How do I assert my exception message with JUnit Te

问题:

I have written a few JUnit tests with @Test annotation. If my test method throws a checked exception and if I want to assert the message along with the exception, is there a way to do so with JUnit @Test annotation? AFAIK, JUnit 4.7 doesn't provide this feature but does any future versions provide it? I know in .NET you can assert the message and the exception class. Looking for similar feature in the Java world.

This is what I want:

@Test (expected = RuntimeException.class, message = "Employee ID is null")
public void shouldThrowRuntimeExceptionWhenEmployeeIDisNull() {}

回答1:

You could use the @Rule annotation with ExpectedException, like this:

@Rule
public ExpectedException expectedEx = ExpectedException.none();

@Test
public void shouldThrowRuntimeExceptionWhenEmployeeIDisNull() throws Exception {
    expectedEx.expect(RuntimeException.class);
    expectedEx.expectMessage("Employee ID is null");
    // do something that should throw the exception...
}

Note that the example in the ExpectedException docs is (currently) wrong - there's no public constructor, so you have to use ExpectedException.none().

回答2:

I like the @Rule answer. However, if for some reason you don't want to use rules. There is a third option.

@Test (expected = RuntimeException.class)
public void myTestMethod()
{
   try
   {
      //Run exception throwing operation here
   }
   catch(RuntimeException re)
   {
      String message = "Employee ID is null";
      assertEquals(message, re.getMessage());
      throw re;
    }
    fail("Employee Id Null exception did not throw!");
  }

回答3:

Do you have to use @Test(expected=SomeException.class)? When we have to assert the actual message of the exception, this is what we do.

@Test
public void myTestMethod()
{
  try
  {
    final Integer employeeId = null;
    new Employee(employeeId);
    fail("Should have thrown SomeException but did not!");
  }
  catch( final SomeException e )
  {
    final String msg = "Employee ID is null";
    assertEquals(msg, e.getMessage());
  }
}

回答4:

Actually, the best usage is with try/catch. Why? Because you can control the place where you expect the exception.

Consider this example:

@Test (expected = RuntimeException.class)
public void someTest() {
   // test preparation
   // actual test
}

What if one day the code is modified and test preparation will throw a RuntimeException? In that case actual test is not even tested and even if it doesn't throw any exception the test will pass.

That is why it is much better to use try/catch than to rely on the annotation.

回答5:

Raystorm had a good answer. I'm not a big fan of Rules either. I do something similar, except that I create the following utility class to help readability and usability, which is one of the big plus'es of annotations in the first place.

Add this utility class:

import org.junit.Assert;

public abstract class ExpectedRuntimeExceptionAsserter {

    private String expectedExceptionMessage;

    public ExpectedRuntimeExceptionAsserter(String expectedExceptionMessage) {
        this.expectedExceptionMessage = expectedExceptionMessage;
    }

    public final void run(){
        try{
            expectException();
            Assert.fail(String.format("Expected a RuntimeException '%s'", expectedExceptionMessage));
        } catch (RuntimeException e){
            Assert.assertEquals("RuntimeException caught, but unexpected message", expectedExceptionMessage, e.getMessage());
        }
    }

    protected abstract void expectException();

}

Then for my unit test, all I need is this code:

@Test
public void verifyAnonymousUserCantAccessPrivilegedResourceTest(){
    new ExpectedRuntimeExceptionAsserter("anonymous user can't access privileged resource"){
        @Override
        protected void expectException() {
            throw new RuntimeException("anonymous user can't access privileged resource");
        }
    }.run(); //passes test; expected exception is caught, and this @Test returns normally as "Passed"
}

回答6:

In JUnit 4.13 (once released) you can do:

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertThrows;

...

@Test
void exceptionTesting() {
  IllegalArgumentException exception = assertThrows(
    IllegalArgumentException.class, 
    () -> { throw new IllegalArgumentException("a message"); }
  );

  assertEquals("a message", exception.getMessage());
}

This also works in JUnit 5 but with different imports:

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;

...

回答7:

If using @Rule, the exception set is applied to all the test methods in the Test class.

回答8:

I like user64141's answer but found that it could be more generalized. Here's my take:

public abstract class ExpectedThrowableAsserter implements Runnable {

    private final Class<? extends Throwable> throwableClass;
    private final String expectedExceptionMessage;

    protected ExpectedThrowableAsserter(Class<? extends Throwable> throwableClass, String expectedExceptionMessage) {
        this.throwableClass = throwableClass;
        this.expectedExceptionMessage = expectedExceptionMessage;
    }

    public final void run() {
        try {
            expectException();
        } catch (Throwable e) {
            assertTrue(String.format("Caught unexpected %s", e.getClass().getSimpleName()), throwableClass.isInstance(e));
            assertEquals(String.format("%s caught, but unexpected message", throwableClass.getSimpleName()), expectedExceptionMessage, e.getMessage());
            return;
        }
        fail(String.format("Expected %s, but no exception was thrown.", throwableClass.getSimpleName()));
    }

    protected abstract void expectException();

}

Note that leaving the "fail" statement within the try block causes the related assertion exception to be caught; using return within the catch statement prevents this.

回答9:

Import the catch-exception library, and use that. It's much cleaner than the ExpectedException rule or a try-catch.

Example form their docs:

import static com.googlecode.catchexception.CatchException.*;
import static com.googlecode.catchexception.apis.CatchExceptionHamcrestMatchers.*;

// given: an empty list
List myList = new ArrayList();

// when: we try to get the first element of the list
catchException(myList).get(1);

// then: we expect an IndexOutOfBoundsException with message "Index: 1, Size: 0"
assertThat(caughtException(),
  allOf(
    instanceOf(IndexOutOfBoundsException.class),
    hasMessage("Index: 1, Size: 0"),
    hasNoCause()
  )
);