How can I use JUnit4 idiomatically to test that some code throws an exception?
While I can certainly do something like this:
@Test
public void testFooThrowsIndexOutOfBoundsException() {
boolean thrown = false;
try {
foo.doStuff();
} catch (IndexOutOfBoundsException e) {
thrown = true;
}
assertTrue(thrown);
}
I recall that there is an annotation or an Assert.xyz or something that is far less kludgy and far more in-the-spirit of JUnit for these sorts of situations.
It depends on the JUnit version and what assert libraries you use.
The original answer for JUnit <= 4.12 was:
@Test(expected = IndexOutOfBoundsException.class)
public void testIndexOutOfBoundsException() {
ArrayList emptyList = new ArrayList();
Object o = emptyList.get(0);
}
Though answer https://stackoverflow.com/a/31826781/2986984 has more options for JUnit <= 4.12.
Reference :
Edit: Now that JUnit 5 and JUnit 4.13 have been released, the best option would be to use Assertions.assertThrows() (for JUnit 5) and Assert.assertThrows() (for JUnit 4.13+). See my other answer for details.
If you haven't migrated to JUnit 5, but can use JUnit 4.7, you can use the ExpectedException Rule:
public class FooTest {
@Rule
public final ExpectedException exception = ExpectedException.none();
@Test
public void doStuffThrowsIndexOutOfBoundsException() {
Foo foo = new Foo();
exception.expect(IndexOutOfBoundsException.class);
foo.doStuff();
}
}
This is much better than @Test(expected=IndexOutOfBoundsException.class) because the test will fail if IndexOutOfBoundsException is thrown before foo.doStuff()
See this article for details.
Be careful using expected exception, because it only asserts that the method threw that exception, not a particular line of code in the test.
I tend to use this for testing parameter validation, because such methods are usually very simple, but more complex tests might better be served with:
try {
methodThatShouldThrow();
fail( "My method didn't throw when I expected it to" );
} catch (MyException expectedException) {
}
Apply judgement.
As answered before, there are many ways of dealing with exceptions in JUnit. But with Java 8 there is another one: using Lambda Expressions. With Lambda Expressions we can achieve a syntax like this:
@Test
public void verifiesTypeAndMessage() {
assertThrown(new DummyService()::someMethod)
.isInstanceOf(RuntimeException.class)
.hasMessage("Runtime exception occurred")
.hasMessageStartingWith("Runtime")
.hasMessageEndingWith("occurred")
.hasMessageContaining("exception")
.hasNoCause();
}
assertThrown accepts a functional interface, whose instances can be created with lambda expressions, method references, or constructor references. assertThrown accepting that interface will expect and be ready to handle an exception.
This is relatively simple yet powerful technique.
Have a look at this blog post describing this technique: http://blog.codeleak.pl/2014/07/junit-testing-exception-with-java-8-and-lambda-expressions.html
The source code can be found here: https://github.com/kolorobot/unit-testing-demo/tree/master/src/test/java/com/github/kolorobot/exceptions/java8
Disclosure: I am the author of the blog and the project.
in junit, there are four ways to test exception.
for junit5.x, you can use assertThrows as following
@Test
public void testFooThrowsIndexOutOfBoundsException() {
Throwable exception = assertThrows(IndexOutOfBoundsException.class, () -> foo.doStuff());
assertEquals("expected messages", exception.getMessage());
}
for junit4.x, use the optional 'expected' attribute of Test annonation
@Test(expected = IndexOutOfBoundsException.class)
public void testFooThrowsIndexOutOfBoundsException() {
foo.doStuff();
}
for junit4.x, use the ExpectedException rule
public class XxxTest {
@Rule
public ExpectedException thrown = ExpectedException.none();
@Test
public void testFooThrowsIndexOutOfBoundsException() {
thrown.expect(IndexOutOfBoundsException.class)
//you can test the exception message like
thrown.expectMessage("expected messages");
foo.doStuff();
}
}
you also can use the classic try/catch way widely used under junit 3 framework
@Test
public void testFooThrowsIndexOutOfBoundsException() {
try {
foo.doStuff();
fail("expected exception was not occured.");
} catch(IndexOutOfBoundsException e) {
//if execution reaches here,
//it indicates this exception was occured.
//so we need not handle it.
}
}
so
for more info, you can read this document and junit5 user guide for details.
tl;dr
post-JDK8 : Use AssertJ or custom lambdas to assert exceptional behaviour.
pre-JDK8 : I will recommend the old good try-catch block. (Don't forget to add a fail() assertion before the catch block)
Regardless of Junit 4 or JUnit 5.
the long story
It is possible to write yourself a do it yourself try-catch block or use the JUnit tools (@Test(expected = ...) or the @Rule ExpectedException JUnit rule feature).
But these ways are not so elegant and don't mix well readability wise with other tools. Moreover, JUnit tooling does have some pitfalls.
The try-catch block you have to write the block around the tested behavior and write the assertion in the catch block, that may be fine but many find that this style interrupts the reading flow of a test. Also, you need to write an Assert.fail at the end of the try block. Otherwise, the test may miss one side of the assertions; PMD, findbugs or Sonar will spot such issues.
The @Test(expected = ...) feature is interesting as you can write less code and then writing this test is supposedly less prone to coding errors. But this approach is lacking in some areas.
Also as the expectation is placed around in the method, depending on how the tested code is written then the wrong part of the test code can throw the exception, leading to false-positive test and I'm not sure that PMD, findbugs or Sonar will give hints on such code.
@Test(expected = WantedException.class)
public void call2_should_throw_a_WantedException__not_call1() {
// init tested
tested.call1(); // may throw a WantedException
// call to be actually tested
tested.call2(); // the call that is supposed to raise an exception
}
The ExpectedException rule is also an attempt to fix the previous caveats, but it feels a bit awkward to use as it uses an expectation style, EasyMock users know very well this style. It might be convenient for some, but if you follow Behaviour Driven Development (BDD) or Arrange Act Assert (AAA) principles the ExpectedException rule won't fit in those writing style. Aside from that it may suffer from the same issue as the @Test way, depending on where you place the expectation.
@Rule ExpectedException thrown = ExpectedException.none()
@Test
public void call2_should_throw_a_WantedException__not_call1() {
// expectations
thrown.expect(WantedException.class);
thrown.expectMessage("boom");
// init tested
tested.call1(); // may throw a WantedException
// call to be actually tested
tested.call2(); // the call that is supposed to raise an exception
}
Even the expected exception is placed before the test statement, it breaks your reading flow if the tests follow BDD or AAA.
Also, see this comment issue on JUnit of the author of ExpectedException. JUnit 4.13-beta-2 even deprecates this mechanism:
Pull request #1519: Deprecate ExpectedException
The method Assert.assertThrows provides a nicer way for verifying exceptions. In addition, the use of ExpectedException is error-prone when used with other rules like TestWatcher because the order of rules is important in that case.
So these above options have all their load of caveats, and clearly not immune to coder errors.
There's a project I became aware of after creating this answer that looks promising, it's catch-exception.
As the description of the project says, it let a coder write in a fluent line of code catching the exception and offer this exception for the latter assertion. And you can use any assertion library like Hamcrest or AssertJ.
A rapid example taken from the home page :
// given: an empty list
List myList = new ArrayList();
// when: we try to get the first element of the list
when(myList).get(1);
// then: we expect an IndexOutOfBoundsException
then(caughtException())
.isInstanceOf(IndexOutOfBoundsException.class)
.hasMessage("Index: 1, Size: 0")
.hasNoCause();
As you can see the code is really straightforward, you catch the exception on a specific line, the then API is an alias that will use AssertJ APIs (similar to using assertThat(ex).hasNoCause()...). At some point the project relied on FEST-Assert the ancestor of AssertJ. EDIT: It seems the project is brewing a Java 8 Lambdas support.
Currently, this library has two shortcomings :
At the time of this writing, it is noteworthy to say this library is based on Mockito 1.x as it creates a mock of the tested object behind the scene. As Mockito is still not updated this library cannot work with final classes or final methods. And even if it was based on Mockito 2 in the current version, this would require to declare a global mock maker (inline-mock-maker), something that may not what you want, as this mock maker has different drawbacks that the regular mock maker.
It requires yet another test dependency.
These issues won't apply once the library supports lambdas. However, the functionality will be duplicated by the AssertJ toolset.
Taking all into account if you don't want to use the catch-exception tool, I will recommend the old good way of the try-catch block, at least up to the JDK7. And for JDK 8 users you might prefer to use AssertJ as it offers may more than just asserting exceptions.
With the JDK8, lambdas enter the test scene, and they have proved to be an interesting way to assert exceptional behaviour. AssertJ has been updated to provide a nice fluent API to assert exceptional behaviour.
And a sample test with AssertJ :
@Test
public void test_exception_approach_1() {
...
assertThatExceptionOfType(IOException.class)
.isThrownBy(() -> someBadIOOperation())
.withMessage("boom!");
}
@Test
public void test_exception_approach_2() {
...
assertThatThrownBy(() -> someBadIOOperation())
.isInstanceOf(Exception.class)
.hasMessageContaining("boom");
}
@Test
public void test_exception_approach_3() {
...
// when
Throwable thrown = catchThrowable(() -> someBadIOOperation());
// then
assertThat(thrown).isInstanceOf(Exception.class)
.hasMessageContaining("boom");
}
With a near-complete rewrite of JUnit 5, assertions have been improved a bit, they may prove interesting as an out of the box way to assert properly exception. But really the assertion API is still a bit poor, there's nothing outside assertThrows.
@Test
@DisplayName("throws EmptyStackException when peeked")
void throwsExceptionWhenPeeked() {
Throwable t = assertThrows(EmptyStackException.class, () -> stack.peek());
Assertions.assertEquals("...", t.getMessage());
}
As you noticed assertEquals is still returning void, and as such doesn't allow chaining assertions like AssertJ.
Also if you remember name clash with Matcher or Assert, be prepared to meet the same clash with Assertions.
I'd like to conclude that today (2017-03-03) AssertJ's ease of use, discoverable API, the rapid pace of development and as a de facto test dependency is the best solution with JDK8 regardless of the test framework (JUnit or not), prior JDKs should instead rely on try-catch blocks even if they feel clunky.
This answer has been copied from another question that don't have the same visibility, I am the same author.
Now that JUnit 5 and JUnit 4.13 have been released, the best option would be to use Assertions.assertThrows() (for JUnit 5) and Assert.assertThrows() (for JUnit 4.13). See
the Junit 5 User Guide.
Here is an example that verifies an exception is thrown, and uses Truth to make assertions on the exception message:
public class FooTest {
@Test
public void doStuffThrowsIndexOutOfBoundsException() {
Foo foo = new Foo();
IndexOutOfBoundsException e = assertThrows(
IndexOutOfBoundsException.class, foo::doStuff);
assertThat(e).hasMessageThat().contains("woops!");
}
}
The advantages over the approaches in the other answers are:
throws clauseA similar method will be added to org.junit Assert in JUnit 4.13.
How about this: catch a very general exception, make sure it makes it out of the catch block, then assert that the class of the exception is what you expect it to be. This assert will fail if a) the exception is of the wrong type (eg. if you got a Null Pointer instead) and b) the exception wasn't ever thrown.
public void testFooThrowsIndexOutOfBoundsException() {
Throwable e = null;
try {
foo.doStuff();
} catch (Throwable ex) {
e = ex;
}
assertTrue(e instanceof IndexOutOfBoundsException);
}
Using an AssertJ assertion, which can be used alongside JUnit:
import static org.assertj.core.api.Assertions.*;
@Test
public void testFooThrowsIndexOutOfBoundsException() {
Foo foo = new Foo();
assertThatThrownBy(() -> foo.doStuff())
.isInstanceOf(IndexOutOfBoundsException.class);
}
It's better than @Test(expected=IndexOutOfBoundsException.class) because it guarantees the expected line in the test threw the exception and lets you check more details about the exception, such as message, easier:
assertThatThrownBy(() ->
{
throw new Exception("boom!");
})
.isInstanceOf(Exception.class)
.hasMessageContaining("boom");
import static com.googlecode.catchexception.apis.BDDCatchException.*;
@Test
public void testFooThrowsIndexOutOfBoundsException() {
when(() -> foo.doStuff());
then(caughtException()).isInstanceOf(IndexOutOfBoundsException.class);
}
eu.codearte.catch-exception:catch-exception:2.0
Update: JUnit5 has an improvement for exceptions testing: assertThrows.
The following example is from: Junit 5 User Guide
@Test
void exceptionTesting() {
Throwable exception = assertThrows(IllegalArgumentException.class, () ->
{
throw new IllegalArgumentException("a message");
});
assertEquals("a message", exception.getMessage());
}
Original answer using JUnit 4.
There are several ways to test that an exception is thrown. I have also discussed the below options in my post How to write great unit tests with JUnit
Set the expected parameter @Test(expected = FileNotFoundException.class).
@Test(expected = FileNotFoundException.class)
public void testReadFile() {
myClass.readFile("test.txt");
}
Using try catch
public void testReadFile() {
try {
myClass.readFile("test.txt");
fail("Expected a FileNotFoundException to be thrown");
} catch (FileNotFoundException e) {
assertThat(e.getMessage(), is("The file test.txt does not exist!"));
}
}
Testing with ExpectedException Rule.
@Rule
public ExpectedException thrown = ExpectedException.none();
@Test
public void testReadFile() throws FileNotFoundException {
thrown.expect(FileNotFoundException.class);
thrown.expectMessage(startsWith("The file test.txt"));
myClass.readFile("test.txt");
}
You could read more about exceptions testing in JUnit4 wiki for Exception testing and bad.robot - Expecting Exceptions JUnit Rule.
To solve the same problem I did set up a small project: http://code.google.com/p/catch-exception/
Using this little helper you would write
verifyException(foo, IndexOutOfBoundsException.class).doStuff();
This is less verbose than the ExpectedException rule of JUnit 4.7. In comparison to the solution provided by skaffman, you can specify in which line of code you expect the exception. I hope this helps.
IMHO, the best way to check for exceptions in JUnit is the try/catch/fail/assert pattern:
// this try block should be as small as possible,
// as you want to make sure you only catch exceptions from your code
try {
sut.doThing();
fail(); // fail if this does not throw any exception
} catch(MyException e) { // only catch the exception you expect,
// otherwise you may catch an exception for a dependency unexpectedly
// a strong assertion on the message,
// in case the exception comes from anywhere an unexpected line of code,
// especially important if your checking IllegalArgumentExceptions
assertEquals("the message I get", e.getMessage());
}
The assertTrue might be a bit strong for some people, so assertThat(e.getMessage(), containsString("the message"); might be preferable.
@Test
void testFooThrowsIndexOutOfBoundsException() {
Throwable exception = expectThrows( IndexOutOfBoundsException.class, foo::doStuff );
assertEquals( "some message", exception.getMessage() );
}
More Infos about JUnit 5 on http://junit.org/junit5/docs/current/user-guide/#writing-tests-assertions
The most flexible and elegant answer for Junit 4 I found in the Mkyong blog. It has the flexibility of the try/catch using the @Rule annotation. I like this approach because you can read specific attributes of a customized exception.
package com.mkyong;
import com.mkyong.examples.CustomerService;
import com.mkyong.examples.exception.NameNotFoundException;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;
import static org.hamcrest.CoreMatchers.containsString;
import static org.hamcrest.CoreMatchers.is;
import static org.hamcrest.Matchers.hasProperty;
public class Exception3Test {
@Rule
public ExpectedException thrown = ExpectedException.none();
@Test
public void testNameNotFoundException() throws NameNotFoundException {
//test specific type of exception
thrown.expect(NameNotFoundException.class);
//test message
thrown.expectMessage(is("Name is empty!"));
//test detail
thrown.expect(hasProperty("errCode")); //make sure getters n setters are defined.
thrown.expect(hasProperty("errCode", is(666)));
CustomerService cust = new CustomerService();
cust.findByName("");
}
}
I tried many of the methods here, but they were either complicated or didn't quite meet my requirements. In fact, one can write a helper method quite simply:
public class ExceptionAssertions {
public static void assertException(BlastContainer blastContainer ) {
boolean caughtException = false;
try {
blastContainer.test();
} catch( Exception e ) {
caughtException = true;
}
if( !caughtException ) {
throw new AssertionFailedError("exception expected to be thrown, but was not");
}
}
public static interface BlastContainer {
public void test() throws Exception;
}
}
Use it like this:
assertException(new BlastContainer() {
@Override
public void test() throws Exception {
doSomethingThatShouldExceptHere();
}
});
Zero dependencies: no need for mockito, no need powermock; and works just fine with final classes.
If you would like a solution which:
Here is a utility function that I wrote:
public final <T extends Throwable> T expectException( Class<T> exceptionClass, Runnable runnable )
{
try
{
runnable.run();
}
catch( Throwable throwable )
{
if( throwable instanceof AssertionError && throwable.getCause() != null )
throwable = throwable.getCause(); //allows testing for "assert x != null : new IllegalArgumentException();"
assert exceptionClass.isInstance( throwable ) : throwable; //exception of the wrong kind was thrown.
assert throwable.getClass() == exceptionClass : throwable; //exception thrown was a subclass, but not the exact class, expected.
@SuppressWarnings( "unchecked" )
T result = (T)throwable;
return result;
}
assert false; //expected exception was not thrown.
return null; //to keep the compiler happy.
}
Use it as follows:
@Test
public void testMyFunction()
{
RuntimeException e = expectException( RuntimeException.class, () ->
{
myFunction();
} );
assert e.getMessage().equals( "I haz fail!" );
}
public void myFunction()
{
throw new RuntimeException( "I haz fail!" );
}
In my case I always get RuntimeException from db, but messages differ. And exception need to be handled respectively. Here is how I tested it:
@Test
public void testThrowsExceptionWhenWrongSku() {
// Given
String articleSimpleSku = "999-999";
int amountOfTransactions = 1;
Exception exception = null;
// When
try {
createNInboundTransactionsForSku(amountOfTransactions, articleSimpleSku);
} catch (RuntimeException e) {
exception = e;
}
// Then
shouldValidateThrowsExceptionWithMessage(exception, MESSAGE_NON_EXISTENT_SKU);
}
private void shouldValidateThrowsExceptionWithMessage(final Exception e, final String message) {
assertNotNull(e);
assertTrue(e.getMessage().contains(message));
}
Just make a Matcher that can be turned off and on, like this:
public class ExceptionMatcher extends BaseMatcher<Throwable> {
private boolean active = true;
private Class<? extends Throwable> throwable;
public ExceptionMatcher(Class<? extends Throwable> throwable) {
this.throwable = throwable;
}
public void on() {
this.active = true;
}
public void off() {
this.active = false;
}
@Override
public boolean matches(Object object) {
return active && throwable.isAssignableFrom(object.getClass());
}
@Override
public void describeTo(Description description) {
description.appendText("not the covered exception type");
}
}
To use it:
add public ExpectedException exception = ExpectedException.none();,
then:
ExceptionMatcher exMatch = new ExceptionMatcher(MyException.class);
exception.expect(exMatch);
someObject.somethingThatThrowsMyException();
exMatch.off();
In JUnit 4 or later you can test the exceptions as follows
@Rule
public ExpectedException exceptions = ExpectedException.none();
this provides a lot of features which can be used to improve our JUnit tests.
If you see the below example I am testing 3 things on the exception.
public class MyTest {
@Rule
public ExpectedException exceptions = ExpectedException.none();
ClassUnderTest classUnderTest;
@Before
public void setUp() throws Exception {
classUnderTest = new ClassUnderTest();
}
@Test
public void testAppleisSweetAndRed() throws Exception {
exceptions.expect(Exception.class);
exceptions.expectMessage("this is the exception message");
exceptions.expectCause(Matchers.<Throwable>equalTo(exceptionCause));
classUnderTest.methodUnderTest("param1", "param2");
}
}
We can use an assertion fail after the method that must return an exception:
try{
methodThatThrowMyException();
Assert.fail("MyException is not thrown !");
} catch (final Exception exception) {
// Verify if the thrown exception is instance of MyException, otherwise throws an assert failure
assertTrue(exception instanceof MyException, "An exception other than MyException is thrown !");
// In case of verifying the error message
MyException myException = (MyException) exception;
assertEquals("EXPECTED ERROR MESSAGE", myException.getMessage());
}
Additionally to what NamShubWriter has said, make sure that:
Do not do this:
@Rule
public ExpectedException expectedException;
@Before
public void setup()
{
expectedException = ExpectedException.none();
}
Finally, this blog post clearly illustrates how to assert that a certain exception is thrown.
Junit4 solution with Java8 is to use this function:
public Throwable assertThrows(Class<? extends Throwable> expectedException, java.util.concurrent.Callable<?> funky) {
try {
funky.call();
} catch (Throwable e) {
if (expectedException.isInstance(e)) {
return e;
}
throw new AssertionError(
String.format("Expected [%s] to be thrown, but was [%s]", expectedException, e));
}
throw new AssertionError(
String.format("Expected [%s] to be thrown, but nothing was thrown.", expectedException));
}
Usage is then:
assertThrows(ValidationException.class,
() -> finalObject.checkSomething(null));
Note that the only limitation is to use a final object reference in lambda expression.
This solution allows to continue test assertions instead of expecting thowable at method level using @Test(expected = IndexOutOfBoundsException.class) solution.
I recomend library assertj-core to handle exception in junit test
In java 8, like this:
//given
//when
Throwable throwable = catchThrowable(() -> anyService.anyMethod(object));
//then
AnyException anyException = (AnyException) throwable;
assertThat(anyException.getMessage()).isEqualTo("........");
assertThat(exception.getCode()).isEqualTo(".......);
Take for example, you want to write Junit for below mentioned code fragment
public int divideByZeroDemo(int a,int b){
return a/b;
}
public void exceptionWithMessage(String [] arr){
throw new ArrayIndexOutOfBoundsException("Array is out of bound");
}
The above code is to test for some unknown exception that may occur and the below one is to assert some exception with custom message.
@Rule
public ExpectedException exception=ExpectedException.none();
private Demo demo;
@Before
public void setup(){
demo=new Demo();
}
@Test(expected=ArithmeticException.class)
public void testIfItThrowsAnyException() {
demo.divideByZeroDemo(5, 0);
}
@Test
public void testExceptionWithMessage(){
exception.expectMessage("Array is out of bound");
exception.expect(ArrayIndexOutOfBoundsException.class);
demo.exceptionWithMessage(new String[]{"This","is","a","demo"});
}
With Java 8 you can create a method taking a code to check and expected exception as parameters:
private void expectException(Runnable r, Class<?> clazz) {
try {
r.run();
fail("Expected: " + clazz.getSimpleName() + " but not thrown");
} catch (Exception e) {
if (!clazz.isInstance(e)) fail("Expected: " + clazz.getSimpleName() + " but " + e.getClass().getSimpleName() + " found", e);
}
}
and then inside your test:
expectException(() -> list.sublist(0, 2).get(2), IndexOutOfBoundsException.class);
Benefits:
@Test(expectedException=IndexOutOfBoundsException.class)
public void testFooThrowsIndexOutOfBoundsException() throws Exception {
doThrow(IndexOutOfBoundsException.class).when(foo).doStuff();
try {
foo.doStuff();
} catch (IndexOutOfBoundsException e) {
assertEquals(IndexOutOfBoundsException .class, ex.getCause().getClass());
throw e;
}
}
Here is another way to check method thrown correct exception or not.
JUnit framework has assertThrows() method:
ArithmeticException exception = assertThrows(ArithmeticException.class, () ->
calculator.divide(1, 0));
assertEquals("/ by zero", exception.getMessage());
org.junit.jupiter.api.Assertions class;org.junit.Assert class;org.junit.jupiter:junit-jupiter-api to your project and you'll get perfectly well working version from JUnit 5.
org.mockito.Mockito.verifywith various parameters to make sure that certain things happened (such that a logger service was called with the correct parameters) before the exception was thrown. – ZeroOne