多线程下的懒汉模式，同步代码块和同步方法

class Single {private Single() {};static Object obj = new Object();private static Single s = null;/** * 用同步代码块，不需要每次都判断锁，效率高 */public static Single getSingle1() {if (s == null) {synchronized (obj) {if (s == null) {s = new Single();}}}return s;}/** * 用同步方法，每次都需要判断锁，效率低 */public static synchronized Single getSingle2() {if (s == null) {s = new Single();}return s;}}