title: When I use JUnit, occur this error. How to solve this error?
body: <p>When I use JUnit, I run into this error:</p>
  
  <blockquote>
    <p>java.lang.NullPointerException    at
    com.mpdidb.mapper.TbUserServiceTest.selectByPrimaryKeyTest(TbUserServiceTest.java:27)
      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.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
      at
    org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
      at
    org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
      at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)    at
    org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:252)
      at
    org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:94)
      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$2.evaluate(ParentRunner.java:268)  at
    org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
      at
    org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
      at org.junit.runners.ParentRunner.run(ParentRunner.java:363)    at
    org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:191)
      at org.junit.runner.JUnitCore.run(JUnitCore.java:137)   at
    com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
      at
    com.intellij.rt.execution.junit.IdeaTestRunner$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)</p>
  </blockquote>
  
  <p>And this is my JUnit code:</p>
  
  <pre><code>@WebAppConfiguration
  @RunWith(SpringJUnit4ClassRunner.class)
  @ContextConfiguration(classes = {RootConfig.class})
  public class TbUserServiceTest
  {
      @Autowired
      private static TbUserService tbUserService;
  
      @Test
      public void selectByPrimaryKeyTest()
      {
          String userId = "wusuodai";
          TbUser tbUser = tbUserService.selectByPrimaryKey(userId);
          System.out.println(tbUser.getUserName());
      }
  }
  </code></pre>
  
  <p>How to solve this error?</p>