package com.example.spring_test.service;

import org.springframework.stereotype.Service;

@Service
public class HelloService {

    public String hello(String who) {
        return "Hello " + who + '!';
    }
}

package com.example.spring_test.controller;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

import com.example.spring_test.service.HelloService;

import lombok.AllArgsConstructor;

@RestController
@AllArgsConstructor
public class HomeController {

    private final HelloService helloService;

    @GetMapping("/hello")
    public String hello(@RequestParam String who) {
        return helloService.hello(who);
    }
}

package com.example.spring_test.test;

import static org.mockito.Mockito.times;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.boot.test.mock.mockito.MockBean;
import org.springframework.test.web.servlet.MockMvc;

import com.example.spring_test.service.HelloService;

@SpringBootTest
@AutoConfigureMockMvc
public class HomeControllerTest {

    @MockBean
    private HelloService helloService;

    @Autowired
    private MockMvc mockMvc;

    @Test
    public void helloTest() throws Exception {
        // given
        final String who = "World";
        final String hello = "Hello World!";
        when(helloService.hello(who)).thenReturn(hello);

        // when
        // then
        mockMvc.perform(get("/hello?who=World"))
            .andExpect(status().isOk())
            .andExpect(content().string(hello));

        verify(helloService, times(1)).hello(who);
    }
}