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.
This question is related to
java
junit
junit5
junit-jupiter
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.
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");} );
}
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());
}
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.
You can use assertThrows()
, But with assertThrows
your assertion will pass even if the thrown exception is of child type.
This is because, JUnit 5 checks exception type by calling Class.isIntance(..)
, Class.isInstance(..)
will return true even if the exception thrown is of the child types.
The workaround for this is to assert on Class:
Throwable throwable = assertThrows(Throwable.class, () -> {
service.readFile("sampleFile.txt");
});
assertEquals(FileNotFoundException.class, throwable.getClass());
An even simpler one liner. No lambda expressions or curly braces required for this example using Java 8 and JUnit 5
import static org.junit.jupiter.api.Assertions.assertThrows;
@Test
void exceptionTesting() {
assertThrows(MyException.class, myStackObject::doStackAction, "custom message if assertion fails...");
// note, no parenthesis on doStackAction ex ::pop NOT ::pop()
}
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(
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());
}
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 :
Source: Stackoverflow.com