@RunWith(MockitoJUnitRunner.class)
public class CustDetMgmtTest {

  private CustDetMgmt custDetMgmt = new CustDetMgmt();

  @Mock
  private CustMgmt custMgmt;

  @Mock
  UserInfo userInfo;

  @Mock
  static ActionContext actionContext;

  @Mock
  static ServletContext servletContext;

  @Mock
  static HttpServletResponse response;

  @Mock
  static HttpServletRequest request;

  Map<String, Object> sessionMap = new HashMap<String, Object>();
  private String custId = "1234";

}

@BeforeClass
public static void setUpBeforeClass() throws Exception {

    ServletActionContext.setContext(actionContext);
    ServletActionContext.setServletContext(servletContext); // NPE Exception here
    ServletActionContext.setResponse(response);

    // Mock the logger calls
    when(response.getWriter()).thenReturn(new PrintWriter(System.out, true));
}

 @Before
 public void setUp() throws Exception {
    sessionMap.put("usersession", userInfo);
    when(userInfo.getUserID()).thenReturn(custId);
    custCaseMgmt.setSessionCtxt(actionContext);
    when(actionContext.getSession()).thenReturn(sessionMap);
  }
}

java.lang.NullPointerException
at org.apache.struts2.ServletActionContext.setServletContext(ServletActionContext.java:148)
at my.package.CustDetMgmtTest.setUpBeforeClass(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:60)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:37)
at java.lang.reflect.Method.invoke(Method.java:611)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.mockito.internal.runners.JUnit45AndHigherRunnerImpl.run(JUnit45AndHigherRunnerImpl.java:37)
at org.mockito.runners.MockitoJUnitRunner.run(MockitoJUnitRunner.java:62)
at junit.framework.JUnit4TestAdapter.run(JUnit4TestAdapter.java:38)
at org.apache.tools.ant.taskdefs.optional.junit.JUnitTestRunner.run(JUnitTestRunner.java:532)
at org.apache.tools.ant.taskdefs.optional.junit.JUnitTestRunner.launch(JUnitTestRunner.java:1179)
at org.apache.tools.ant.taskdefs.optional.junit.JUnitTestRunner.main(JUnitTestRunner.java:1030)