How to test that no exception is thrown

Question

I know that one way to do it would be   Test public void foo        try            execute code that you expect not to throw Exceptions       catch Exception e          fail  quot Should not have thrown any exception quot            Is there any cleaner way of doing this   Probably using Junit s  Rule

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To test a scenario with a void method like void testMeWell   throws SomeException       to not throw an exception  Junit5 assertDoesNotThrow    - gt        testMeWell

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Java 8 makes this a lot easier  and Kotlin Scala doubly so   We can write a little utility class  class MyAssertions    public static void assertDoesNotThrow FailingRunnable action       try        action run             catch Exception ex         throw new Error  expected action not to throw  but it did    ex                FunctionalInterface interface FailingRunnable   void run   throws Exception     and then your code becomes simply    Test public void foo      MyAssertions assertDoesNotThrow    - gt          execute code that you expect not to throw Exceptions          If you dont have access to Java-8  I would use a painfully old java facility  aribitrary code blocks and a simple comment    setup Component component   new Component       act configure component      assert    assert does not throw      component doSomething        And finally  with kotlin  a language I ve recently fallen in love with   fun     - gt  Any   shouldNotThrow          try   invoke     catch  ex   Exception   throw Error  expected not to throw    ex      Test fun  when foo happens should not throw                     code that shouldn t throw     shouldNotThrow       Though there is a lot of room to fiddle with exactly how you want to express this  I was always a fan of fluent assertions     Regarding      You re approaching this the wrong way  Just test your functionality  if an exception is thrown the test will automatically fail  If no exception is thrown  your tests will all turn up green    This is correct in principle but incorrect in conclusion    Java allows exceptions for flow of control  This is done by the JRE runtime itself in APIs like Double parseDouble via a NumberFormatException and Paths get via a InvalidPathException    Given you ve written a component that validates Number strings for Double ParseDouble  maybe using a Regex  maybe a hand-written parser  or perhaps something that embeds some other domain rules that restricts the range of a double to something specific  how best to test this component  I think an obvious test would be to assert that  when the resulting string is parsed  no exception is thrown  I would write that test using either the above assertDoesNotThrow or   comment   code  block  Something like   Test public void given validator accepts string result should be interpretable by doubleParseDouble        setup   String input    12 34E 26    a string double with domain significance      act   boolean isValid   component validate input       assert -- using the library  assertJ   my personal favourite    assertThat isValid  describedAs input     was considered valid by component   isTrue      assertDoesNotThrow    - gt  Double parseDouble input        I would also encourage you to parameterize this test on input using Theories or Parameterized so that you can more easily re-use this test for other inputs  Alternatively  if you want to go exotic  you could go for a test-generation tool  and this   TestNG has better support for parameterized tests    What I find particularly disagreeable is the recommendation of using  Test expectedException IllegalArgumentException class   this exception is dangerously broad  If your code changes such that the component under test s constructor has if constructorArgument  lt   0  throw IllegalArgumentException    and your test was supplying 0 for that argument because it was convenient --and this is very common  because good generating test data is a surprisingly hard problem--  then your test will be green-bar even though it tests nothing  Such a test is worse than useless

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You can create any kind of your own assertions based on assertions from junit   static void assertDoesNotThrow Executable executable        assertDoesNotThrow executable   must not throw      static void assertDoesNotThrow Executable executable  String message        try           executable execute          catch  Throwable err            fail message             And test     the following will succeed assertDoesNotThrow   - gt methodMustNotThrow 1    assertDoesNotThrow   - gt methodMustNotThrow 1    fail with specific message  facepalm      the following will fail assertDoesNotThrow   - gt methodMustNotThrow 2    assertDoesNotThrow   - gt   throw new Exception  Hello world       Fail  must not trow      Generally speaking there is possibility to instantly fail  bla bla bla   the test in any scenarios  in any place where it makes sense  For instance use it in a try catch block to fail if anything is thrown in the test case   try methodMustNotThrow 1   catch Throwable e  fail  must not throw       or try methodMustNotThrow 1   catch Throwable e  Assertions fail  must not throw       This is the sample of the method we test  supposing we have such a method that must not fail under specific circumstances  but it can fail   void methodMustNotThrow int x  throws Exception      if  x    1  return      throw new Exception        The above method is a simple sample  But this works for complex situations  where the failure is not so obvious   There are the imports   import org junit jupiter api Assertions  import org junit jupiter api Test  import org junit jupiter api function Executable  import static org junit jupiter api Assertions

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Although this post is 6 years old now  however  a lot has changed in the Junit world  With Junit5  you can now use  org junit jupiter api Assertions assertDoesNotThrow     Ex   public void thisMethodDoesNotThrowException       System out println  Hello There        Test public void test thisMethodDoesNotThrowException      org junit jupiter api Assertions assertDoesNotThrow          - gt  thisMethodDoesNotThrowException              Hope it will help people who are using newer version of Junit5

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You can do it by using a  Rule and then call method reportMissingExceptionWithMessage as shown below  This is Scala code

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This may not be the best way but it definitely makes sure that exception is not thrown from the code block that is being tested   import org assertj core api Assertions  import org junit Test   public class AssertionExample         Test     public void testNoException            assertNoException                   private void assertException            Assertions assertThatThrownBy this  doNotThrowException  isInstanceOf Exception class              private void assertNoException            Assertions assertThatThrownBy    - gt  assertException    isInstanceOf AssertionError class              private void doNotThrowException              This method will never throw exception

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With AssertJ fluent assertions 3 7 0   Assertions assertThatCode    - gt  toTest method         doesNotThrowAnyException

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JUnit 5  Jupiter  provides three functions to check exception absence presence     assertAll      emsp     Asserts that all supplied executables  emsp     do not throw exceptions     assertDoesNotThrow      emsp     Asserts that execution of the  emsp     supplied executable supplier  emsp     does not throw any kind of exception      emsp     This function is available  emsp     since JUnit 5 2 0  29 April 2018      assertThrows      emsp     Asserts that execution of the supplied executable  emsp     throws an exception of the expectedType  emsp     and returns the exception   Example  package test mycompany myapp mymodule   import static org junit jupiter api Assertions     import org junit jupiter api Test   class MyClassTest         Test     void when string has been constructed then myFunction does not throw             String myString    this string has been constructed           assertAll    - gt  MyClass myFunction myString                Test     void when string has been constructed then myFunction does not throw  junit v520             String myString    this string has been constructed           assertDoesNotThrow    - gt  MyClass myFunction myString                Test     void when string is null then myFunction throws IllegalArgumentException             String myString   null          assertThrows              IllegalArgumentException class                 - gt  MyClass myFunction myString

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You re approaching this the wrong way  Just test your functionality  if an exception is thrown the test will automatically fail  If no exception is thrown  your tests will all turn up green   I have noticed this question garners interest from time to time so I ll expand a little   Background to unit testing  When you re unit testing it s important to define to yourself what you consider a unit of work  Basically  an extraction of your codebase that may or may not include multiple methods or classes that represents a single piece of functionality   Or  as defined in The art of Unit Testing  2nd Edition by Roy Osherove  page 11      A unit test is an automated piece of code that invokes the unit of work being tested  and then checks some assumptions about a single end result of that unit  A unit test is almost always written using a unit testing framework  It can be written easily and runs quickly  It s trustworthy  readable  and maintainable  It s consistent in its results as long as production code hasn t changed    What is important to realize is that one unit of work usually isn t just one method but at the very basic level it is one method and after that it is encapsulated by other unit of works     Ideally you should have a test method for each separate unit of work so you can always immediately view where things are going wrong  In this example there is a basic method called getUserById   which will return a user and there is a total of 3 unit of works   The first unit of work should test whether or not a valid user is being returned in the case of valid and invalid input  Any exceptions that are being thrown by the datasource have to be handled here  if no user is present there should be a test that demonstrates that an exception is thrown when the user can t be found  A sample of this could be the IllegalArgumentException which is caught with the  Test expected   IllegalArgumentException class  annotation   Once you have handled all your usecases for this basic unit of work  you move up a level  Here you do exactly the same  but you only handle the exceptions that come from the level right below the current one  This keeps your testing code well structured and allows you to quickly run through the architecture to find where things go wrong  instead of having to hop all over the place   Handling a tests  valid and faulty input  At this point it should be clear how we re going to handle these exceptions  There are 2 types of input  valid input and faulty input  the input is valid in the strict sense  but it s not correct    When you work with valid input you re setting the implicit expectancy that whatever test you write  will work    Such a method call can look like this  existingUserById ShouldReturn UserObject  If this method fails  e g   an exception is thrown  then you know something went wrong and you can start digging   By adding another test  nonExistingUserById ShouldThrow IllegalArgumentException  that uses the faulty input and expects an exception you can see whether your method does what it is supposed to do with wrong input    TL DR  You were trying to do two things in your test  check for valid and faulty input  By splitting this into two method that each do one thing  you will have much clearer tests and a much better overview of where things go wrong   By keeping the layered unit of works in mind you can also reduce the amount of tests you need for a layer that is higher in the hierarchy because you don t have to account for every thing that might have gone wrong in the lower layers  the layers below the current one are a virtual guarantee that your dependencies work and if something goes wrong  it s in your current layer  assuming the lower layers don t throw any errors themselves

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I stumbled upon this because of SonarQube s rule  squid S2699    Add at least one assertion to this test case    I had a simple test whose only goal was to go through without throwing exceptions   Consider this simple code   public class Printer        public static void printLine final String line            System out println line             What kind of assertion can be added to test this method  Sure  you can make a try-catch around it  but that is only code bloat   The solution comes from JUnit itself   In case no exception is thrown and you want to explicitly illustrate this behaviour  simply add expected as in the following example    Test expected   Test None class    no exception expected     public void test printLine         Printer printLine  line        Test None class is the default for the expected value

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If you are unlucky enough to catch all errors in your code  You can stupidly do  class DumpTest       Exception ex       Test     public void testWhatEver             try               thisShouldThrowError              catch  Exception e                ex   e                    assertEquals null ex

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The following fails the test for all exceptions  checked or unchecked    Test public void testMyCode          try           runMyTestCode          catch  Throwable t            throw new Error  fail

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If you want to test that whether your test target consumes the exception  Just leave the test as  mock collaborator using jMock2     Test public void consumesAndLogsExceptions   throws Exception        context checking new Expectations                           oneOf collaborator  doSth                will throwException new NullPointerException                             target doSth         The test would pass if your target does consume the exception thrown  otherwise the test would fail   If you want to test your exception consumption logic  things get more complex  I suggest delegating the consumption to a collaborator which could be mocked  Therefore the test could be    Test public void consumesAndLogsExceptions   throws Exception       Exception e   new NullPointerException        context checking new Expectations                           allowing collaborator  doSth                will throwException e                 oneOf consumer  consume e                          target doSth         But sometimes it s over-designed if you just want to log it  In this case  this article http   java dzone com articles monitoring-declarative-transac  http   blog novoj net 2008 09 20 testing-aspect-pointcuts-is-there-an-easy-way   may help if you insist tdd in this case

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I faced the same situation  I needed to check that exception is thrown when it should  and only when it should  Ended up using the exception handler to my benefit with the following code      try           functionThatMightThrowException        catch  Exception e           Assert fail  quot should not throw exception quot              RestOfAssertions     The main benefit for me was that it is quite straight forward and to check the other way of the  quot if and only if quot  is really easy in this same structure

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JUnit5 adds the assertAll   method for this exact purpose   assertAll     - gt  foo       source  JUnit 5 API

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You can expect that exception is not thrown by creating a rule    Rule public ExpectedException expectedException   ExpectedException none

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Use assertNull        Test public void foo         try             execute code that you expect not to throw Exceptions        catch  Exception e           assertNull e

[java] How to test that no exception is thrown?

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