一、遇到的问题

1. 在请求时get方法路径传参，接收时，用枚举接收，出现参数绑定错误

请求路径：http://localhost:9104/api/sent/test2?type=0

后端代码：

@GetMapping("/test2")
    public String openNewFile2(FileDTO param) {

        System.out.println("=====" + param);
        return "222";
    }

@Data
public class FileDTO {

    private SortTypeEnum type;
}

枚举类：

@Getter
@AllArgsConstructor
public enum SortTypeEnum {
    /**
     * 生序
     */
    ASC(0,"生序"),

    /**
     * 降序
     */
    DESC(1,"降序"),
    ;

    @JsonValue
    private final Integer code;
    private final String Label;
}

org.springframework.validation.BindException: org.springframework.validation.BeanPropertyBindingResult

org.springframework.validation.BindException: org.springframework.validation.BeanPropertyBindingResult: 1 errors\nField error in object 'fileDTO' on field 'type': rejected value [0]; codes [typeMismatch.fileDTO.type,typeMismatch.type,typeMismatch.com.example.demoes.enums.SortTypeEnum,typeMismatch]; arguments [org.springframework.context.support.DefaultMessageSourceResolvable: codes [fileDTO.type,type]; arguments []; default message [type]]; default message [Failed to convert property value of type 'java.lang.String' to required type 'com.example.demoes.enums.SortTypeEnum' for property 'type'; nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [com.example.demoes.enums.SortTypeEnum] for value '0'; nested exception is java.lang.IllegalArgumentException: No enum constant com.example.demoes.enums.SortTypeEnum.0]\r\n\tat org.springframework.web.method.annotation.ModelAttributeMethodProcessor.resolveArgument(ModelAttributeMethodProcessor.java:164)\r\n\tat org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:121)\r\n\tat org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:167)\r\n\tat org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:134)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:105)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:878)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:792)\r\n\tat org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)\r\n\tat org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040)\r\n\tat org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943)\r\n\tat org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)\r\n\tat org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)\r\n\tat javax.servlet.http.HttpServlet.service(HttpServlet.java:626)\r\n\tat org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)\r\n\tat javax.servlet.http.HttpServlet.service(HttpServlet.java:733)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)\r\n\tat org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)\r\n\tat org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)\r\n\tat org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)\r\n\tat org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)\r\n\tat org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)\r\n\tat org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)\r\n\tat org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)\r\n\tat org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202)\r\n\tat org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)\r\n\tat org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)\r\n\tat org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)\r\n\tat org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)\r\n\tat org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)\r\n\tat org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)\r\n\tat org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:373)\r\n\tat org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)\r\n\tat org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)\r\n\tat org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1589)\r\n\tat org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)\r\n\tat java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)\r\n\tat java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)\r\n\tat org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)\r\n\tat java.lang.Thread.run(Thread.java:745)\r\n

经过排查发现，在StringToEnumConverterFactory---convert

public T convert(String source) {
    return source.isEmpty() ? null : Enum.valueOf(this.enumType, source.trim());
}

这个代码的时候，出现错误，上边代码是通过枚举的name去转换成枚举，实现参数绑定。

如果页面传枚举下标就会出错。

正确请求方式：

http://localhost:9104/api/sent/test2?type=ASC

这样就能绑定到参数上了。

2. 如果是post请求，就可以需要body传参

请求路径：

http://localhost:9104/api/sent/test4

body参数：

{
    "type":1
}

后端代码：

@PostMapping("/test4")
    public String openNewFile5(@RequestBody FileDTO param) {

        System.out.println("=====" + param);
        return "222";
    }

二、源码分析：

1. get参数绑定

在项目启动时会初始化默认的参数转换类：org.springframework.boot.autoconfigure.BackgroundPreinitializer.ConversionServiceInitializer

private static class ConversionServiceInitializer implements Runnable {
        private ConversionServiceInitializer() {
        }
// 会执行这个方法，初始化
        public void run() {
            new DefaultFormattingConversionService();
        }
    }

public DefaultFormattingConversionService(@Nullable StringValueResolver embeddedValueResolver, boolean registerDefaultFormatters) {
        if (embeddedValueResolver != null) {
            this.setEmbeddedValueResolver(embeddedValueResolver);
        }
        // 开始注册默认的
        DefaultConversionService.addDefaultConverters(this);
//这个方法也会执行
        if (registerDefaultFormatters) {
            addDefaultFormatters(this);
        }

    }

继续走到下面org.springframework.core.convert.support.DefaultConversionService#addDefaultConverters

这个方法会注册很多数据类型转换类

 public static void addDefaultConverters(ConverterRegistry converterRegistry) {
        addScalarConverters(converterRegistry);
        addCollectionConverters(converterRegistry);
        converterRegistry.addConverter(new ByteBufferConverter((ConversionService)converterRegistry));
        converterRegistry.addConverter(new StringToTimeZoneConverter());
        converterRegistry.addConverter(new ZoneIdToTimeZoneConverter());
        converterRegistry.addConverter(new ZonedDateTimeToCalendarConverter());
        converterRegistry.addConverter(new ObjectToObjectConverter());
        converterRegistry.addConverter(new IdToEntityConverter((ConversionService)converterRegistry));
        converterRegistry.addConverter(new FallbackObjectToStringConverter());
        converterRegistry.addConverter(new ObjectToOptionalConverter((ConversionService)converterRegistry));
    }

在spring容器启动过程中，有个后置处理器

org.springframework.boot.SpringApplication#postProcessApplicationContext

protected void postProcessApplicationContext(ConfigurableApplicationContext context) {
        if (this.beanNameGenerator != null) {
            context.getBeanFactory().registerSingleton("org.springframework.context.annotation.internalConfigurationBeanNameGenerator", this.beanNameGenerator);
        }

        if (this.resourceLoader != null) {
            if (context instanceof GenericApplicationContext) {
                ((GenericApplicationContext)context).setResourceLoader(this.resourceLoader);
            }

            if (context instanceof DefaultResourceLoader) {
                ((DefaultResourceLoader)context).setClassLoader(this.resourceLoader.getClassLoader());
            }
        }
//  这里会获取上边实现*Factory的转换类，放入conversionService字段中
        if (this.addConversionService) {
            context.getBeanFactory().setConversionService(ApplicationConversionService.getSharedInstance());
        }

    }

这里会有166个转换类，被加载好放进去

下面进行请求访问

第一次请求会初始化servlet

org.springframework.web.servlet.DispatcherServlet#initStrategies---

注册本地的转换

initLocaleResolver(context);

会创建bean--org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver

将上边启动时创建好的ConversionService放入下边的字段中

org.springframework.beans.PropertyEditorRegistrySupport#setConversionService

由请求转发类到参数解析

org.springframework.web.method.support.HandlerMethodArgumentResolverComposite#resolveArgument

public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer, NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
        HandlerMethodArgumentResolver resolver = this.getArgumentResolver(parameter);
        if (resolver == null) {
            throw new IllegalArgumentException("Unsupported parameter type [" + parameter.getParameterType().getName() + "]. supportsParameter should be called first.");
        } else {
// 走这里参数解析
            return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
        }
    }

绑定参数

this.bindRequestParameters(binder, webRequest);

protected void bindRequestParameters(WebDataBinder binder, NativeWebRequest request) {
		ServletRequest servletRequest = request.getNativeRequest(ServletRequest.class);
		Assert.state(servletRequest != null, "No ServletRequest");
		ServletRequestDataBinder servletBinder = (ServletRequestDataBinder) binder;
// 下面参数绑定，先从请求中获取参数
		servletBinder.bind(servletRequest);
	}

org.springframework.web.bind.ServletRequestDataBinder#bind

public void bind(ServletRequest request) {
// 请求获取参数
        MutablePropertyValues mpvs = new ServletRequestParameterPropertyValues(request);
        MultipartRequest multipartRequest = (MultipartRequest)WebUtils.getNativeRequest(request, MultipartRequest.class);
        if (multipartRequest != null) {
            this.bindMultipart(multipartRequest.getMultiFileMap(), mpvs);
        }

        this.addBindValues(mpvs, request);
        this.doBind(mpvs);
    }

从请求获取参数的方法，都是用String接收的

org.springframework.web.util.WebUtils#getParametersStartingWith