How do you assert that a certain exception is thrown in JUnit 4 tests?
JavaExceptionJunitJunit4AssertJava Problem Overview
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.
Java Solutions
Solution 1 - Java
It depends on the JUnit version and what assert libraries you use.
- For JUnit5 and 4.13 see answer https://stackoverflow.com/a/2935935/2986984
- If you use assertJ or google-truth, see answer https://stackoverflow.com/a/41019785/2986984
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 :
Solution 2 - Java
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.
Solution 3 - Java
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.
Solution 4 - Java
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.
Solution 5 - Java
in junit, there are four ways to test exception.
junit5.x
-
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()); }
junit4.x
-
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
- if you like junit 5, then you should like the 1st one
- the 2nd way is used when you only want test the type of exception
- the first and last two are used when you want test exception message further
- if you use junit 3, then the 4th one is preferred
-
for more info, you can read this document and junit5 user guide for details.
Solution 6 - Java
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 afail()
assertion before thecatch
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 anAssert.fail
at the end of thetry
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.-
If the test needs to check additional things on the exception like the cause or the message (good exception messages are really important, having a precise exception type may not be enough).
-
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 theExpectedException
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 usingassertThat(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 returningvoid
, and as such doesn't allow chaining assertions like AssertJ.Also if you remember name clash with
Matcher
orAssert
, be prepared to meet the same clash withAssertions
.
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.
Solution 7 - Java
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:
- Built into JUnit
- You get a useful exception message if the code in the lambda doesn't throw an exception, and a stacktrace if it throws a different exception
- Concise
- Allows your tests to follow Arrange-Act-Assert
- You can precisely indicate what code you are expecting to throw the exception
- You don't need to list the expected exception in the
throws
clause - You can use the assertion framework of your choice to make assertions about the caught exception
Solution 8 - Java
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);
}
Solution 9 - Java
Update: JUnit5 has an improvement for exceptions testing: assertThrows
.
The following example is from: Junit 5 User Guide
@Test
void exceptionTesting() {
IllegalArgumentException 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.
Solution 10 - Java
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");
Solution 11 - Java
BDD Style Solution: JUnit 4 + Catch Exception + AssertJ
import static com.googlecode.catchexception.apis.BDDCatchException.*;
@Test
public void testFooThrowsIndexOutOfBoundsException() {
when(() -> foo.doStuff());
then(caughtException()).isInstanceOf(IndexOutOfBoundsException.class);
}
Dependencies
eu.codearte.catch-exception:catch-exception:2.0
Solution 12 - Java
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.
Solution 13 - Java
You can also do this:
@Test
public void testFooThrowsIndexOutOfBoundsException() {
try {
foo.doStuff();
assert false;
} catch (IndexOutOfBoundsException e) {
assert true;
}
}
Solution 14 - Java
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.
Solution 15 - Java
JUnit 5 Solution
@Test
void testFooThrowsIndexOutOfBoundsException() {
IndexOutOfBoundsException 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
Solution 16 - Java
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("");
}
}
Solution 17 - Java
JUnit has built-in support for this, with an "expected" attribute.
Solution 18 - Java
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.
Solution 19 - Java
Java 8 solution
If you would like a solution which:
- Utilizes Java 8 lambdas
- Does not depend on any JUnit magic
- Allows you to check for multiple exceptions within a single test method
- Checks for an exception being thrown by a specific set of lines within your test method instead of any unknown line in the entire test method
- Yields the actual exception object that was thrown so that you can further examine it
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!" );
}
Solution 20 - Java
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));
}
Solution 21 - Java
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();
Solution 22 - Java
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());
}
Solution 23 - Java
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.
- The Type of exception thrown
- The exception Message
- The cause of 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");
}
}
Solution 24 - Java
Additionally to what NamShubWriter has said, make sure that:
- The ExpectedException instance is public (Related Question)
- The ExpectedException isn't instantiated in say, the @Before method. This post clearly explains all the intricacies of JUnit's order of execution.
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.
Solution 25 - Java
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.
Solution 26 - Java
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(".......);
Solution 27 - Java
JUnit framework has assertThrows()
method:
ArithmeticException exception = assertThrows(ArithmeticException.class, () ->
calculator.divide(1, 0));
assertEquals("/ by zero", exception.getMessage());
-
for JUnit 5 it's in
org.junit.jupiter.api.Assertions
class; -
for JUnit 4.13 it's in
org.junit.Assert
class; -
for earlier versions of JUnit 4: just add reference on
org.junit.jupiter:junit-jupiter-api
to your project and you'll get perfectly well working version from JUnit 5.
Solution 28 - Java
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"});
}
Solution 29 - Java
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:
- not relying on any library
- localised check - more precise and allows to have multiple assertions like this within one test if needed
- easy to use
Solution 30 - Java
@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.
Solution 31 - Java
My solution using Java 8 lambdas:
public static <T extends Throwable> T assertThrows(Class<T> expected, ThrowingRunnable action) throws Throwable {
try {
action.run();
Assert.fail("Did not throw expected " + expected.getSimpleName());
return null; // never actually
} catch (Throwable actual) {
if (!expected.isAssignableFrom(actual.getClass())) { // runtime '!(actual instanceof expected)'
System.err.println("Threw " + actual.getClass().getSimpleName()
+ ", which is not a subtype of expected "
+ expected.getSimpleName());
throw actual; // throw the unexpected Throwable for maximum transparency
} else {
return (T) actual; // return the expected Throwable for further examination
}
}
}
You have to define a FunctionalInterface, because Runnable
doesn't declare the required throws
.
@FunctionalInterface
public interface ThrowingRunnable {
void run() throws Throwable;
}
The method can be used as follows:
class CustomException extends Exception {
public final String message;
public CustomException(final String message) { this.message = message;}
}
CustomException e = assertThrows(CustomException.class, () -> {
throw new CustomException("Lorem Ipsum");
});
assertEquals("Lorem Ipsum", e.message);
Solution 32 - Java
There are two ways of writing test case
-
Annotate the test with the exception which is thrown by the method. Something like this
@Test(expected = IndexOutOfBoundsException.class)
-
You can simply catch the exception in the test class using the try catch block and assert on the message that is thrown from the method in test class.
try{ } catch(exception to be thrown from method e) { assertEquals("message", e.getmessage()); }
I hope this answers your query Happy learning...
Solution 33 - Java
I wanted to comment with my solution to this problem, which avoided needing any of the exception related JUnit code.
I used assertTrue(boolean) combined with try/catch to look for my expected exception to be thrown. Here's an example:
public void testConstructor() {
boolean expectedExceptionThrown;
try {
// Call constructor with bad arguments
double a = 1;
double b = 2;
double c = a + b; // In my example, this is an invalid option for c
new Triangle(a, b, c);
expectedExceptionThrown = false; // because it successfully constructed the object
}
catch(IllegalArgumentException e) {
expectedExceptionThrown = true; // because I'm in this catch block
}
catch(Exception e) {
expectedExceptionThrown = false; // because it threw an exception but not the one expected
}
assertTrue(expectedExceptionThrown);
}
Solution 34 - Java
try {
my method();
fail( "This method must thrwo" );
} catch (Exception ex) {
assertThat(ex.getMessage()).isEqual(myErrormsg);
}