Singleton

public class Singleton {

/**

* volatile关键字确保，当instance变量被初始化成Singleton实例时，多个线程正确的处理instance变量

*/

private volatile static Singleton instance;

private Singleton() {}

public static Singleton getInstance() {

//第一次检查

if(instance == null) {

synchronized (Singleton.class) {

//第二次检查，确保instance为空时才实例化对象

if(instance == null) {

instance = new Singleton();

}

}

}

return instance;

}

}