步骤1：在顶级pom.xml添加常用工具依赖。

<!-- apache 工具类 -->
<dependency>
	<groupId>commons-codec</groupId>
	<artifactId>commons-codec</artifactId>
	<version>1.11</version>
</dependency>
<dependency>
	<groupId>org.apache.commons</groupId>
	<artifactId>commons-lang3</artifactId>
	<version>3.4</version>
</dependency>
<dependency>
	<groupId>org.apache.commons</groupId>
	<artifactId>commons-io</artifactId>
	<version>1.3.2</version>
</dependency>

步骤2：创建UserService接口和显示类

public interface UserService {
    /**
     * 判断用户名是否存在
     */
    public boolean queryUsernameIsExist(String username);
}

@Service
public class UserServiceImpl implements UserService {
    @Autowired
    public UsersMapper usersMapper;
    @Transactional(propagation = Propagation.SUPPORTS)
    @Override
    public boolean queryUsernameIsExist(String username) {
        Example userExample = new Example(Users.class);
        Example.Criteria userCriteria = userExample.createCriteria();
        userCriteria.andEqualTo("username", username);
        Users result = usersMapper.selectOneByExample(userExample);
        return result == null ? false : true;
    }
}

步骤3：返回自定义响应数据结构

import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.databind.ObjectMapper;
/**
 * 
 * @Title: IMOOCJSONResult.java
 * @Package com.imooc.utils
 * @Description: 自定义响应数据结构
 * 				本类可提供给 H5/ios/安卓/公众号/小程序 使用
 * 				前端接受此类数据（json object)后，可自行根据业务去实现相关功能
 * 
 * 				200：表示成功
 * 				500：表示错误，错误信息在msg字段中
 * 				501：bean验证错误，不管多少个错误都以map形式返回
 * 				502：拦截器拦截到用户token出错
 * 				555：异常抛出信息
 * 				556: 用户qq校验异常
 */
public class JSONResult {

    // 定义jackson对象
    private static final ObjectMapper MAPPER = new ObjectMapper();

    // 响应业务状态
    private Integer status;

    // 响应消息
    private String msg;

    // 响应中的数据
    private Object data;
    
    @JsonIgnore
    private String ok;	// 不使用

    public static JSONResult build(Integer status, String msg, Object data) {
        return new JSONResult(status, msg, data);
    }

    public static JSONResult build(Integer status, String msg, Object data, String ok) {
        return new JSONResult(status, msg, data, ok);
    }
    
    public static JSONResult ok(Object data) {
        return new JSONResult(data);
    }

    public static JSONResult ok() {
        return new JSONResult(null);
    }
    
    public static JSONResult errorMsg(String msg) {
        return new JSONResult(500, msg, null);
    }
    
    public static JSONResult errorMap(Object data) {
        return new JSONResult(501, "error", data);
    }
    
    public static JSONResult errorTokenMsg(String msg) {
        return new JSONResult(502, msg, null);
    }
    
    public static JSONResult errorException(String msg) {
        return new JSONResult(555, msg, null);
    }
    
    public static JSONResult errorUserQQ(String msg) {
        return new JSONResult(556, msg, null);
    }

    public JSONResult() {

    }

    public JSONResult(Integer status, String msg, Object data) {
        this.status = status;
        this.msg = msg;
        this.data = data;
    }
    
    public JSONResult(Integer status, String msg, Object data, String ok) {
        this.status = status;
        this.msg = msg;
        this.data = data;
        this.ok = ok;
    }

    public JSONResult(Object data) {
        this.status = 200;
        this.msg = "OK";
        this.data = data;
    }

    public Boolean isOK() {
        return this.status == 200;
    }

    public Integer getStatus() {
        return status;
    }

    public void setStatus(Integer status) {
        this.status = status;
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }

	public String getOk() {
		return ok;
	}

	public void setOk(String ok) {
		this.ok = ok;
	}

}

步骤4：创建UserController访问接口类

@RestController
@RequestMapping("passport")
public class UserController {
    @Autowired
    private UserService userService;
    @GetMapping("/usernameIsExist")
    public JSONResult usernameIsExist(@RequestParam String username) {
        // 1. 判断用户名不能为空
        if (StringUtils.isBlank(username)) {
            return JSONResult.errorMsg("用户名不能为空");
        }
        // 2. 查找注册的用户名是否存在
        boolean isExist = userService.queryUsernameIsExist(username);
        if (isExist) {
            return JSONResult.errorMsg("用户名已经存在");
        }
        // 3. 请求成功，用户名没有重复
        return JSONResult.ok();
    }
}

步骤5：运行结果