Polling for Changes in Java

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Polling for Changes
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A sample-based assertion repeatedly samples some visible effect of the system through a probe, waiting for the probe to detect that the system has entered an expected state There are two aspects to the process of sampling: polling the system and failure reporting, and probing the system for a given state Separating the two helps us think clearly about the behavior, and different tests can reuse the polling with different probes Poller is an example of how to poll a system It repeatedly calls its probe, with a short delay between samples, until the system is ready or the poller times out The poller drives a probe that actually checks the target system, which we ve abstracted behind a Probe interface
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public interface Probe { boolean isSatisfied(); void sample(); void describeFailureTo(Description d); }
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The probe s sample() method takes a snapshot of the system state that the test is interested in The isSatisfied() method returns true if that state meets the test s acceptance criteria To simplify the poller logic, we allow isSatisfied() to be called before sample()
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public class Poller { private long timeoutMillis; private long pollDelayMillis; // constructors and accessors to configure the timeout [ ] public void check(Probe probe) throws InterruptedException { Timeout timeout = new Timeout(timeoutMillis); while (! probeisSatisfied()) { if (timeouthasTimedOut()) { throw new AssertionError(describeFailureOf(probe)); } Threadsleep(pollDelayMillis); probesample(); } } private String describeFailureOf(Probe probe) { [ ] }
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This simple implementation delegates synchronization with the system to the probe A more sophisticated version might implement synchronization in the poller, so it could be shared between probes The similarity to NotificationTrace is obvious, and we could have pulled out a common abstract structure, but we wanted to keep the designs clear for now To poll, for example, for the length of a le, we would write this line in a test:
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assertEventually(fileLength("datatxt", is(greaterThan(2000))));
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This wraps up the construction of our sampling code in a more expressive assertion The helper methods to implement this are:
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public static void assertEventually(Probe probe) throws InterruptedException { new Poller(1000L, 100L)check(probe); } public static Probe fileLength(String path, final Matcher<Integer> matcher) { final File file = new File(path); return new Probe() { private long lastFileLength = NOT_SET; public void sample() { lastFileLength = filelength(); } public boolean isSatisfied() { return lastFileLength != NOT_SET && matchermatches(lastFileLength); } public void describeFailureTo(Description d) { dappendText("length was ")appendValue(lastFileLength); } }; }
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Separating the act of sampling from checking whether the sample is satisfactory makes the structure of the probe clearer We can hold on to the sample result to report the unsatisfactory result we found if there s a failure
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Testing Asynchronous Code
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Timing Out
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Finally we show the Timeout class that the two example assertion classes use It packages up time checking and synchronization:
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public class Timeout { private final long endTime; public Timeout(long duration) { thisendTime = SystemcurrentTimeMillis() + duration; } public boolean hasTimedOut() { return timeRemaining() <= 0; } public void waitOn(Object lock) throws InterruptedException { long waitTime = timeRemaining(); if (waitTime > 0) lockwait(waitTime); } private long timeRemaining() { return endTime - SystemcurrentTimeMillis(); } }
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Retro tting a Probe
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We can now rewrite the test from the introduction Instead of making an assertion about the current holding of a stock, the test must wait for the holding of the stock to reach the expected level within an acceptable time limit
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@Test public void buyAndSellOfSameStockOnSameDayCancelsOutOurHolding() { Date tradeDate = new Date(); send(aTradeEvent()ofType(BUY)onDate(tradeDate)forStock("A")withQuantity(10)); send(aTradeEvent()ofType(SELL)onDate(tradeDate)forStock("A")withQuantity(10)); assertEventually(holdingOfStock("A", tradeDate, equalTo(0))); }
Previously, the holdingOfStock() method returned a value to be compared Now it returns a Probe that samples the system s holding and returns if it meets the acceptance criteria de ned by a Hamcrest matcher in this case equalTo(0)