JUnit 5 How to assert an exception is thrown

Question

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

User · Answer

In Java 8 and JUnit 5  Jupiter  we can assert for exceptions as follows  Using org junit jupiter api Assertions assertThrows     public static  lt  T extends Throwable   T assertThrows Class lt  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                - gt                 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-

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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     - gt                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     - gt                expectEx constructErrorMessage  sample    error                    assertEquals  Invalid parametercount  expected 3  passed 2   exception getMessage

User · Answer

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     - gt            throw new IllegalArgumentException  a message                assertEquals  a message   exception getMessage

User · Answer

You can use assertThrows    which allows you to test multiple exceptions within the same test  With support for lambdas in Java 8  this is the canonical way to test for exceptions in JUnit  Per the JUnit docs  import static org junit jupiter api Assertions assertThrows    Test void exceptionTesting         MyException thrown   assertThrows             MyException class                - gt  myObject doThing                quot Expected doThing   to throw  but it didn t quot              assertTrue thrown getMessage   contains  quot Stuff quot

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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  lt T extends Throwable gt  T expectThrows

User · Answer

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

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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     - gt        service readFile  quot sampleFile txt quot        assertEquals FileNotFoundException class  throwable getClass

User · Answer

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                - gt  objectName yourMethod  WRONG

User · Answer

I think this is an even simpler example  List lt String gt  emptyList   new ArrayList lt  gt     Optional lt String gt  opt2   emptyList stream   findFirst    assertThrows NoSuchElementException class     - gt  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

User · Answer

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

[java] JUnit 5: How to assert an exception is thrown?

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