設計模式--單例模式--餓漢、懶漢、方法鎖、類鎖（雙判斷）和靜態內部類模式

1、餓漢

/**
 * 餓漢式
 */
public class Singleton {

    private static Singleton instance = new Singleton();//實在餓的慌，先吃了（先創建實例）

    private Singleton(){}

    public static Singleton getInstance() {
        return instance;
    }
}

2、懶漢

/**
 * 懶漢式
 */
public class Singleton {

    private static Singleton instance;//我還不餓，先不吃（不創建實例）

    private Singleton(){}

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();//這會兒我餓了，我吃（需要時創建實例）
        }
        return instance;
    }
}

3、方法鎖

/**
 * 懶漢式（synchronized 方發鎖 ）線程安全，人云效率低
 */
public class Singleton {

    private static Singleton instance;

    private Singleton(){}

    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

4、類鎖（雙判斷）

/**
 * 雙重判斷 類鎖 synchronized (Singleton.class) 線程安全 效率高
 */
public class Singleton {

    private static Singleton instance = null;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (instance == null) {//一次判斷
            synchronized (Singleton.class) {//鎖
                if (instance == null) {//二次判斷
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

5、靜態內部類

/**
 * 靜態內部類單例模式，線程安全?
 */
public class Singleton {

    private Singleton(){}

    public static Singleton getInstance() {
        return SingletonHolder.instance;
    }

    /**
     * 靜態內部類
     */
    private static class SingletonHolder{
        private static final Singleton instance = new Singleton();//靜態內部類內部實例化
    }

}