JUnit 5: How to assert an exception is thrown?

问题:

Is there a better way to assert that a method throws an exception in JUnit 5?

Currently, I have to use an @Rule in order to verify that my test throws an exception, but this doesn't work for the cases where I expect multiple methods to throw exceptions in my test.

回答1:

You can use assertThrows(), which allows you to test multiple exceptions within the same test. With support for lambdas in Java 8, this is probably going to become the canonical way to test for exceptions in JUnit.

Per the JUnit docs:

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

...

@Test
void exceptionTesting() {
    Executable closureContainingCodeToTest = () -> throw new IllegalArgumentException("a message");

    assertThrows(IllegalArgumentException.class, closureContainingCodeToTest, "a message");
}

回答2:

In Java 8 and JUnit 5 (Jupiter) we can assert for exceptions as follows. Using org.junit.jupiter.api.Assertions.assertThrows

public static < T extends Throwable > T assertThrows(Class< T > expectedType, Executable executable)

Asserts that execution of the supplied executable throws an exception of the expectedType and returns the exception.

If no exception is thrown, or if an exception of a different type is thrown, this method will fail.

If you do not want to perform additional checks on the exception instance, simply ignore the return value.

@Test
public void itShouldThrowNullPointerExceptionWhenBlahBlah() {
    assertThrows(NullPointerException.class,
            ()->{
            //do whatever you want to do here
            //ex : objectName.thisMethodShoulThrowNullPointerExceptionForNullParameter(null);
            });
}

That approach will use the Functional Interface Executable in org.junit.jupiter.api.

Refer :

• http://junit.org/junit5/docs/current/user-guide/#writing-tests-assertions
• http://junit.org/junit5/docs/5.0.0-M2/api/org/junit/jupiter/api/Executable.html
• http://junit.org/junit5/docs/5.0.0-M4/api/org/junit/jupiter/api/Assertions.html#assertThrows-java.lang.Class-org.junit.jupiter.api.function.Executable-

回答3:

They've changed it in JUnit 5 (expected: InvalidArgumentException, actual: invoked method) and code looks like this one:

@Test
public void wrongInput() {
    Throwable exception = assertThrows(InvalidArgumentException.class,
            ()->{objectName.yourMethod("WRONG");} );
}

回答4:

Now Junit5 provides a way to assert the exceptions

You can test both general exceptions and customized exceptions

A general exception scenario:

ExpectGeneralException.java

public void validateParameters(Integer param ) {
    if (param == null) {
        throw new NullPointerException("Null parameters are not allowed");
    }
}

ExpectGeneralExceptionTest.java

@Test
@DisplayName("Test assert NullPointerException")
void testGeneralException(TestInfo testInfo) {
    final ExpectGeneralException generalEx = new ExpectGeneralException();

     NullPointerException exception = assertThrows(NullPointerException.class, () -> {
            generalEx.validateParameters(null);
        });
    assertEquals("Null parameters are not allowed", exception.getMessage());
}

You can find a sample to test CustomException here : assert exception code sample

ExpectCustomException.java

public String constructErrorMessage(String... args) throws InvalidParameterCountException {
    if(args.length!=3) {
        throw new InvalidParameterCountException("Invalid parametercount: expected=3, passed="+args.length);
    }else {
        String message = "";
        for(String arg: args) {
            message += arg;
        }
        return message;
    }
}

ExpectCustomExceptionTest.java

@Test
@DisplayName("Test assert exception")
void testCustomException(TestInfo testInfo) {
    final ExpectCustomException expectEx = new ExpectCustomException();

     InvalidParameterCountException exception = assertThrows(InvalidParameterCountException.class, () -> {
            expectEx.constructErrorMessage("sample ","error");
        });
    assertEquals("Invalid parametercount: expected=3, passed=2", exception.getMessage());
}

回答5:

I think this is an even simpler example

List<String> emptyList = new ArrayList<>();
Optional<String> opt2 = emptyList.stream().findFirst();
assertThrows(NoSuchElementException.class, () -> opt2.get());

Calling get() on an optional containing an empty ArrayList will throw a NoSuchElementException. assertThrows declares the expected exception and provides a lambda supplier (takes no arguments and returns a value).

Thanks to @prime for his answer which I hopefully elaborated on.

回答6:

You can use assertThrows(). My example is taken from the docs http://junit.org/junit5/docs/current/user-guide/

import org.junit.jupiter.api.Test;

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

....

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

回答7:

Actually I think there is a error in the documentation for this particular example. The method that is intended is expectThrows

public static void assertThrows(
public static <T extends Throwable> T expectThrows(

回答8:

Here is an easy way.

@Test
void exceptionTest() {

   try{
        model.someMethod("invalidInput");
        fail("Exception Expected!");
   }
   catch(SpecificException e){

        assertTrue(true);
   }
   catch(Exception e){
        fail("wrong exception thrown");
   }

}

It only succeeds when the Exception you expect is thrown.