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今天测试了一下搭建一个新的SpringMVC项目
然后测试Controller返回String类型，加上了@ResponseBody，访问之后是可以正常访问到返回内容
然后我改成返回Map类型，访问之后直接报错。
在这之前首先我是：
1.没有配置StringHttpMessageConvertor
2.maven没有把json包引入
严重: Servlet.service() for servlet [dispatcher] in context with path [/ElecEmp] threw exception [Request processing failed; nested exception is java.lang.IllegalArgumentException: No converter found for return value of type: class java.util.HashMap] with root cause
java.lang.IllegalArgumentException: No converter found for return value of type: class java.util.HashMap
at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:187)
at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:174)
at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:81)
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:132)
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827)
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738)
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
错误信息里看到没有converter用来处理HashMap
/**
* Writes the given return type to the given output message.
* @param value the value to write to the output message
* @param returnType the type of the value
* @param inputMessage the input messages. Used to inspect the {@code Accept} header.
* @param outputMessage the output message to write to
* @throws IOException thrown in case of I/O errors
* @throws HttpMediaTypeNotAcceptableException thrown when the conditions indicated by {@code Accept} header on
* the request cannot be met by the message converters
*/
@SuppressWarnings("unchecked")
protected <T> void writeWithMessageConverters(T value, MethodParameter returnType,
ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
Object outputValue;
Class<?> valueType;
Type declaredType;
if (value instanceof CharSequence) {
outputValue = value.toString();
valueType = String.class;
declaredType = String.class;
}
else {
outputValue = value;
valueType = getReturnValueType(outputValue, returnType);
declaredType = getGenericType(returnType);
}
HttpServletRequest request = inputMessage.getServletRequest();
List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);
List<MediaType> producibleMediaTypes = getProducibleMediaTypes(request, valueType, declaredType);
if (outputValue != null && producibleMediaTypes.isEmpty()) {
throw new IllegalArgumentException("No converter found for return value of type: " + valueType);
}
..............
看到报错的地方是因为producibleMediaTypes的值为空。getProducibleMediaTypes是获取produces的，看下源码：
/**
* Returns the media types that can be produced:
* <ul>
* <li>The producible media types specified in the request mappings, or
* <li>Media types of configured converters that can write the specific return value, or
* <li>{@link MediaType#ALL}
* </ul>
* @since 4.2
*/
@SuppressWarnings("unchecked")
protected List<MediaType> getProducibleMediaTypes(HttpServletRequest request, Class<?> valueClass, Type declaredType) {
Set<MediaType> mediaTypes = (Set<MediaType>) request.getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
if (!CollectionUtils.isEmpty(mediaTypes)) {
return new ArrayList<MediaType>(mediaTypes);
}
else if (!this.allSupportedMediaTypes.isEmpty()) {
List<MediaType> result = new ArrayList<MediaType>();
for (HttpMessageConverter<?> converter : this.messageConverters) {
if (converter instanceof GenericHttpMessageConverter && declaredType != null) {
if (((GenericHttpMessageConverter<?>) converter).canWrite(declaredType, valueClass, null)) {
result.addAll(converter.getSupportedMediaTypes());
}
}
else if (converter.canWrite(valueClass, null)) {
result.addAll(converter.getSupportedMediaTypes());
}
}
return result;
}
else {
return Collections.singletonList(MediaType.ALL);
}
}
请求的mediaType是从request中获取的，用google浏览器查看请求，看到请求头默认是：
Accept:
text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
因此mediaTypes的值不为空，到了下面的逻辑，是先获取全部支持的mediaType，我这边获取到
[application/octet-stream, text/plain, application/xml, text/xml, application/x-www-form-urlencoded, application/*+xml, multipart/form-data, */*]
而且messageConverter的值如下：
这些值如何获取呢，看源码：
/**
* Return the media types supported by all provided message converters sorted
* by specificity via {@link MediaType#sortBySpecificity(List)}.
*/
private static List<MediaType> getAllSupportedMediaTypes(List<HttpMessageConverter<?>> messageConverters) {
Set<MediaType> allSupportedMediaTypes = new LinkedHashSet<MediaType>();
for (HttpMessageConverter<?> messageConverter : messageConverters) {
allSupportedMediaTypes.addAll(messageConverter.getSupportedMediaTypes());
}
List<MediaType> result = new ArrayList<MediaType>(allSupportedMediaTypes);
MediaType.sortBySpecificity(result);
return Collections.unmodifiableList(result);
}
很明显，意思是获取messageConverters支持的mediatype，关键是看messageConverters是什么值，我看了一下源码，调用了此方法的往上追溯是Resolver，Adapter方法
也就是适配器，用来查询适配的controller，再往上到顶，是WebMvcConfigurationSupport（带有@Bean），调用了
/**
* Provides access to the shared {@link HttpMessageConverter}s used by the
* {@link RequestMappingHandlerAdapter} and the
* {@link ExceptionHandlerExceptionResolver}.
* This method cannot be overridden.
* Use {@link #configureMessageConverters(List)} instead.
* Also see {@link #addDefaultHttpMessageConverters(List)} that can be
* used to add default message converters.
*/
protected final List<HttpMessageConverter<?>> getMessageConverters() {
if (this.messageConverters == null) {
this.messageConverters = new ArrayList<HttpMessageConverter<?>>();
configureMessageConverters(this.messageConverters);
if (this.messageConverters.isEmpty()) {
addDefaultHttpMessageConverters(this.messageConverters);
}
extendMessageConverters(this.messageConverters);
}
return this.messageConverters;
}
configureMessageConverters是抽象方法，我觉得应该是实现了此方法的类可以对messageConverters做新的配置。
假如没有做配置，那么就还是空，因为开头我说了我没有配置messageConverters，所以这里拿到的messageConverters是空的。所以会调用addDefaultHttpMessageConverters
源码：
/**
* Adds a set of default HttpMessageConverter instances to the given list.
* Subclasses can call this method from {@link #configureMessageConverters(List)}.
* @param messageConverters the list to add the default message converters to
*/
protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
StringHttpMessageConverter stringConverter = new StringHttpMessageConverter();
stringConverter.setWriteAcceptCharset(false);
messageConverters.add(new ByteArrayHttpMessageConverter());
messageConverters.add(stringConverter);
messageConverters.add(new ResourceHttpMessageConverter());
messageConverters.add(new SourceHttpMessageConverter<Source>());
messageConverters.add(new AllEncompassingFormHttpMessageConverter());
if (romePresent) {
messageConverters.add(new AtomFeedHttpMessageConverter());
messageConverters.add(new RssChannelHttpMessageConverter());
}
if (jackson2XmlPresent) {
ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.xml().applicationContext(this.applicationContext).build();
messageConverters.add(new MappingJackson2XmlHttpMessageConverter(objectMapper));
}
else if (jaxb2Present) {
messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
}
if (jackson2Present) {
ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.json().applicationContext(this.applicationContext).build();
messageConverters.add(new MappingJackson2HttpMessageConverter(objectMapper));
}
else if (gsonPresent) {
messageConverters.add(new GsonHttpMessageConverter());
}
}
所以默认的话MessageConverter有5个ByteArrayHttpMessageConverter，stringConverter，ResourceHttpMessageConverter，SourceHttpMessageConverter，AllEncompassingFormHttpMessageConverter
源码里也看到了：
private static boolean romePresent =
ClassUtils.isPresent("com.rometools.rome.feed.WireFeed", WebMvcConfigurationSupport.class.getClassLoader());
private static final boolean jaxb2Present =
ClassUtils.isPresent("javax.xml.bind.Binder", WebMvcConfigurationSupport.class.getClassLoader());
private static final boolean jackson2Present =
ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper", WebMvcConfigurationSupport.class.getClassLoader()) &&
ClassUtils.isPresent("com.fasterxml.jackson.core.JsonGenerator", WebMvcConfigurationSupport.class.getClassLoader());
private static final boolean jackson2XmlPresent =
ClassUtils.isPresent("com.fasterxml.jackson.dataformat.xml.XmlMapper", WebMvcConfigurationSupport.class.getClassLoader());
private static final boolean gsonPresent =
ClassUtils.isPresent("com.google.gson.Gson", WebMvcConfigurationSupport.class.getClassLoader());
.....................
/**
* Determine whether the {@link Class} identified by the supplied name is present
* and can be loaded. Will return {@code false} if either the class or
* one of its dependencies is not present or cannot be loaded.
* @param className the name of the class to check
* @param classLoader the class loader to use
* (may be {@code null}, which indicates the default class loader)
* @return whether the specified class is present
*/
public static boolean isPresent(String className, ClassLoader classLoader) {
try {
forName(className, classLoader);
return true;
}
catch (Throwable ex) {
// Class or one of its dependencies is not present...
return false;
}
}
这些值的定义，所以假如maven没有配json相关的jar包，MessageConverter也就没有相应的处理json的converter。
所以回到开始那里，因为没有一个converter可以写hashmap到响应体里，所以就会报错。
所以结论就是，只要配置了json相关的jar包，就可以返回json类型的数据。比如com.fasterxml.jackson