Spring Boot控制器测试，空指针异常

我正在学习制作API。我正在尝试对我的Get请求运行测试，但是在我的测试类中一直得到Null Pointer Exception。我错过了什么？

我正在使用Spring Boot / Maven / Java11。据我所知，这是一个非常基本的设置，因为我刚刚开始涉足Spring Boot。

请参见下面的代码：

控制器测试等级：

package ControllerTest;

import org.junit.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.web.servlet.MockMvc;
import org.springframework.test.web.servlet.ResultMatcher;
import org.springframework.test.web.servlet.request.MockHttpServletRequestBuilder;
import org.springframework.test.web.servlet.request.MockMvcRequestBuilders;
import org.springframework.test.web.servlet.result.MockMvcResultMatchers;

public class StatControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @Test
    public void testGetResponse()
            throws Exception {

        MockHttpServletRequestBuilder builder = MockMvcRequestBuilders
                .get("/get");

        ResultMatcher contentMatcher = MockMvcResultMatchers.content()
                .string("GET Response");

        mockMvc.perform(builder).andExpect(contentMatcher).andExpect(MockMvcResultMatchers.status().isOk());
    }
}

控制器类别：

package apiPackage;

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;

@RestController
public class StatController {

    @RequestMapping("/")
    public String index(){
        return "Welcome to the SilverSnacks Stat Sheet!";
    }

    @GetMapping("/get")
    public @ResponseBody ResponseEntity<String> get(){
        return new ResponseEntity<String>("GET Response", HttpStatus.OK);
    }

    @GetMapping("/get/{id}")
    public @ResponseBody ResponseEntity<String> getById(@PathVariable String id){
        return new ResponseEntity<String>("GET Response:" + id, HttpStatus.OK);
    }

    @PutMapping("/put")
    public @ResponseBody ResponseEntity<String> put(){
        return new ResponseEntity<String>("PUT Response", HttpStatus.OK);
    }

    @PostMapping("/post")
    public @ResponseBody ResponseEntity<String> post(){
        return new ResponseEntity<String>("POST Response", HttpStatus.OK);
    }

    @DeleteMapping("/delete")
    public @ResponseBody ResponseEntity<String> delete(){
        return new ResponseEntity<String>("DELETE Response", HttpStatus.OK);
    }

    @PatchMapping("/patch")
    public @ResponseBody ResponseEntity<String> patch(){
        return new ResponseEntity<String>("PATCH Response", HttpStatus.OK);
    }
}

错误输出：

java.lang.NullPointerException
    at ControllerTest.StatControllerTest.testGetResponse(StatControllerTest.java:26)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    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.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)