redis> EXISTS job    # job 不存在
(integer) 0
redis> SETNX job "programmer" # job 设置成功
(integer) 1
redis> SETNX job "code-farmer" # 尝试覆盖 job ，失败
(integer) 0
redis> GET job     # 没有被覆盖
"programmer"

redis> GETSET db mongodb # 没有旧值，返回 nil
(nil)
redis> GET db
"mongodb"
redis> GETSET db redis  # 返回旧值 mongodb
"mongodb"
redis> GET db
"redis"

/**
 * 使用redis锁来控制并发抢单
 * @author fuyuwei
 */
public class OrderBiz {
 public int createOrder(){
  // 下单之前的参数、合法性校验这里就不在演示
  OrderLock<Boolean> orderLock = new RedisOrderLock<Boolean>("pro-12345678901");
  boolean isSyn = orderLock.isSyn(new OrderLockBiz<Boolean>(){
   @Override
   public Boolean createOrder() {
    // 省去创建订单逻辑
    return null;
   }
  });
  if(!isSyn){
   BizLogger.info("创建订单失败");
  }
  return 0;
 }
}

public interface OrderLock<T> {
 public boolean isSyn(OrderLockBiz<T> orderBiz);

}

public interface OrderLockBiz<T> {
 public T createOrder();
}

public class RedisOrderLock<T> implements OrderLock<T> {

 // 锁等待超时，防止线程饥饿，永远没有入锁执行代码的机会 
 public static final long timeout = 10000;//ms

 // 锁持有超时，防止线程在入锁以后，无限的执行下去，让锁无法释放 
 public static final long expireMsecs = 10000;// ms

 public String lockKey = "orderLockKey";

 public Jedis jedis;

 private static volatile JedisPool jedisPool;

 public RedisOrderLock(String lockKey) {
  this.lockKey = lockKey;
 }
 /**
  * 初始化redis 
  * @return
  */
 public Jedis getInstance() {
  if(jedisPool == null) {
   synchronized(RedisOrderLock.class) {
    if(jedisPool == null) {
     JedisPoolConfig config = new JedisPoolConfig();
     config.setMaxIdle(100);
     jedisPool = new JedisPool(config,"localhost",6379, 3000,"test");
    }
   }
  }
  return jedisPool.getResource();
 }

 /**
  * 线程安全的业务逻辑处理
  */
 @Override
 public boolean isSyn(OrderLockBiz<T> orderBiz) {
  jedis = this.getInstance();
  try {
   // 获取到锁
   if(acquire(jedis)){
    // 执行创建订单逻辑
    orderBiz.createOrder();
   }else{
    BizLogger.info("waiting other thread creating");
   }
  } catch (Exception e) {
   BizLogger.error(e,"acquire lock failre");
  }finally{
   // 解锁
   this.releaseLock(jedis);
  }
  return false;
 }

 /**
  * accqure lock
  * @param jedis
  * @return
  * @throws InterruptedException
  */
 public synchronized boolean acquire(Jedis jedis){
  boolean locked = false;
  while(timeout > 0){
   long expires = System.currentTimeMillis() + expireMsecs + 1;
   // 10秒之后锁到期
   String expiresStr = String.valueOf(expires);
   // 获取到锁
   if(jedis.setnx(lockKey, expiresStr) == 1){
    locked = true;
    return locked;
   }
   // 没有获取到锁
   String oldValue = jedis.get(lockKey);
   // expireMsecs（10秒）锁的有效期内无法进入if判断，如果锁超时了
   if(oldValue != null 
     && Long.parseLong(oldValue) < System.currentTimeMillis()){
    // 如果锁超时重新设置
    String oldValue_ = jedis.getSet(lockKey, expiresStr);
    // 值相同说明是同一个线程的操作，获取锁成功
    if(Long.valueOf(oldValue_) == Long.valueOf(oldValue)){
     locked = true;
    }else{
     // 被其他线程抢先获取锁
     locked = false;
    }
   }
   // 锁没有超时，继续等待
   return false;
  }
 }
 /**
  * 释放锁
  * @param jedis
  */
 public synchronized void releaseLock(Jedis jedis){
  try {
   long current = System.currentTimeMillis(); 
   // 避免删除非自己获取得到的锁
   if (current < Long.valueOf(jedis.get(lockKey)))
    jedis.del(lockKey);
  } catch (Exception e) {
   e.printStackTrace();
  }finally{
   // 把用完的连接放到连接池汇中供其他线程调用
   jedisPool.returnResource(jedis);
  }
 }
}