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

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

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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 responseBodyConverter() {
        StringHttpMessageConverter converter = new StringHttpMessageConverter(
                Charset.forName("UTF-8"));
        return converter;
    }

    @Override
    protected void configureMessageConverters(List> 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

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 responseBodyConverter() {
        StringHttpMessageConverter converter = new StringHttpMessageConverter(
                Charset.forName("UTF-8"));
        return converter;
    }

    @Override
    public void configureMessageConverters(List> 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> 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 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