大家用过spring mvc的肯定都用过@RequestBody和@ResponseBody注解吧,你了解这个的原理吗？这篇文章我们就来说下它是怎么实现json转换的。
首先来看一个类RequestResponseBodyMethodProcessor,这个类继承了AbstractMessageConverterMethodProcessor,我们来看看这个类的构造方法
protected AbstractMessageConverterMethodProcessor(List<HttpMessageConverter<?>> converters,
@Nullable ContentNegotiationManager manager, @Nullable List<Object> requestResponseBodyAdvice) {
super(converters, requestResponseBodyAdvice);
this.contentNegotiationManager = (manager != null ? manager : new ContentNegotiationManager());
this.pathStrategy = initPathStrategy(this.contentNegotiationManager);
this.safeExtensions.addAll(this.contentNegotiationManager.getAllFileExtensions());
this.safeExtensions.addAll(WHITELISTED_EXTENSIONS);
}再到这个类的父类AbstractMessageConverterMethodArgumentResolver里,看一下这个类的构造方法
public AbstractMessageConverterMethodArgumentResolver(List<HttpMessageConverter<?>> converters,
@Nullable List<Object> requestResponseBodyAdvice) {
Assert.notEmpty(converters, "'messageConverters' must not be empty");
this.messageConverters = converters;
this.allSupportedMediaTypes = getAllSupportedMediaTypes(converters);
this.advice = new RequestResponseBodyAdviceChain(requestResponseBodyAdvice);
}这里有几个重要的属性
protected final List<HttpMessageConverter<?>> messageConverters;
protected final List<MediaType> allSupportedMediaTypes;
private final RequestResponseBodyAdviceChain advice;messageConverters:处理消息的HttpMessageConverter
allSupportedMediaTypes:所有支持的MediaType
advice:所有处理注解的advice,也是拦截器链
我们来看一下RequestResponseBodyAdviceChain这个类
class RequestResponseBodyAdviceChain implements RequestBodyAdvice, ResponseBodyAdvice<Object>这个类会处理@RequestBody和@ResponseBody注解,并组成一个链,由HttpMessageConverter处理。
@Nullable
private <T> Object processBody(@Nullable Object body, MethodParameter returnType, MediaType contentType,
Class<? extends HttpMessageConverter<?>> converterType,
ServerHttpRequest request, ServerHttpResponse response) {
for (ResponseBodyAdvice<?> advice : getMatchingAdvice(returnType, ResponseBodyAdvice.class)) {
if (advice.supports(returnType, converterType)) {
body = ((ResponseBodyAdvice<T>) advice).beforeBodyWrite((T) body, returnType,
contentType, converterType, request, response);
}
}
return body;
}在AbstractMessageConverterMethodArgumentResolver的readWithMessageConverters方法里调用HttpMessageConverter处理消息
@Nullable
protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
MediaType contentType;
boolean noContentType = false;
try {
contentType = inputMessage.getHeaders().getContentType();
}
catch (InvalidMediaTypeException ex) {
throw new HttpMediaTypeNotSupportedException(ex.getMessage());
}
if (contentType == null) {
noContentType = true;
contentType = MediaType.APPLICATION_OCTET_STREAM;
}
Class<?> contextClass = parameter.getContainingClass();
Class<T> targetClass = (targetType instanceof Class ? (Class<T>) targetType : null);
if (targetClass == null) {
ResolvableType resolvableType = ResolvableType.forMethodParameter(parameter);
targetClass = (Class<T>) resolvableType.resolve();
}
HttpMethod httpMethod = (inputMessage instanceof HttpRequest ? ((HttpRequest) inputMessage).getMethod() : null);
Object body = NO_VALUE;
EmptyBodyCheckingHttpInputMessage message;
try {
message = new EmptyBodyCheckingHttpInputMessage(inputMessage);
for (HttpMessageConverter<?> converter : this.messageConverters) {
Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
GenericHttpMessageConverter<?> genericConverter =
(converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);
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);
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;
}主要就是取出所有的HttpMessageConverter,然后调用read方法读取消息,再处理MediaType,返回body。
在拦截器链执行后,HttpMessageConverter处理的消息就是返回的http消息了。
现在我们来说下MappingJackson2HttpMessageConverter这个类。先来看下类的结构
MappingJackson2HttpMessageConverter
MappingJackson2HttpMessageConverter实现了HttpMessageConverter接口。HttpMessageConverter接口有canRead和canWrite方法。我们来看下AbstractJackson2HttpMessageConverter都有哪些属性
/**
* The default charset used by the converter.
*/
public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;
protected ObjectMapper objectMapper;很简单,它就是通过ObjectMapper完成的json转换的。
我们来看下canRead方法
public boolean canRead(Type type, @Nullable Class<?> contextClass, @Nullable MediaType mediaType) {
if (!canRead(mediaType)) {
return false;
}
JavaType javaType = getJavaType(type, contextClass);
AtomicReference<Throwable> causeRef = new AtomicReference<>();
if (this.objectMapper.canDeserialize(javaType, causeRef)) {
return true;
}
logWarningIfNecessary(javaType, causeRef.get());
return false;
}再来看下canWrite方法
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;
}其实就是通过ObjectMapper判断是否能被转换成json的。
再来看下read方法
public Object read(Type type, @Nullable Class<?> contextClass, HttpInputMessage inputMessage)
throws IOException, HttpMessageNotReadableException {
JavaType javaType = getJavaType(type, contextClass);
return readJavaType(javaType, inputMessage);
}再到readJavaType方法
private Object readJavaType(JavaType javaType, HttpInputMessage inputMessage) throws IOException {
try {
if (inputMessage instanceof MappingJacksonInputMessage) {
Class<?> deserializationView = ((MappingJacksonInputMessage) inputMessage).getDeserializationView();
if (deserializationView != null) {
return this.objectMapper.readerWithView(deserializationView).forType(javaType).
readValue(inputMessage.getBody());
}
}
return this.objectMapper.readValue(inputMessage.getBody(), javaType);
}
catch (InvalidDefinitionException ex) {
throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);
}
catch (JsonProcessingException ex) {
throw new HttpMessageNotReadableException("JSON parse error: " + ex.getOriginalMessage(), ex, inputMessage);
}
}看到ObjectMapper.readValue方法就知道了,其实就是用ObjectMapper把json转换成实体对象的。
再看一下write方法
public final void write(final T t, @Nullable final Type type, @Nullable MediaType contentType,
HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
final HttpHeaders headers = outputMessage.getHeaders();
addDefaultHeaders(headers, t, contentType);
if (outputMessage instanceof StreamingHttpOutputMessage) {
StreamingHttpOutputMessage streamingOutputMessage = (StreamingHttpOutputMessage) outputMessage;
streamingOutputMessage.setBody(outputStream -> writeInternal(t, type, new HttpOutputMessage() {
@Override
public OutputStream getBody() {
return outputStream;
}
@Override
public HttpHeaders getHeaders() {
return headers;
}
}));
}
else {
writeInternal(t, type, outputMessage);
outputMessage.getBody().flush();
}
}再到writeInternal方法
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);
}
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);
}
}其实也是通过ObjectMapper把实体对象转换成json的。
MappingJackson2HttpMessageConverter就分析到这里了。