SpringMVC源碼總結（七）mvc:annotation-driven中的HttpMessageConverter

這一篇文章主要介紹下HttpMessageConverter整個註冊過程包含自定義的HttpMessageConverter，然後對一些HttpMessageConverter進行具體介紹。 

HttpMessageConverter接口介紹： 

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;

}

從HttpInputMessage中讀取數據： T read(Class<? extends T> clazz, HttpInputMessage inputMessage)，前提clazz能夠通過canRead(clazz，mediaType)測試。 
向HttpOutputMessage中寫入數據：void write(T t, MediaType contentType, HttpOutputMessage outputMessage)，前提能夠通過canWrite方法。 

簡單舉例： 
如StringHttpMessageConverter，read方法就是根據編碼類型將HttpInputMessage中的數據變爲字符串。write方法就是根據編碼類型將字符串數據寫入HttpOutputMessage中。 

HttpMessageConverter的使用場景： 
它主要是用來轉換request的內容到一定的格式，轉換輸出的內容的到response。 
看下自定義的使用方式： 

<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>

首先還是在對mvc:annotation-driven解析的AnnotationDrivenBeanDefinitionParser中，有這麼一個方法： 

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

獲取所有的HttpMessageConverter，最終設置到RequestMappingHandlerAdapter的private List<HttpMessageConverter<?>> messageConverters屬性上。看下具體的獲取過程： 

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;
    }

該過程第一步： 
解析並獲取我們自定義的HttpMessageConverter， 
該過程第二步： 
<mvc:message-converters register-defaults="true">有一個register-defaults屬性，當爲true時，仍然註冊默認的HttpMessageConverter，當爲false則不註冊，僅僅使用用戶自定義的HttpMessageConverter。 

獲取完畢，便會將這些HttpMessageConverter設置進RequestMappingHandlerAdapter的messageConverters屬性中。 

然後就是它的使用過程，HttpMessageConverter主要針對那些不會返回view視圖的response： 
即含有方法含有@ResponseBody或者返回值爲HttpEntity等類型的，它們都會用到HttpMessageConverter。以@ResponseBody舉例： 
首先先決定由哪個HandlerMethodReturnValueHandler來處理返回值，由於是@ResponseBody所以將會由RequestResponseBodyMethodProcessor來處理，然後就是如下的寫入： 

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);
    }

選取一個合適的content-type，再由這個content-type和返回類型來選取合適的HttpMessageConverter，找到合適的HttpMessageConverter後，便調用它的write方法。 

接下來就說一說一些具體的HttpMessageConverter。 

AbstractHttpMessageConverter：提供了進一步的抽象，將是否支持相應的MediaType這一共有的功能實現，它的子類只需關心是否支持返回類型。 

AbstractHttpMessageConverter子類-StringHttpMessageConverter：如用於處理字符串到response中，這就要涉及編碼問題，這一過程在本系列的第四篇文章中做過詳細說明，這裏跳過。 

AbstractHttpMessageConverter子類-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());
    }

}

源碼就很清晰明瞭。它專門負責byte[]類型的轉換。 

AbstractHttpMessageConverter子類-MappingJacksonHttpMessageConverter：用於轉換Object到json字符串類型。已過時,使用的是http://jackson.codehaus.org中Jackson 1.x的ObjectMapper，取代者爲MappingJackson2HttpMessageConverter。依賴爲： 

<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子類-MappingJackson2HttpMessageConverter： 
它所使用的json轉換器是http://jackson.codehaus.org中Jackson 2.x的ObjectMapper。 
依賴的jar包爲有3個，jackson-databind和它的兩個依賴jackson-annotations、jackson-core，但是有了jackson-databind的pom文件會去自動下載它的依賴，所以只需增添jackson-databind的pom即可獲取上述3個jar包： 

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

接下來便說道：在註冊HttpMessageConverter過程中的一些問題： 

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));
            }
        }

這段代碼是在註冊默認的HttpMessageConverter，但是個別HttpMessageConverter也是有條件的。即相應的jar包存在，纔會去註冊它。如MappingJackson2HttpMessageConverter，if (jackson2Present) { 
messageConverters.add(createConverterDefinition(MappingJackson2HttpMessageConverter.class, source));當jackson2Present爲true時纔會註冊。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());

也就是當com.fasterxml.jackson.databind.ObjectMapper和com.fasterxml.jackson.core.JsonGenerator存在在classpath中才會去加載MappingJackson2HttpMessageConverter。 

同理，MappingJacksonHttpMessageConverter的判斷如下： 

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

所以當我們程序沒法轉換json時，你就需要考慮是否已經把MappingJacksonHttpMessageConverter或者MappingJackson2HttpMessageConverter的依賴加進來了，官方推薦使用MappingJackson2HttpMessageConverter。

轉載自：http://blog.csdn.net/z69183787/article/details/52817067