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本文主要研究一下Elasticsearch的ReleasableLock

ReleasableLock

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/common/util/concurrent/ReleasableLock.java

public class ReleasableLock implements Releasable {    private final Lock lock;    // a per-thread count indicating how many times the thread has entered the lock; only works if assertions are enabled    private final ThreadLocal
    
      holdingThreads;    public ReleasableLock(Lock lock) {        this.lock = lock;        if (Assertions.ENABLED) {            holdingThreads = new ThreadLocal<>();        } else {            holdingThreads = null;        }    }    @Override    public void close() {        lock.unlock();        assert removeCurrentThread();    }    public ReleasableLock acquire() throws EngineException {        lock.lock();        assert addCurrentThread();        return this;    }    private boolean addCurrentThread() {        final Integer current = holdingThreads.get();        holdingThreads.set(current == null ? 1 : current + 1);        return true;    }    private boolean removeCurrentThread() {        final Integer count = holdingThreads.get();        assert count != null && count > 0;        if (count == 1) {            holdingThreads.remove();        } else {            holdingThreads.set(count - 1);        }        return true;    }    public boolean isHeldByCurrentThread() {        if (holdingThreads == null) {            throw new UnsupportedOperationException("asserts must be enabled");        }        final Integer count = holdingThreads.get();        return count != null && count > 0;    }}复制代码
    

• ReleasableLock实现了Releasable接口(close方法)；它的构造器要求输入Lock参数，只有在开启了assertions的条件下才会初始化holdingThreads；isHeldByCurrentThread方法判断调用线程是否正在使用lock
• acquire方法首先调用lock的lock方法，然后利用assert来断言addCurrentThread方法，该方法会增加调用线程正在使用lock的次数
• close方法首先调用lock的unlock方法，然后利用assert来断言removeCurrentThread方法，该方法会减少调用线程正在使用lock的次数

ReleasableLockTests

elasticsearch-7.0.1/server/src/test/java/org/elasticsearch/common/util/concurrent/ReleasableLockTests.java

public class ReleasableLockTests extends ESTestCase {    /**     * Test that accounting on whether or not a thread holds a releasable lock is correct. Previously we had a bug where on a re-entrant     * lock that if a thread entered the lock twice we would declare that it does not hold the lock after it exits its first entrance but     * not its second entrance.     *     * @throws BrokenBarrierException if awaiting on the synchronization barrier breaks     * @throws InterruptedException   if awaiting on the synchronization barrier is interrupted     */    public void testIsHeldByCurrentThread() throws BrokenBarrierException, InterruptedException {        final ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();        final ReleasableLock readLock = new ReleasableLock(readWriteLock.readLock());        final ReleasableLock writeLock = new ReleasableLock(readWriteLock.writeLock());        final int numberOfThreads = scaledRandomIntBetween(1, 32);        final int iterations = scaledRandomIntBetween(1, 32);        final CyclicBarrier barrier = new CyclicBarrier(1 + numberOfThreads);        final List
    
      threads = new ArrayList<>();        for (int i = 0; i < numberOfThreads; i++) {            final Thread thread = new Thread(() -> {                try {                    barrier.await();                } catch (final BrokenBarrierException | InterruptedException e) {                    throw new RuntimeException(e);                }                for (int j = 0; j < iterations; j++) {                    if (randomBoolean()) {                        acquire(readLock, writeLock);                    } else {                        acquire(writeLock, readLock);                    }                }                try {                    barrier.await();                } catch (final BrokenBarrierException | InterruptedException e) {                    throw new RuntimeException(e);                }            });            threads.add(thread);            thread.start();        }        barrier.await();        barrier.await();        for (final Thread thread : threads) {            thread.join();        }    }    private void acquire(final ReleasableLock lockToAcquire, final ReleasableLock otherLock) {        try (@SuppressWarnings("unused") Releasable outer = lockToAcquire.acquire()) {            assertTrue(lockToAcquire.isHeldByCurrentThread());            assertFalse(otherLock.isHeldByCurrentThread());            try (@SuppressWarnings("unused") Releasable inner = lockToAcquire.acquire()) {                assertTrue(lockToAcquire.isHeldByCurrentThread());                assertFalse(otherLock.isHeldByCurrentThread());            }            // previously there was a bug here and this would return false            assertTrue(lockToAcquire.isHeldByCurrentThread());            assertFalse(otherLock.isHeldByCurrentThread());        }        assertFalse(lockToAcquire.isHeldByCurrentThread());        assertFalse(otherLock.isHeldByCurrentThread());    }}复制代码
    

• ReleasableLockTests使用多线程随机执行acquire，该方法断言lockToAcquire被当前线程持有，而otherLock不被当前线程持有

Cache.CacheSegment

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/common/cache/Cache.java

private static class CacheSegment
    
      {        // read/write lock protecting mutations to the segment        ReadWriteLock segmentLock = new ReentrantReadWriteLock();        ReleasableLock readLock = new ReleasableLock(segmentLock.readLock());        ReleasableLock writeLock = new ReleasableLock(segmentLock.writeLock());        Map
     
      >> map = new HashMap<>();        SegmentStats segmentStats = new SegmentStats();        /**         * get an entry from the segment; expired entries will be returned as null but not removed from the cache until the LRU list is         * pruned or a manual {@link Cache#refresh()} is performed however a caller can take action using the provided callback         *         * @param key       the key of the entry to get from the cache         * @param now       the access time of this entry         * @param isExpired test if the entry is expired         * @param onExpiration a callback if the entry associated to the key is expired         * @return the entry if there was one, otherwise null         */        Entry
      
        get(K key, long now, Predicate
       
        
         > isExpired, Consumer
         
          
           > onExpiration) { CompletableFuture
           
            
             > future; try (ReleasableLock ignored = readLock.acquire()) { future = map.get(key); } if (future != null) { Entry
             
               entry; try { entry = future.get(); } catch (ExecutionException e) { assert future.isCompletedExceptionally(); segmentStats.miss(); return null; } catch (InterruptedException e) { throw new IllegalStateException(e); } if (isExpired.test(entry)) { segmentStats.miss(); onExpiration.accept(entry); return null; } else { segmentStats.hit(); entry.accessTime = now; return entry; } } else { segmentStats.miss(); return null; } } /** * put an entry into the segment * * @param key the key of the entry to add to the cache * @param value the value of the entry to add to the cache * @param now the access time of this entry * @return a tuple of the new entry and the existing entry, if there was one otherwise null */ Tuple
              
               
                , Entry
                
                 > put(K key, V value, long now) { Entry
                 
                   entry = new Entry<>(key, value, now); Entry
                  
                    existing = null; try (ReleasableLock ignored = writeLock.acquire()) { try { CompletableFuture
                   
                    
                     > future = map.put(key, CompletableFuture.completedFuture(entry)); if (future != null) { existing = future.handle((ok, ex) -> { if (ok != null) { return ok; } else { return null; } }).get(); } } catch (ExecutionException | InterruptedException e) { throw new IllegalStateException(e); } } return Tuple.tuple(entry, existing); } /** * remove an entry from the segment * * @param key the key of the entry to remove from the cache * @param onRemoval a callback for the removed entry */ void remove(K key, Consumer
                     
                      
                       
                        >> onRemoval) { CompletableFuture
                        
                         
                          > future; try (ReleasableLock ignored = writeLock.acquire()) { future = map.remove(key); } if (future != null) { segmentStats.eviction(); onRemoval.accept(future); } } /** * remove an entry from the segment iff the future is done and the value is equal to the * expected value * * @param key the key of the entry to remove from the cache * @param value the value expected to be associated with the key * @param onRemoval a callback for the removed entry */ void remove(K key, V value, Consumer
                          
                           
                            
                             >> onRemoval) { CompletableFuture
                             
                              
                               > future; boolean removed = false; try (ReleasableLock ignored = writeLock.acquire()) { future = map.get(key); try { if (future != null) { if (future.isDone()) { Entry
                               
                                 entry = future.get(); if (Objects.equals(value, entry.value)) { removed = map.remove(key, future); } } } } catch (ExecutionException | InterruptedException e) { throw new IllegalStateException(e); } } if (future != null && removed) { segmentStats.eviction(); onRemoval.accept(future); } } private static class SegmentStats { private final LongAdder hits = new LongAdder(); private final LongAdder misses = new LongAdder(); private final LongAdder evictions = new LongAdder(); void hit() { hits.increment(); } void miss() { misses.increment(); } void eviction() { evictions.increment(); } } }复制代码
                               
                              
                             
                            
                           
                          
                         
                        
                       
                      
                     
                    
                   
                  
                 
                
               
              
             
            
           
          
         
        
       
      
     
    

• CacheSegment使用ReentrantReadWriteLock的readLock及writeLock创建了两个ReleasableLock，一个为readLock，一个为writeLock；由于ReleasableLock实现了Releasable接口(close方法)，而该接口继承了java.lang.AutoCloseable接口，因而可以直接利用try with resources语法来自动close，从而释放锁

小结

• ReleasableLock实现了Releasable接口(close方法)；它的构造器要求输入Lock参数，只有在开启了assertions的条件下才会初始化holdingThreads；isHeldByCurrentThread方法判断调用线程是否正在使用lock
• acquire方法首先调用lock的lock方法，然后利用assert来断言addCurrentThread方法，该方法会增加调用线程正在使用lock的次数
• close方法首先调用lock的unlock方法，然后利用assert来断言removeCurrentThread方法，该方法会减少调用线程正在使用lock的次数

ReleasableLock实现了Releasable接口(close方法)，而该接口继承了java.lang.AutoCloseable接口，因而可以直接利用try with resources语法来自动close，从而释放锁

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