1 public class Singleton {
 2
 3     private static Singleton uniqueInstance = null;
 4
 5
 6
 7     private Singleton() {
 8
 9        // Exists only to defeat instantiation.
10
11     }
12
13
14
15     public static Singleton getInstance() {
16
17        if (uniqueInstance == null) {
18
19            uniqueInstance = new Singleton();
20
21        }
22
23        return uniqueInstance;
24
25     }
26
27     // Other methods...
28
29 }

 1 //饿汉式单例类.在类初始化时，已经自行实例化
 2  public class Singleton1 {
 3      //私有的默认构造子
 4      private Singleton1() {}
 5      //已经自行实例化
 6      private static final Singleton1 single = new Singleton1();
 7      //静态工厂方法
 8      public static Singleton1 getInstance() {
 9          return single;
10      }
11  }

 1 //懒汉式单例类.在第一次调用的时候实例化
 2  public class Singleton2 {
 3      //私有的默认构造子
 4      private Singleton2() {}
 5      //注意，这里没有final
 6      private static Singleton2 single=null;
 7      //静态工厂方法
 8      public synchronized  static Singleton2 getInstance() {
 9           if (single == null) {
10               single = new Singleton2();
11           }
12          return single;
13      }
14  }

 1 import java.util.HashMap;
 2  import java.util.Map;
 3  //登记式单例类.
 4  //类似Spring里面的方法，将类名注册，下次从里面直接获取。
 5  public class Singleton3 {
 6      private static Map<String,Singleton3> map = new HashMap<String,Singleton3>();
 7      static{
 8          Singleton3 single = new Singleton3();
 9          map.put(single.getClass().getName(), single);
10      }
11      //保护的默认构造子
12      protected Singleton3(){}
13      //静态工厂方法,返还此类惟一的实例
14      public static Singleton3 getInstance(String name) {
15          if(name == null) {
16              name = Singleton3.class.getName();
17              System.out.println("name == null"+"--->name="+name);
18          }
19          if(map.get(name) == null) {
20              try {
21                  map.put(name, (Singleton3) Class.forName(name).newInstance());
22              } catch (InstantiationException e) {
23                  e.printStackTrace();
24              } catch (IllegalAccessException e) {
25                  e.printStackTrace();
26              } catch (ClassNotFoundException e) {
27                  e.printStackTrace();
28              }
29          }
30          return map.get(name);
31      }
32      //一个示意性的商业方法
33      public String about() {
34          return "Hello, I am RegSingleton.";
35      }
36      public static void main(String[] args) {
37          Singleton3 single3 = Singleton3.getInstance(null);
38          System.out.println(single3.about());
39      }
40  }

 1 //单例对象的初始化同步
 2 public class GlobalConfig {
 3     private static GlobalConfig instance = null;
 4     private Vector properties = null;
 5     private GlobalConfig() {
 6       //Load configuration information from DB or file
 7       //Set values for properties
 8     }
 9     private static synchronized void syncInit() {
10       if (instance == null) {
11         instance = new GlobalConfig();
12       }
13     }
14     public static GlobalConfig getInstance() {
15       if (instance == null) {
16         syncInit();
17       }
18       return instance;
19     }
20     public Vector getProperties() {
21       return properties;
22     }
23   }

 1 public class GlobalConfig {
 2  private static GlobalConfig instance;
 3  private Vector properties = null;
 4  private boolean isUpdating = false;
 5  private int readCount = 0;
 6  private GlobalConfig() {
 7    //Load configuration information from DB or file
 8       //Set values for properties
 9  }
10  private static synchronized void syncInit() {
11   if (instance == null) {
12    instance = new GlobalConfig();
13   }
14  }
15  public static GlobalConfig getInstance() {
16   if (instance==null) {
17    syncInit();
18   }
19   return instance;
20  }
21  public synchronized void update(String p_data) {
22   syncUpdateIn();
23   //Update properties
24  }
25  private synchronized void syncUpdateIn() {
26   while (readCount > 0) {
27    try {
28     wait();
29    } catch (Exception e) {
30    }
31   }
32  }
33  private synchronized void syncReadIn() {
34   readCount++;
35  }
36  private synchronized void syncReadOut() {
37   readCount--;
38   notifyAll();
39  }
40  public Vector getProperties() {
41   syncReadIn();
42   //Process data
43   syncReadOut();
44   return properties;
45  }
46   }

 1 public class GlobalConfig {
 2     private static GlobalConfig instance = null;
 3     private Vector properties = null;
 4     private GlobalConfig() {
 5       //Load configuration information from DB or file
 6       //Set values for properties
 7     }
 8     private static synchronized void syncInit() {
 9       if (instance = null) {
10         instance = new GlobalConfig();
11       }
12     }
13     public static GlobalConfig getInstance() {
14       if (instance = null) {
15         syncInit();
16       }
17       return instance;
18     }
19     public Vector getProperties() {
20       return properties;
21     }
22     public void updateProperties() {
23       //Load updated configuration information by new a GlobalConfig object
24       GlobalConfig shadow = new GlobalConfig();
25       properties = shadow.getProperties();
26     }
27   }