NullPointerException when testing resource in dropwizard with mockito

I built a restful API with dropwizard. And now I want to test it with what dropwizard document provided.

public class BlogCommentResourceTest {
private static final BlogCommentDAO dao = mock(BlogCommentDAO.class);
private final BlogComment blogComment = new BlogComment(1, "url", "comment", new Date(), "name");
@ClassRule
public static final ResourceTestRule resources = ResourceTestRule.builder()
        .addResource(new BlogCommentResource(dao)).build();
List<BlogComment> list = new ArrayList<BlogComment>();

@Before
public void setUp(){
    list.add(blogComment);

    when(dao.getLast()).thenReturn(blogComment);
    when(dao.getAll(POJOs.BlogCommentPOJO.toString())).thenReturn(list);
}
@After
public void tearDown(){
    reset(dao);
}
@Test
public void getLast(){
    assertThat(resources.client().target("/blogcomments").request().get(List.class))
            .isEqualTo(list);
    verify(dao.getAll(POJOs.BlogCommentPOJO.toString()), times(1));
}

}

This is the method I want to test.

enter code here

And error message is shown as below:

java.lang.NullPointerException
at dao.DAOBase.getAll(DAOBase.java:37)
at dao.BlogCommentDAO.getAll(BlogCommentDAO.java:19)
at resource.BlogCommentResourceTest.setUp(BlogCommentResourceTest.java:29)
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.RunBefores.evaluate(RunBefores.java:24)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
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 io.dropwizard.testing.junit.ResourceTestRule$1.evaluate(ResourceTestRule.java:202)
at org.junit.rules.RunRules.evaluate(RunRules.java:20)
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:119)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:42)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:234)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:74)
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 com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

Somebody know what happened? Why it is the DAOBase that throw a null pointer? My unit test for all of dao shows they works well.