设计模式之单例模式

1.lazy instantiaze 懒汉式（线程非安全）

public class Singleton {
    private static Singleton instance = null;
    private Singleton(){}

    /**
     * lazy instantiaze 懒汉式
    * */
    public static Singleton getInstance() {
        if (instance == null)
            instance = new Singleton();
        return instance;// == null ? new Singleton() : instance;
    }

}

2.lazy instantiaze 懒汉式（线程安全）

public class Singleton {
    private static Singleton instance = null;
    private Singleton(){}

    /**
     * lazy instantiaze 懒汉式
    * */
    public static synchronized Singleton getInstance() {//加同步锁，但是每次想要得到实例的时候都要同步，降低了效率
        if (instance == null)
            instance = new Singleton();
        return instance;// == null ? new Singleton() : instance;
    }

}

3.eagerly 饥汉式

class Singleton2 {
    private static Singleton2 instance = new Singleton2();
    private Singleton2(){}
    /**
     * eagerly 饥汉式
    * */
    public Singleton2 getInstance() {
        return instance;
    }
}

4.double-checked locking: 双重检查加锁

class Sington3 {
    private static Sington3 instance;
    private Sington3(){}
    /**
     * double-checked locking: 双重检查加锁
     * */
    public static Sington3 getInstance() {
        if (instance == null) {
            synchronized (Sington3.class) {
                if (instance == null)
                    instance = new Sington3();
            }
        }
        return instance;
    }
}