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同步客户端执行HTTP请求，在底层HTTP客户端库（如JDK HttpURLConnection，Apache HttpComponents等）上公开简单的模板方法API。RestTemplate除了支持频率较低的情况的通用交换和执行方法之外，还通过HTTP方法提供常见方案的模板。
注意：从5.0开始，非阻塞，反应式org.springframework.web.reactive.client.WebClient提供了RestTemplate的现代替代方案，同时有效支持同步和异步以及流方案。 RestTemplate将在未来版本中弃用，并且不会在未来添加主要的新功能。
1、构造函数
public RestTemplate() {
this.messageConverters.add(new ByteArrayHttpMessageConverter());
this.messageConverters.add(new StringHttpMessageConverter());
this.messageConverters.add(new ResourceHttpMessageConverter(false));
try {
this.messageConverters.add(new SourceHttpMessageConverter<>());
}
catch (Error err) {
// Ignore when no TransformerFactory implementation is available
}
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());
}
this.uriTemplateHandler = initUriTemplateHandler();
}1.1、初始化HttpMessageConverter
策略接口，代表一个可以转换HTTP请求和响应的转换器。
public interface HttpMessageConverter<T> {
//指示此转换器是否可以读取给定的类。
//clazz - 测试可读性的类
//mediaType - 要读取的媒体类型（如果未指定，则可以为null）; 通常是Content-Type标头的值。
boolean canRead(Class<?> clazz, @Nullable MediaType mediaType);
//指示此转换器是否可以写入给定的类。
boolean canWrite(Class<?> clazz, @Nullable MediaType mediaType);
//返回此转换器支持的MediaType对象列表。
List<MediaType> getSupportedMediaTypes();
//从给定的输入消息中读取给定类型的对象，并返回它。
T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
throws IOException, HttpMessageNotReadableException;
//将给定对象写入给定的输出消息。
void write(T t, @Nullable MediaType contentType, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException;
}在默认构造函数会初始化很多HttpMessageConverter实例，这些实例会在三处地方起到作用：
一是这些实例被封装在AcceptHeaderRequestCallback中，当执行http请求时会根据responseType来选取canRead()方法支持的HttpMessageConverter，通过getSupportedMediaTypes()方法返回的MediaType设置http头Accept。
private class AcceptHeaderRequestCallback implements RequestCallback {
@Nullable
private final Type responseType;
public AcceptHeaderRequestCallback(@Nullable Type responseType) {
this.responseType = responseType;
}
@Override
public void doWithRequest(ClientHttpRequest request) throws IOException {
if (this.responseType != null) {
List<MediaType> allSupportedMediaTypes = getMessageConverters().stream()
.filter(converter -> canReadResponse(this.responseType, converter))
.flatMap(this::getSupportedMediaTypes)
.distinct()
.sorted(MediaType.SPECIFICITY_COMPARATOR)
.collect(Collectors.toList());
if (logger.isDebugEnabled()) {
logger.debug("Accept=" + allSupportedMediaTypes);
}
request.getHeaders().setAccept(allSupportedMediaTypes);
}
}
private boolean canReadResponse(Type responseType, HttpMessageConverter<?> converter) {
Class<?> responseClass = (responseType instanceof Class ? (Class<?>) responseType : null);
if (responseClass != null) {
return converter.canRead(responseClass, null);
}
else if (converter instanceof GenericHttpMessageConverter) {
GenericHttpMessageConverter<?> genericConverter = (GenericHttpMessageConverter<?>) converter;
return genericConverter.canRead(responseType, null, null);
}
return false;
}
private Stream<MediaType> getSupportedMediaTypes(HttpMessageConverter<?> messageConverter) {
return messageConverter.getSupportedMediaTypes()
.stream()
.map(mediaType -> {
if (mediaType.getCharset() != null) {
return new MediaType(mediaType.getType(), mediaType.getSubtype());
}
return mediaType;
});
}
}二是当请求时POST/PUT操作时，会使用HttpEntityRequestCallback这种AcceptHeaderRequestCallback，他除了可以设置http头Accept(通过继承的方式)外，还会将请求体写入ClientHttpRequest中，写入流程是从构造方法实例化的HttpMessageConverter中选出canWrite()方法支持的实例，然后调用write()方法写入。
private class HttpEntityRequestCallback extends AcceptHeaderRequestCallback {
private final HttpEntity<?> requestEntity;
public HttpEntityRequestCallback(@Nullable Object requestBody) {
this(requestBody, null);
}
public HttpEntityRequestCallback(@Nullable Object requestBody, @Nullable Type responseType) {
super(responseType);
if (requestBody instanceof HttpEntity) {
this.requestEntity = (HttpEntity<?>) requestBody;
}
else if (requestBody != null) {
this.requestEntity = new HttpEntity<>(requestBody);
}
else {
this.requestEntity = HttpEntity.EMPTY;
}
}
@Override
@SuppressWarnings("unchecked")
public void doWithRequest(ClientHttpRequest httpRequest) throws IOException {
super.doWithRequest(httpRequest);
Object requestBody = this.requestEntity.getBody();
if (requestBody == null) {
HttpHeaders httpHeaders = httpRequest.getHeaders();
HttpHeaders requestHeaders = this.requestEntity.getHeaders();
if (!requestHeaders.isEmpty()) {
requestHeaders.forEach((key, values) -> httpHeaders.put(key, new LinkedList<>(values)));
}
if (httpHeaders.getContentLength() < 0) {
httpHeaders.setContentLength(0L);
}
}
else {
Class<?> requestBodyClass = requestBody.getClass();
Type requestBodyType = (this.requestEntity instanceof RequestEntity ?
((RequestEntity<?>)this.requestEntity).getType() : requestBodyClass);
HttpHeaders httpHeaders = httpRequest.getHeaders();
HttpHeaders requestHeaders = this.requestEntity.getHeaders();
MediaType requestContentType = requestHeaders.getContentType();
for (HttpMessageConverter<?> messageConverter : getMessageConverters()) {
if (messageConverter instanceof GenericHttpMessageConverter) {
GenericHttpMessageConverter<Object> genericConverter =
(GenericHttpMessageConverter<Object>) messageConverter;
if (genericConverter.canWrite(requestBodyType, requestBodyClass, requestContentType)) {
if (!requestHeaders.isEmpty()) {
requestHeaders.forEach((key, values) -> httpHeaders.put(key, new LinkedList<>(values)));
}
logBody(requestBody, requestContentType, genericConverter);
genericConverter.write(requestBody, requestBodyType, requestContentType, httpRequest);
return;
}
}
else if (messageConverter.canWrite(requestBodyClass, requestContentType)) {
if (!requestHeaders.isEmpty()) {
requestHeaders.forEach((key, values) -> httpHeaders.put(key, new LinkedList<>(values)));
}
logBody(requestBody, requestContentType, messageConverter);
((HttpMessageConverter<Object>) messageConverter).write(
requestBody, requestContentType, httpRequest);
return;
}
}
String message = "No HttpMessageConverter for [" + requestBodyClass.getName() + "]";
if (requestContentType != null) {
message += " and content type [" + requestContentType + "]";
}
throw new RestClientException(message);
}
}
private void logBody(Object body, @Nullable MediaType mediaType, HttpMessageConverter<?> converter) {
if (logger.isDebugEnabled()) {
if (mediaType != null) {
logger.debug("Writing [" + body + "] as \"" + mediaType + "\"");
}
else {
String classname = converter.getClass().getName();
logger.debug("Writing [" + body + "] with " + classname);
}
}
}
}三是这些实例被封装在HttpMessageConverterExtractor中在发送http请求后用来解析http响应消息为相应的实体类。
//响应提取器使用给定的实体转换器将响应转换为类型T.
public class HttpMessageConverterExtractor<T> implements ResponseExtractor<T> {
private final Type responseType;
@Nullable
private final Class<T> responseClass;
private final List<HttpMessageConverter<?>> messageConverters;
private final Log logger;
//使用给定的响应类型和消息转换器创建HttpMessageConverterExtractor的新实例。 给定的转换器必须支持响应类型。
public HttpMessageConverterExtractor(Class<T> responseType, List<HttpMessageConverter<?>> messageConverters) {
this((Type) responseType, messageConverters);
}
public HttpMessageConverterExtractor(Type responseType, List<HttpMessageConverter<?>> messageConverters) {
this(responseType, messageConverters, LogFactory.getLog(HttpMessageConverterExtractor.class));
}
@SuppressWarnings("unchecked")
HttpMessageConverterExtractor(Type responseType, List<HttpMessageConverter<?>> messageConverters, Log logger) {
Assert.notNull(responseType, "'responseType' must not be null");
Assert.notEmpty(messageConverters, "'messageConverters' must not be empty");
this.responseType = responseType;
this.responseClass = (responseType instanceof Class ? (Class<T>) responseType : null);
this.messageConverters = messageConverters;
this.logger = logger;
}
@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()) {
ResolvableType resolvableType = ResolvableType.forType(this.responseType);
logger.debug("Reading to [" + resolvableType + "]");
}
return (T) genericMessageConverter.read(this.responseType, null, responseWrapper);
}
}
if (this.responseClass != null) {
if (messageConverter.canRead(this.responseClass, contentType)) {
if (logger.isDebugEnabled()) {
String className = this.responseClass.getName();
logger.debug("Reading to [" + className + "] as \"" + contentType + "\"");
}
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 + "]");
}
//根据“Content-Type”标头确定响应的Content-Type，否则默认为MediaType.APPLICATION_OCTET_STREAM。
@Nullable
protected MediaType getContentType(ClientHttpResponse response) {
MediaType contentType = response.getHeaders().getContentType();
if (contentType == null) {
if (logger.isTraceEnabled()) {
logger.trace("No content-type, using 'application/octet-stream'");
}
contentType = MediaType.APPLICATION_OCTET_STREAM;
}
return contentType;
}
}1.2、初始化UriTemplateHandler
初始化一个URI_COMPONENT编码格式的DefaultUriBuilderFactory。
private static DefaultUriBuilderFactory initUriTemplateHandler() {
DefaultUriBuilderFactory uriFactory = new DefaultUriBuilderFactory();
uriFactory.setEncodingMode(EncodingMode.URI_COMPONENT); // for backwards compatibility..
return uriFactory;
}
//定义使用变量扩展URI模板的方法。
public interface UriTemplateHandler {
//使用URI变量数组展开给定的URI模板。
URI expand(String uriTemplate, Map<String, ?> uriVariables);
URI expand(String uriTemplate, Object... uriVariables);
}public interface UriBuilderFactory extends UriTemplateHandler {
//使用给定的URI模板初始化构建器。
UriBuilder uriString(String uriTemplate);
//使用默认设置创建URI构建器。
UriBuilder builder();
}在发起一个http请求求会先使用这个UriTemplateHandler将模板url和uri变量解析成一个URI然后交给doExecute()方法发起请求。
@Override
@Nullable
public <T> T execute(String url, HttpMethod method, @Nullable RequestCallback requestCallback,
@Nullable ResponseExtractor<T> responseExtractor, Object... uriVariables) throws RestClientException {
URI expanded = getUriTemplateHandler().expand(url, uriVariables);
return doExecute(expanded, method, requestCallback, responseExtractor);
}下面是DefaultUriBuilderFactory#expand(java.lang.String, java.util.Map<java.lang.String,?>)方法实现：
public URI expand(String uriTemplate, Map<String, ?> uriVars) {
return uriString(uriTemplate).build(uriVars);
}
public UriBuilder uriString(String uriTemplate) {
return new DefaultUriBuilder(uriTemplate);
}URI的创建使用DefaultUriBuilder的build()方法。
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@Override
public URI build(Map<String, ?> uriVars) {
if (!defaultUriVariables.isEmpty()) {
Map<String, Object> map = new HashMap<>();
map.putAll(defaultUriVariables);
map.putAll(uriVars);
uriVars = map;
}
if (encodingMode.equals(EncodingMode.VALUES_ONLY)) {
uriVars = UriUtils.encodeUriVariables(uriVars);
}
UriComponents uric = this.uriComponentsBuilder.build().expand(uriVars);
return createUri(uric);
}
private URI createUri(UriComponents uric) {
if (encodingMode.equals(EncodingMode.URI_COMPONENT)) {
uric = uric.encode();
}
return URI.create(uric.toString());
}从代码上面代码可以看出，URI的创建又是使用了uriComponentsBuilder.build().expand(uriVars);这个uriComponentsBuilder是在DefaultUriBuilder构造方法完成初始化的，如下：
private class DefaultUriBuilder implements UriBuilder {
private final UriComponentsBuilder uriComponentsBuilder;
public DefaultUriBuilder(String uriTemplate) {
this.uriComponentsBuilder = initUriComponentsBuilder(uriTemplate);
}
private UriComponentsBuilder initUriComponentsBuilder(String uriTemplate) {
UriComponentsBuilder result;
if (StringUtils.isEmpty(uriTemplate)) {
result = baseUri != null ? baseUri.cloneBuilder() : UriComponentsBuilder.newInstance();
}
else if (baseUri != null) {
UriComponentsBuilder builder = UriComponentsBuilder.fromUriString(uriTemplate);
UriComponents uri = builder.build();
result = uri.getHost() == null ? baseUri.cloneBuilder().uriComponents(uri) : builder;
}
else {
//使用正则分解出uriTemplate各个部分赋给UriComponentsBuilder对应的字段
result = UriComponentsBuilder.fromUriString(uriTemplate);
}
if (encodingMode.equals(EncodingMode.TEMPLATE_AND_VALUES)) {
result.encode();
}
parsePathIfNecessary(result);
return result;
}
private void parsePathIfNecessary(UriComponentsBuilder result) {
//EncodingMode.URI_COMPONENT在RestTemplate中是默认编码模式
//如果存在scheme-specific-part 从中分离path
if (parsePath && encodingMode.equals(EncodingMode.URI_COMPONENT)) {
UriComponents uric = result.build();
String path = uric.getPath();
result.replacePath(null);
for (String segment : uric.getPathSegments()) {
result.pathSegment(segment);
}
if (path != null && path.endsWith("/")) {
result.path("/");
}
}
}
...
}关于URI各部分含义请参考Uri详解之——Uri结构与代码提取，URI介绍。
build()方法返回一个UriComponents实例，表示URI组件的不可变集合，将组件类型映射到String值。
public UriComponents build() {
return build(false);
}
public UriComponents build(boolean encoded) {
return buildInternal(encoded ?
EncodingHint.FULLY_ENCODED :
this.encodeTemplate ? EncodingHint.ENCODE_TEMPLATE : EncodingHint.NONE);
}
private UriComponents buildInternal(EncodingHint hint) {
UriComponents result;
if (this.ssp != null) {
//带有scheme-specific-part部分的使用OpaqueUriComponents
result = new OpaqueUriComponents(this.scheme, this.ssp, this.fragment);
}
else {
HierarchicalUriComponents uric = new HierarchicalUriComponents(this.scheme, this.fragment,
this.userInfo, this.host, this.port, this.pathBuilder.build(), this.queryParams,
hint == EncodingHint.FULLY_ENCODED);
result = hint == EncodingHint.ENCODE_TEMPLATE ? uric.encodeTemplate(this.charset) : uric;
}
//默认为空
if (!this.uriVariables.isEmpty()) {
result = result.expand(name -> this.uriVariables.getOrDefault(name, UriTemplateVariables.SKIP_VALUE));
}
return result;
}UriComponents#expand(java.util.Map<java.lang.String,?>)方法使用给定映射中的值替换所有URI模板变量。
public final UriComponents expand(Map<String, ?> uriVariables) {
Assert.notNull(uriVariables, "'uriVariables' must not be null");
return expandInternal(new MapTemplateVariables(uriVariables));
}expandInternal()方法是抽象方法由子类实现，它的参数是一个UriTemplateVariables，内部会调用它getValue()方法获取模板参数值。对于Map<String, ?>类型模板参数值会将其封装成MapTemplateVariables，getValue()方法会根据map的key获取参数值，所以map类型模板参数变量的顺序不重要。对于可变参数类型的模板参数值会将其封装成VarArgsTemplateVariables，getValue()方法是根据可变参数值的顺序返回参数值，所以模板参数变量的顺序应该与可变参数一致，并且变量名称不重要。
private static class MapTemplateVariables implements UriTemplateVariables {
private final Map<String, ?> uriVariables;
public MapTemplateVariables(Map<String, ?> uriVariables) {
this.uriVariables = uriVariables;
}
@Override
@Nullable
public Object getValue(@Nullable String name) {
if (!this.uriVariables.containsKey(name)) {
throw new IllegalArgumentException("Map has no value for '" + name + "'");
}
return this.uriVariables.get(name);
}
}
private static class VarArgsTemplateVariables implements UriTemplateVariables {
private final Iterator<Object> valueIterator;
public VarArgsTemplateVariables(Object... uriVariableValues) {
this.valueIterator = Arrays.asList(uriVariableValues).iterator();
}
@Override
@Nullable
public Object getValue(@Nullable String name) {
if (!this.valueIterator.hasNext()) {
throw new IllegalArgumentException("Not enough variable values available to expand '" + name + "'");
}
return this.valueIterator.next();
}
}HierarchicalUriComponents#expandInternal()方法。
@Override
protected HierarchicalUriComponents expandInternal(UriTemplateVariables uriVariables) {
Assert.state(!this.encodeState.equals(EncodeState.FULLY_ENCODED),
"URI components already encoded, and could not possibly contain '{' or '}'.");
String schemeTo = expandUriComponent(getScheme(), uriVariables, this.variableEncoder);
String fragmentTo = expandUriComponent(getFragment(), uriVariables, this.variableEncoder);
String userInfoTo = expandUriComponent(this.userInfo, uriVariables, this.variableEncoder);
String hostTo = expandUriComponent(this.host, uriVariables, this.variableEncoder);
String portTo = expandUriComponent(this.port, uriVariables, this.variableEncoder);
PathComponent pathTo = this.path.expand(uriVariables, this.variableEncoder);
MultiValueMap<String, String> queryParamsTo = expandQueryParams(uriVariables);
return new HierarchicalUriComponents(schemeTo, fragmentTo, userInfoTo,
hostTo, portTo, pathTo, queryParamsTo, this.encodeState, this.variableEncoder);
}
private MultiValueMap<String, String> expandQueryParams(UriTemplateVariables variables) {
int size = this.queryParams.size();
MultiValueMap<String, String> result = new LinkedMultiValueMap<>(size);
UriTemplateVariables queryVariables = new QueryUriTemplateVariables(variables);
this.queryParams.forEach((key, values) -> {
String name = expandUriComponent(key, queryVariables, this.variableEncoder);
List<String> expandedValues = new ArrayList<>(values.size());
for (String value : values) {
expandedValues.add(expandUriComponent(value, queryVariables, this.variableEncoder));
}
result.put(name, expandedValues);
});
return CollectionUtils.unmodifiableMultiValueMap(result);
}expandUriComponent()方法用于使用参数值替换模板变量。
@Nullable
static String expandUriComponent(@Nullable String source, UriTemplateVariables uriVariables,
@Nullable UnaryOperator<String> encoder) {
if (source == null) {
return null;
}
if (source.indexOf('{') == -1) {
return source;
}
if (source.indexOf(':') != -1) {
source = sanitizeSource(source);
}
Matcher matcher = NAMES_PATTERN.matcher(source);
StringBuffer sb = new StringBuffer();
while (matcher.find()) {
String match = matcher.group(1);
String varName = getVariableName(match);
Object varValue = uriVariables.getValue(varName);
if (UriTemplateVariables.SKIP_VALUE.equals(varValue)) {
continue;
}
String formatted = getVariableValueAsString(varValue);
formatted = encoder != null ? encoder.apply(formatted) : Matcher.quoteReplacement(formatted);
matcher.appendReplacement(sb, formatted);
}
matcher.appendTail(sb);
return sb.toString();
}2、执行http请求
RestTemplate的execute()方法负责发送http请求并解析响应的结果，这个方法首先使用上面提到的UriTemplateHandler将url模板和模板参数转换成URI，然后传递给doExecute()方法完成整个流程的调用。
@Nullable
protected <T> T doExecute(URI url, @Nullable HttpMethod method, @Nullable RequestCallback requestCallback,
@Nullable ResponseExtractor<T> responseExtractor) throws RestClientException {
Assert.notNull(url, "URI is required");
Assert.notNull(method, "HttpMethod is required");
ClientHttpResponse response = null;
try {
ClientHttpRequest request = createRequest(url, method);
if (requestCallback != null) {
requestCallback.doWithRequest(request);
}
response = request.execute();
handleResponse(url, method, response);
return (responseExtractor != null ? responseExtractor.extractData(response) : null);
}
catch (IOException ex) {
String resource = url.toString();
String query = url.getRawQuery();
resource = (query != null ? resource.substring(0, resource.indexOf('?')) : resource);
throw new ResourceAccessException("I/O error on " + method.name() +
" request for \"" + resource + "\": " + ex.getMessage(), ex);
}
finally {
if (response != null) {
response.close();
}
}
}2.1、构建ClientHttpRequest
在抽象类HttpAccessor中定义了创建ClientHttpRequest的方法和配置ClientHttpRequestFactory的方法，RestTemplate中有个构造方法可以方便的配置ClientHttpRequestFactory，如果没有配置默认使用SimpleClientHttpRequestFactory。
public RestTemplate(ClientHttpRequestFactory requestFactory) {
this();
setRequestFactory(requestFactory);
}下面是创建ClientHttpRequest的方法。
protected ClientHttpRequest createRequest(URI url, HttpMethod method) throws IOException {
ClientHttpRequest request = getRequestFactory().createRequest(url, method);
if (logger.isDebugEnabled()) {
logger.debug("HTTP " + method.name() + " " + url);
}
return request;
}创建ClientHttpRequest的任务直接交给了ClientHttpRequestFactory，这个接口只有唯一的方法：
@FunctionalInterface
public interface ClientHttpRequestFactory {
//为指定的URI和HTTP方法创建新的ClientHttpRequest。
//可以写入请求信息，然后通过调用ClientHttpRequest.execute()来执行。
ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException;
}SimpleClientHttpRequestFactory#createRequest()方法的实现：
@Override
public ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException {
HttpURLConnection connection = openConnection(uri.toURL(), this.proxy);
prepareConnection(connection, httpMethod.name());
//默认true
if (this.bufferRequestBody) {
return new SimpleBufferingClientHttpRequest(connection, this.outputStreaming);
}
else {
return new SimpleStreamingClientHttpRequest(connection, this.chunkSize, this.outputStreaming);
}
}SimpleClientHttpRequestFactory底层是使用JDK标准库HttpURLConnection来构建SimpleBufferingClientHttpRequest。
Spring除了提供默认的SimpleClientHttpRequestFactory，还提供OkHttp3ClientHttpRequestFactory底层使用OkHttp3.x框架发送http请求，Netty4ClientHttpRequestFactory底层使用Netty4，HttpComponentsClientHttpRequestFactory底层使用Apache HttpComponents 4.3或更高版本。
2.2、应用RequestCallback
在上一步创建一个ClientHttpRequest后，会调用它的doWithRequest()，AcceptHeaderRequestCallback会设置请求的accept头，HttpEntityRequestCallback写请求体对象到请求流中，前面已给出具体实现不再说了。
2.3、发送http请求
发送http请求是通过调用ClientHttpRequest的execute()方法完成的，下面看一下ClientHttpRequest的体系结构，然后分析一下SimpleBufferingClientHttpRequest的实现原理。
AbstractClientHttpRequest是ClientHttpRequest的抽象基础，确保不会多次写入header和body并且只支持一次http当请求通过assertNotExecuted()方法的限制。此外还实现了HttpOutputMessage接口法，这个接口会在上面提到的HttpEntityRequestCallback中用于完成POST和PUT请求的消息体写入。
public abstract class AbstractClientHttpRequest implements ClientHttpRequest {
private final HttpHeaders headers = new HttpHeaders();
private boolean executed = false;
@Override
public final HttpHeaders getHeaders() {
return (this.executed ? HttpHeaders.readOnlyHttpHeaders(this.headers) : this.headers);
}
@Override
public final OutputStream getBody() throws IOException {
//不可以多次调用写body
assertNotExecuted();
return getBodyInternal(this.headers);
}
@Override
public final ClientHttpResponse execute() throws IOException {
//http请求不能执行多次
assertNotExecuted();
ClientHttpResponse result = executeInternal(this.headers);
this.executed = true;
return result;
}
//断言此请求尚未执行execute()方法
protected void assertNotExecuted() {
Assert.state(!this.executed, "ClientHttpRequest already executed");
}
//返回主体的抽象模板方法。
protected abstract OutputStream getBodyInternal(HttpHeaders headers) throws IOException;
//将给定headers和内容写入HTTP请求的抽象模板方法。
protected abstract ClientHttpResponse executeInternal(HttpHeaders headers) throws IOException;
}AbstractBufferingClientHttpRequest实现了getBodyInternal()方法返回一个ByteArrayOutputStream用于缓存写入http请求体，这意味着executeInternal()方法在执行请求时就可以设置contentLength通过ByteArrayOutputStream的字节大小，因此使用它小心内存溢出。
abstract class AbstractBufferingClientHttpRequest extends AbstractClientHttpRequest {
private ByteArrayOutputStream bufferedOutput = new ByteArrayOutputStream(1024);
@Override
protected OutputStream getBodyInternal(HttpHeaders headers) throws IOException {
return this.bufferedOutput;
}
@Override
protected ClientHttpResponse executeInternal(HttpHeaders headers) throws IOException {
byte[] bytes = this.bufferedOutput.toByteArray();
if (headers.getContentLength() < 0) {
headers.setContentLength(bytes.length);
}
ClientHttpResponse result = executeInternal(headers, bytes);
this.bufferedOutput = new ByteArrayOutputStream(0);
return result;
}
//将给定heanders和body写入HTTP请求的抽象模板方法。
protected abstract ClientHttpResponse executeInternal(HttpHeaders headers, byte[] bufferedOutput)
throws IOException;
}SimpleBufferingClientHttpRequest会缓存请求体的内容，然后一次性发送。
final class SimpleBufferingClientHttpRequest extends AbstractBufferingClientHttpRequest {
private final HttpURLConnection connection;
private final boolean outputStreaming;
SimpleBufferingClientHttpRequest(HttpURLConnection connection, boolean outputStreaming) {
this.connection = connection;
this.outputStreaming = outputStreaming;
}
@Override
public String getMethodValue() {
return this.connection.getRequestMethod();
}
@Override
public URI getURI() {
try {
return this.connection.getURL().toURI();
}
catch (URISyntaxException ex) {
throw new IllegalStateException("Could not get HttpURLConnection URI: " + ex.getMessage(), ex);
}
}
@Override
protected ClientHttpResponse executeInternal(HttpHeaders headers, byte[] bufferedOutput) throws IOException {
addHeaders(this.connection, headers);
// JDK <1.8 doesn't support getOutputStream with HTTP DELETE
if (getMethod() == HttpMethod.DELETE && bufferedOutput.length == 0) {
this.connection.setDoOutput(false);
}
//bufferedOutput总是代表消息体的大小，显示设置提高flush效率
if (this.connection.getDoOutput() && this.outputStreaming) {
this.connection.setFixedLengthStreamingMode(bufferedOutput.length);
}
this.connection.connect();
if (this.connection.getDoOutput()) {
//一次将请求发送完毕
FileCopyUtils.copy(bufferedOutput, this.connection.getOutputStream());
}
else {
// 在无output场景中立即触发请求
this.connection.getResponseCode();
}
return new SimpleClientHttpResponse(this.connection);
}
//将给定的heanders添加到给定的HTTP连接。
static void addHeaders(HttpURLConnection connection, HttpHeaders headers) {
headers.forEach((headerName, headerValues) -> {
if (HttpHeaders.COOKIE.equalsIgnoreCase(headerName)) { // RFC 6265
String headerValue = StringUtils.collectionToDelimitedString(headerValues, "; ");
connection.setRequestProperty(headerName, headerValue);
}
else {
for (String headerValue : headerValues) {
String actualHeaderValue = headerValue != null ? headerValue : "";
connection.addRequestProperty(headerName, actualHeaderValue);
}
}
});
}
}2.4、异常状态的处理
handleResponse()方法当http相应的状态码是4xx或5xx则会调用ResponseErrorHandler处理这个响应。
protected void handleResponse(URI url, HttpMethod method, ClientHttpResponse response) throws IOException {
ResponseErrorHandler errorHandler = getErrorHandler();
boolean hasError = errorHandler.hasError(response);
if (logger.isDebugEnabled()) {
try {
int code = response.getRawStatusCode();
HttpStatus status = HttpStatus.resolve(code);
logger.debug("Response " + (status != null ? status : code));
}
catch (IOException ex) {
// ignore
}
}
if (hasError) {
errorHandler.handleError(url, method, response);
}
}啊RestTemplate内部默认使用DefaultResponseErrorHandler来处理异常响应。
public class DefaultResponseErrorHandler implements ResponseErrorHandler {
@Override
public boolean hasError(ClientHttpResponse response) throws IOException {
int rawStatusCode = response.getRawStatusCode();
HttpStatus statusCode = HttpStatus.resolve(rawStatusCode);
return (statusCode != null ? hasError(statusCode) : hasError(rawStatusCode));
}
protected boolean hasError(HttpStatus statusCode) {
return statusCode.isError();
}
protected boolean hasError(int unknownStatusCode) {
HttpStatus.Series series = HttpStatus.Series.resolve(unknownStatusCode);
//4xx || 5xx
return (series == HttpStatus.Series.CLIENT_ERROR || series == HttpStatus.Series.SERVER_ERROR);
}
@Override
public void handleError(ClientHttpResponse response) throws IOException {
HttpStatus statusCode = HttpStatus.resolve(response.getRawStatusCode());
if (statusCode == null) {
throw new UnknownHttpStatusCodeException(response.getRawStatusCode(), response.getStatusText(),
response.getHeaders(), getResponseBody(response), getCharset(response));
}
handleError(response, statusCode);
}
protected void handleError(ClientHttpResponse response, HttpStatus statusCode) throws IOException {
String statusText = response.getStatusText();
HttpHeaders headers = response.getHeaders();
byte[] body = getResponseBody(response);
Charset charset = getCharset(response);
switch (statusCode.series()) {
case CLIENT_ERROR:
throw HttpClientErrorException.create(statusCode, statusText, headers, body, charset);
case SERVER_ERROR:
throw HttpServerErrorException.create(statusCode, statusText, headers, body, charset);
default:
throw new UnknownHttpStatusCodeException(statusCode.value(), statusText, headers, body, charset);
}
}
@Deprecated
protected HttpStatus getHttpStatusCode(ClientHttpResponse response) throws IOException {
HttpStatus statusCode = HttpStatus.resolve(response.getRawStatusCode());
if (statusCode == null) {
throw new UnknownHttpStatusCodeException(response.getRawStatusCode(), response.getStatusText(),
response.getHeaders(), getResponseBody(response), getCharset(response));
}
return statusCode;
}
protected byte[] getResponseBody(ClientHttpResponse response) {
try {
return FileCopyUtils.copyToByteArray(response.getBody());
}
catch (IOException ex) {
// ignore
}
return new byte[0];
}
@Nullable
protected Charset getCharset(ClientHttpResponse response) {
HttpHeaders headers = response.getHeaders();
MediaType contentType = headers.getContentType();
return (contentType != null ? contentType.getCharset() : null);
}
}2.5、提取response数据转换成自定义实体
在调用ClientHttpRequest的execute()方法后返回了一个封装http response的ClientHttpResponse，ResponseExtractor用来从ClientHttpResponse提取数据转换成泛型参数所代表的对象。
public <T> ResponseExtractor<ResponseEntity<T>> responseEntityExtractor(Type responseType) {
return new ResponseEntityResponseExtractor<>(responseType);
}ResponseEntityResponseExtractor内部委托给HttpMessageConverterExtractor。
private class ResponseEntityResponseExtractor<T> implements ResponseExtractor<ResponseEntity<T>> {
@Nullable
private final HttpMessageConverterExtractor<T> delegate;
public ResponseEntityResponseExtractor(@Nullable Type responseType) {
if (responseType != null && Void.class != responseType) {
this.delegate = new HttpMessageConverterExtractor<>(responseType, getMessageConverters(), logger);
}
else {
this.delegate = null;
}
}
@Override
public ResponseEntity<T> extractData(ClientHttpResponse response) throws IOException {
if (this.delegate != null) {
T body = this.delegate.extractData(response);
return ResponseEntity.status(response.getRawStatusCode()).headers(response.getHeaders()).body(body);
}
else {
return ResponseEntity.status(response.getRawStatusCode()).headers(response.getHeaders()).build();
}
}
}HttpMessageConverterExtractor在1.1小节看过了这里不再啰嗦。
3、拦截器
拦截器可以在发送http请求之前对请求进行访问或修改，这一切对用户是透明的，只需要编写我们的ClientHttpRequestInterceptor即可。
@FunctionalInterface
public interface ClientHttpRequestInterceptor {
ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution)
throws IOException;
}3.1、拦截器原理
RestTemplate的父类InterceptingHttpAccessor通过调用setInterceptors()方法提供了添加拦截器的功能。
public void setInterceptors(List<ClientHttpRequestInterceptor> interceptors) {
// Take getInterceptors() List as-is when passed in here
if (this.interceptors != interceptors) {
this.interceptors.clear();
this.interceptors.addAll(interceptors);
AnnotationAwareOrderComparator.sort(this.interceptors);
}
}InterceptingHttpAccessor也重写了getRequestFactory()方法，目的是如果配置了拦截器，则使用InterceptingClientHttpRequestFactory，这个类重写了父类的createRequest()方法返回InterceptingClientHttpRequest，这个类持有拦截器的引用。
InterceptingHttpAccessor#getRequestFactory()方法：
@Override
public ClientHttpRequestFactory getRequestFactory() {
List<ClientHttpRequestInterceptor> interceptors = getInterceptors();
if (!CollectionUtils.isEmpty(interceptors)) {
ClientHttpRequestFactory factory = this.interceptingRequestFactory;
if (factory == null) {
factory = new InterceptingClientHttpRequestFactory(super.getRequestFactory(), interceptors);
this.interceptingRequestFactory = factory;
}
return factory;
}
else {
return super.getRequestFactory();
}
}public class InterceptingClientHttpRequestFactory extends AbstractClientHttpRequestFactoryWrapper {
private final List<ClientHttpRequestInterceptor> interceptors;
public InterceptingClientHttpRequestFactory(ClientHttpRequestFactory requestFactory,
@Nullable List<ClientHttpRequestInterceptor> interceptors) {
super(requestFactory);
this.interceptors = (interceptors != null ? interceptors : Collections.emptyList());
}
@Override
protected ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod, ClientHttpRequestFactory requestFactory) {
return new InterceptingClientHttpRequest(requestFactory, this.interceptors, uri, httpMethod);
}
}应为拦截器中可以获取到request的body，所以InterceptingClientHttpRequest继承于AbstractBufferingClientHttpRequest，拦截器是在executeInternal()方法中起作用的。
@Override
protected final ClientHttpResponse executeInternal(HttpHeaders headers, byte[] bufferedOutput) throws IOException {
InterceptingRequestExecution requestExecution = new InterceptingRequestExecution();
return requestExecution.execute(this, bufferedOutput);
}
private class InterceptingRequestExecution implements ClientHttpRequestExecution {
private final Iterator<ClientHttpRequestInterceptor> iterator;
public InterceptingRequestExecution() {
this.iterator = interceptors.iterator();
}
@Override
public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException {
if (this.iterator.hasNext()) {
ClientHttpRequestInterceptor nextInterceptor = this.iterator.next();
//拦截器起作用后内部会调用execution.execute()方法继续调用这个方法使下个拦截器起作用
return nextInterceptor.intercept(request, body, this);
}
else {
//直到所有拦截器拦截完毕，使用原requestFactory创建的ClientHttpRequest完成请求
HttpMethod method = request.getMethod();
Assert.state(method != null, "No standard HTTP method");
ClientHttpRequest delegate = requestFactory.createRequest(request.getURI(), method);
request.getHeaders().forEach((key, value) -> delegate.getHeaders().addAll(key, value));
if (body.length > 0) {
if (delegate instanceof StreamingHttpOutputMessage) {
StreamingHttpOutputMessage streamingOutputMessage = (StreamingHttpOutputMessage) delegate;
streamingOutputMessage.setBody(outputStream -> StreamUtils.copy(body, outputStream));
}
else {
StreamUtils.copy(body, delegate.getBody());
}
}
return delegate.execute();
}
}
}3.2、Spring提供的拦截器实现
BasicAuthenticationInterceptor可以对http header添加basic-auth提供客户端的Basic认证。
public class BasicAuthenticationInterceptor implements ClientHttpRequestInterceptor {
private final String username;
private final String password;
@Nullable
private final Charset charset;
public BasicAuthenticationInterceptor(String username, String password) {
this(username, password, null);
}
public BasicAuthenticationInterceptor(String username, String password, @Nullable Charset charset) {
Assert.doesNotContain(username, ":", "Username must not contain a colon");
this.username = username;
this.password = password;
this.charset = charset;
}
@Override
public ClientHttpResponse intercept(
HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException {
HttpHeaders headers = request.getHeaders();
if (!headers.containsKey(HttpHeaders.AUTHORIZATION)) {
headers.setBasicAuth(this.username, this.password, this.charset);
}
return execution.execute(request, body);
}
}