How can I test exception in completable future?

Question

I have been converting some code to be asynchronous. The original unit test used the annotation @Test(expected = MyExcpetion.class) but I don't think this will work because the exception I want to assert on is wrapped in java.util.concurrent.ExcutionException . I did try calling my future like this but my assertion is still failing and I don't love that I had to add in return null

myApiCall.get(123).exceptionally((ex) -> {
 assertEquals(ex.getCause(),MyCustomException.class)
 return null
}

I also tried this flavor but still not working

myApiCall.get(123).exceptionally((ex) -> {
 assertThat(ex.getCause())
  .isInstanceOF(MyException.class)
  .hasMessage("expected message etc")
 return null;
}

My API just throws exception if it can't find id. How should I be properly testing this? Can I use that original annotation in anyway?

my api call reaches out to db when run. In this test I am setting up my future to return an error so it doesn't actually try to communicate with anything. the code under test looks like this

 public class myApiCall  { 
   public completableFuture get(final String id){
   return myService.getFromDB(id)
    .thenApply( 
       //code here looks at result and if happy path then returns it after 
       //doing some transformation 
      //otherwise it throws exception 
   )
  }
 }

in the unit test I force myService.getFromDB(id) to return bad data so I can test exception and also keep this a unit test don't reach out to db etc.

Answer 1

Assume that you have a class and you want to test a method which returns a completable future:

  public class A {
    private final Api api;

    public A(Api api) { this.api = api;}

    public CompletableFuture<Void> execute(Integer input) {
       final CompletableFuture<Void> future = api.execute(input)
                .thenApplyAsync(result -> doSomething())
                .exceptionally(ex -> doFailure());
       return future;
    }
 }

To test the execution of "doSomething()" then you may use mockito and do the following:

     // prepare test
     final Api api = mock(Api.class)
     final A a = new A(api);
     when(api.execute(any(Integer.class)))
          .thenReturn(CompletableFuture.completedFuture(null));

     // execute
     final CompletableFuture<Void> result = a.execute(input);

     // validate
     ...
         

To test "doFailure" do the following:

when(api.execute(any(Integer.class))).thenAnswer(answer -> {
     CompletableFuture<Void> future = new CompletableFuture<>();
     future.completeExceptionally(new RuntimeException());
     return future;
});

// execute
final CompletableFuture<Void> result = a.execute(input);

// validate
assertTrue(result.isCompletedExceptionally());

          

Answer 2

Let's assume your API throws if called with 0:

public static CompletableFuture<Integer> apiCall(int id) {
  return CompletableFuture.supplyAsync(() -> {
    if (id == 0) throw new RuntimeException("Please not 0!!");
    else return id;
  });
}

You can test that it works as expected with the following code (I'm using TestNG but I suspect it won't be too difficult to translate into a JUnit test):

@Test public void test_ok() throws Exception {
  CompletableFuture<Integer> result = apiCall(1);
  assertEquals(result.get(), (Integer) 1);
}

@Test(expectedExceptions = ExecutionException.class,
      expectedExceptionsMessageRegExp = ".*RuntimeException.*Please not 0!!")
public void test_ex() throws Throwable {
  CompletableFuture<Integer> result = apiCall(0);
  result.get();
}

Note that the second test uses the fact that the ExecutionException message will contain the original exception type and message and captures the expectation with a regex. If you can't do that with JUnit, you can call result.get() in a try/catch block and call throw e.getCause(); in the catch block. In other words, something like this:

@Test(expectedExceptions = RuntimeException.class,
      expectedExceptionsMessageRegExp = "Please not 0!!")
public void test_ex() throws Throwable {
  CompletableFuture<Integer> result = apiCall(0);
  try {
    result.get();
  } catch (ExecutionException e) {
    throw e.getCause();
  }
}