java中的几种锁:synchronized,ReentrantLock,ReentrantReadWriteLock已基本可以满足编程需求,但其粒度都太大,同一时刻只有一个线程能进入同步块,这对于某些高并发的场景并不适用。本文实现了一个基于KEY(主键)的互斥锁,具有更细的粒度,在缓存或其他基于KEY的场景中有很大的用处。下面将讲解这个锁的设计和实现
(关于这个锁的讨论贴:KeyLock讨论贴-CSDN)
设想这么一个场景:转账
private int[] accounts; // 账户数组,其索引为账户ID,内容为金额 public boolean transfer(int from, int to, int money) { if (accounts[from] < money) return false; accounts[from] -= money; accounts[to] += money; return true; }
从from中转出金额到to中。可能同时会有很多个线程同时调用这个转账方法,为保证原子性,保证金额不会出错,必须为这个方法加个锁,防止对共享变量accounts的并发修改。
加锁后的代码如下:
private int[] accounts; // 账户数组,其索引为账户ID,内容为金额 private Lock lock = new ReentrantLock(); public boolean transfer(int from, int to, int money) { lock.lock(); try { if (accounts[from] < money) return false; accounts[from] -= money; accounts[to] += money; return true; } finally { lock.unlock(); } }
好了,加锁后这个代码就能保证金额不出错了。但问题又出现了,一次只能执行一个转账过程!意思就是A给B转账的时候,C要给D转账也得等A给B转完了才能开始转。这就有点扯蛋了,就像只有一个柜台,所有人必须排队等前面的处理完了才能到自己,效率太低。
解决这种情况有一个方案:A给B转账的时候只锁定A和B的账户,使其转账期间不能再有其他针对A和B账户的操作,但其他账户的操作可以并行发生。类似于如下场景:
public boolean transfer(int from, int to, int money) { lock.lock(from, to); try { if (accounts[from] < money) return false; accounts[from] -= money; accounts[to] += money; return true; } finally { lock.unlock(from, to); } }
但很显然,JAVA并没有为我们提供这样的锁(也有可能是我没找到。。。)
于是,就在这样的需求下我花了整一天来实现了这个锁——KeyLock(代码量很短,但多线程的东西真的很让人头疼)
不同于synchronized等锁,KeyLock是对所需处理的数据的KEY(主键)进行加锁,只要是对不同key操作,其就可以并行处理,大大提高了线程的并行度(最后有几个锁的对比测试)
总结下就是:对相同KEY操作的线程互斥,对不同KEY操作的线程可以并行
KeyLock有如下几个特性:
1、细粒度,高并行性
2、可重入
3、公平锁
4、加锁开销比ReentrantLock大,适用于处理耗时长、key范围大的场景
KeyLock代码如下(注释很少,因为我也不知道该怎么写清楚,能看懂就看,懒得看的直接用就行):
public class KeyLock<K> { // 保存所有锁定的KEY及其信号量 private final ConcurrentMap<K, Semaphore> map = new ConcurrentHashMap<K, Semaphore>(); // 保存每个线程锁定的KEY及其锁定计数 private final ThreadLocal<Map<K, LockInfo>> local = new ThreadLocal<Map<K, LockInfo>>() { @Override protected Map<K, LockInfo> initialValue() { return new HashMap<K, LockInfo>(); } }; /** * 锁定key,其他等待此key的线程将进入等待,直到调用{@link #unlock(K)} * 使用hashcode和equals来判断key是否相同,因此key必须实现{@link #hashCode()}和 * {@link #equals(Object)}方法 * * @param key */ public void lock(K key) { if (key == null) return; LockInfo info = local.get().get(key); if (info == null) { Semaphore current = new Semaphore(1); current.acquireUninterruptibly(); Semaphore previous = map.put(key, current); if (previous != null) previous.acquireUninterruptibly(); local.get().put(key, new LockInfo(current)); } else { info.lockCount++; } } /** * 释放key,唤醒其他等待此key的线程 * @param key */ public void unlock(K key) { if (key == null) return; LockInfo info = local.get().get(key); if (info != null && --info.lockCount == 0) { info.current.release(); map.remove(key, info.current); local.get().remove(key); } } /** * 锁定多个key * 建议在调用此方法前先对keys进行排序,使用相同的锁定顺序,防止死锁发生 * @param keys */ public void lock(K[] keys) { if (keys == null) return; for (K key : keys) { lock(key); } } /** * 释放多个key * @param keys */ public void unlock(K[] keys) { if (keys == null) return; for (K key : keys) { unlock(key); } } private static class LockInfo { private final Semaphore current; private int lockCount; private LockInfo(Semaphore current) { this.current = current; this.lockCount = 1; } } }
KeyLock使用示例:
private int[] accounts; private KeyLock<Integer> lock = new KeyLock<Integer>(); public boolean transfer(int from, int to, int money) { Integer[] keys = new Integer[] {from, to}; Arrays.sort(keys); //对多个key进行排序,保证锁定顺序防止死锁 lock.lock(keys); try { //处理不同的from和to的线程都可进入此同步块 if (accounts[from] < money) return false; accounts[from] -= money; accounts[to] += money; return true; } finally { lock.unlock(keys); } }
好,工具有了,接下来就是测试了,为了测出并行度,我把转账过程延长了,加了个sleep(2),使每个转账过程至少要花2毫秒(这只是个demo,真实环境下对数据库操作也很费时)。
测试代码如下:
//场景:多线程并发转账 public class Test { private final int[] account; // 账户数组,其索引为账户ID,内容为金额 public Test(int count, int money) { account = new int[count]; Arrays.fill(account, money); } boolean transfer(int from, int to, int money) { if (account[from] < money) return false; account[from] -= money; try { Thread.sleep(2); } catch (Exception e) { } account[to] += money; return true; } int getAmount() { int result = 0; for (int m : account) result += m; return result; } public static void main(String[] args) throws Exception { int count = 100; //账户个数 int money = 10000; //账户初始金额 int threadNum = 8; //转账线程数 int number = 10000; //转账次数 int maxMoney = 1000; //随机转账最大金额 Test test = new Test(count, money); //不加锁 // Runner runner = test.new NonLockRunner(maxMoney, number); //加synchronized锁 // Runner runner = test.new SynchronizedRunner(maxMoney, number); //加ReentrantLock锁 // Runner runner = test.new ReentrantLockRunner(maxMoney, number); //加KeyLock锁 Runner runner = test.new KeyLockRunner(maxMoney, number); Thread[] threads = new Thread[threadNum]; for (int i = 0; i < threadNum; i++) threads[i] = new Thread(runner, "thread-" + i); long begin = System.currentTimeMillis(); for (Thread t : threads) t.start(); for (Thread t : threads) t.join(); long time = System.currentTimeMillis() - begin; System.out.println("类型:" + runner.getClass().getSimpleName()); System.out.printf("耗时:%dms\n", time); System.out.printf("初始总金额:%d\n", count * money); System.out.printf("终止总金额:%d\n", test.getAmount()); } // 转账任务 abstract class Runner implements Runnable { final int maxMoney; final int number; private final Random random = new Random(); private final AtomicInteger count = new AtomicInteger(); Runner(int maxMoney, int number) { this.maxMoney = maxMoney; this.number = number; } @Override public void run() { while(count.getAndIncrement() < number) { int from = random.nextInt(account.length); int to; while ((to = random.nextInt(account.length)) == from) ; int money = random.nextInt(maxMoney); doTransfer(from, to, money); } } abstract void doTransfer(int from, int to, int money); } // 不加锁的转账 class NonLockRunner extends Runner { NonLockRunner(int maxMoney, int number) { super(maxMoney, number); } @Override void doTransfer(int from, int to, int money) { transfer(from, to, money); } } // synchronized的转账 class SynchronizedRunner extends Runner { SynchronizedRunner(int maxMoney, int number) { super(maxMoney, number); } @Override synchronized void doTransfer(int from, int to, int money) { transfer(from, to, money); } } // ReentrantLock的转账 class ReentrantLockRunner extends Runner { private final ReentrantLock lock = new ReentrantLock(); ReentrantLockRunner(int maxMoney, int number) { super(maxMoney, number); } @Override void doTransfer(int from, int to, int money) { lock.lock(); try { transfer(from, to, money); } finally { lock.unlock(); } } } // KeyLock的转账 class KeyLockRunner extends Runner { private final KeyLock<Integer> lock = new KeyLock<Integer>(); KeyLockRunner(int maxMoney, int number) { super(maxMoney, number); } @Override void doTransfer(int from, int to, int money) { Integer[] keys = new Integer[] {from, to}; Arrays.sort(keys); lock.lock(keys); try { transfer(from, to, money); } finally { lock.unlock(keys); } } } }
最最重要的测试结果:
(8线程对100个账户随机转账总共10000次):
类型:NonLockRunner(不加锁)
耗时:2482ms
初始总金额:1000000
终止总金额:998906(无法保证原子性)
类型:SynchronizedRunner(加synchronized锁)
耗时:20872ms
初始总金额:1000000
终止总金额:1000000
类型:ReentrantLockRunner(加ReentrantLock锁)
耗时:21588ms
初始总金额:1000000
终止总金额:1000000
类型:KeyLockRunner(加KeyLock锁)
耗时:2831ms
初始总金额:1000000
终止总金额:1000000
转载:http://blog.csdn.net/icebamboo_moyun/article/details/9391915