Displaying the Failure in Java

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We added an auctionFailed() method to AuctionEventListener while unittesting AuctionMessageTranslator This triggers a compiler warning in AuctionSniper, so we added an empty implementation to keep going Now it s time to make it work, which turns out to be easy We write some tests in AuctionSniperTest for the new state transitions, for example:
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@Test public void reportsFailedIfAuctionFailsWhenBidding() { ignoringAuction(); allowingSniperBidding(); expectSniperToFailWhenItIs("bidding"); snipercurrentPrice(123, 45, PriceSourceFromOtherBidder); sniperauctionFailed(); } private void expectSniperToFailWhenItIs(final String state) { contextchecking(new Expectations() {{ atLeast(1)of(sniperListener)sniperStateChanged( new SniperSnapshot(ITEM_ID, 00, 0, SniperStateFAILED)); when(sniperStateis(state)); }}); }
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Disconnecting the Sniper We ve added a couple more helper methods: ignoringAuction() says that we don t care what happens to auction, allowing events to pass through so we can get to the failure; and, expectSniperToFailWhenItIs() describes what a failure should look like, including the previous state of the Sniper All we have to do is add a failed() transition to SniperSnapshot and use it in the new method
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public class AuctionSniper implements AuctionEventListener { public void auctionFailed() { snapshot = snapshotfailed(); listenersannounce()sniperStateChanged(snapshot); } [ ] public class SniperSnapshot { public SniperSnapshot failed() { return new SniperSnapshot(itemId, 0, 0, SniperStateFAILED); } [ ]
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This displays the failure, as we can see in Figure 191
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The Sniper shows a failed auction
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The end-to-end test, however, still fails The synchronization hook we added reveals that we haven t disconnected the Sniper from receiving further events from the auction
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Disconnecting the Sniper
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We turn off a Sniper by removing its AuctionMessageTranslator from its Chat s set of MessageListeners We can do this safely while processing a message because Chat stores its listeners in a thread-safe copy on write collection One obvious place to do this is within processMessage() in AuctionMessageTranslator, which receives the Chat as an argument, but we have two doubts about this First, as we pointed out in 12, constructing a real Chat is painful Most of the mocking frameworks support creating a mock class, but it makes us uncomfortable because then we re de ning a relationship with an implementation, not a role we re being too precise about our dependencies Second, we might be assigning too many responsibilities to AuctionMessageTranslator; it would have to translate the message and decide what to do when it fails
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Our alternative approach is to attach another object to the translator that implements this disconnection policy, using the infrastructure we already have for notifying AuctionEventListeners
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public final class XMPPAuction implements Auction { public XMPPAuction(XMPPConnection connection, String auctionJID) { AuctionMessageTranslator translator = translatorFor(connection); thischat = connectiongetChatManager()createChat(auctionJID, translator); addAuctionEventListener(chatDisconnectorFor(translator)); } private AuctionMessageTranslator translatorFor(XMPPConnection connection) { return new AuctionMessageTranslator(connectiongetUser(), auctionEventListenersannounce()); }
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private AuctionEventListener chatDisconnectorFor(final AuctionMessageTranslator translator) { return new AuctionEventListener() { public void auctionFailed() { chatremoveMessageListener(translator); } public void auctionClosed(// empty method public void currentPrice( // empty method }; } [ ]
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The end-to-end test, as far as it goes, passes
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The issue in this design episode is not the fundamental complexity of the feature, which is constant, but how we divide it up The design we chose (attaching a disconnection listener) could be argued to be more complicated than its alternative (detaching the chat within the translator) It certainly takes more lines of code, but that s not the only metric Instead, we re emphasizing the single responsibility principle, which means each object does just one thing well and the system behavior comes from how we assemble those objects Sometimes this feels as if the behavior we re looking for is always somewhere else (as Gertrude Stein said, There is no there there ), which can be frustrating for developers not used to the style Our experience, on the other hand, is that focused responsibilities make the code more maintainable because we don t have to cut through unrelated functionality to get to the piece we need See 6 for a longer discussion
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