java几种单例模式

1:饿汉

public class SingletonExample {
	private static SingletonExample _instance = new SingletonExample();
 
	public static SingletonExample getInstance() {
		return _instance;
	}
 
	private SingletonExample() {
	}
}

2:懒汉

public class SingletonExample {
	
	private static SingletonExample _instance = null;
	 
	public static synchronized SingletonExample getInstance() {
		if(_instance == null){
			_instance = new SingletonExample();
		}
		return _instance;
	}
 
	private SingletonExample() {
	}

}

3:懒汉，不安全

public class SingletonExample {
	
	private static SingletonExample _instance = null;
	 
	public static SingletonExample getInstance() {
		if(_instance == null){
			synchronized (SingletonExample.class){
				if(_instance == null){
					_instance = new SingletonExample();
				}
			}
		}
		return _instance;
	}
 
	private SingletonExample() {
	}

}

4:懒汉，内部类

public class SingletonExample {
	
	private static class SingletonExampleHolder{
		private final static SingletonExample _instance = new SingletonExample();
	}
	 
	public static SingletonExample getInstance() {
		return SingletonExampleHolder._instance;
	}
 
	private SingletonExample() {
	}

}

5:登记式，可继承

public class SingletonExample {
	
	static private HashMap<String, Object> singletonMap = new HashMap<String, Object>();

    static {
    	SingletonExample x = new SingletonExample();
    	singletonMap.put(x.getClass().getName(), x);
    }

    protected SingletonExample() {
    }

    public static SingletonExample getInstance(String name) {
        if (name == null) {
            name = "*.*.SingletonExample";
        }

        if (singletonMap.get(name) == null) {
            try {
            	singletonMap.put(name, Class.forName(name).newInstance());
            } catch (ClassNotFoundException cnf) {
                System.out.println("Couldn't find class " + name);
            } catch (InstantiationException ie) {
                System.out.println("Couldn't instantiate an object of type "+ name);
            } catch (IllegalAccessException ia) {
                System.out.println("Couldn't access class " + name);
            }
        }
        return (SingletonExample) (singletonMap.get(name));
    }
    
}

class SingletonChild extends SingletonExample{
	
	public SingletonChild(){}
	
	public static SingletonChild getInstance(){
		return (SingletonChild)SingletonExample.getInstance("*.*.SingletonChild");
	}
	
}