java單例設計模式

實現方法一

package com.hs.pattern.singleton;

/**
 * 優點：實現簡單
 * 缺點：線程不安全
 * 例如：當兩個線程都運行到if( singleton == null )，而singleton確實爲空，則兩個線程都會創建一個實例
 * @author Administrator
 *
 */
public class Singleton1 {

    private static Singleton1 singleton = null;
    
    private Singleton1(){}
    
    public Singleton1 getInstance(){
        if( singleton == null ){
            singleton = new Singleton1();
        }
        return singleton;
    }
}

實現方法二

package com.hs.pattern.singleton;

/**
 * 懶漢模式
 * 優點：線程安全、lazy-loading
 * 缺點：每次調用getInstance方法都要獲取鎖，效率低
 * @author Administrator
 *
 */
public class Singleton2 {

    private static Singleton2 singleton = null;
    
    private Singleton2(){}
    
    public synchronized Singleton2 getInstance(){
        if( singleton == null ){
            singleton = new Singleton2();
        }
        return singleton;
    }
}

實現方法三

package com.hs.pattern.singleton;

/**
 * 雙重檢查鎖
 * 優點：只有singleton爲空、需要創建實例時才需要獲取鎖，效率高且線程安全
 * 缺點：如果需要實現序列化，則可以通過序列化、反序列化獲取多個實例
 * @author Administrator
 *
 */
public class Singleton3 {

    private static Singleton3 singleton = null;
    
    private Singleton3(){}
    
    public Singleton3 getInstance(){
        
        if( singleton == null ){
            synchronized (Singleton3.class) {
                if( singleton == null ){
                    singleton = new Singleton3();
                }
            }
        }
        
        return singleton;
    }
}

實現方案四

package com.hs.pattern.singleton;

/**
 * 餓漢模式
 * 優點：實現簡單、線程安全
 * 缺點：沒有實現lazy-loading
 * @author Administrator
 *
 */
public class Singleton4 {

    private static Singleton4 singleton = new Singleton4();
    
    private Singleton4(){}
    
    public Singleton4 getInstance(){
        return singleton;
    }
    
}

實現方案五

package com.hs.pattern.singleton;

/**
 * 靜態內部類
 * 優點：由於SingletonHolder是靜態的，所以只有首次調用時纔會初始化，因爲SingletonHolder是私有內部類，
 *      所以只有調用Singleton5.getInstance()方法是纔會初始化，從而實現lazy-loading
 * 缺點：如果需要實現序列化，則可以通過序列化、反序列化獲取多個實例
 * @author Administrator
 *
 */
public class Singleton5 {

    private Singleton5(){}
    
    private static class SingletonHolder{
        
        public final static Singleton5 singleton = new Singleton5();
        
    }
    
    public static Singleton5 getInstance(){
        return SingletonHolder.singleton;
    }
    
}

實現方案六

package com.hs.pattern.singleton;

/**
 * 枚舉類實現單例
 * 優點：對於序列化、反序列化，因爲每個枚舉類型和每個枚舉變量在jvm中都是唯一的，
 *      即Java在序列化和反序列化枚舉時做了特殊的規定，枚舉的writeObject、
 *      readObject、readObjectNoData、writeReplace和readResolve
 *      等方法是被編譯器禁用的，因此也不存在實現序列化接口後調用readObject會破壞單例的問題。
 * 缺點：實現比較複雜
 * @author Administrator
 *
 */
public class Singleton6 {

    private Singleton6(){}
    
    public static Singleton6 getInstance(){
        return SingletonEnum.INSTANCE.getSingleton();
    }
    
    private static enum SingletonEnum{
        
        INSTANCE;
        
        private Singleton6 singleton ;
        
        private SingletonEnum(){
            singleton = new Singleton6();
        }
        
        public Singleton6 getSingleton(){
            return singleton;
        }
        
    }
    
}