Runaway Tests in Java

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Unfortunately, the new version of the test is still unreliable, even though we re now sampling for a result The assertion is waiting for the holding to become zero, which is what we started out with, so it s possible for the test to pass before the system has even begun processing This test can run ahead of the system without actually testing anything
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Lost Updates The worst aspect of runaway tests is that they give false positive results, so broken code looks like it s working We don t often review tests that pass, so it s easy to miss this kind of failure until something breaks down the line Even more tricky, the code might have worked when we rst wrote it, as the tests happened to synchronize correctly during development, but now it s broken and we can t tell
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Beware of Tests That Return the System to the Same State
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Be careful when an asynchronous test asserts that the system returns to a previous state Unless it also asserts that the system enters an intermediate state before asserting the initial state, the test will run ahead of the system
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To stop the test running ahead of the system, we must add assertions that wait for the system to enter an intermediate state Here, for example, we make sure that the rst trade event has been processed before asserting the effect of the second event:
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@Test public void buyAndSellOfSameStockOnSameDayCancelsOutOurHolding() { Date tradeDate = new Date(); send(aTradeEvent()ofType(BUY)onDate(tradeDate)forStock("A")withQuantity(10)); assertEventually(holdingOfStock("A", tradeDate, equalTo(10))); send(aTradeEvent()ofType(SELL)onDate(tradeDate)forStock("A")withQuantity(10)); assertEventually(holdingOfStock("A", tradeDate, equalTo(0))); }
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Similarly, in 14, we check all the displayed states in the acceptance tests for the Auction Sniper user interface:
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auctionreportPrice(1098, 97, ApplicationRunnerSNIPER_XMPP_ID); applicationhasShownSniperIsWinning(); auctionannounceClosed(); applicationhasShownSniperHasWon();
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We want to make sure that the sniper has responded to each message before continuing on to the next one
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Lost Updates
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A signi cant difference between tests that sample and those that listen for events is that polling can miss state changes that are later overwritten, Figure 271
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Testing Asynchronous Code
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A test that polls can miss changes in the system under test
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If the test can record noti cations from the system, it can look through its records to nd signi cant noti cations
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A test that records noti cations will not lose updates
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To be reliable, a sampling test must make sure that its system is stable before triggering any further interactions Sampling tests need to be structured as a series of phases, as shown in Figure 273 In each phase, the test sends a stimulus to prompt a change in the observable state of the system, and then waits until that change becomes visible or times out
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Phases of a sampling test
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Testing That an Action Has No Effect This shows the limits of how precise we can be with a sampling test All the test can do between stimulate and sample is wait We can write more reliable tests by not confusing the different steps in the loop and only triggering further changes once we ve detected that the system is stable by observing a change in its sampled state
Testing That an Action Has No Effect
Asynchronous tests look for changes in a system, so to test that something has not changed takes a little ingenuity Synchronous tests don t have this problem because they completely control the execution of the tested code After invoking the target object, synchronous tests can query its state or check that it hasn t made any unexpected calls to its neighbors If an asynchronous test waits for something not to happen, it cannot even be sure that the system has started before it checks the result For example, if we want to show that trades in another region are not counted in the stock holding, then this test:
@Test public void doesNotShowTradesInOtherRegions() { send(aTradeEvent()ofType(BUY)forStock("A")withQuantity(10) inTradingRegion(OTHER_REGION)); assertEventually(holdingOfStock("A", tradeDate, equalTo(0))); }
cannot tell whether the system has correctly ignored the trade or just not received it yet The most obvious workaround is for the test to wait for a xed period of time and then check that the unwanted event did not occur Unfortunately, this makes the test run slowly even when successful, and so breaks our rule of succeed fast Instead, the test should trigger a behavior that is detectable and use that to detect that the system has stabilized The skill here is in picking a behavior that will not interfere with the test s assertions and that will complete after the tested behavior For example, we could add another trade event to the regions example This shows that the out-of-region event is excluded because its quantity is not included in the total holding
@Test public void doesNotShowTradesInOtherRegions() { send(aTradeEvent()ofType(BUY)forStock("A")withQuantity(10) inTradingRegion(OTHER_REGION)); send(aTradeEvent()ofType(BUY)forStock("A")withQuantity(66) inTradingRegion(SAME_REGION)); assertEventually(holdingOfStock("A", tradeDate, equalTo(66))); }
Of course, this test assumes that trade events are processed in sequence, not in parallel, so that the second event cannot overtake the rst and give a false positive That s why such tests are not completely black box but have to make assumptions about the structure of the system This might make these tests