深入浅出Spring Boot接口

如何优雅的写 Controller 层代码？
瞧瞧，人家这后端API接口写得，那叫一个巴适~，再看看我的，像坨屎！

统一返回格式

定义一个业务CODE枚举类

public enum ResultCodeEnum {

    SUCCESS(1, "成功"),
    //参数错误

    ILLEGAL_PARAMETER(10001, "非法参数");

    private Integer code;
    private String msg;

    ResultCodeEnum(Integer code, String msg) {
        this.code = code;
        this.msg = msg;
    }

    public Integer getCode() {
        return code;
    }
    
    public String getMsg() {
        return msg;
    }
}
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定义统一返回类

public class R<T> implements Serializable {

    private T data;
    private Integer code;
    private String msg;

    public static <T> R<T> ok(T data) {
        R<T> r = new R<>();
        r.setData(data);
        return r;
    }

    public static <T> R<T> ok(ResultCodeEnum codeEnum, T data) {
        R<T> r = new R<>();
        r.setData(data);
        r.setCode(codeEnum.getCode());
        r.setMsg(codeEnum.getMsg());
        return r;
    }

    public static <T> R<T> fail(Integer code, String msg) {
        R<T> r = new R<>();
        r.setCode(code);
        r.setMsg(msg);
        return r;
    }

    public static <T> R<T> fail(ResultCodeEnum codeEnum) {
        R<T> r = new R<>();
        r.setCode(codeEnum.getCode());
        r.setMsg(codeEnum.getMsg());
        return r;
    }
	//省略部分代码
}
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这样每个接口都需要使用R.ok()包裹返回值，下面我们通过一些配置来实现自动包裹。

定义一个封装注解

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
public @interface Result {
}
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ResponseBodyAdvice接口

Allows customizing the response after the execution of an @ResponseBody or a ResponseEntity controller method but before the body is written with an HttpMessageConverter.

这个接口作用在Controller返回结果之后和written with an HttpMessageConverter之前。

true if beforeBodyWrite should be invoked; false otherwise

重写supports方法 ，返回true调用beforeBodyWrite 。

    @Override
    public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
        //方法所在class
        Class<?> methodClass = returnType.getContainingClass();
        //获取注解
        Result r = methodClass.getAnnotation(Result.class);
        //如果不存在不处理
        if (ObjectUtils.isEmpty(r)) {
            return false;
        }
        //如果已经是R返回false，如果不是R返回true
        boolean assignableFrom = returnType.getParameterType().isAssignableFrom(R.class);
        return !assignableFrom;
    }
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当接口返回String类型报错如下

R cannot be cast to java.lang.String
	at org.springframework.http.converter.StringHttpMessageConverter.addDefaultHeaders
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Invoked after an HttpMessageConverter is selected and just before its write method is invoked.

beforeBodyWrite 调用在HttpMessageConverter选择后和write 方法调用之前。

重写beforeBodyWrite方法

    @Override
    public Object beforeBodyWrite(Object body, MethodParameter returnType,
                                  MediaType selectedContentType,
                                  Class<? extends HttpMessageConverter<?>> selectedConverterType,
                                  ServerHttpRequest request,
                                  ServerHttpResponse response) {

        if (body instanceof String) {
            try {
                ObjectMapper objectMapper = new ObjectMapper();
                //修改返回类型
                response.getHeaders().add("Content-Type", MediaType.APPLICATION_JSON_VALUE);
                R<Object> r = R.ok(body);
                return objectMapper.writeValueAsString(r);
            } catch (JsonProcessingException e) {
                e.printStackTrace();
            }
        }
        return R.ok(body);
    }
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到这里我们就不需要使用R.ok来包裹返回类型了。

selectedContentType和selectedConverterType是选定的ContentType和ConverterType

打印一下这两个值

        logger.info("selectedContentType:" + selectedContentType);
        logger.info("selectedConverterType:" + selectedConverterType);
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输出

//String
selectedConverterType:class org.springframework.http.converter.StringHttpMessageConverter
selectedContentType:text/html
//其他
selectedContentType:application/json
selectedConverterType:class org.springframework.http.converter.json.MappingJackson2HttpMessageConverter
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那么谁调用的 beforeBodyWrite 这个方法呢 ？

beforeBodyWrite 调用过程

在方法内部打一个断点定位到RequestResponseBodyAdviceChain，processBody中141行调用beforeBodyWrite

				body = ((ResponseBodyAdvice<T>) advice).beforeBodyWrite((T) body, returnType,
						contentType, converterType, request, response);
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beforeBodyWrite中116行调用processBody

		return processBody(body, returnType, contentType, converterType, request, response);
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AbstractMessageConverterMethodProcessor中268行调用了beforeBodyWrite

					body = getAdvice().beforeBodyWrite(body, returnType, selectedMediaType,
							(Class<? extends HttpMessageConverter<?>>) converter.getClass(),
							inputMessage, outputMessage);
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全局异常处理

迭代到这里，只需要在类上加入@Result注解就可以实现接口的封装处理，那么异常的情况呢？

在ResultBodyAdvice中加入这一段代码，通过code和msg来统一处理异常。

    @ExceptionHandler(Exception.class)
    public R runtimeExceptionHandle(Throwable e) {
        if (RuntimeException.class.equals(e.getClass())) {
            return R.fail(ResultCodeEnum.SERVER_ERROR);
        } else if (IllegalArgumentException.class.equals(e.getClass())) {
            return R.fail(ResultCodeEnum.ILLEGAL_PARAMETER);
        } else {
            return R.fail(3000, "其他错误");
        }
    }
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参数校验

SpringBoot 还在用 if 校验参数？那你真的太low了，老司机都是这么玩的！

目的：消灭 if 条件判断。

约定：Get请求不传body，Post请求通过body传参。
RequestParam绑定url参数，RequestBody 绑定body体中参数。

@RequestParam("id") Integer id
@RequestBody String name
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@Validated

添加依赖

        <dependency>
            <groupId>org.springframework.bootgroupId>
            <artifactId>spring-boot-starter-validationartifactId>
        dependency>
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public class Hello6VO {
    @Min(value = 10, message = "最小值10")
    private Integer id;

    @Min(value = 5, message = "最小值5")
    private Integer id2;
}
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    @GetMapping("hello6")
    public void hello6(@Validated Hello6VO vo) {
        System.out.println(vo.getId());
    }

    @PostMapping("hello7")
    public void hello7(@RequestBody @Validated  Hello6VO vo) {
        System.out.println(vo.getId());
    }
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捕获失败异常

else if (e instanceof org.springframework.validation.BindException) {
            BindException exception = (org.springframework.validation.BindException) e;
            //优先处理字段错误
            List<FieldError> fieldErrorsList = exception.getBindingResult().getFieldErrors();
            List<String> fieldErrorMessageList = new ArrayList<>(fieldErrorsList.size());
            //获取默认提示信息列表
            for (FieldError fieldError : fieldErrorsList) {
                //字段名
                String field = fieldError.getField();
                //提示信息
                String message = fieldError.getDefaultMessage();
                //拒绝的值
                Object rejectedValue = fieldError.getRejectedValue();
                String str = "字段:" + field + " 提示信息:" + message + " 拒绝的值:" + rejectedValue;
                fieldErrorMessageList.add(str);
            }
            return R.fail(ResultCodeEnum.VALIDATE_FAILED.getCode(), fieldErrorMessageList.toString());
        }
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输出

{
  "data": null,
  "code": 1002,
  "msg": "[字段:id 提示信息:最小值10 拒绝的值:1, 字段:id2 提示信息:最小值5 拒绝的值:2]"
}
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分组校验

通用思想

后端思想篇：设计好接口的36个锦囊！

性能优化

7种方式，教你提升 SpringBoot 项目的吞吐量
Spring Boot 程序优化的 14 个小妙招！
实战总结！18种接口优化方案的总结