.getInstance();         System.out.println(mySingleton1+":"+mySingleton1.hashCode());     }

測試代碼:起10個線程,每個線程都是一個單獨的實例             for(int i=0;i<10;i++){                         Thread t = new Thread(new MyThread(),String.valueOf(i));             t.start();             }

看結(jié)果: 證明多線程多實例下,獲取的確實是同一實例 com.machome.singleton.MySingleton1@ca0b6:827574 com.machome.singleton.MySingleton1@ca0b6:827574 com.machome.singleton.MySingleton1@ca0b6:827574 com.machome.singleton.MySingleton1@ca0b6:827574 com.machome.singleton.MySingleton1@ca0b6:827574 com.machome.singleton.MySingleton1@ca0b6:827574 com.machome.singleton.MySingleton1@ca0b6:827574 com.machome.singleton.MySingleton1@ca0b6:827574 com.machome.singleton.MySingleton1@ca0b6:827574 com.machome.singleton.MySingleton1@ca0b6:827574

2.單實例多線程

單例類和線程類都不變

就測試代碼改一下,起10個線程,都引用同一實例         MyThread myThread= new MyThread();         for(int i=0;i<10;i++){                         Thread t = new Thread(myThread,String.valueOf(i));             t.start();             }

看結(jié)果: com.machome.singleton.MySingleton1@1b67f74:28737396 com.machome.singleton.MySingleton1@1b67f74:28737396 com.machome.singleton.MySingleton1@1b67f74:28737396 com.machome.singleton.MySingleton1@1b67f74:28737396 com.machome.singleton.MySingleton1@1b67f74:28737396 com.machome.singleton.MySingleton1@1b67f74:28737396 com.machome.singleton.MySingleton1@1b67f74:28737396 com.machome.singleton.MySingleton1@1b67f74:28737396 com.machome.singleton.MySingleton1@1b67f74:28737396 com.machome.singleton.MySingleton1@1b67f74:28737396

• 有狀態(tài)單例下的多線程

有狀態(tài)單例 public class MySingleton1 {     private static final MySingleton1 instance =new MySingleton1();     private MySingleton1(){}       public static MySingleton1 getInstance() {         return instance;     }     // 狀態(tài)     private String name;     private int age;         public String getName() {         return name;     }     public void setName(String name) {         this.name = name;     }     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;     }      }

線程類: 在線程中獲取單例,并且在第1號線程和第5號線程分別改變單例的狀態(tài) public class MyThreadForStateful implements Runnable{         public void run(){         MySingleton1 mySingleton1=null;         mySingleton1 = MySingleton1.getInstance();                 if(Thread.currentThread().getName().equals("1")){             mySingleton1.setName("mac");             mySingleton1.setAge(11);         }else if(Thread.currentThread().getName().equals("5")){             mySingleton1.setName("silly");             mySingleton1.setAge(22);         }                 System.out.println(Thread.currentThread().getName()                 +":"+mySingleton1+":"                 +mySingleton1.getName()+mySingleton1.getAge());     } }

測試代碼:         for(int i=0;i<10;i++){                        Thread t = new Thread(new MyThreadForStateful(),String.valueOf(i));             t.start();             } 執(zhí)行效果:  證明單例的狀態(tài)是被多實例共享的 0:com.machome.singleton.MySingleton1@ca0b6:null0 1:com.machome.singleton.MySingleton1@ca0b6:mac11 2:com.machome.singleton.MySingleton1@ca0b6:mac11 3:com.machome.singleton.MySingleton1@ca0b6:mac11 4:com.machome.singleton.MySingleton1@ca0b6:mac11 5:com.machome.singleton.MySingleton1@ca0b6:silly22 6:com.machome.singleton.MySingleton1@ca0b6:silly22 7:com.machome.singleton.MySingleton1@ca0b6:silly22 8:com.machome.singleton.MySingleton1@ca0b6:silly22 9:com.machome.singleton.MySingleton1@ca0b6:silly22

結(jié)論:只要多線程能成功獲取單例(即多線程獲得的單例實例是相同的),必然能共享單例的狀態(tài)