title: Java NullPointerException with Mocked Object
body: <p>I have a unit test I am writing for a class, and this class uses a ConfigurationManager I wrote to get configuration from a TOML object (and therefore from a TOML file). </p>
  
  <p><strong>The Code</strong></p>
  
  <p>Here is the relevant ConfigurationManager code:</p>
  
  <pre><code>public class ConfigurationManager {
      // DEFAULT_LOGGING_LEVEL should represent level to use in production environment.
      private final static Level DEFAULT_LOGGING_LEVEL = Level.FINE;
      private final static String LOG_LEVEL_ENV_PROPERTY = "LOG_LEVEL";
      private final static String TOML_FILE_NAME = "config.toml";
      private final static String LOCAL_ENV_PROPERTY = "local";
      private final static String ENV_PROPERTY = "MY_ENV";
      private static Logger CM_LOGGER;
  
      private Environment environment;
  
      public ConfigurationManager() {
          environment = new Environment();
          CM_LOGGER = new LoggerUtil(ConfigurationManager.class.getName(), getLoggingLevel()).getLogger();
      }
  
      public File getTomlFile() throws URISyntaxException, FileNotFoundException {
          URI uri = getConfigURI();
          File tomlFile = new File(uri);
          if (!tomlFile.exists()) {
              String err = TOML_FILE_NAME + " does not exist!";
              CM_LOGGER.severe(err);
              throw new FileNotFoundException(err);
          }
          return tomlFile;
      }
  
      /**
       * @return A URI representing the path to the config
       * @throws URISyntaxException if getConfigURI encounters bad URI syntax.
       */
      private URI getConfigURI() throws URISyntaxException {
          return getClass().getClassLoader().getResource(TOML_FILE_NAME).toURI();
      }
  
      /**
       * Method for getting the app configuration as a toml object.
       *
       * @return A toml object built from the config.toml file.
       * @throws URISyntaxException    if getConfigURI encounters bad URI syntax.
       * @throws FileNotFoundException if getTomlFile can't find the config.toml file.
       */
      public Toml getConfigToml() throws URISyntaxException, FileNotFoundException {
          return new Toml().read(getTomlFile());
      }
  }
  </code></pre>
  
  <p>And here is the code that calls this Configuration Manager:</p>
  
  <pre><code>public class Listener {
      // Initializations
      private static final String CONFIG_LISTENER_THREADS = "listenerThreads";
      private static final String DEFAULT_LISTENER_THREADS = "1";
  
      /**
       * Constructor for getting app properties and initializing executor service with thread pool based on app prop.
       */
      @PostConstruct
      void init() throws FileNotFoundException, URISyntaxException {
          ConfigurationManager configurationManager = new ConfigurationManager();
              int listenerThreads = Integer.parseInt(configurationManager.getConfigToml()
                      .getString(CONFIG_LISTENER_THREADS, DEFAULT_LISTENER_THREADS));  
          this.executorService = Executors.newFixedThreadPool(listenerThreads);
          LOGGER.config("Listener executorService threads: " + listenerThreads);
      }
  ...
  }
  </code></pre>
  
  <p>And here is the test for that code (See comment to see where NPE is triggered):</p>
  
  <pre><code>public class ListenerTests {
  
      @Mock
      ConfigurationManager mockCM;
  
      @InjectMocks
      Listener listener = new Listener();
  
      @Before
      public void setUp(){
          MockitoAnnotations.initMocks(this);
      }
  
      @Test
      public void testListener_ShouldInit() throws FileNotFoundException, URISyntaxException {
          when(mockCM.getConfigToml().getString(any(), any())).thenReturn("5"); // !!!NPE TRIGGERED BY THIS LINE!!!
          listener.init();
          verify(mockCM, times(1)).getConfigToml().getString(any(), any());
      }
  }
  </code></pre>
  
  <p><strong>The Problem</strong></p>
  
  <p>I get a NullPointerException as follows </p>
  
  <pre><code>java.lang.NullPointerException
      at com.bose.source_account_listener.ListenerTests.testListener_ShouldInit(ListenerTests.java:37)
      at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
      at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
      at java.lang.reflect.Method.invoke(Method.java:498)
      at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
      at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
      at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
      at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
      at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
      at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
      at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
      at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
      at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
      at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
      at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
      at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
      at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
      at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
      at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
      at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
      at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
      at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
      at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
  </code></pre>
  
  <p>I have a guess as to the problem here but I'm not sure how to validate my guess. My guess is that I am mocking the ConfigurationManager, but the mock doesn't know that the ConfigurationManager can generate a TOML file which has a getString method, so I end up getting the NPE. This makes me think that I need a mock TOML for my mock ConfigurationManager, but I am neither certain this is the case or that this is the proper solution. </p>
  
  <p>I am new to Mockito and Java in general so any help would be appreciated.</p>