1 public class test_test { 2     private test_test(){} 3     private static final test_test instance=new test_test(); 4     public static test_test getInstance(){ 5         return instance; 6     } 7  8 } 9 10 public class test_test{11     private test_test(){}12     private volatile static  test_test instance=null;13     public  static  test_test getInstance(){14         if(instance==null){15             synchronized(test_test.class){16                 if(instance==null){17                     return new test_test();                18                 }19             }            20         }21     }    22 }23 24 public class test_test{25     private test_test(){}26     //Java的静态内部类27     private static class test_test setInstance(){28         private static final test_test instance=new test_test();        29     }30     public static  test_test getInstance(){31         return setInstance.instance;32     }33 }