android junit getApplication 为空，怎么回事？

就是我想搭建测试环境，但是getApplication为空，gradle环境配置也已经配置了，应该没有漏吧？找了很多网上所说的配置，但是还是失败，如果不用getApplication()，它是可以通过测试的。

public class CallServiceTest extends ServiceTestCase<CallService> {



    public CallServiceTest() {

        super(CallService.class);

    }



    @Override

    public void setUp() throws Exception {

        super.setUp();

        getApplication().onLowMemory();

    }



    public void testSomething() throws Exception {

        System.out.println("1");

        assertEquals(1, 1);

        System.out.println("2");

    }

}

gradle：

dependencies {

    compile fileTree(dir: 'libs', include: ['*.jar'])



    androidTestCompile 'com.android.support:support-annotations:23.1.0'

    androidTestCompile 'com.android.support.test:runner:0.4.1'

    androidTestCompile 'com.android.support.test:rules:0.4.1'

    androidTestCompile 'com.android.support.test.espresso:espresso-core:2.2.1'

    androidTestCompile 'com.android.support.test.espresso:espresso-intents:2.2.1'

    androidTestCompile 'com.android.support.test.espresso:espresso-web:2.2.1'

    compile 'com.android.support:appcompat-v7:23.1.0'

    testCompile 'junit:junit:4.12'

...

}



 defaultConfig {

        applicationId "com.revenco.av_talk"

        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"

        minSdkVersion 17

        targetSdkVersion 18

        versionCode 1

        versionName "1.0"

    }



    testOptions {

        unitTests.returnDefaultValues = true

    }

    //ADD THESE LINES:

    packagingOptions {

        exclude 'LICENSE.txt'

    }

报的错：
java.lang.NullPointerException
at com.revenco.test.CallServiceTest.setUp(CallServiceTest.java:24)
at junit.framework.TestCase.runBare(TestCase.java:139)
at junit.framework.TestResult$1.protect(TestResult.java:122)
at junit.framework.TestResult.runProtected(TestResult.java:142)
at junit.framework.TestResult.run(TestResult.java:125)
at junit.framework.TestCase.run(TestCase.java:129)
at junit.framework.TestSuite.runTest(TestSuite.java:252)
at junit.framework.TestSuite.run(TestSuite.java:247)
at org.junit.internal.runners.JUnit38ClassRunner.run(JUnit38ClassRunner.java:86)
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)