SpringBoot2.X 遭遇 No converter found for return value of type: class java.util.LinkedHashMap

Love The Way You Lie 2022-05-11 13:36 177阅读 0赞

前几天，在项目上线时遭遇了 No converter found for return value of type: class java.util.LinkedHashMap 异常，异常的糟心唉。。。

**场景重现**：  
return 基本类型木有问题，但是return 对象的时候就报出了该问题，  
**网上答案汇总**：  
1、缺失Jackson 依赖，如果有 `spring-boot-starter-web`依赖可排除该问题  
2、是否是 POJO （get set 方法是否有）  
`注`： 我使用lombok插件，同样排除了此项可能性  
3、注解是否是 @RestController

--------------------

以上可能性完全排除，由于上线紧急，暂时采用了  
`HttpServletReponse.getWriter().print()` 的方式

昨天在朋友帮助下，推测出是Swagger2 的问题，便没有在继续进行下去，然而一小时前采用一个demo进行测试排查是，发现并不是，继续昨天的思路，问题可能出现在拦截器那里，于是乎，开始从拦截器分析，就在刚刚，终于分析出了问题所在…

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— 2018/11/07 凌晨 00:30 重新改写，(上周，今晚抽空改写)在一位网友(微信昵称 rookie\_cc )的帮助下发现之前的文章存在巨大漏洞

**问题出现原因**：  
*`extends WebMvcConfigurationSupport`*，这种方式会屏蔽springboot `@EnableAutoConfiguration`中的设置

我的源码是这样的：

/**
     * 国际化默认语言配置及编码方式修改
     * @author fxbin
     * @version v1.0
     * @since 2018/10/23 14:38
     */
    @Configuration
    public class I18nConfig extends WebMvcConfigurationSupport {
    
        @Bean
        public LocaleResolver localeResolver() {
            CookieLocaleResolver localeResolver = new CookieLocaleResolver();
            //localeResolver.setCookieName("lang");
            //设置默认区域
            localeResolver.setDefaultLocale(new Locale("en", "US"));
            //设置cookie有效期.
            localeResolver.setCookieMaxAge(-1);
    
            return localeResolver;
        }
    
        @Bean
        public HttpMessageConverter<String> responseBodyConverter() {
            StringHttpMessageConverter converter = new StringHttpMessageConverter(
                    Charset.forName("UTF-8"));
            return converter;
        }
    
        @Override
        protected void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
            super.configureMessageConverters(converters);
            converters.add(responseBodyConverter());
        }
    
        /**
         * 解决SpringBoot2.X 加入拦截器后swagger2不能访问问题
         * @author fxbin
         * @param registry
         */
        @Override
        public void addInterceptors(InterceptorRegistry registry) {
            registry.addInterceptor(new LocaleChangeInterceptor())
                    .excludePathPatterns("/swagger-resources/**", "/webjars/**", "/v2/**", "/swagger-ui.html/**");
        }
    
        @Override
        public void addResourceHandlers(ResourceHandlerRegistry registry) {
            registry.addResourceHandler("swagger-ui.html")
                    .addResourceLocations("classpath:/META-INF/resources/");
            registry.addResourceHandler("/webjars/**")
                    .addResourceLocations("classpath:/META-INF/resources/webjars/");
        }
    }

查看Spring Boot启动WebMVC自动配置的条件发现 ：

@Configuration
    @ConditionalOnWebApplication(type = Type.SERVLET)
    @ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })
    @ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
    @AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
    @AutoConfigureAfter({ DispatcherServletAutoConfiguration.class,
    		ValidationAutoConfiguration.class })
    public class WebMvcAutoConfiguration {

`@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)`，只有当容器中没有WebMvcConfigurationSupport这个类型的组件的时候，才会启动自动配置。

所以当我们继承`WebMvcConfigurationSupport`之后，除非在继承类中重写了一系列有关WebMVC的配置，否则可能就会遇到静态资源访问不到，返回数据不成功这些一系列问题了。

鉴于SpringBoot2.X 中 `WebMvcConfigurerAdapter` 已经过时，所以可以实现 `WebMvcConfigurer` 接口来达到相同的目的  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z4YmluMTIz_size_16_color_FFFFFF_t_70]

我的实现代码如下

/**
     * 国际化默认语言配置及编码方式修改
     * @author fxbin
     * @version v1.1
     * @since 2018/8/23 14:38
     * @see org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration
     * @see org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter
     */
    @Configuration
    public class I18nConfig implements WebMvcConfigurer {
    
        @Bean
        public LocaleResolver localeResolver() {
            CookieLocaleResolver localeResolver = new CookieLocaleResolver();
            //localeResolver.setCookieName("lang");
            //设置默认区域
            localeResolver.setDefaultLocale(new Locale("en", "US"));
            //设置cookie有效期.
            localeResolver.setCookieMaxAge(-1);
    
            return localeResolver;
        }
    
        @Bean
        public HttpMessageConverter<String> responseBodyConverter() {
            StringHttpMessageConverter converter = new StringHttpMessageConverter(
                    Charset.forName("UTF-8"));
            return converter;
        }
    
        @Override
        public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
            converters.add(responseBodyConverter());
        }
    }

总结：  
1、@EnableWebMvc=WebMvcConfigurationSupport，使用了@EnableWebMvc注解等于扩展了WebMvcConfigurationSupport但是没有重写任何方法。`@EnableWebMvc` 和`WebMvcConfigurationSupport` 二者只能选一个，如果同时实现，则会失效 （感谢一位网友热心帮助，开始忽略了这个问题）  
2、继承`WebMvcConfigurationSupport`， 需要重写一系列有关WebMVC的配置，否则可能就会遇到静态资源访问不到，返回数据不成功这些一系列问题了（我就是掉在此坑），  
因为springboot已经实例化了`WebMvcConfigurationSupport`， 如果使用`@EnableWebMvc` 注解或者继承`WebMvcConfigurationSupport`会覆盖默认实现， 一般建议还是不覆盖默认的好  
3、实现`WebMvcConfigurer` 接口  
示例：

import com.alibaba.fastjson.serializer.SerializerFeature;
    import com.alibaba.fastjson.support.config.FastJsonConfig;
    import com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter;
    import org.springframework.context.annotation.Configuration;
    import org.springframework.format.FormatterRegistry;
    import org.springframework.format.datetime.DateFormatter;
    import org.springframework.http.MediaType;
    import org.springframework.http.converter.HttpMessageConverter;
    import org.springframework.web.servlet.ModelAndView;
    import org.springframework.web.servlet.config.annotation.CorsRegistry;
    import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
    import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
    import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
    import org.springframework.web.servlet.handler.HandlerInterceptorAdapter;
    
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import java.time.LocalDate;
    import java.util.ArrayList;
    import java.util.List;
    
    /**
     * 实现WebMvcConfigurer接口的部分示例...
     *
     * @author fxbin
     * @version v1.0
     * @since 2018/11/7 1:21
     */
    @Configuration
    public class WebConfig implements WebMvcConfigurer {
    
        /**
         * 添加类型转换器和格式化器
         * @author fxbin
         * @param registry
         */
        @Override
        public void addFormatters(FormatterRegistry registry) {
            registry.addFormatterForFieldType(LocalDate.class, new DateFormatter());
        }
    
        /**
         * 跨域支持
         * @author fxbin
         * @param registry
         */
        @Override
        public void addCorsMappings(CorsRegistry registry) {
            registry.addMapping("/**")
                    .allowedOrigins("*")
                    .allowCredentials(true)
                    .allowedMethods("GET", "POST", "DELETE", "PUT")
                    .maxAge(3600 * 24);
        }
    
        /**
         * 添加静态资源--过滤swagger-api (开源的在线API文档)
         * @author fxbin
         * @param registry
         */
        @Override
        public void addResourceHandlers(ResourceHandlerRegistry registry) {
            //过滤swagger
            registry.addResourceHandler("swagger-ui.html")
                    .addResourceLocations("classpath:/META-INF/resources/");
    
            registry.addResourceHandler("/webjars/**")
                    .addResourceLocations("classpath:/META-INF/resources/webjars/");
    
            registry.addResourceHandler("/swagger-resources/**")
                    .addResourceLocations("classpath:/META-INF/resources/swagger-resources/");
    
            registry.addResourceHandler("/swagger/**")
                    .addResourceLocations("classpath:/META-INF/resources/swagger*");
    
            registry.addResourceHandler("/v2/api-docs/**")
                    .addResourceLocations("classpath:/META-INF/resources/v2/api-docs/");
    
        }
    
        /**
         * 配置消息转换器--这里我用的是alibaba 开源的 fastjson
         * @author fxbin
         * @param converters
         */
        @Override
        public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
            //1.需要定义一个convert转换消息的对象;
            FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter();
            //2.添加fastJson的配置信息，比如：是否要格式化返回的json数据;
            FastJsonConfig fastJsonConfig = new FastJsonConfig();
            fastJsonConfig.setSerializerFeatures(SerializerFeature.PrettyFormat,
                    SerializerFeature.WriteMapNullValue,
                    SerializerFeature.WriteNullStringAsEmpty,
                    SerializerFeature.DisableCircularReferenceDetect,
                    SerializerFeature.WriteNullListAsEmpty,
                    SerializerFeature.WriteDateUseDateFormat);
            //3处理中文乱码问题
            List<MediaType> fastMediaTypes = new ArrayList<>();
            fastMediaTypes.add(MediaType.APPLICATION_JSON_UTF8);
            //4.在convert中添加配置信息.
            fastJsonHttpMessageConverter.setSupportedMediaTypes(fastMediaTypes);
            fastJsonHttpMessageConverter.setFastJsonConfig(fastJsonConfig);
            //5.将convert添加到converters当中.
            converters.add(fastJsonHttpMessageConverter);
        }
    
        /**
         * 添加自定义的拦截器
         * @author fxbin
         * @param registry
         */
        @Override
        public void addInterceptors(InterceptorRegistry registry) {
            registry.addInterceptor(new MyInterceptor()).addPathPatterns("/**");
        }
    }
    
    
    /**
     * 拦截器
     * @author fxbin
     * @version v1.0
     * @since 2018/11/7 1:25
     */
    class MyInterceptor extends HandlerInterceptorAdapter {
        @Override
        public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
            System.out.println("Interceptor preHandler method is running !");
            return super.preHandle(request, response, handler);
        }
    
        @Override
        public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
            System.out.println("Interceptor postHandler method is running !");
            super.postHandle(request, response, handler, modelAndView);
        }
    
        @Override
        public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
            System.out.println("Interceptor afterCompletion method is running !");
            super.afterCompletion(request, response, handler, ex);
        }
    
        @Override
        public void afterConcurrentHandlingStarted(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
            System.out.println("Interceptor afterConcurrentHandlingStarted method is running !");
            super.afterConcurrentHandlingStarted(request, response, handler);
        }
    }

— end —

*`如有问题，请在评论区留言交流，谢谢`*

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