使用注解
@RunWith(SpringRunner.class)
@SpringBootTest(classes = StudyTestApplication.class)

注意当测试类包名与启动类包名相同或者在其子包下。使用@SpringBootTest 不需要在括号内注明启动类

package test;

import com.studytest.StudyTestApplication;
import com.studytest.service.UserService;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

@RunWith(SpringRunner.class)
//包名与启动类不同，在SpringBootTest中注明启动类
@SpringBootTest(classes = StudyTestApplication.class)
//@SpringBootTest
public class UserServiceTest {
    @Autowired
    UserService userService;

    @Test
    public void testAdd() {
        System.out.println("testAdd");
        userService.add();
    }

}