java实现表锁行锁

背景

今天做需求时遇到一个统计场景，接口将用户请求记录缓存在concurrentHashmap，其中用户名作为map的Key，value为统计结果类的对象，更新此map的时候使用分段锁(通过用户名取hash值定位对应的锁)确保在相对良好性能下使得value更新线程安全。此外通过定时任务2秒一次将缓存的map保存到redis数据库后再清空map，这意味着定时任务执行的某个时候需要暂停所有写入map操作，由于map写入是使用分段锁，意味着需要阻塞所有获取分段锁线程，类似于mysql某些修改表结构的时候会阻塞行锁一样。查阅资料后发现java并无相关实现，这里手动实现一个。

分段独占锁

1. 首先实现分段独占锁，在表-行锁中充当行锁的功能
2. 代码如下，通过对key取hash值来定位具体的某行(锁)

public class SegReentrantLock{private List<Lock> lockList;public SegReentrantLock() {this(10);}public SegReentrantLock(int size) {if (size < 1){throw new IllegalArgumentException("size must great than 0");}this.lockList = new ArrayList<>(size);for (int i = 0; i < size; i++) {this.lockList.add( new ReentrantLock() );}}public void lock(String key){int lockIndex = this.getLockIndex(key);this.lockList.get(lockIndex).lock();}public void unlock(String key){int lockIndex = this.getLockIndex(key);this.lockList.get(lockIndex).unlock();}private int getLockIndex(String key){int lockIndex = key.hashCode() % this.lockList.size();return Math.abs(lockIndex);}public boolean tryLock(String key, long time, TimeUnit timeUnit) throws InterruptedException {int lockIndex = key.hashCode() % this.lockList.size();if (time < 1 || timeUnit == null){return this.lockList.get(lockIndex).tryLock();}return this.lockList.get(lockIndex).tryLock(time, timeUnit);}public boolean tryLock(String key) throws InterruptedException {return this.tryLock(key, 0, TimeUnit.SECONDS);}
}

行表锁

1. 在实现了分段锁的基础上，通过jdk自带StampedLock来模拟表锁。
2. 获取行锁的时候，先获取StampedLock的读锁，再获取行锁，释放同理
3. 获取表锁的时候，直接获取StampedLock的写锁即可，如对性能有高要求，需减少获取表锁的饥饿现象，可通过StampedLock的tryOptimisticRead方法将加锁逻辑修改为“读的过程中也允许获取写锁后写入”的模式。
4. 行表锁代码如下

public class TableRowsLock {//表锁private final StampedLock STAMPED_LOCK = new StampedLock();//行锁private SegReentrantLock rowLocks;public TableRowsLock() {this(10);}public TableRowsLock(int rows) {rowLocks = new SegReentrantLock(rows);}/*** @description:释放表锁* @param time* @param timeUnit* @return long 时间戳，为0表示加锁失败*/public long tryLockTable(int time, TimeUnit timeUnit) throws InterruptedException {return STAMPED_LOCK.tryWriteLock(time, timeUnit);}public long tryLockTable() throws InterruptedException {return STAMPED_LOCK.tryWriteLock(0, TimeUnit.SECONDS);}/*** @description:释放表锁* @param stamp 获取表锁返回的时间戳* @see #tryLockTable(int, TimeUnit) * @return void*/public void unLockTable(long stamp){STAMPED_LOCK.unlockWrite(stamp);}/*** @description:加行锁* @param key* @param time* @param timeUnit* @return long 时间戳，为0表示加锁失败*/public long tryLockRow(String key, int time, TimeUnit timeUnit) throws InterruptedException {//先设置表锁为已读状态long stamp = STAMPED_LOCK.tryReadLock(time, timeUnit);//失败，直接返回if (stamp == 0) return stamp;//锁行boolean lockedRow = rowLocks.tryLock(key, time, timeUnit);if (!lockedRow){STAMPED_LOCK.unlockRead(stamp);return 0;}return stamp;}public long tryLockRow(String key) throws InterruptedException {return tryLockRow(key, 0, TimeUnit.SECONDS);}/*** @description:释放行锁* @param key* @param stamp 获取行锁获得的时间戳* @see #tryLockRow(String, int, TimeUnit)*/public void unlockRow(String key, long stamp){STAMPED_LOCK.unlockRead(stamp);rowLocks.unlock(key);}
}

测试代码

public static void main(String[] args) {//测试行锁释放和未释放情况下获取表锁testBlockingTableLock();//测试表锁释放和未释放情况下获取行锁
//        testBlockingRowLock();}//测试行锁释放和未释放情况下获取表锁private static void testBlockingTableLock(){TableRowsLock tableLock = new TableRowsLock(2);new Thread(() -> {try {long stamp1 = tableLock.tryLockRow("1");System.out.println("未加表锁与行锁时获取行锁1，结果:" + stamp1);long stamp2 = tableLock.tryLockRow("2");System.out.println("未加表锁只加行锁时获取行锁2，结果:" + stamp2);TimeUnit.SECONDS.sleep(2);//释放行锁tableLock.unlockRow("1", stamp1);tableLock.unlockRow("2", stamp2);} catch (Exception ex){ex.printStackTrace();}}).start();new Thread(() -> {try {TimeUnit.MILLISECONDS.sleep(500);//获取表锁long stamp = tableLock.tryLockTable();System.out.println("行锁未释放,获取表锁，结果:" + stamp);TimeUnit.SECONDS.sleep(3);stamp = tableLock.tryLockTable();System.out.println("行锁全部释放后 再次获取表锁,结果:" + stamp);} catch (InterruptedException e) {e.printStackTrace();}}).start();}//测试表锁释放和未释放情况下获取行锁private static void testBlockingRowLock(){TableRowsLock tableLock = new TableRowsLock(2);new Thread(() -> {try {long stamp = tableLock.tryLockTable();System.out.println("未加行锁时获取表锁，结果:" + stamp);TimeUnit.SECONDS.sleep(2);//释放表锁tableLock.unLockTable(stamp);} catch (Exception ex){ex.printStackTrace();}}).start();new Thread(() -> {try {TimeUnit.MILLISECONDS.sleep(500);//获取行锁long stamp = tableLock.tryLockRow("1");System.out.println("表锁未释放,获取行锁，结果:" + stamp);//表锁释放后获取行锁TimeUnit.SECONDS.sleep(3);stamp = tableLock.tryLockRow("1");System.out.println("表锁释放后,再次获取行锁，结果:" + stamp);} catch (Exception e) {e.printStackTrace();}}).start();}

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