HBase行锁原理及实现

protected RowLock getRowLockInternal(byte[] row, boolean waitForLock) throws IOException {
    HashedBytes rowKey = new HashedBytes(row);
    RowLockContext rowLockContext = new RowLockContext(rowKey);

    // loop until we acquire the row lock (unless !waitForLock)
    while (true) {
      RowLockContext existingContext = lockedRows.putIfAbsent(rowKey, rowLockContext);
      if (existingContext == null) {
        // Row is not already locked by any thread, use newly created context.
        break;
      } else if (existingContext.ownedByCurrentThread()) {
        // Row is already locked by current thread, reuse existing context instead.
        rowLockContext = existingContext;
        break;
      } else {
        if (!waitForLock) {
          return null;
        }
        try {
          // Row is already locked by some other thread, give up or wait for it
          if (!existingContext.latch.await(this.rowLockWaitDuration, TimeUnit.MILLISECONDS)) {
            throw new IOException("Timed out waiting for lock for row: " + rowKey);
          }
        } catch (InterruptedException ie) {
          LOG.warn("Thread interrupted waiting for lock on row: " + rowKey);
          InterruptedIOException iie = new InterruptedIOException();
          iie.initCause(ie);
          throw iie;
        }
      }
    }

    // allocate new lock for this thread
    return rowLockContext.newLock();
  }

public static class RowLock {
    @VisibleForTesting final RowLockContext context;
    private boolean released = false;

    @VisibleForTesting RowLock(RowLockContext context) {
      this.context = context;
    }

    /**
     * Release the given lock.  If there are no remaining locks held by the current thread
     * then unlock the row and allow other threads to acquire the lock.
     * @throws IllegalArgumentException if called by a different thread than the lock owning thread
     */
    public void release() {
      if (!released) {
        context.releaseLock();
        released = true;
      }
    }
  }

@VisibleForTesting class RowLockContext {
    private final HashedBytes row;
　　//通过计数以及CountDownLatch实现对行锁的condition。这里之所以将countdownlatch设置为一，是因为hbase自己也不知道到底有多少condition来竞争锁，所以加一个计数lockCount,
　　//当lockCount为零时，再把latch.coutDown。否则会在getRowLockInternal中await。

    private final CountDownLatch latch = new CountDownLatch(1);
    private final Thread thread;
    private int lockCount = 0;

    RowLockContext(HashedBytes row) {
      this.row = row;
      this.thread = Thread.currentThread();
    }

    boolean ownedByCurrentThread() {
      return thread == Thread.currentThread();
    }

    RowLock newLock() {
      lockCount++;
      return new RowLock(this);
    }

    void releaseLock() {
      if (!ownedByCurrentThread()) {
        throw new IllegalArgumentException("Lock held by thread: " + thread
          + " cannot be released by different thread: " + Thread.currentThread());
      }
      lockCount--;
      if (lockCount == 0) {
        // no remaining locks by the thread, unlock and allow other threads to access
        RowLockContext existingContext = lockedRows.remove(row);
        if (existingContext != this) {
          throw new RuntimeException(
              "Internal row lock state inconsistent, should not happen, row: " + row);
        }
        latch.countDown();
      }
    }
  }

// map from a locked row to the context for that lock including:
  // - CountDownLatch for threads waiting on that row
  // - the thread that owns the lock (allow reentrancy)
  // - reference count of (reentrant) locks held by the thread
  // - the row itself
  private final ConcurrentHashMap<HashedBytes, RowLockContext> lockedRows =
      new ConcurrentHashMap<HashedBytes, RowLockContext>();

 /**
   * If the given list of row locks is not null, releases all locks.
   */
  public void releaseRowLocks(List<RowLock> rowLocks) {
    if (rowLocks != null) {
      for (RowLock rowLock : rowLocks) {
        rowLock.release();
      }
      rowLocks.clear();
    }
  }