在开发当中需要经常用到synchronized保证代码线程安全，在竞争条件下会阻塞等待资源，如果允许竞争不到资源返回失败，就可以使用cas减少阻塞时间。先来看一个cas的单例模式。

public class NonBlock {private static volatile NonBlock nonBlock;private static AtomicBoolean atomicBoolean = new AtomicBoolean(false);public static NonBlock getInstance() {if (nonBlock == null) {if (atomicBoolean.compareAndSet(false, true)) {nonBlock = new NonBlock();}}return nonBlock;}
}

在这个单例模式中，不同于synchronized的阻塞，多线程环境下，getInstance确保只会创建一个对象的情况下，可能返回的nonBlock是一个空对象。但，如果允许返回空对象的情况下，

	public class RedPacket {private long balance;private int num;public RedPacket(long balance, int num) {this.balance = balance;this.num = num;}public long get() {if (balance < 1 || num < 1) {return -1;}if (num == 1) {long result = balance;balance = 0;num--;return result;}long average = balance / num;long result = ThreadLocalRandom.current().nextLong(1, average * 2);balance -= result;num--;return result;}

我们还可以使用cas达到非阻塞的目的，这样能保证线程安全，出现竞争情况就提示抢失败，确点就是提示抢失败还可能余额大于0，先来不一定能抢到，后来人还能抢。

public class RedPacket {private long balance;private AtomicInteger num;public RedPacket(long balance, int num) {this.balance = balance;this.num = new AtomicInteger(num);}public long get() {int number = num.get();long balan = balance;if (balan < 1 || number < 1) {return -1;}if (num.compareAndSet(number, number - 1)) {if (number - 1 == 0) {balance = 0;return balan;}long average = balan / number;long result = ThreadLocalRandom.current().nextLong(1, average * 2);balance -= result;}return -1;}}