记一个低级错误

Java: jdk1.8.0_144

做测试时犯下了一个极其低级的错误，记录下来以警示自己。原计划测试线程池运算的时间和预期值，代码如下：

public class StudyCompletableFuture {
    private static int processors = Runtime.getRuntime().availableProcessors();
    static final int MAX_SIZE = 100000;
    private static final ExecutorService executorService = Executors.newFixedThreadPool(processors);

    public static Pair<Long, List<String>> executorService() throws InterruptedException {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        List<String> list = new ArrayList<>();
        for (int index = 0; index < MAX_SIZE; index++) {
            final String value = String.valueOf(index);
            executorService.execute(() -> {
                list.add(value);
            });
        }
        executorService.shutdown();
        executorService.awaitTermination(Integer.MAX_VALUE, TimeUnit.DAYS);
        stopWatch.stop();
        return ImmutablePair.of(stopWatch.getTime(), list);
    }

    @Test
    public void testExecutorService() throws Exception {
        Pair<Long, List<String>> pair = StudyCompletableFuture.executorService();
        System.out.println("executorService:" + pair.getLeft());
        Assert.assertEquals(StudyCompletableFuture.MAX_SIZE, pair.getRight().size());
    }
}

最终结果list的size总是不如预期，错误如下：

java.lang.AssertionError: expected:<100000> but was:<99746>
    at org.junit.Assert.fail(Assert.java:88)
    at org.junit.Assert.failNotEquals(Assert.java:834)
    at org.junit.Assert.assertEquals(Assert.java:645)
    at org.junit.Assert.assertEquals(Assert.java:631)
    at org.lxp.java8.StudyCompletableFuture.testExecutorService(StudyCompletableFuture.java:72)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

猜测一：ExecutorService丢失任务，导致少数thread未得到执行机会

猜测二：ExecutorService在shutdown时拒绝接收新任务，导致少数thread未得得到执行机会

最后在线程中记下日志才现其实所有线程都得到了执行机会，但因为使用了非线程安全的ArrayList导致返回值不符合预期，更换为Vector后一切问题都完美解决，最终代码如下：

public class StudyCompletableFuture {
    private static int processors = Runtime.getRuntime().availableProcessors();
    static final int MAX_SIZE = 100000;
    private static final ExecutorService executorService = Executors.newFixedThreadPool(processors);

    public static Pair<Long, List<String>> executorService() throws InterruptedException {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        List<String> list = new Vector<>();
        for (int index = 0; index < MAX_SIZE; index++) {
            final String value = String.valueOf(index);
            executorService.execute(() -> {
                list.add(value);
            });
        }
        executorService.shutdown();
        executorService.awaitTermination(Integer.MAX_VALUE, TimeUnit.DAYS);
        stopWatch.stop();
        return ImmutablePair.of(stopWatch.getTime(), list);
    }

    @Test
    public void testExecutorService() throws Exception {
        Pair<Long, List<String>> pair = StudyCompletableFuture.executorService();
        System.out.println("executorService:" + pair.getLeft());
        Assert.assertEquals(StudyCompletableFuture.MAX_SIZE, pair.getRight().size());
    }
}