• Ehcache BlockingCache 源码分析


    BlockingCache是对Ehcache进行的扩展,BlockingCache内置了读写锁,不需要用户显示调用。

    要彻底分析BlockingCache的原理,需要首先来看一下它内部用到的一些类。

    //锁的管理器接口
    public interface CacheLockProvider {
        /**
         * 根据key获取锁
         * 这个实现需要保证给定相同的key必须返回同一把锁
         */
        Sync getSyncForKey(Object key);
    }
    public interface StripedReadWriteLock extends CacheLockProvider {
    
        /**
         * 根据key获得Java的ReadWriteLock
         */
        ReadWriteLock getLockForKey(Object key);
    
        /**
         * 获取所有锁
         */
        List<ReadWriteLockSync> getAllSyncs();
    
    }

    上面的俩个接口是用来管理锁的,待会我们再看具体实现,再来看一下锁的接口。

    public interface Sync {
        /**
         * Acquire lock of LockType.READ or WRITE
         * @param type the lock type to acquire
         */
        void lock(LockType type);
    
        /**
         * Tries to acquire a LockType.READ or WRITE for a certain period
         * @param type the lock type to acquire
         * @param msec timeout
         * @return true if the lock got acquired, false otherwise
         * @throws InterruptedException Should the thread be interrupted
         */
        boolean tryLock(LockType type, long msec) throws InterruptedException;
    
        /**
         * Releases the lock held by the current Thread.
         * In case of a LockType.WRITE, should the lock not be held by the current Thread, nothing happens 
         * @param type the lock type to acquire
         */
        void unlock(LockType type);
        
        /**
         * Returns true is this is lock is held at given level by the current thread.
         * 
         * @param type the lock type to test
         * @return true if the lock is held
         */
        boolean isHeldByCurrentThread(LockType type);
    }
    /**
     * 这个类是Sync接口的实现类,底层用到了Java原生的ReentrantReadWriteLock
     * ReentrantReadWriteLock是读写锁,不了解的可以先去学习一下这个类的使用方法
     * 不然会很难理解,其实ReadWriteLockSync的类的方法都是调用ReentrantReadWriteLock。
     */
    public class ReadWriteLockSync implements Sync {
    
        //Java的读写锁,其实这个类底层都是调用它.
        private final ReentrantReadWriteLock rrwl;
    
        public ReadWriteLockSync() {
            this(new ReentrantReadWriteLock());
        }
    
        public ReadWriteLockSync(ReentrantReadWriteLock lock) {
            this.rrwl = lock;
        }
        /**
         * 根据类型获取锁.
         */
        public void lock(final LockType type) {
            getLock(type).lock();
        }
    
        /**
         * 在给定时间内尝试获取锁.
         */
        public boolean tryLock(final LockType type, final long msec) throws InterruptedException {
            return getLock(type).tryLock(msec, TimeUnit.MILLISECONDS);
        }
    
        /**
         * 释放锁.
         */
        public void unlock(final LockType type) {
            getLock(type).unlock();
        }
    
        //根据枚举类型获取写入锁OR读取锁.
        private Lock getLock(final LockType type) {
            switch (type) {
                case READ:
                    return rrwl.readLock();
                case WRITE:
                    return rrwl.writeLock();
                default:
                    throw new IllegalArgumentException("We don't support any other lock type than READ or WRITE!");
            }
        }
    
        public ReadWriteLock getReadWriteLock() {
          return rrwl;
        }
        
        /**
         * 判断当前线程是否获取了写入锁.
         */
        public boolean isHeldByCurrentThread(LockType type) {
            switch (type) {
                case READ:
                    throw new UnsupportedOperationException("Querying of read lock is not supported.");
                case WRITE:
                    return rrwl.isWriteLockedByCurrentThread();
                default:
                    throw new IllegalArgumentException("We don't support any other lock type than READ or WRITE!");
            }
        }
    }

    StripedReadWriteLockSync类是负责管理锁的,会根据传入的key的hash值计算出数组的下标,原理与hashmap一样。

    /**
     * 这个类是锁管理器的实现类,用来管理锁.
     * 这个类的实现原理就是内部维护一个锁的数组(默认2048个)
     * 然后根据传入的key通过hash算法获取到一个int类型的值,这个值就是锁数组的下标.
     * 这样就可以针对不同的key分别锁定提高并发效率
     */
    public class StripedReadWriteLockSync implements StripedReadWriteLock {
    
        /**
         * 默认锁的数量必须是2的幂
         */
        public static final int DEFAULT_NUMBER_OF_MUTEXES = 2048;
    
        //锁的数组
        private final ReadWriteLockSync[] mutexes;
        //锁的list
        private final List<ReadWriteLockSync> mutexesAsList;
    
        public StripedReadWriteLockSync() {
            this(DEFAULT_NUMBER_OF_MUTEXES);
        }
    
        /**
         * 构造函数,初始化锁.
         */
        public StripedReadWriteLockSync(int numberOfStripes) {
            if ((numberOfStripes & (numberOfStripes - 1)) != 0) {
                throw new CacheException("Cannot create a CacheLockProvider with a non power-of-two number of stripes");
            }
            if (numberOfStripes == 0) {
                throw new CacheException("A zero size CacheLockProvider does not have useful semantics.");
            }
    
            //初始化数组.
            mutexes = new ReadWriteLockSync[numberOfStripes];
    
            //初始化ReadWriteLockSync锁放入数组.
            for (int i = 0; i < mutexes.length; i++) {
                mutexes[i] = new ReadWriteLockSync();
            }
            mutexesAsList = Collections.unmodifiableList(Arrays.asList(mutexes));
        }
    
        /**
         * 根据key获取锁,相同的key获取到同一把锁。
         */
        public ReadWriteLockSync getSyncForKey(final Object key) {
            //根据key的hash算法在与数组的长度计算出数组下标,这个算法会保证得到的int值在0-数组长度之内不会越界.
            int lockNumber = ConcurrencyUtil.selectLock(key, mutexes.length);
            return mutexes[lockNumber];
        }
    
        /**
         * 根据key获取锁
         */
        public ReadWriteLock getLockForKey(final Object key) {
            int lockNumber = ConcurrencyUtil.selectLock(key, mutexes.length);
            return mutexes[lockNumber].getReadWriteLock();
        }
    
        /**
         * Returns all internal syncs
         * @return all internal syncs
         */
        public List<ReadWriteLockSync> getAllSyncs() {
            return mutexesAsList;
        }
    }

    BlockingCache核心代码分析,在使用它的时候有一点需要注意,如果一个线程get方法获取element,当获取不到时会返回null,这时线程并没有释放写入锁,这点一定要注意。

    所以必须要调用blockingCache.put(new Element(key, null)) 来释放锁。

    /**
     * A blocking decorator for an Ehcache, backed by a {@link Ehcache}.
     */
    public class BlockingCache extends EhcacheDecoratorAdapter {
    
        /**
         * The amount of time to block a thread before a LockTimeoutException is thrown
         */
        protected volatile int timeoutMillis;
    
        private final int stripes;
    
        //线程安全的引用AtomicReference
        private final AtomicReference<CacheLockProvider> cacheLockProviderReference;
    
        private final OperationObserver<GetOutcome> getObserver = operation(GetOutcome.class).named("get").of(this).tag("blocking-cache").build();
    
        /**
         * Creates a BlockingCache which decorates the supplied cache.
         */
        public BlockingCache(final Ehcache cache, int numberOfStripes) throws CacheException {
            super(cache);
            this.stripes = numberOfStripes;
            this.cacheLockProviderReference = new AtomicReference<CacheLockProvider>();
        }
    
        /**
         * Creates a BlockingCache which decorates the supplied cache.
         */
        public BlockingCache(final Ehcache cache) throws CacheException {
            this(cache, StripedReadWriteLockSync.DEFAULT_NUMBER_OF_MUTEXES);
        }
    
        private CacheLockProvider getCacheLockProvider() {
            CacheLockProvider provider = cacheLockProviderReference.get();
            while (provider == null) {
                cacheLockProviderReference.compareAndSet(null, createCacheLockProvider());
                provider = cacheLockProviderReference.get();
            }
            return provider;
        }
    
        //初始化StripedReadWriteLockSync
        private CacheLockProvider createCacheLockProvider() {
            Object context = underlyingCache.getInternalContext();
            if (underlyingCache.getCacheConfiguration().isTerracottaClustered() && context != null) {
                return (CacheLockProvider) context;
            } else {
                return new StripedReadWriteLockSync(stripes);
            }
        }
    
        /**
         * Retrieve the EHCache backing cache
         */
        protected Ehcache getCache() {
            return underlyingCache;
        }
    
        /**
         * 获取元素
         */
        @Override
        public Element get(final Object key) throws RuntimeException, LockTimeoutException {
            getObserver.begin();
    
            //通过key获取锁,不同的key在大多数情况获取的锁都是不同的,所以性能会很好.
            Sync lock = getLockForKey(key);
    
            //获取读取锁,读取锁是可以并发读的,所以效率会很好.
            acquiredLockForKey(key, lock, LockType.READ);
    
            Element element;
            try {
                //在真正的cache里获取元素
                element = underlyingCache.get(key);
            } finally {
                //释放读取锁
                lock.unlock(LockType.READ);
            }
    
            //如果元素为空,则获取写入锁,写入锁不可以并发进入的。
            if (element == null) {
                acquiredLockForKey(key, lock, LockType.WRITE);
    
                //再次获取元素,如果为空则直接返回,注意当前线程并没有释放锁.这里一定要注意。
                element = underlyingCache.get(key);
                if (element != null) {
                    //如果元素不为空,说明已经有线程在其它时刻put进去元素了,那么直接释放锁就OK了。
                    lock.unlock(LockType.WRITE);
                    getObserver.end(GetOutcome.HIT);
                } else {
                    getObserver.end(GetOutcome.MISS_AND_LOCKED);
                }
                return element;
            } else {
                getObserver.end(GetOutcome.HIT);
                return element;
            }
        }
    
       
        private void acquiredLockForKey(final Object key, final Sync lock, final LockType lockType) {
            if (timeoutMillis > 0) {
                try {
                     //如果设置了超时时间,那么在给定时间内获取不到锁就抛异常.
                    boolean acquired = lock.tryLock(lockType, timeoutMillis);
                    if (!acquired) {
                        StringBuilder message = new StringBuilder("Lock timeout. Waited more than ")
                                .append(timeoutMillis)
                                .append("ms to acquire lock for key ")
                                .append(key).append(" on blocking cache ").append(underlyingCache.getName());
                        throw new LockTimeoutException(message.toString());
                    }
                } catch (InterruptedException e) {
                    throw new LockTimeoutException("Got interrupted while trying to acquire lock for key " + key, e);
                }
            } else {
                //获取锁,获取不到就会一直等待知道获取到锁。
                lock.lock(lockType);
            }
        }
    
        protected Sync getLockForKey(final Object key) {
            return getCacheLockProvider().getSyncForKey(key);
        }
    
        /**
         * put元素自动释放锁,主要看doAndReleaseWriteLock.
         */
        @Override
        public void put(final Element element) {
    
            doAndReleaseWriteLock(new PutAction<Void>(element) {
                @Override
                public Void put() {
                    if (element.getObjectValue() != null) {
                        underlyingCache.put(element);
                    } else {
                        underlyingCache.remove(element.getObjectKey());
                    }
                    return null;
                }
            });
        }
    
    
        private <V> V doAndReleaseWriteLock(PutAction<V> putAction) {
    
            if (putAction.element == null) {
                return null;
            }
    
            Object key = putAction.element.getObjectKey();
    
            //根据key获取锁.
            Sync lock = getLockForKey(key);
    
            //判断一下当前线程是否已经获取了写入锁,如果已经获取到锁,那么说明当前线程是执行get方法是元素为null时获取到锁的.
            //否则说明是另一个新的线程直接调用put的方法,所以需要重新获取锁.
            if (!lock.isHeldByCurrentThread(LockType.WRITE)) {
                lock.lock(LockType.WRITE);
            }
            try {
                return putAction.put();
            } finally {
                //在这里释放锁.
                lock.unlock(LockType.WRITE);
            }
        }
    
    
    
        private abstract static class PutAction<V> {
    
            private final Element element;
    
            private PutAction(Element element) {
                this.element = element;
            }
            abstract V put();
        }
    }
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  • 原文地址:https://www.cnblogs.com/daxin/p/4289928.html
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