JAVA 之 七种单例模式

1、饿汉式单例类

class EagerSingleton {   
    private static final EagerSingleton instance = new EagerSingleton();   
    private EagerSingleton() { }   
  
    public static EagerSingleton getInstance() {  
        return instance;   
    }     
}  

2、懒汉式单例类

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

　　或者

public static LazySingleton getInstance() {   
    if (instance == null) {  
        synchronized (LazySingleton.class) {  
            instance = new LazySingleton();   
        }  
    }  
    return instance;   
}  

3、双重检查锁定-懒汉式单例

class LazySingleton {   
    private volatile static LazySingleton instance = null;   
  
    private LazySingleton() { }   
  
    public static LazySingleton getInstance() {   
        //第一重判断  
        if (instance == null) {  
            //锁定代码块  
            synchronized (LazySingleton.class) {  
                //第二重判断  
                if (instance == null) {  
                    instance = new LazySingleton(); //创建单例实例  
                }  
            }  
        }  
        return instance;   
    }  
}  

4、Initialization Demand Holder (IoDH)静态内部类之单例模式

class Singleton {  
    private Singleton() {  
    }  
      
    private static class HolderClass {  
            private final static Singleton instance = new Singleton();  
    }  
      
    public static Singleton getInstance() {  
        return HolderClass.instance;  
    }  
     
}

IoDH可以实现延迟加载，又可以保证线程安全，不影响系统性能，不失为一种最好的Java语言单例模式实现方式（其缺点是与编程语言本身的特性相关，很多面向对象语言不支持IoDH）。

5、序列化和反序列化 -- 单例模式

public class SingletonTwo implements Serializable{

	/**
	 * 序列化与反序列化方式
	 */
	private static final long serialVersionUID = 1L;
	
	private static class SingletonHan{
		private static final SingletonTwo single = new SingletonTwo();
	}
	
	private SingletonTwo(){}
	
	public static SingletonTwo getInstance(){
		return SingletonHan.single;
	}
	
	/**
	 * 反序列化接口
	 * @return
	 * @throws ObjectStreamException
	 */
	protected Object readResolve() throws ObjectStreamException{
		return SingletonHan.single; 
	}
}

　　

6、使用static代码块 

public class Singleton {

	private static Singleton instance = null;
	
	private Singleton(){}
	/**
	 * 静态代码块实现
	 */
	static{
		instance = new Singleton();
	} 
	
	public static Singleton getInstance(){
		return instance;
	}
}

　　

7、使用enum枚举数据类型

/**
 * enum枚举实现单例模式
 * @author MIXCS
 *
 */
public class SingletonEnum {

	public enum EnumSingle{
		singleton;
		private SingletonEnum instance;
		
		private EnumSingle(){
			instance = new SingletonEnum();
		}
		
		public SingletonEnum getSingele(){
			return instance;
		}
	}
	
	public static SingletonEnum getInstance(){
		return EnumSingle.singleton.getSingele();
	}
}