高并发数据结构Disruptor解析（4）

RingBuffer

RingBuffer类是Disruptor核心的数据结构类。它是一个环状的Buffer，上面的槽（slot）可以保存一个个Event。下面是Disruptor中RingBuffer类继承关系：

除了实现之前提到过的Sequenced和Cursored接口，这里还涉及到了DataProvider这个接口。

public interface DataProvider<T>{    T get(long sequence);}

它只有一个方法get，这个方法就是获取某个sequence对应的对象，对象类型在这里是抽象的（T）。这个方法对于RingBuffer会在两个地方调用，第一个是在生产时，这个Event对象需要被生产者获取往里面填充数据。第二个是在消费时，获取这个Event对象用于消费。
EventSequencer接口没有自己的方法，只是为了将Sequencer和DataProvider合起来。
EventSink代表RingBuffer是一个以Event槽为基础的数据结构。同时实现EventSequencer和EventSink代表RingBuffer是一个以Event槽为基础元素保存的数据结构。
EventSink接口的主要方法都是发布Event，发布一个Event的流程是：申请下一个Sequence->申请成功则获取对应槽的Event->初始化并填充对应槽的Event->发布Event。这里，初始化，填充Event是通过实现EventTranslator，EventTranslatorOneArg，EventTranslatorTwoArg，EventTranslatorThreeArg，EventTranslatorVararg这些EventTranslator来做的。我们看下EventTranslator，EventTranslatorOneArg和EventTranslatorVararg的源码:

public interface EventTranslator<T>{    /**     * Translate a data representation into fields set in given event     *     * @param event    into which the data should be translated.     * @param sequence that is assigned to event.     */    void translateTo(final T event, long sequence);}public interface EventTranslatorOneArg<T, A>{    /**     * Translate a data representation into fields set in given event     *     * @param event    into which the data should be translated.     * @param sequence that is assigned to event.     * @param arg0     The first user specified argument to the translator     */    void translateTo(final T event, long sequence, final A arg0);}public interface EventTranslatorVararg<T>{    /**     * Translate a data representation into fields set in given event     *     * @param event    into which the data should be translated.     * @param sequence that is assigned to event.     * @param args     The array of user arguments.     */    void translateTo(final T event, long sequence, final Object... args);}

他们由生产者用户实现，将Event初始化并填充。在发布一条Event的时候，这些Translator的translate方法会被调用。在translate方法初始化并填充Event。对于EventTranslator，translate方法只接受Event和Sequence作为参数，对于其他的，都还会接受一个或多个参数用来初始化并填充Event。
EventSink接口是用来发布Event的，在发布的同时，调用绑定的Translator来初始化并填充Event。EventSink接口的大部分方法接受不同的Translator来处理Event：

public interface EventSink<E> {    /**     * 申请下一个Sequence->申请成功则获取对应槽的Event->利用translator初始化并填充对应槽的Event->发布Event     * @param translator translator用户实现，用于初始化Event，这里是不带参数Translator     */     void publishEvent(EventTranslator<E> translator);    /**     * 尝试申请下一个Sequence->申请成功则获取对应槽的Event->利用translator初始化并填充对应槽的Event->发布Event     * 若空间不足，则立即失败返回     * @param translator translator用户实现，用于初始化Event，这里是不带参数Translator     * @return 成功true，失败false     */     boolean tryPublishEvent(EventTranslator<E> translator);     <A> void publishEvent(EventTranslatorOneArg<E, A> translator, A arg0);     <A> boolean tryPublishEvent(EventTranslatorOneArg<E, A> translator, A arg0);     <A, B> void publishEvent(EventTranslatorTwoArg<E, A, B> translator, A arg0, B arg1);     <A, B> boolean tryPublishEvent(EventTranslatorTwoArg<E, A, B> translator, A arg0, B arg1);     <A, B, C> void publishEvent(EventTranslatorThreeArg<E, A, B, C> translator, A arg0, B arg1, C arg2);     <A, B, C> boolean tryPublishEvent(EventTranslatorThreeArg<E, A, B, C> translator, A arg0, B arg1, C arg2);     void publishEvent(EventTranslatorVararg<E> translator, Object... args);     boolean tryPublishEvent(EventTranslatorVararg<E> translator, Object... args);    /**     * 包括申请多个Sequence->申请成功则获取对应槽的Event->利用每个translator初始化并填充每个对应槽的Event->发布Event     * @param translators     */     void publishEvents(EventTranslator<E>[] translators);     void publishEvents(EventTranslator<E>[] translators, int batchStartsAt, int batchSize);     boolean tryPublishEvents(EventTranslator<E>[] translators);     boolean tryPublishEvents(EventTranslator<E>[] translators, int batchStartsAt, int batchSize);     <A> void publishEvents(EventTranslatorOneArg<E, A> translator, A[] arg0);     <A> void publishEvents(EventTranslatorOneArg<E, A> translator, int batchStartsAt, int batchSize, A[] arg0);     <A> boolean tryPublishEvents(EventTranslatorOneArg<E, A> translator, A[] arg0);     <A> boolean tryPublishEvents(EventTranslatorOneArg<E, A> translator, int batchStartsAt, int batchSize, A[] arg0);     <A, B> void publishEvents(EventTranslatorTwoArg<E, A, B> translator, A[] arg0, B[] arg1);     <A, B> void publishEvents(            EventTranslatorTwoArg<E, A, B> translator, int batchStartsAt, int batchSize, A[] arg0,            B[] arg1);     <A, B> boolean tryPublishEvents(EventTranslatorTwoArg<E, A, B> translator, A[] arg0, B[] arg1);     <A, B> boolean tryPublishEvents(            EventTranslatorTwoArg<E, A, B> translator, int batchStartsAt, int batchSize,            A[] arg0, B[] arg1);     <A, B, C> void publishEvents(EventTranslatorThreeArg<E, A, B, C> translator, A[] arg0, B[] arg1, C[] arg2);     <A, B, C> void publishEvents(            EventTranslatorThreeArg<E, A, B, C> translator, int batchStartsAt, int batchSize,            A[] arg0, B[] arg1, C[] arg2);     <A, B, C> boolean tryPublishEvents(EventTranslatorThreeArg<E, A, B, C> translator, A[] arg0, B[] arg1, C[] arg2);     <A, B, C> boolean tryPublishEvents(            EventTranslatorThreeArg<E, A, B, C> translator, int batchStartsAt,            int batchSize, A[] arg0, B[] arg1, C[] arg2);     void publishEvents(EventTranslatorVararg<E> translator, Object[]... args);     void publishEvents(EventTranslatorVararg<E> translator, int batchStartsAt, int batchSize, Object[]... args);     boolean tryPublishEvents(EventTranslatorVararg<E> translator, Object[]... args);     boolean tryPublishEvents(EventTranslatorVararg<E> translator, int batchStartsAt, int batchSize, Object[]... args);}

接下来到我们的主要环节，RingBuffer类。与之前相似，RingBuffer也是做了缓冲行填充。
RingBuffer类中保存了整个RingBuffer每个槽（entry或者slot）的Event对象，对应的field是private final Object[] entries;，这些对象只在RingBuffer初始化时被建立，之后就是修改这些对象（初始化Event和填充Event），并不会重新建立新的对象。RingBuffer可以有多生产者和消费者，所以这个entries会被多线程访问频繁的，但不会修改（因为不会重新建立新的对象，这个数组保存的是对对象的具体引用，所以不会变）。但是我们要避免他们和被修改的对象读取到同一个缓存行，避免缓存行失效重新读取。
我们看源代码：

abstract class RingBufferPad{    protected long p1, p2, p3, p4, p5, p6, p7;}abstract class RingBufferFields<E> extends RingBufferPad{    //Buffer数组填充    private static final int BUFFER_PAD;    //Buffer数组起始基址    private static final long REF_ARRAY_BASE;    //2^n=每个数组对象引用所占空间，这个n就是REF_ELEMENT_SHIFT    private static final int REF_ELEMENT_SHIFT;    private static final Unsafe UNSAFE = Util.getUnsafe();    static    {        //Object数组引用长度，32位为4字节，64位为8字节        final int scale = UNSAFE.arrayIndexScale(Object[].class);        if (4 == scale)        {            REF_ELEMENT_SHIFT = 2;        }        else if (8 == scale)        {            REF_ELEMENT_SHIFT = 3;        }        else        {            throw new IllegalStateException("Unknown pointer size");        }        //需要填充128字节，缓存行长度一般是128字节        BUFFER_PAD = 128 / scale;        // Including the buffer pad in the array base offset        REF_ARRAY_BASE = UNSAFE.arrayBaseOffset(Object[].class) + (BUFFER_PAD << REF_ELEMENT_SHIFT);    }    private final long indexMask;    private final Object[] entries;    protected final int bufferSize;    protected final Sequencer sequencer;    RingBufferFields(        EventFactory<E> eventFactory,        Sequencer sequencer)    {        this.sequencer = sequencer;        this.bufferSize = sequencer.getBufferSize();        //保证buffer大小不小于1        if (bufferSize < 1)        {            throw new IllegalArgumentException("bufferSize must not be less than 1");        }        //保证buffer大小为2的n次方        if (Integer.bitCount(bufferSize) != 1)        {            throw new IllegalArgumentException("bufferSize must be a power of 2");        }        //m % 2^n  <=>  m & (2^n - 1)        this.indexMask = bufferSize - 1;        /**         * 结构：缓存行填充，避免频繁访问的任一entry与另一被修改的无关变量写入同一缓存行         * --------------         * *   数组头   * BASE         * *   Padding  * 128字节         * * reference1 * SCALE         * * reference2 * SCALE         * * reference3 * SCALE         * ..........         * *   Padding  * 128字节         * --------------         */        this.entries = new Object[sequencer.getBufferSize() + 2 * BUFFER_PAD];        //利用eventFactory初始化RingBuffer的每个槽        fill(eventFactory);    }    private void fill(EventFactory<E> eventFactory)    {        for (int i = 0; i < bufferSize; i++)        {            entries[BUFFER_PAD + i] = eventFactory.newInstance();        }    }    @SuppressWarnings("unchecked")    protected final E elementAt(long sequence)    {        return (E) UNSAFE.getObject(entries, REF_ARRAY_BASE + ((sequence & indexMask) << REF_ELEMENT_SHIFT));    }}

注释中提到对于entries数组的缓存行填充，申请的数组大小为实际需要大小加上2 * BUFFER_PAD，所占空间就是2*128字节。由于数组中的元素经常访问，所以将数组中的实际元素两边各加上128字节的padding防止false sharing。
所以，初始化RingBuffer内所有对象时，从下标BUFFER_PAD开始，到BUFFER_PAD+bufferSize-1为止。取出某一sequence的对象，也是BUFFER_PAD开始算0,这里的：return (E) UNSAFE.getObject(entries, REF_ARRAY_BASE + ((sequence & indexMask) << REF_ELEMENT_SHIFT));
代表取出entries对象，地址为REF_ARRAY_BASE + ((sequence & indexMask) << REF_ELEMENT_SHIFT)的对象。这里是个对象引用，地址是以REF_ARRAY_BASE 为基址（数组基址+数组头+引用偏移），每个引用占用2^REF_ELEMENT_SHIFT个字节，sequence 对bufferSize取模乘以2^REF_ELEMENT_SHIFT。
接下来看可以供用户调用的具体的构造方法，RingBuffer在Disruptor包外部不能直接调用其构造方法，用户只能用静态方法创建：

    /**     * Construct a RingBuffer with the full option set.     *     * @param eventFactory to newInstance entries for filling the RingBuffer     * @param sequencer    sequencer to handle the ordering of events moving through the RingBuffer.     * @throws IllegalArgumentException if bufferSize is less than 1 or not a power of 2     */    RingBuffer(        EventFactory<E> eventFactory,        Sequencer sequencer)    {        super(eventFactory, sequencer);    }    /**     * Create a new multiple producer RingBuffer with the specified wait strategy.     *     * @param factory      used to create the events within the ring buffer.     * @param bufferSize   number of elements to create within the ring buffer.     * @param waitStrategy used to determine how to wait for new elements to become available.     * @throws IllegalArgumentException if bufferSize is less than 1 or not a power of 2     * @see MultiProducerSequencer     */    public static <E> RingBuffer<E> createMultiProducer(        EventFactory<E> factory,        int bufferSize,        WaitStrategy waitStrategy)    {        MultiProducerSequencer sequencer = new MultiProducerSequencer(bufferSize, waitStrategy);        return new RingBuffer<E>(factory, sequencer);    }    /**     * Create a new multiple producer RingBuffer using the default wait strategy  {@link BlockingWaitStrategy}.     *     * @param factory    used to create the events within the ring buffer.     * @param bufferSize number of elements to create within the ring buffer.     * @throws IllegalArgumentException if <tt>bufferSize</tt> is less than 1 or not a power of 2     * @see MultiProducerSequencer     */    public static <E> RingBuffer<E> createMultiProducer(EventFactory<E> factory, int bufferSize)    {        return createMultiProducer(factory, bufferSize, new BlockingWaitStrategy());    }    /**     * Create a new single producer RingBuffer with the specified wait strategy.     *     * @param factory      used to create the events within the ring buffer.     * @param bufferSize   number of elements to create within the ring buffer.     * @param waitStrategy used to determine how to wait for new elements to become available.     * @throws IllegalArgumentException if bufferSize is less than 1 or not a power of 2     * @see SingleProducerSequencer     */    public static <E> RingBuffer<E> createSingleProducer(        EventFactory<E> factory,        int bufferSize,        WaitStrategy waitStrategy)    {        SingleProducerSequencer sequencer = new SingleProducerSequencer(bufferSize, waitStrategy);        return new RingBuffer<E>(factory, sequencer);    }    /**     * Create a new single producer RingBuffer using the default wait strategy  {@link BlockingWaitStrategy}.     *     * @param factory    used to create the events within the ring buffer.     * @param bufferSize number of elements to create within the ring buffer.     * @throws IllegalArgumentException if <tt>bufferSize</tt> is less than 1 or not a power of 2     * @see MultiProducerSequencer     */    public static <E> RingBuffer<E> createSingleProducer(EventFactory<E> factory, int bufferSize)    {        return createSingleProducer(factory, bufferSize, new BlockingWaitStrategy());    }    /**     * Create a new Ring Buffer with the specified producer type (SINGLE or MULTI)     *     * @param producerType producer type to use {@link ProducerType}.     * @param factory      used to create events within the ring buffer.     * @param bufferSize   number of elements to create within the ring buffer.     * @param waitStrategy used to determine how to wait for new elements to become available.     * @throws IllegalArgumentException if bufferSize is less than 1 or not a power of 2     */    public static <E> RingBuffer<E> create(        ProducerType producerType,        EventFactory<E> factory,        int bufferSize,        WaitStrategy waitStrategy)    {        switch (producerType)        {            case SINGLE:                return createSingleProducer(factory, bufferSize, waitStrategy);            case MULTI:                return createMultiProducer(factory, bufferSize, waitStrategy);            default:                throw new IllegalStateException(producerType.toString());        }    }

用户组装一个RingBuffer需要如下元素：实现EventFactory的Event的工厂，实现Sequencer的生产者，等待策略waitStrategy还有bufferSize。
接下来里面方法的实现都比较简单，这里不再赘述