SpringMVC源码总结（七）mvc:annotation-driven中的HttpMessageConverter

这一篇文章主要介绍下HttpMessageConverter整个注册过程包含自定义的HttpMessageConverter，然后对一些HttpMessageConverter进行具体介绍。

HttpMessageConverter接口介绍：

public interface HttpMessageConverter<T> {

	/**
	 * Indicates whether the given class can be read by this converter.
	 * @param clazz the class to test for readability
	 * @param mediaType the media type to read, can be {@code null} if not specified.
	 * Typically the value of a {@code Content-Type} header.
	 * @return {@code true} if readable; {@code false} otherwise
	 */
	boolean canRead(Class<?> clazz, MediaType mediaType);

	/**
	 * Indicates whether the given class can be written by this converter.
	 * @param clazz the class to test for writability
	 * @param mediaType the media type to write, can be {@code null} if not specified.
	 * Typically the value of an {@code Accept} header.
	 * @return {@code true} if writable; {@code false} otherwise
	 */
	boolean canWrite(Class<?> clazz, MediaType mediaType);

	/**
	 * Return the list of {@link MediaType} objects supported by this converter.
	 * @return the list of supported media types
	 */
	List<MediaType> getSupportedMediaTypes();

	/**
	 * Read an object of the given type form the given input message, and returns it.
	 * @param clazz the type of object to return. This type must have previously been passed to the
	 * {@link #canRead canRead} method of this interface, which must have returned {@code true}.
	 * @param inputMessage the HTTP input message to read from
	 * @return the converted object
	 * @throws IOException in case of I/O errors
	 * @throws HttpMessageNotReadableException in case of conversion errors
	 */
	T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException;

	/**
	 * Write an given object to the given output message.
	 * @param t the object to write to the output message. The type of this object must have previously been
	 * passed to the {@link #canWrite canWrite} method of this interface, which must have returned {@code true}.
	 * @param contentType the content type to use when writing. May be {@code null} to indicate that the
	 * default content type of the converter must be used. If not {@code null}, this media type must have
	 * previously been passed to the {@link #canWrite canWrite} method of this interface, which must have
	 * returned {@code true}.
	 * @param outputMessage the message to write to
	 * @throws IOException in case of I/O errors
	 * @throws HttpMessageNotWritableException in case of conversion errors
	 */
	void write(T t, MediaType contentType, HttpOutputMessage outputMessage)
			throws IOException, HttpMessageNotWritableException;

}

从HttpInputMessage中读取数据： T read(Class<? extends T> clazz, HttpInputMessage inputMessage)，前提clazz能够通过canRead(clazz，mediaType)测试。
向HttpOutputMessage中写入数据：void write(T t, MediaType contentType, HttpOutputMessage outputMessage)，前提能够通过canWrite方法。

简单举例：
如StringHttpMessageConverter，read方法就是根据编码类型将HttpInputMessage中的数据变为字符串。write方法就是根据编码类型将字符串数据写入HttpOutputMessage中。

HttpMessageConverter的使用场景：
它主要是用来转换request的内容到一定的格式，转换输出的内容的到response。
看下自定义的使用方式：

<mvc:annotation-driven>
		<mvc:message-converters register-defaults="true">
			<bean class="org.springframework.http.converter.StringHttpMessageConverter">
				<constructor-arg value="UTF-8"/>
			</bean>
		</mvc:message-converters>
	</mvc:annotation-driven>

首先还是在对mvc:annotation-driven解析的AnnotationDrivenBeanDefinitionParser中，有这么一个方法：

ManagedList<?> messageConverters = getMessageConverters(element, source, parserContext);

获取所有的HttpMessageConverter，最终设置到RequestMappingHandlerAdapter的private List<HttpMessageConverter<?>> messageConverters属性上。看下具体的获取过程：

private ManagedList<?> getMessageConverters(Element element, Object source, ParserContext parserContext) {
		Element convertersElement = DomUtils.getChildElementByTagName(element, "message-converters");
		ManagedList<? super Object> messageConverters = new ManagedList<Object>();
		if (convertersElement != null) {
			messageConverters.setSource(source);
			for (Element beanElement : DomUtils.getChildElementsByTagName(convertersElement, "bean", "ref")) {
				Object object = parserContext.getDelegate().parsePropertySubElement(beanElement, null);
				messageConverters.add(object);
			}
		}

		if (convertersElement == null || Boolean.valueOf(convertersElement.getAttribute("register-defaults"))) {
			messageConverters.setSource(source);
			messageConverters.add(createConverterDefinition(ByteArrayHttpMessageConverter.class, source));

			RootBeanDefinition stringConverterDef = createConverterDefinition(StringHttpMessageConverter.class, source);
			stringConverterDef.getPropertyValues().add("writeAcceptCharset", false);
			messageConverters.add(stringConverterDef);

			messageConverters.add(createConverterDefinition(ResourceHttpMessageConverter.class, source));
			messageConverters.add(createConverterDefinition(SourceHttpMessageConverter.class, source));
			messageConverters.add(createConverterDefinition(AllEncompassingFormHttpMessageConverter.class, source));

			if (romePresent) {
				messageConverters.add(createConverterDefinition(AtomFeedHttpMessageConverter.class, source));
				messageConverters.add(createConverterDefinition(RssChannelHttpMessageConverter.class, source));
			}
			if (jaxb2Present) {
				messageConverters.add(createConverterDefinition(Jaxb2RootElementHttpMessageConverter.class, source));
			}
			if (jackson2Present) {
				messageConverters.add(createConverterDefinition(MappingJackson2HttpMessageConverter.class, source));
			}
			else if (jacksonPresent) {
				messageConverters.add(createConverterDefinition(
						org.springframework.http.converter.json.MappingJacksonHttpMessageConverter.class, source));
			}
		}
		return messageConverters;
	}

该过程第一步：
解析并获取我们自定义的HttpMessageConverter，
该过程第二步：
<mvc:message-converters register-defaults="true">有一个register-defaults属性，当为true时，仍然注册默认的HttpMessageConverter，当为false则不注册，仅仅使用用户自定义的HttpMessageConverter。

获取完毕，便会将这些HttpMessageConverter设置进RequestMappingHandlerAdapter的messageConverters属性中。

然后就是它的使用过程，HttpMessageConverter主要针对那些不会返回view视图的response：
即含有方法含有@ResponseBody或者返回值为HttpEntity等类型的，它们都会用到HttpMessageConverter。以@ResponseBody举例：
首先先决定由哪个HandlerMethodReturnValueHandler来处理返回值，由于是@ResponseBody所以将会由RequestResponseBodyMethodProcessor来处理，然后就是如下的写入：

protected <T> void writeWithMessageConverters(T returnValue, MethodParameter returnType,
			ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
			throws IOException, HttpMediaTypeNotAcceptableException {

		Class<?> returnValueClass = returnValue.getClass();
		HttpServletRequest servletRequest = inputMessage.getServletRequest();
		List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(servletRequest);
		List<MediaType> producibleMediaTypes = getProducibleMediaTypes(servletRequest, returnValueClass);

		Set<MediaType> compatibleMediaTypes = new LinkedHashSet<MediaType>();
		for (MediaType requestedType : requestedMediaTypes) {
			for (MediaType producibleType : producibleMediaTypes) {
				if (requestedType.isCompatibleWith(producibleType)) {
					compatibleMediaTypes.add(getMostSpecificMediaType(requestedType, producibleType));
				}
			}
		}
		if (compatibleMediaTypes.isEmpty()) {
			throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
		}

		List<MediaType> mediaTypes = new ArrayList<MediaType>(compatibleMediaTypes);
		MediaType.sortBySpecificityAndQuality(mediaTypes);

		MediaType selectedMediaType = null;
		for (MediaType mediaType : mediaTypes) {
			if (mediaType.isConcrete()) {
				selectedMediaType = mediaType;
				break;
			}
			else if (mediaType.equals(MediaType.ALL) || mediaType.equals(MEDIA_TYPE_APPLICATION)) {
				selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
				break;
			}
		}

		if (selectedMediaType != null) {
			selectedMediaType = selectedMediaType.removeQualityValue();
			for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
				if (messageConverter.canWrite(returnValueClass, selectedMediaType)) {
					((HttpMessageConverter<T>) messageConverter).write(returnValue, selectedMediaType, outputMessage);
					if (logger.isDebugEnabled()) {
						logger.debug("Written [" + returnValue + "] as \"" + selectedMediaType + "\" using [" +
								messageConverter + "]");
					}
					return;
				}
			}
		}
		throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
	}

选取一个合适的content-type，再由这个content-type和返回类型来选取合适的HttpMessageConverter，找到合适的HttpMessageConverter后，便调用它的write方法。

接下来就说一说一些具体的HttpMessageConverter。

AbstractHttpMessageConverter：提供了进一步的抽象，将是否支持相应的MediaType这一共有的功能实现，它的子类只需关心是否支持返回类型。

AbstractHttpMessageConverter子类-StringHttpMessageConverter：如用于处理字符串到response中，这就要涉及编码问题，这一过程在本系列的第四篇文章中做过详细说明，这里跳过。

AbstractHttpMessageConverter子类-ByteArrayHttpMessageConverter：

public class ByteArrayHttpMessageConverter extends AbstractHttpMessageConverter<byte[]> {

	/** Creates a new instance of the {@code ByteArrayHttpMessageConverter}. */
	public ByteArrayHttpMessageConverter() {
		super(new MediaType("application", "octet-stream"), MediaType.ALL);
	}

	@Override
	public boolean supports(Class<?> clazz) {
		return byte[].class.equals(clazz);
	}

	@Override
	public byte[] readInternal(Class<? extends byte[]> clazz, HttpInputMessage inputMessage) throws IOException {
		long contentLength = inputMessage.getHeaders().getContentLength();
		ByteArrayOutputStream bos =
				new ByteArrayOutputStream(contentLength >= 0 ? (int) contentLength : StreamUtils.BUFFER_SIZE);
		StreamUtils.copy(inputMessage.getBody(), bos);
		return bos.toByteArray();
	}

	@Override
	protected Long getContentLength(byte[] bytes, MediaType contentType) {
		return (long) bytes.length;
	}

	@Override
	protected void writeInternal(byte[] bytes, HttpOutputMessage outputMessage) throws IOException {
		StreamUtils.copy(bytes, outputMessage.getBody());
	}

}

源码就很清晰明了。它专门负责byte[]类型的转换。

AbstractHttpMessageConverter子类-MappingJacksonHttpMessageConverter：用于转换Object到json字符串类型。已过时,使用的是http://jackson.codehaus.org中Jackson 1.x的ObjectMapper，取代者为MappingJackson2HttpMessageConverter。依赖为：

<dependency> 
		<groupId>org.codehaus.jackson</groupId> 
		<artifactId>jackson-core-asl</artifactId> 
		<version>1.9.11</version> 
	</dependency> 
	
	<dependency> 
		<groupId>org.codehaus.jackson</groupId> 
		<artifactId>jackson-mapper-asl</artifactId> 
		<version>1.9.11</version> 
	</dependency> 
	

AbstractHttpMessageConverter子类-MappingJackson2HttpMessageConverter：
它所使用的json转换器是http://jackson.codehaus.org中Jackson 2.x的ObjectMapper。
依赖的jar包为有3个，jackson-databind和它的两个依赖jackson-annotations、jackson-core，但是有了jackson-databind的pom文件会去自动下载它的依赖，所以只需增添jackson-databind的pom即可获取上述3个jar包：

<dependency>
	<dependency>
	    <groupId>com.fasterxml.jackson.core</groupId>
	    <artifactId>jackson-databind</artifactId>
	    <version>2.4.2</version> 
	</dependency>

接下来便说道：在注册HttpMessageConverter过程中的一些问题：

if (convertersElement == null || Boolean.valueOf(convertersElement.getAttribute("register-defaults"))) {
			messageConverters.setSource(source);
			messageConverters.add(createConverterDefinition(ByteArrayHttpMessageConverter.class, source));

			RootBeanDefinition stringConverterDef = createConverterDefinition(StringHttpMessageConverter.class, source);
			stringConverterDef.getPropertyValues().add("writeAcceptCharset", false);
			messageConverters.add(stringConverterDef);

			messageConverters.add(createConverterDefinition(ResourceHttpMessageConverter.class, source));
			messageConverters.add(createConverterDefinition(SourceHttpMessageConverter.class, source));
			messageConverters.add(createConverterDefinition(AllEncompassingFormHttpMessageConverter.class, source));

			if (romePresent) {
				messageConverters.add(createConverterDefinition(AtomFeedHttpMessageConverter.class, source));
				messageConverters.add(createConverterDefinition(RssChannelHttpMessageConverter.class, source));
			}
			if (jaxb2Present) {
				messageConverters.add(createConverterDefinition(Jaxb2RootElementHttpMessageConverter.class, source));
			}
			if (jackson2Present) {
				messageConverters.add(createConverterDefinition(MappingJackson2HttpMessageConverter.class, source));
			}
			else if (jacksonPresent) {
				messageConverters.add(createConverterDefinition(
						org.springframework.http.converter.json.MappingJacksonHttpMessageConverter.class, source));
			}
		}

这段代码是在注册默认的HttpMessageConverter，但是个别HttpMessageConverter也是有条件的。即相应的jar包存在，才会去注册它。如MappingJackson2HttpMessageConverter，if (jackson2Present) {
messageConverters.add(createConverterDefinition(MappingJackson2HttpMessageConverter.class, source));当jackson2Present为true时才会注册。而jackson2Present的值如下：

private static final boolean jackson2Present =
			ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper", AnnotationDrivenBeanDefinitionParser.class.getClassLoader()) &&
					ClassUtils.isPresent("com.fasterxml.jackson.core.JsonGenerator", AnnotationDrivenBeanDefinitionParser.class.getClassLoader());

也就是当com.fasterxml.jackson.databind.ObjectMapper和com.fasterxml.jackson.core.JsonGenerator存在在classpath中才会去加载MappingJackson2HttpMessageConverter。

同理，MappingJacksonHttpMessageConverter的判断如下：

private static final boolean jacksonPresent =
			ClassUtils.isPresent("org.codehaus.jackson.map.ObjectMapper", AnnotationDrivenBeanDefinitionParser.class.getClassLoader()) &&
					ClassUtils.isPresent("org.codehaus.jackson.JsonGenerator", AnnotationDrivenBeanDefinitionParser.class.getClassLoader());

所以当我们程序没法转换json时，你就需要考虑是否已经把MappingJacksonHttpMessageConverter或者MappingJackson2HttpMessageConverter的依赖加进来了，官方推荐使用MappingJackson2HttpMessageConverter。