AbstractMessageConverterMethodArgumentResolver的介绍
这里使用这个类来引入是不准确的需要修改
该类的结构如下：
这个类的处理流程如下：
/**
* Create the method argument value of the expected parameter type by reading
* from the given HttpInputMessage.
* @param <T> the expected type of the argument value to be created
* @param inputMessage the HTTP input message representing the current request
* @param parameter the method parameter descriptor
* @param targetType the target type, not necessarily the same as the method
* parameter type, e.g. for {@code HttpEntity<String>}.
* @return the created method argument value
* @throws IOException if the reading from the request fails
* @throws HttpMediaTypeNotSupportedException if no suitable message converter is found
*/
@SuppressWarnings("unchecked")
@Nullable
protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
MediaType contentType;
boolean noContentType = false;
try {
// 从request的Headers中获取Content-type内容。
contentType = inputMessage.getHeaders().getContentType();
}
catch (InvalidMediaTypeException ex) {
throw new HttpMediaTypeNotSupportedException(ex.getMessage());
}
if (contentType == null) {
noContentType = true;
// 如果contentType为空，则默认使用二进制的APPLICATION_OCTET_STREAM
contentType = MediaType.APPLICATION_OCTET_STREAM;
}
// 获取Controller的类型。
Class<?> contextClass = parameter.getContainingClass();
// 转化的目标类型。
Class<T> targetClass = (targetType instanceof Class ? (Class<T>) targetType : null);
if (targetClass == null) {
ResolvableType resolvableType = ResolvableType.forMethodParameter(parameter);
targetClass = (Class<T>) resolvableType.resolve();
}
// 获取请求方式。
HttpMethod httpMethod = (inputMessage instanceof HttpRequest ? ((HttpRequest) inputMessage).getMethod() : null);
Object body = NO_VALUE;
EmptyBodyCheckingHttpInputMessage message;
try {
message = new EmptyBodyCheckingHttpInputMessage(inputMessage);
// 从messageConverters中循环遍历是否存在支持转换的Converter，将转化之后的结果返回。
for (HttpMessageConverter<?> converter : this.messageConverters) {
Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
GenericHttpMessageConverter<?> genericConverter =
(converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);
// 如果converter符合调用条件，则执行转换操作。
if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :
(targetClass != null && converter.canRead(targetClass, contentType))) {
if (message.hasBody()) {
HttpInputMessage msgToUse =
getAdvice().beforeBodyRead(message, parameter, targetType, converterType);
// 调用converter的read方法将字符串转成对应的对象。
body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :
((HttpMessageConverter<T>) converter).read(targetClass, msgToUse));
body = getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType);
}
else {
body = getAdvice().handleEmptyBody(null, message, parameter, targetType, converterType);
}
break;
}
}
}
catch (IOException ex) {
throw new HttpMessageNotReadableException("I/O error while reading input message", ex, inputMessage);
}
if (body == NO_VALUE) {
if (httpMethod == null || !SUPPORTED_METHODS.contains(httpMethod) ||
(noContentType && !message.hasBody())) {
return null;
}
throw new HttpMediaTypeNotSupportedException(contentType, this.allSupportedMediaTypes);
}
MediaType selectedContentType = contentType;
Object theBody = body;
LogFormatUtils.traceDebug(logger, traceOn -> {
String formatted = LogFormatUtils.formatValue(theBody, !traceOn);
return "Read \"" + selectedContentType + "\" to [" + formatted + "]";
});
return body;
}
上面就是转换器的工作的入口。
/**
* Invoke the method and handle the return value through one of the
* configured {@link HandlerMethodReturnValueHandler HandlerMethodReturnValueHandlers}.
* @param webRequest the current request
* @param mavContainer the ModelAndViewContainer for this request
* @param providedArgs "given" arguments matched by type (not resolved)
*/
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
setResponseStatus(webRequest);
if (returnValue == null) {
if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
disableContentCachingIfNecessary(webRequest);
mavContainer.setRequestHandled(true);
return;
}
}
else if (StringUtils.hasText(getResponseStatusReason())) {
mavContainer.setRequestHandled(true);
return;
}
mavContainer.setRequestHandled(false);
Assert.state(this.returnValueHandlers != null, "No return value handlers");
try {
this.returnValueHandlers.handleReturnValue(
returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
}
catch (Exception ex) {
if (logger.isTraceEnabled()) {
logger.trace(formatErrorForReturnValue(returnValue), ex);
}
throw ex;
}
}
/**
* Iterate over registered {@link HandlerMethodReturnValueHandler HandlerMethodReturnValueHandlers} and invoke the one that supports it.
* @throws IllegalStateException if no suitable {@link HandlerMethodReturnValueHandler} is found.
*/
@Override
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
if (handler == null) {
throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
}
handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}
@Override
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
mavContainer.setRequestHandled(true);
ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
// Try even with null return value. ResponseBodyAdvice could get involved.
// 这里会调用HttpMessageConverter的write操作。
writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}
writeWithMessageConverters方法如下：
/**
* 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 the {@code Accept} header on the request cannot be met by the message converters
* @throws HttpMessageNotWritableException thrown if a given message cannot
* be written by a converter, or if the content-type chosen by the server
* has no compatible converter.
*/
@SuppressWarnings({"rawtypes", "unchecked"})
protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
Object body;
Class<?> valueType;
Type targetType;
if (value instanceof CharSequence) {
body = value.toString();
valueType = String.class;
targetType = String.class;
}
else {
body = value;
valueType = getReturnValueType(body, returnType);
targetType = GenericTypeResolver.resolveType(getGenericType(returnType), returnType.getContainingClass());
}
if (isResourceType(value, returnType)) {
outputMessage.getHeaders().set(HttpHeaders.ACCEPT_RANGES, "bytes");
if (value != null && inputMessage.getHeaders().getFirst(HttpHeaders.RANGE) != null &&
outputMessage.getServletResponse().getStatus() == 200) {
Resource resource = (Resource) value;
try {
List<HttpRange> httpRanges = inputMessage.getHeaders().getRange();
outputMessage.getServletResponse().setStatus(HttpStatus.PARTIAL_CONTENT.value());
body = HttpRange.toResourceRegions(httpRanges, resource);
valueType = body.getClass();
targetType = RESOURCE_REGION_LIST_TYPE;
}
catch (IllegalArgumentException ex) {
outputMessage.getHeaders().set(HttpHeaders.CONTENT_RANGE, "bytes */" + resource.contentLength());
outputMessage.getServletResponse().setStatus(HttpStatus.REQUESTED_RANGE_NOT_SATISFIABLE.value());
}
}
}
MediaType selectedMediaType = null;
MediaType contentType = outputMessage.getHeaders().getContentType();
boolean isContentTypePreset = contentType != null && contentType.isConcrete();
if (isContentTypePreset) {
if (logger.isDebugEnabled()) {
logger.debug("Found 'Content-Type:" + contentType + "' in response");
}
selectedMediaType = contentType;
}
else {
HttpServletRequest request = inputMessage.getServletRequest();
List<MediaType> acceptableTypes = getAcceptableMediaTypes(request);
List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);
if (body != null && producibleTypes.isEmpty()) {
throw new HttpMessageNotWritableException(
"No converter found for return value of type: " + valueType);
}
List<MediaType> mediaTypesToUse = new ArrayList<>();
for (MediaType requestedType : acceptableTypes) {
for (MediaType producibleType : producibleTypes) {
if (requestedType.isCompatibleWith(producibleType)) {
mediaTypesToUse.add(getMostSpecificMediaType(requestedType, producibleType));
}
}
}
if (mediaTypesToUse.isEmpty()) {
if (body != null) {
throw new HttpMediaTypeNotAcceptableException(producibleTypes);
}
if (logger.isDebugEnabled()) {
logger.debug("No match for " + acceptableTypes + ", supported: " + producibleTypes);
}
return;
}
MediaType.sortBySpecificityAndQuality(mediaTypesToUse);
for (MediaType mediaType : mediaTypesToUse) {
if (mediaType.isConcrete()) {
selectedMediaType = mediaType;
break;
}
else if (mediaType.isPresentIn(ALL_APPLICATION_MEDIA_TYPES)) {
selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
break;
}
}
if (logger.isDebugEnabled()) {
logger.debug("Using '" + selectedMediaType + "', given " +
acceptableTypes + " and supported " + producibleTypes);
}
}
if (selectedMediaType != null) {
selectedMediaType = selectedMediaType.removeQualityValue();
for (HttpMessageConverter<?> converter : this.messageConverters) {
GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
(GenericHttpMessageConverter<?>) converter : null);
if (genericConverter != null ?
((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) :
converter.canWrite(valueType, selectedMediaType)) {
body = getAdvice().beforeBodyWrite(body, returnType, selectedMediaType,
(Class<? extends HttpMessageConverter<?>>) converter.getClass(),
inputMessage, outputMessage);
if (body != null) {
Object theBody = body;
LogFormatUtils.traceDebug(logger, traceOn ->
"Writing [" + LogFormatUtils.formatValue(theBody, !traceOn) + "]");
addContentDispositionHeader(inputMessage, outputMessage);
if (genericConverter != null) {
genericConverter.write(body, targetType, selectedMediaType, outputMessage);
}
else {
((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Nothing to write: null body");
}
}
return;
}
}
}
if (body != null) {
Set<MediaType> producibleMediaTypes =
(Set<MediaType>) inputMessage.getServletRequest()
.getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
if (isContentTypePreset || !CollectionUtils.isEmpty(producibleMediaTypes)) {
throw new HttpMessageNotWritableException(
"No converter for [" + valueType + "] with preset Content-Type '" + contentType + "'");
}
throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
}
}
有些类需要同时支持参数的解析和参数的写入操作比如HttpEntityMethodProcessor和RequestResponseBodyMethodProcessor所以它们会同时具有readWithMessageConverters和writeWithMessageConverters两个方法。
类的结构图如下
具体则是根据解析及写入对象的需求来决定的。
这里会详细分析下HttpMessageConverter的工作原理
HttpMessageConverter的工作原理
SpringMVC在初始化的时候自动注册了一些转换器，代码如下：
/**
* Returns a {@link RequestMappingHandlerAdapter} for processing requests
* through annotated controller methods. Consider overriding one of these
* other more fine-grained methods:
* <ul>
* <li>{@link #addArgumentResolvers} for adding custom argument resolvers.
* <li>{@link #addReturnValueHandlers} for adding custom return value handlers.
* <li>{@link #configureMessageConverters} for adding custom message converters.
* </ul>
*/
@Bean
public RequestMappingHandlerAdapter requestMappingHandlerAdapter(
@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,
@Qualifier("mvcConversionService") FormattingConversionService conversionService,
@Qualifier("mvcValidator") Validator validator) {
RequestMappingHandlerAdapter adapter = createRequestMappingHandlerAdapter();
adapter.setContentNegotiationManager(contentNegotiationManager);
// 在这里注册了默认的MessageConverters
adapter.setMessageConverters(getMessageConverters());
adapter.setWebBindingInitializer(getConfigurableWebBindingInitializer(conversionService, validator));
adapter.setCustomArgumentResolvers(getArgumentResolvers());
adapter.setCustomReturnValueHandlers(getReturnValueHandlers());
if (jackson2Present) {
adapter.setRequestBodyAdvice(Collections.singletonList(new JsonViewRequestBodyAdvice()));
adapter.setResponseBodyAdvice(Collections.singletonList(new JsonViewResponseBodyAdvice()));
}
AsyncSupportConfigurer configurer = new AsyncSupportConfigurer();
configureAsyncSupport(configurer);
if (configurer.getTaskExecutor() != null) {
adapter.setTaskExecutor(configurer.getTaskExecutor());
}
if (configurer.getTimeout() != null) {
adapter.setAsyncRequestTimeout(configurer.getTimeout());
}
adapter.setCallableInterceptors(configurer.getCallableInterceptors());
adapter.setDeferredResultInterceptors(configurer.getDeferredResultInterceptors());
return adapter;
}
/**
* Provides access to the shared {@link HttpMessageConverter HttpMessageConverters}
* used by the {@link RequestMappingHandlerAdapter} and the
* {@link ExceptionHandlerExceptionResolver}.
* <p>This method cannot be overridden; use {@link #configureMessageConverters} instead.
* Also see {@link #addDefaultHttpMessageConverters} for adding default message converters.
*/
protected final List<HttpMessageConverter<?>> getMessageConverters() {
if (this.messageConverters == null) {
this.messageConverters = new ArrayList<>();
configureMessageConverters(this.messageConverters);
if (this.messageConverters.isEmpty()) {
// 注册默认的HttpMessageConverters
addDefaultHttpMessageConverters(this.messageConverters);
}
extendMessageConverters(this.messageConverters);
}
return this.messageConverters;
}
/**
* Adds a set of default HttpMessageConverter instances to the given list.
* Subclasses can call this method from {@link #configureMessageConverters}.
* @param messageConverters the list to add the default message converters to
*/
protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
messageConverters.add(new ByteArrayHttpMessageConverter());
messageConverters.add(new StringHttpMessageConverter());
messageConverters.add(new ResourceHttpMessageConverter());
messageConverters.add(new ResourceRegionHttpMessageConverter());
try {
messageConverters.add(new SourceHttpMessageConverter<>());
}
catch (Throwable ex) {
// Ignore when no TransformerFactory implementation is available...
}
messageConverters.add(new AllEncompassingFormHttpMessageConverter());
if (romePresent) {
messageConverters.add(new AtomFeedHttpMessageConverter());
messageConverters.add(new RssChannelHttpMessageConverter());
}
if (jackson2XmlPresent) {
Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.xml();
if (this.applicationContext != null) {
builder.applicationContext(this.applicationContext);
}
messageConverters.add(new MappingJackson2XmlHttpMessageConverter(builder.build()));
}
else if (jaxb2Present) {
messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
}
if (jackson2Present) {
Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.json();
if (this.applicationContext != null) {
builder.applicationContext(this.applicationContext);
}
messageConverters.add(new MappingJackson2HttpMessageConverter(builder.build()));
}
else if (gsonPresent) {
messageConverters.add(new GsonHttpMessageConverter());
}
else if (jsonbPresent) {
messageConverters.add(new JsonbHttpMessageConverter());
}
if (jackson2SmilePresent) {
Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.smile();
if (this.applicationContext != null) {
builder.applicationContext(this.applicationContext);
}
messageConverters.add(new MappingJackson2SmileHttpMessageConverter(builder.build()));
}
if (jackson2CborPresent) {
Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.cbor();
if (this.applicationContext != null) {
builder.applicationContext(this.applicationContext);
}
messageConverters.add(new MappingJackson2CborHttpMessageConverter(builder.build()));
}
}
所有的转换器都来自同一个实现接口HttpMessageConverter，该接口的内容如下：
（1）、canRead：判断一个给定的类型和请求类型是否可以进行读取。比如JSON -> Object
/**
* 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, @Nullable MediaType mediaType);
（2）、canWrite：判断一个给定的类型和请求类型是否可以进行写入。比如Object -> JSON
/**
* 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, @Nullable MediaType mediaType);
（3）、getSupportedMediaTypes：获取支持的MediaTypes。
/**
* Return the list of {@link MediaType} objects supported by this converter.
* @return the list of supported media types, potentially an immutable copy
*/
List<MediaType> getSupportedMediaTypes();
（4）、read：执行读取操作。从某个InputStream中读取数据并将数据转成T
/**
* Read an object of the given type from 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;
（5）、write：执行写入操作，将数据T写入到某个OutputStream
/**
* 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, @Nullable MediaType contentType, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException;
HttpMessageConverter这个接口还有一个子接口GenericHttpMessageConverter用于处理部分解析存在泛型的情况。目前使用的JSON解析器就是使用了这个接口来进行实现。
接下来逐个说明下实现类的原理：
（1）、ByteArrayHttpMessageConverter
该类负责读取二进制格式的数据和写出二进制格式的数据。
该类的构造函数如下：
/**
* Create a new instance of the {@code ByteArrayHttpMessageConverter}.
*/
public ByteArrayHttpMessageConverter() {
super(MediaType.APPLICATION_OCTET_STREAM, MediaType.ALL);
}
会调用父类AbstractHttpMessageConverter的构造函数。
/**
* Construct an {@code AbstractHttpMessageConverter} with multiple supported media types.
* @param supportedMediaTypes the supported media types
*/
protected AbstractHttpMessageConverter(MediaType... supportedMediaTypes) {
setSupportedMediaTypes(Arrays.asList(supportedMediaTypes));
}
/**
* Set the list of {@link MediaType} objects supported by this converter.
*/
public void setSupportedMediaTypes(List<MediaType> supportedMediaTypes) {
Assert.notEmpty(supportedMediaTypes, "MediaType List must not be empty");
this.supportedMediaTypes = new ArrayList<>(supportedMediaTypes);
}
在创建该类的对象的时候会注册MediaType.APPLICATION_OCTET_STREAM, MediaType.ALL这两个类型，表明该类支持这两种类型的数据的读取和写入。
读操作如下：
canRead方法
/**
* This implementation checks if the given class is {@linkplain #supports(Class) supported},
* and if the {@linkplain #getSupportedMediaTypes() supported media types}
* {@linkplain MediaType#includes(MediaType) include} the given media type.
*/
@Override
public boolean canRead(Class<?> clazz, @Nullable MediaType mediaType) {
// supports在子类中进行覆盖
return supports(clazz) && canRead(mediaType);
}
@Override
public boolean supports(Class<?> clazz) {
// 判断参数类型是否是byte[]
return byte[].class == clazz;
}
/**
* Returns {@code true} if any of the {@linkplain #setSupportedMediaTypes(List)
* supported} media types {@link MediaType#includes(MediaType) include} the
* given media type.
* @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 the supported media types include the media type,
* or if the media type is {@code null}
*/
protected boolean canRead(@Nullable MediaType mediaType) {
// 如果为null则表示该转换器不用必须需要mediaType。
if (mediaType == null) {
return true;
}
// 判断该mediaType是否在该转换器的支持的类型列表中。也就是MediaType.APPLICATION_OCTET_STREAM, MediaType.ALL这两个类型。
for (MediaType supportedMediaType : getSupportedMediaTypes()) {
if (supportedMediaType.includes(mediaType)) {
return true;
}
}
return false;
}
读取操作如下：
/**
* This implementation simple delegates to {@link #readInternal(Class, HttpInputMessage)}.
* Future implementations might add some default behavior, however.
*/
@Override
public final T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
throws IOException, HttpMessageNotReadableException {
// 抽象方法，子类提供实现。
return readInternal(clazz, inputMessage);
}
@Override
public byte[] readInternal(Class<? extends byte[]> clazz, HttpInputMessage inputMessage) throws IOException {
// 获取数据长度
long contentLength = inputMessage.getHeaders().getContentLength();
// 使用ByteArrayOutputStream来读取inputMessage中的数据。
ByteArrayOutputStream bos =
new ByteArrayOutputStream(contentLength >= 0 ? (int) contentLength : StreamUtils.BUFFER_SIZE);
StreamUtils.copy(inputMessage.getBody(), bos);
return bos.toByteArray();
}
写操作如下：
/**
* This implementation checks if the given class is
* {@linkplain #supports(Class) supported}, and if the
* {@linkplain #getSupportedMediaTypes() supported} media types
* {@linkplain MediaType#includes(MediaType) include} the given media type.
*/
@Override
public boolean canWrite(Class<?> clazz, @Nullable MediaType mediaType) {
// 写操作判断方式其实与读操作一致，这里就不再说明了。
return supports(clazz) && canWrite(mediaType);
}
/**
* Returns {@code true} if the given media type includes any of the
* {@linkplain #setSupportedMediaTypes(List) supported media types}.
* @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 the supported media types are compatible with the media type,
* or if the media type is {@code null}
*/
protected boolean canWrite(@Nullable MediaType mediaType) {
if (mediaType == null || MediaType.ALL.equalsTypeAndSubtype(mediaType)) {
return true;
}
for (MediaType supportedMediaType : getSupportedMediaTypes()) {
if (supportedMediaType.isCompatibleWith(mediaType)) {
return true;
}
}
return false;
}
write方法：
/**
* This implementation sets the default headers by calling {@link #addDefaultHeaders},
* and then calls {@link #writeInternal}.
*/
@Override
public final void write(final T t, @Nullable MediaType contentType, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException {
final HttpHeaders headers = outputMessage.getHeaders();
addDefaultHeaders(headers, t, contentType);
// 处理异步情况，此处不做介绍。
if (outputMessage instanceof StreamingHttpOutputMessage) {
StreamingHttpOutputMessage streamingOutputMessage = (StreamingHttpOutputMessage) outputMessage;
streamingOutputMessage.setBody(outputStream -> writeInternal(t, new HttpOutputMessage() {
@Override
public OutputStream getBody() {
return outputStream;
}
@Override
public HttpHeaders getHeaders() {
return headers;
}
}));
}
else {
// 执行真正的写入操作。
writeInternal(t, outputMessage);
// 刷新流。
outputMessage.getBody().flush();
}
}
@Override
protected void writeInternal(byte[] bytes, HttpOutputMessage outputMessage) throws IOException {
// 将bytes写入到output中。
StreamUtils.copy(bytes, outputMessage.getBody());
}
（2）、StringHttpMessageConverter
与byte操作一致，不过只是类型换成String。
@Override
public boolean supports(Class<?> clazz) {
// 判断转换的参数类型是否是String。
return String.class == clazz;
}
读取操作：
@Override
protected String readInternal(Class<? extends String> clazz, HttpInputMessage inputMessage) throws IOException {
// 获取字符集
Charset charset = getContentTypeCharset(inputMessage.getHeaders().getContentType());
// 从body中获取String数据。
return StreamUtils.copyToString(inputMessage.getBody(), charset);
}
写入操作：
@Override
protected void writeInternal(String str, HttpOutputMessage outputMessage) throws IOException {
HttpHeaders headers = outputMessage.getHeaders();
if (this.writeAcceptCharset && headers.get(HttpHeaders.ACCEPT_CHARSET) == null) {
headers.setAcceptCharset(getAcceptedCharsets());
}
// 获取字符集
Charset charset = getContentTypeCharset(headers.getContentType());
// 将字符串写入到OutputStream中。
StreamUtils.copy(str, charset, outputMessage.getBody());
}
（3）、MappingJackson2XmlHttpMessageConverter
XML解析器。
与JSON解析器一起说明。
（4）、MappingJackson2HttpMessageConverter
JSON解析器。
XML和JSON解析器同样都使用了同一个父类来进行读取和写入操作的。
判断是否可以读取操作如下：
@Override
public boolean canRead(Class<?> clazz, @Nullable MediaType mediaType) {
return canRead(clazz, null, mediaType);
}
@Override
public boolean canRead(Type type, @Nullable Class<?> contextClass, @Nullable MediaType mediaType) {
if (!canRead(mediaType)) {
return false;
}
// 获取需要转成的java类
JavaType javaType = getJavaType(type, contextClass);
AtomicReference<Throwable> causeRef = new AtomicReference<>();
// 使用Jackson的类来判断是否可以序列化。
if (this.objectMapper.canDeserialize(javaType, causeRef)) {
return true;
}
logWarningIfNecessary(javaType, causeRef.get());
return false;
}
执行读取操作：
@Override
public Object read(Type type, @Nullable Class<?> contextClass, HttpInputMessage inputMessage)
throws IOException, HttpMessageNotReadableException {
JavaType javaType = getJavaType(type, contextClass);
// 执行JSON字符串->obj的过程。
return readJavaType(javaType, inputMessage);
}
判断是否可以写入操作：
@Override
public boolean canWrite(Class<?> clazz, @Nullable MediaType mediaType) {
if (!canWrite(mediaType)) {
return false;
}
AtomicReference<Throwable> causeRef = new AtomicReference<>();
if (this.objectMapper.canSerialize(clazz, causeRef)) {
return true;
}
logWarningIfNecessary(clazz, causeRef.get());
return false;
}
执行写入操作：
@Override
protected void writeInternal(Object object, @Nullable Type type, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException {
MediaType contentType = outputMessage.getHeaders().getContentType();
JsonEncoding encoding = getJsonEncoding(contentType);
JsonGenerator generator = this.objectMapper.getFactory().createGenerator(outputMessage.getBody(), encoding);
try {
writePrefix(generator, object);
Object value = object;
Class<?> serializationView = null;
FilterProvider filters = null;
JavaType javaType = null;
if (object instanceof MappingJacksonValue) {
MappingJacksonValue container = (MappingJacksonValue) object;
value = container.getValue();
serializationView = container.getSerializationView();
filters = container.getFilters();
}
if (type != null && TypeUtils.isAssignable(type, value.getClass())) {
javaType = getJavaType(type, null);
}
ObjectWriter objectWriter = (serializationView != null ?
this.objectMapper.writerWithView(serializationView) : this.objectMapper.writer());
if (filters != null) {
objectWriter = objectWriter.with(filters);
}
if (javaType != null && javaType.isContainerType()) {
objectWriter = objectWriter.forType(javaType);
}
SerializationConfig config = objectWriter.getConfig();
if (contentType != null && contentType.isCompatibleWith(MediaType.TEXT_EVENT_STREAM) &&
config.isEnabled(SerializationFeature.INDENT_OUTPUT)) {
objectWriter = objectWriter.with(this.ssePrettyPrinter);
}
// 执行obj->json的操作。
objectWriter.writeValue(generator, value);
writeSuffix(generator, object);
generator.flush();
}
catch (InvalidDefinitionException ex) {
throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);
}
catch (JsonProcessingException ex) {
throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getOriginalMessage(), ex);
}
}