Disruptor技術調研之配置EventHandler執行順序

一、handlerA必需要在handlerB前執行

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

二、handlerA必須在handlerB前執行

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

三、舉例：註冊handler1,handler2，而且配置handlerWithBarrier在handler1,handler2執行完成後執行

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

四、舉例：註冊handler1,handler2，而且配置handlerWithBarrier在handler1,handler2執行完成後執行。handler1和handler2能夠經過and註冊到同一個group

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

五、舉例：調用after配置執行順序時，須要先經過handleEventsWith註冊到Disruptor

@Test(expected = IllegalArgumentException.class)

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

}

六、舉例：EvilEqualsEventHandler重寫了equals和hashCode方法，下述的handler1和handler2具備相同的equals和hashCode返回值。可是Disruptor內部的handlers的註冊時經過IdentityHashMap存儲的，也就是根據對象的引用是否相等來判斷是否已註冊，而非對象是否相等。

@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)).) 

大體的意思是該類也是實現了Map接口，可是比較鍵值時使用reference-equality替代了object-equality比較的方法。只有當k1==k2才被認爲是相同鍵值。普通的實現了Map接口的鍵值比較相同時是經過k1==null ? k2==null : k1.equals(k2)來確認的。