Mocking static methods with Mockito

Question

I ve written a factory to produce java sql Connection objects   public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory         Override public Connection getConnection             try               return DriverManager getConnection                 catch  SQLException e                throw new RuntimeException e                       I d like to validate the parameters passed to DriverManager getConnection  but I don t know how to mock a static method  I m using JUnit 4 and Mockito for my test cases  Is there a good way to mock verify this specific use-case

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Observation   When you call static method within a static entity  you need to change the class in  PrepareForTest   For e g     securityAlgo   MessageDigest getInstance SECURITY ALGORITHM     For the above code if you need to mock MessageDigest class  use    PrepareForTest MessageDigest class    While if you have something like below     public class CustomObjectRule        object   DatatypeConverter printHexBinary MessageDigest getInstance SECURITY ALGORITHM                digest message getBytes ENCODING          then  you d need to prepare the class this code resides in     PrepareForTest CustomObjectRule class    And then mock the method     PowerMockito mockStatic MessageDigest class   PowerMockito when MessageDigest getInstance Mockito anyString            thenThrow new RuntimeException

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The typical strategy for dodging static methods that you have no way of avoiding using  is by creating wrapped objects and using the wrapper objects instead   The wrapper objects become facades to the real static classes  and you do not test those   A wrapper object could be something like  public class Slf4jMdcWrapper       public static final Slf4jMdcWrapper SINGLETON   new Slf4jMdcWrapper         public String myApisToTheSaticMethodsInSlf4jMdcStaticUtilityClass             return MDC getWhateverIWant              Finally  your class under test can use this singleton object by  for example   having a default constructor for real life use   public class SomeClassUnderTest       final Slf4jMdcWrapper myMockableObject           constructor used by CDI or whatever real life use case        public myClassUnderTestContructor             this myMockableObject   Slf4jMdcWrapper SINGLETON                 constructor used in tests       myClassUnderTestContructor Slf4jMdcWrapper myMock            this myMockableObject   myMock            And here you have a class that can easily be tested  because you do not  directly use a class with static methods   If you are using CDI and can make use of the  Inject annotation then it is even easier  Just make your Wrapper bean  ApplicationScoped  get that thing injected as a collaborator  you do not even need messy constructors for testing   and go on with the mocking

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Mocking of static methods in Mockito is possible since Mockito 3 4 0  For more details see  https   github com mockito mockito releases tag v3 4 0 https   github com mockito mockito issues 1013  In your case  something like this     Test   public void testStaticMockWithVerification   throws SQLException       try  MockedStatic lt DriverManager gt  dummy   Mockito mockStatic DriverManager class           DatabaseConnectionFactory factory   new MySQLDatabaseConnectionFactory          dummy when    - gt  DriverManager getConnection  quot arg1 quot    quot arg2 quot    quot arg3 quot             thenReturn new Connection                      factory getConnection           dummy verify    - gt  DriverManager getConnection eq  quot arg1 quot    eq  quot arg2 quot    eq  quot arg3 quot                 NOTE  this feature requires mockito-inline dependency

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As mentioned before you can not mock static methods with mockito    If changing your testing framework is not an option you can do the following   Create an interface for DriverManager  mock this interface  inject it via some kind of dependency injection and verify on that mock

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For those who use JUnit 5  Powermock is not an option  You ll require the following dependencies to successfully mock a static method with just Mockito  testCompile    group   org mockito   name   mockito-core             version   3 6 0  testCompile    group   org mockito   name   mockito-junit-jupiter    version   3 6 0  testCompile    group   org mockito   name   mockito-inline           version   3 6 0   mockito-junit-jupiter add supports for JUnit 5  And support for mocking static methods is provided by mockito-inline dependency  Example   Test void returnUtilTest         assertEquals  quot foo quot   UtilClass staticMethod  quot foo quot          try  MockedStatic lt UtilClass gt  classMock   mockStatic UtilClass class              classMock when    - gt  UtilClass staticMethod  quot foo quot    thenReturn  quot bar quot             assertEquals  quot bar quot   UtilClass staticMethod  quot foo quot                  assertEquals  quot foo quot   UtilClass staticMethod  quot foo quot        The try-with-resource block is used to make the static mock remains temporary  so it s mocked only within that scope  When not using a try block  make sure to close the scoped mock  once you are done with the assertions  MockedStatic lt UtilClass gt  classMock   mockStatic UtilClass class  classMock when    - gt  UtilClass staticMethod  quot foo quot    thenReturn  quot bar quot    assertEquals  quot bar quot   UtilClass staticMethod  quot foo quot     classMock close     Mocking void methods  When mockStatic is called on a class  all the static void methods in that class automatically get mocked to doNothing

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You can do it with a little bit of refactoring   public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory         Override public Connection getConnection             try               return  getConnection    some params                catch  SQLException e                throw new RuntimeException e                          method to forward parameters  enabling mocking  extension  etc     Connection  getConnection    some params     throws SQLException           return DriverManager getConnection    some params                Then you can extend your class MySQLDatabaseConnectionFactory to return a mocked connection  do assertions on the parameters  etc   The extended class can reside within the test case  if it s located in the same package  which I encourage you to do   public class MockedConnectionFactory extends MySQLDatabaseConnectionFactory        Connection  getConnection    some params     throws SQLException           if  some param    something  throw new InvalidParameterException               consider mocking some methods with when yourMock something    thenReturn value          return Mockito mock Connection class

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For mocking static functions i was able to do it that way   create a wrapper function in some helper class object   using a name variant might be beneficial for keeping things separated and maintainable   use this wrapper in your codes   Yes  codes need to be realized with testing in mind   mock the wrapper function   wrapper code snippet  not really functional  just for illustration  class myWrapperClass         def myWrapperFunction                 return theOriginalFunction              of course having multiple such functions accumulated in a single wrapper class might be beneficial in terms of code reuse

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Use JMockit framework  It worked for me  You don t have to write statements for mocking DBConenction getConnection   method  Just the below code is enough    Mock below is mockit Mock package  Connection jdbcConnection   Mockito mock Connection class    MockUp lt DBConnection gt  mockUp   new MockUp lt DBConnection gt                   DBConnection singleton   new DBConnection                  Mock             public DBConnection getInstance                      return singleton                              Mock             public Connection getConnection                     return jdbcConnection

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I had a similar issue  The accepted answer did not work for me  until I made the change    PrepareForTest TheClassThatContainsStaticMethod class   according to PowerMock s documentation for mockStatic   And I don t have to use BDDMockito    My class   public class SmokeRouteBuilder       public static String smokeMessageId             try               return InetAddress getLocalHost   getHostAddress              catch  UnknownHostException e                log error  Exception occurred while fetching localhost address   e               return UUID randomUUID   toString                        My test class    RunWith PowerMockRunner class   PrepareForTest SmokeRouteBuilder class  public class SmokeRouteBuilderTest        Test     public void testSmokeMessageId exception   throws UnknownHostException           UUID id   UUID randomUUID             mockStatic InetAddress class           mockStatic UUID class           when InetAddress getLocalHost    thenThrow UnknownHostException class           when UUID randomUUID    thenReturn id            assertEquals id toString    SmokeRouteBuilder smokeMessageId

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Since that method is static  it already has everything you need to use it  so it defeats the purpose of mocking   Mocking the static methods is considered to be a bad practice   If you try to do that  it means there is something wrong with the way you want to perform testing   Of course you can use PowerMockito or any other framework capable of doing that  but try to rethink your approach   For example  try to mock provide the objects  which that static method consumes instead

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Mockito cannot capture static methods  but since Mockito 2 14 0 you can simulate it by creating invocation instances of static methods   Example  extracted from their tests    public class StaticMockingExperimentTest extends TestBase        Foo mock   Mockito mock Foo class       MockHandler handler   Mockito mockingDetails mock  getMockHandler        Method staticMethod      InvocationFactory RealMethodBehavior realMethod   new InvocationFactory RealMethodBehavior              Override         public Object call   throws Throwable               return null                         Before     public void before   throws Throwable           staticMethod   Foo class getDeclaredMethod  staticMethod   String class               Test     public void verify static method   throws Throwable             register staticMethod call on mock         Invocation invocation   Mockito framework   getInvocationFactory   createInvocation mock  withSettings   build Foo class   staticMethod  realMethod                   some arg            handler handle invocation              verify staticMethod on mock           Mockito cannot capture static methods so we will simulate this scenario in 3 steps            1  Call standard  verify  method  Internally  it will add verificationMode to the thread local state              Effectively  we indicate to Mockito that right now we are about to verify a method call on this mock          verify mock             2  Create the invocation instance using the new public API             Mockito cannot capture static methods but we can create an invocation instance of that static invocation         Invocation verification   Mockito framework   getInvocationFactory   createInvocation mock  withSettings   build Foo class   staticMethod  realMethod                   some arg              3  Make Mockito handle the static method invocation             Mockito will find verification mode in thread local state and will try verify the invocation         handler handle verification              verify zero times  method with different argument         verify mock  times 0            Invocation differentArg   Mockito framework   getInvocationFactory   createInvocation mock  withSettings   build Foo class   staticMethod  realMethod                   different arg            handler handle differentArg               Test     public void stubbing static method   throws Throwable             register staticMethod call on mock         Invocation invocation   Mockito framework   getInvocationFactory   createInvocation mock  withSettings   build Foo class   staticMethod  realMethod                   foo            handler handle invocation              register stubbing         when null  thenReturn  hey               validate stubbed return value         assertEquals  hey   handler handle invocation            assertEquals  hey   handler handle invocation               default null value is returned if invoked with different argument         Invocation differentArg   Mockito framework   getInvocationFactory   createInvocation mock  withSettings   build Foo class   staticMethod  realMethod                   different arg            assertEquals null  handler handle differentArg               static class Foo            private final String arg           public Foo String arg                this arg   arg                     public static String staticMethod String arg                return                         Override         public String toString                 return  foo     arg                      Their goal is not to directly support static mocking  but to improve its public APIs so that other libraries  like Powermockito  don t have to rely on internal APIs or directly have to duplicate some Mockito code   source      Disclaimer  Mockito team thinks that the road to hell is paved with static methods  However  Mockito s job is not to protect your code from static methods  If you don   t like your team doing static mocking  stop using Powermockito in your organization  Mockito needs to evolve as a toolkit with an opinionated vision on how Java tests should be written  e g  don t mock statics      However  Mockito is not dogmatic  We don t want to block unrecommended use cases like static mocking  It s just not our job

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Use PowerMockito on top of Mockito   Example code    RunWith PowerMockRunner class   PrepareForTest DriverManager class  public class Mocker         Test     public void shouldVerifyParameters   throws Exception              given         PowerMockito mockStatic DriverManager class           BDDMockito given DriverManager getConnection       willReturn                  when         sut execute       System Under Test  sut             then         PowerMockito verifyStatic            DriverManager getConnection                More information    Why doesn  39 t Mockito mock static methods

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I also wrote a combination of Mockito and AspectJ  https   github com iirekm varia tree develop ajmock  Your example becomes   when    - gt  DriverManager getConnection       thenReturn

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There is an easy solution by using java FunctionalInterface and then add that interface as dependency for the class you are trying to unit test

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To mock static method you should use a Powermock look at  https   github com powermock powermock wiki MockStatic  Mockito doesn t provide this functionality   You can read nice a article about mockito  http   refcardz dzone com refcardz mockito

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