How to test method using sleep () function with Java?

Question

I have the following method, and I'm trying my best to get 100% code coverage.

public final class SleepingHelper {
    public static void sleepInMillis(Duration timeOfNextTry) {
        try {
            Thread.sleep(timeOfNextTry.toMillis());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

The question is how to make an Thread.sleepexception be thrown?

Edit: since it was marked as a duplicate, I'm still wondering what I will claim in the test? Another question is more general.

Answer 1

You need to interrupt it from another thread. For example:

 Thread t = new Thread() {
     public void run () {
        SleeperMillis.sleepInMillis(new Duration(10000000l));
     }
 }.start();
 Thread.sleep(100); // let the other thread start
 t.interrupt;

Answer 2

You do not need to actually interrupt the stream. You can use PowerMockito to mock a static methodThread.sleep()

@RunWith(PowerMockRunner.class)
@PrepareForTest(Thread.class)
public class TestClass {

    @Test
    public void testSleepInMillis() throws Exception {
        PowerMockito.mockStatic(Thread.class);
        PowerMockito.doThrow(new InterruptedException ()).when(Thread.class);

        try {
            SleepHelper.sleepInMillis(11);
            fail("expected exception");
        } catch (InterruptedException e) {
            System.out.println("all good");
        }

    }

Answer 3

, , , Thread.sleep , .

Mocking - .

Btw, , , , Thread.sleep , . Thread.sleep(some magic number goes here) .

Answer 4

. 100% . :

@Test
public void testException() throws Exception {

    // Capture the system error stream, so that we can test that the expected exception is printed.
    ByteArrayOutputStream capturedErrors = new ByteArrayOutputStream();
    System.setErr(new PrintStream(capturedErrors));

    // Create a new thread on which to run the candidate method.
    Thread thread = new Thread() {
        @Override
        public void run() {
            SleepingHelper.sleepInMillis(Duration.ofMillis(10));
        }
    };

    // Start the second thread.
    thread.start();

    // Interrupt the second thread. (The candidate method hasn't finished yet. It takes 10 milliseconds to run.)
    thread.interrupt();

    // Wait for the thread to die (and write out the stack-trace).
    thread.join();

    // Test that the expected exception stack trace was printed to the system error stream.
    // The output should start with the exception name.
    String output = capturedErrors.toString();
    int lengthOfExceptionName = "java.lang.InterruptedException".length();
    assertEquals(output.substring(0, lengthOfExceptionName), "java.lang.InterruptedException");
}