RestTemplate是Spring提供的访问Rest服务的客户端，它简化了和http服务器的交互。
HTTP 协议特点是纯文本协议，其媒体类型MediaType可以为text/html、text/xml、application/json等，HTTP消息必须使用content-type进行自我描述，否则不能区分媒体类型。
RestTemplate使用HTTP消息转换器HttpMessageConverter，根据消息的媒体类型进行消息解析。在REST服务端与客户端，消息传输时需要进行消息相应的序列化与反序列化：
反序列化：文本（通讯） -> 对象（程序使用）
序列化：对象 -> 文本HttpMessageConverter
HttpMessageConverter是一个策略接口，指定了转换器的基本规范，接口源码：
/**
* Strategy interface that specifies a converter that can convert from and to HTTP requests and responses.
*
* @author Arjen Poutsma
* @author Juergen Hoeller
* @since 3.0
*/
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, @Nullable 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, @Nullable 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 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;
/**
* 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;
}canRead用于表明给定class是否可以被当前转换器重写。getSupportedMediaTypes用于获取当前转换器支持的媒体类型，read和write为反序列化与序列化。
MappingJackson2HttpMessageConverter是一个最终的转换器类，其结构图如下：
根据MappingJackson2HttpMessageConverter源码分析：
/**
* Implementation of {@link org.springframework.http.converter.HttpMessageConverter} that can read and
* write JSON using <a href="http://wiki.fasterxml.com/JacksonHome">Jackson 2.x's</a> {@link ObjectMapper}.
*
* <p>This converter can be used to bind to typed beans, or untyped {@code HashMap} instances.
*
* <p>By default, this converter supports {@code application/json} and {@code application/*+json}
* with {@code UTF-8} character set. This can be overridden by setting the
* {@link #setSupportedMediaTypes supportedMediaTypes} property.
*
* <p>The default constructor uses the default configuration provided by {@link Jackson2ObjectMapperBuilder}.
*
* <p>Compatible with Jackson 2.9 and higher, as of Spring 5.0.
*
* @author Arjen Poutsma
* @author Keith Donald
* @author Rossen Stoyanchev
* @author Juergen Hoeller
* @author Sebastien Deleuze
* @since 3.1.2
*/
public class MappingJackson2HttpMessageConverter extends AbstractJackson2HttpMessageConverter {
@Nullable
private String jsonPrefix;
/**
* Construct a new {@link MappingJackson2HttpMessageConverter} using default configuration
* provided by {@link Jackson2ObjectMapperBuilder}.
*/
public MappingJackson2HttpMessageConverter() {
this(Jackson2ObjectMapperBuilder.json().build());
}
/**
* Construct a new {@link MappingJackson2HttpMessageConverter} with a custom {@link ObjectMapper}.
* You can use {@link Jackson2ObjectMapperBuilder} to build it easily.
* @see Jackson2ObjectMapperBuilder#json()
*/
public MappingJackson2HttpMessageConverter(ObjectMapper objectMapper) {
super(objectMapper, MediaType.APPLICATION_JSON, new MediaType("application", "*+json"));
}
/**
* Specify a custom prefix to use for this view's JSON output.
* Default is none.
* @see #setPrefixJson
*/
public void setJsonPrefix(String jsonPrefix) {
this.jsonPrefix = jsonPrefix;
}
/**
* Indicate whether the JSON output by this view should be prefixed with ")]}', ". Default is false.
* <p>Prefixing the JSON string in this manner is used to help prevent JSON Hijacking.
* The prefix renders the string syntactically invalid as a script so that it cannot be hijacked.
* This prefix should be stripped before parsing the string as JSON.
* @see #setJsonPrefix
*/
public void setPrefixJson(boolean prefixJson) {
this.jsonPrefix = (prefixJson ? ")]}', " : null);
}
@Override
@SuppressWarnings("deprecation")
protected void writePrefix(JsonGenerator generator, Object object) throws IOException {
if (this.jsonPrefix != null) {
generator.writeRaw(this.jsonPrefix);
}
String jsonpFunction =
(object instanceof MappingJacksonValue ? ((MappingJacksonValue) object).getJsonpFunction() : null);
if (jsonpFunction != null) {
generator.writeRaw("/**/");
generator.writeRaw(jsonpFunction + "(");
}
}
@Override
@SuppressWarnings("deprecation")
protected void writeSuffix(JsonGenerator generator, Object object) throws IOException {
String jsonpFunction =
(object instanceof MappingJacksonValue ? ((MappingJacksonValue) object).getJsonpFunction() : null);
if (jsonpFunction != null) {
generator.writeRaw(");");
}
}
}判断是否可读可写
public interface HttpMessageConverter<T> {
boolean canRead(Class<?> clazz, @Nullable MediaType mediaType);
boolean canWrite(Class<?> clazz, @Nullable MediaType mediaType);
}判断是否可读可写逻辑，综合了媒体类型和canDeserialize考量，其中canDeserialize用于判断当前objectMapper是否可以序列化给定类型：
@Override
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;
}
@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;
}调用的方法中都根据当前转换器支持的媒体类型做了判断：
protected boolean canRead(@Nullable MediaType mediaType) {
if (mediaType == null) {
return true;
}
for (MediaType supportedMediaType : getSupportedMediaTypes()) {
if (supportedMediaType.includes(mediaType)) {
return true;
}
}
return false;
} protected boolean canWrite(@Nullable MediaType mediaType) {
if (mediaType == null || MediaType.ALL.equals(mediaType)) {
return true;
}
for (MediaType supportedMediaType : getSupportedMediaTypes()) {
if (supportedMediaType.isCompatibleWith(mediaType)) {
return true;
}
}
return false;
}getSupportedMediaTypes返回的是一个只读列表：
@Override
public List<MediaType> getSupportedMediaTypes() {
return Collections.unmodifiableList(this.supportedMediaTypes);
}当前支持的媒体类型
public interface HttpMessageConverter<T> {
List<MediaType> getSupportedMediaTypes();
}MappingJackson2HttpMessageConverter其中构造器：
/**
* Construct a new {@link MappingJackson2HttpMessageConverter} with a custom {@link ObjectMapper}.
* You can use {@link Jackson2ObjectMapperBuilder} to build it easily.
* @see Jackson2ObjectMapperBuilder#json()
*/
public MappingJackson2HttpMessageConverter(ObjectMapper objectMapper) {
super(objectMapper, MediaType.APPLICATION_JSON, new MediaType("application", "*+json"));
}在构造时声明了支持的媒体类型MediaType.APPLICATION_JSON, new MediaType("application", "*+json")。
其父类直接将其定义的媒体类型设置为了supportedMediaTypes属性：
protected AbstractJackson2HttpMessageConverter(ObjectMapper objectMapper, MediaType... supportedMediaTypes) {
this(objectMapper);
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);
}序列化和反序列化
序列化和反序列化实质上都是调用的objectMapper的读写方法：
@Override
public Object read(Type type, @Nullable Class<?> contextClass, HttpInputMessage inputMessage)
throws IOException, HttpMessageNotReadableException {
JavaType javaType = getJavaType(type, contextClass);
return readJavaType(javaType, inputMessage);
}
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);
}
} @Override
protected void writeInternal(Object object, @Nullable Type type, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException {
...
try {
...
ObjectWriter objectWriter;
if (serializationView != null) {
objectWriter = this.objectMapper.writerWithView(serializationView);
}
else if (filters != null) {
objectWriter = this.objectMapper.writer(filters);
}
else {
objectWriter = this.objectMapper.writer();
}
...
}
}RestTemplate如何选择HttpMessageConverter
RestTemplate可能对应多个HttpMessageConverter，那么如何确定使用哪个HttpMessageConverter：
public class RestTemplate extends InterceptingHttpAccessor implements RestOperations {
...
private final List<HttpMessageConverter<?>> messageConverters = new ArrayList<>();
...
/**
* Create a new instance of the {@link RestTemplate} using default settings.
* Default {@link HttpMessageConverter}s are initialized.
*/
public RestTemplate() {
this.messageConverters.add(new ByteArrayHttpMessageConverter());
this.messageConverters.add(new StringHttpMessageConverter());
this.messageConverters.add(new ResourceHttpMessageConverter(false));
this.messageConverters.add(new SourceHttpMessageConverter<>());
this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());
if (romePresent) {
this.messageConverters.add(new AtomFeedHttpMessageConverter());
this.messageConverters.add(new RssChannelHttpMessageConverter());
}
if (jackson2XmlPresent) {
this.messageConverters.add(new MappingJackson2XmlHttpMessageConverter());
}
else if (jaxb2Present) {
this.messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
}
if (jackson2Present) {
this.messageConverters.add(new MappingJackson2HttpMessageConverter());
}
else if (gsonPresent) {
this.messageConverters.add(new GsonHttpMessageConverter());
}
else if (jsonbPresent) {
this.messageConverters.add(new JsonbHttpMessageConverter());
}
if (jackson2SmilePresent) {
this.messageConverters.add(new MappingJackson2SmileHttpMessageConverter());
}
if (jackson2CborPresent) {
this.messageConverters.add(new MappingJackson2CborHttpMessageConverter());
}
}
...
/**
* Create a new instance of the {@link RestTemplate} using the given list of
* {@link HttpMessageConverter} to use
* @param messageConverters the list of {@link HttpMessageConverter} to use
* @since 3.2.7
*/
public RestTemplate(List<HttpMessageConverter<?>> messageConverters) {
Assert.notEmpty(messageConverters, "At least one HttpMessageConverter required");
this.messageConverters.addAll(messageConverters);
}
/**
* Return the list of message body converters.
* <p>The returned {@link List} is active and may get appended to.
*/
public List<HttpMessageConverter<?>> getMessageConverters() {
return this.messageConverters;
}
...
}RestTemplate构建时，默认添加内置 HttpMessageConvertor 实现：
this.messageConverters.add(new ByteArrayHttpMessageConverter());
this.messageConverters.add(new StringHttpMessageConverter());
this.messageConverters.add(new ResourceHttpMessageConverter(false));
this.messageConverters.add(new SourceHttpMessageConverter<>());
this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());然后有条件地添加第三方库HttpMessageConvertor的整合实现，例如：
if (romePresent) {
this.messageConverters.add(new AtomFeedHttpMessageConverter());
this.messageConverters.add(new RssChannelHttpMessageConverter());
}
...分析RestTemplate解析数据确定转换器的过程，以get为例：
@Override
@Nullable
public <T> T getForObject(String url, Class<T> responseType, Object... uriVariables) throws RestClientException {
RequestCallback requestCallback = acceptHeaderRequestCallback(responseType);
HttpMessageConverterExtractor<T> responseExtractor =
new HttpMessageConverterExtractor<>(responseType, getMessageConverters(), logger);
return execute(url, HttpMethod.GET, requestCallback, responseExtractor, uriVariables);
}HttpMessageConverterExtractor参数getMessageConverters()就是当前RestTemplate的messageConverters属性，跟踪代码可以发现最终解析的方法为extractData：
@Override
@SuppressWarnings({"unchecked", "rawtypes", "resource"})
public T extractData(ClientHttpResponse response) throws IOException {
MessageBodyClientHttpResponseWrapper responseWrapper = new MessageBodyClientHttpResponseWrapper(response);
if (!responseWrapper.hasMessageBody() || responseWrapper.hasEmptyMessageBody()) {
return null;
}
MediaType contentType = getContentType(responseWrapper);
try {
for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
if (messageConverter instanceof GenericHttpMessageConverter) {
GenericHttpMessageConverter<?> genericMessageConverter =
(GenericHttpMessageConverter<?>) messageConverter;
if (genericMessageConverter.canRead(this.responseType, null, contentType)) {
if (logger.isDebugEnabled()) {
logger.debug("Reading [" + this.responseType + "] as \"" +
contentType + "\" using [" + messageConverter + "]");
}
return (T) genericMessageConverter.read(this.responseType, null, responseWrapper);
}
}
if (this.responseClass != null) {
if (messageConverter.canRead(this.responseClass, contentType)) {
if (logger.isDebugEnabled()) {
logger.debug("Reading [" + this.responseClass.getName() + "] as \"" +
contentType + "\" using [" + messageConverter + "]");
}
return (T) messageConverter.read((Class) this.responseClass, responseWrapper);
}
}
}
}
catch (IOException | HttpMessageNotReadableException ex) {
throw new RestClientException("Error while extracting response for type [" +
this.responseType + "] and content type [" + contentType + "]", ex);
}
throw new RestClientException("Could not extract response: no suitable HttpMessageConverter found " +
"for response type [" + this.responseType + "] and content type [" + contentType + "]");
}方法中首先获取了contentType：
private MediaType getContentType(ClientHttpResponse response) {
MediaType contentType = response.getHeaders().getContentType();
if (contentType == null) {
if (logger.isTraceEnabled()) {
logger.trace("No Content-Type header found, defaulting to application/octet-stream");
}
contentType = MediaType.APPLICATION_OCTET_STREAM;
}
return contentType;
}然后对RestTemplate支持的messageConverts进行了循环，当遇到支持当前媒体类型的转换器时，直接读取并返回：
for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
if (messageConverter instanceof GenericHttpMessageConverter) {
GenericHttpMessageConverter<?> genericMessageConverter =
(GenericHttpMessageConverter<?>) messageConverter;
if (genericMessageConverter.canRead(this.responseType, null, contentType)) {
if (logger.isDebugEnabled()) {
logger.debug("Reading [" + this.responseType + "] as \"" +
contentType + "\" using [" + messageConverter + "]");
}
return (T) genericMessageConverter.read(this.responseType, null, responseWrapper);
}
}这样产生了顺序和优先级，当多个converter都支持当前媒体类型读取时，在this.messageConverters中位置越靠前，则会优先读取。
