Disruptor技术调研之配置EventHandler执行顺序

一、handlerA必需要在handlerB前执行

dw.handleEventsWith(A).then(B);

二、handlerA必须在handlerB前执行

dw.after(A).handleEventsWith(B);

三、举例：注册handler1,handler2，而且配置handlerWithBarrier在handler1,handler2执行完成后执行

disruptor.handleEventsWith(handler1, handler2);
disruptor.after(handler1, handler2).handleEventsWith(handlerWithBarrier);

四、举例：注册handler1,handler2，而且配置handlerWithBarrier在handler1,handler2执行完成后执行。handler1和handler2能够经过and注册到同一个group

disruptor.handleEventsWith(handler1);
final EventHandlerGroup<TestEvent> handler2Group = disruptor.handleEventsWith(handler2);
disruptor.after(handler1).and(handler2Group).handleEventsWith(handlerWithBarrier);

五、举例：调用after配置执行顺序时，须要先经过handleEventsWith注册到Disruptor

@Test(expected = IllegalArgumentException.class)

public void shouldThrowExceptionIfHandlerIsNotAlreadyConsuming()
    throws Exception
{
    disruptor.after(createDelayedEventHandler()).handleEventsWith(createDelayedEventHandler());

}

六、举例：EvilEqualsEventHandler重写了equals和hashCode方法，下述的handler1和handler2具备相同的equals和hashCode返回值。可是Disruptor内部的handlers的注册时经过IdentityHashMap存储的，也就是根据对象的引用是否相等来判断是否已注册，而非对象是否相等。

@Test(expected = IllegalArgumentException.class)
public void shouldTrackEventHandlersByIdentityNotEquality()
    throws Exception
{
    EvilEqualsEventHandler handler1 = new EvilEqualsEventHandler();
    EvilEqualsEventHandler handler2 = new EvilEqualsEventHandler();

    disruptor.handleEventsWith(handler1);
    disruptor.after(handler2);

}

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java.util.IdentityHashMap<K, V>

This class implements the Map interface with a hash table, using reference-equality in place of object-equality when comparing keys (and values). In other words, in an IdentityHashMap, two keys k1 and k2 are considered equal if and only if (k1==k2). (In normal Map implementations (like HashMap) two keys k1 and k2 are considered equal if and only if (k1==null ? k2==null : k1.equals(k2)).) 

大体的意思是该类也是实现了Map接口，可是比较键值时使用reference-equality替代了object-equality比较的方法。只有当k1==k2才被认为是相同键值。普通的实现了Map接口的键值比较相同时是经过k1==null ? k2==null : k1.equals(k2)来确认的。