OKHttp原始碼分析3
1 概述
上篇文章,我們詳細分析了OKHttp中Request的建立和傳送過程。其中sendRequest(), readResponse(), followUpRequest()三個關鍵方法在底層HttpEngine中實現。革命尚未成功,我們接下來在這篇文章中分析HttpEngine中的這三個方法。由於底層HttpEngine涉及到很多Http協議方面東西,對Http協議不熟悉的同學可以先閱讀我的這篇文章 Http協議簡介
2 sendRequest()原始碼分析
sendRequest()方法是client向server傳送request的主要方法。它先對request的header添加了一些預設欄位,如keep-alive。然後對cache進行處理,判斷是否可以直接使用cache。如果不行,才真正傳送網路request。
public void sendRequest() throws RequestException, RouteException, IOException {
if (cacheStrategy != null) return; // Already sent.
if (transport != null) throw new IllegalStateException();
// 對header的處理,利用app中使用者構造的原始request
// 主要是對header進行新增。如新增"Connection: Keep-Alive"首部。後面單獨分析
Request request = networkRequest(userRequest);
// 對cache的處理
InternalCache responseCache = Internal.instance.internalCache(client);
// 利用request為key,從cache中取出response
Response cacheCandidate = responseCache != null
? responseCache.get(request)
: null;
// 判斷cache是否可用,利用Expires,Last-Modified,Date,Age等header欄位,後面詳細分析
long now = System.currentTimeMillis();
cacheStrategy = new CacheStrategy.Factory(now, request, cacheCandidate).get();
// cache可用或網路被禁止使用則networkRequest為null
networkRequest = cacheStrategy.networkRequest;
// cache不可用,則cacheResponse為null。對應情況有,不允許使用cache,沒有對應cache,cache過期需要重新驗證
cacheResponse = cacheStrategy.cacheResponse;
if (responseCache != null) {
responseCache.trackResponse(cacheStrategy);
}
if (cacheCandidate != null && cacheResponse == null) {
// cache已過期,不可用,關閉它
closeQuietly(cacheCandidate.body());
}
if (networkRequest != null) {
// networkRequest不為空,代表cache不可用,且網路可用
// 從這兒可以看出,cache可用時會直接使用cache,不可用才走網路資料。這也是符合Http常規做法的。
if (connection == null) {
// 連線到server,直接連線或通過代理均可
connect();
}
// 構造HttpTransport,與傳送request到網路中去有關
transport = Internal.instance.newTransport(connection, this);
// 將start line,headers,body寫入到buffer中,以等待發送出去
if (callerWritesRequestBody && permitsRequestBody(networkRequest) && requestBodyOut == null) {
// 從request的header中獲取content-length
long contentLength = OkHeaders.contentLength(request);
if (bufferRequestBody) {
// bufferRequestBody表示body在記憶體中了,這可能是多次傳送重試等情況
// content-length太大
if (contentLength > Integer.MAX_VALUE) {
throw new IllegalStateException("Use setFixedLengthStreamingMode() or "
+ "setChunkedStreamingMode() for requests larger than 2 GiB.");
}
if (contentLength != -1) {
// content-length已知,是個準確值
// 可以將start line和header寫入HttpConnection中,此處涉及到Http報文結構和傳送,後面重點講解
transport.writeRequestHeaders(networkRequest);
// 構造request body的buffer,長度為content-length
requestBodyOut = new RetryableSink((int) contentLength);
} else {
// content-length還不確定,此時不能設定content-length首部,因為它還不確定。
// 要等到整個body準備好後,才能計算出content-length
requestBodyOut = new RetryableSink();
}
} else {
transport.writeRequestHeaders(networkRequest);
requestBodyOut = transport.createRequestBody(networkRequest, contentLength);
}
}
} else {
// networkRequest為null,要麼cache可用,要麼網路被禁止使用
if (connection != null) {
// 回收網路connection,並關閉它
Internal.instance.recycle(client.getConnectionPool(), connection);
connection = null;
}
if (cacheResponse != null) {
// cache可用。可用代表有此request的cache response,且沒有過期
this.userResponse = cacheResponse.newBuilder()
.request(userRequest)
.priorResponse(stripBody(priorResponse))
.cacheResponse(stripBody(cacheResponse))
.build();
} else {
// 網路被禁止使用,自己構造一個504的response,gateway timeout
this.userResponse = new Response.Builder()
.request(userRequest)
.priorResponse(stripBody(priorResponse))
.protocol(Protocol.HTTP_1_1)
.code(504)
.message("Unsatisfiable Request (only-if-cached)")
.body(EMPTY_BODY)
.build();
}
// 將利用cache或自己生成的504response,進行gzip壓縮
// 前面提到過,request的headers中聲明瞭支援gzip壓縮,故response中最好加入gzip壓縮。
userResponse = unzip(userResponse);
}
}
我們接下來分析sendRequest()中使用到的一些比較重要的方法。networkRequest()方法作用為,在原有的request基礎上新增一些header。從這些header中我們可以看出,OKHttp預設是使用Keep-Alive,response body支援gzip壓縮,支援Cookie的使用。看到了吧,分析底層程式碼有助於我們對Http協議的理解和對OKHttp特性的掌握。
private Request networkRequest(Request request) throws IOException {
Request.Builder result = request.newBuilder();
// 利用url解析出host,然後新增host header。它指明瞭server地址
if (request.header("Host") == null) {
result.header("Host", Util.hostHeader(request.httpUrl()));
}
// 新增Connection首部,Keep-Alive表示持久連線,一次request和response完成後,HTTP並不立刻關閉。
if (request.header("Connection") == null) {
result.header("Connection", "Keep-Alive");
}
// 新增Accept-Encoding首部,gzip表示可接收gzip格式的壓縮編碼
if (request.header("Accept-Encoding") == null) {
transparentGzip = true;
result.header("Accept-Encoding", "gzip");
}
// 處理cookie header
CookieHandler cookieHandler = client.getCookieHandler();
if (cookieHandler != null) {
// 將使用者構建的原始request中的header弄成Map結構
Map<String, List<String>> headers = OkHeaders.toMultimap(result.build().headers(), null);
// 從URI中解析出cookie,並新增到Map中,其key為"Cookie"
Map<String, List<String>> cookies = cookieHandler.get(request.uri(), headers);
// 新增Cookie和Cookie2 header
OkHeaders.addCookies(result, cookies);
}
// 新增User-Agent header,它表示client端是啥東西,比如瀏覽器
// 對於OKHttp來說,就是okhttp和它的版本號
if (request.header("User-Agent") == null) {
result.header("User-Agent", Version.userAgent());
}
return result.build();
}
下面分析下cache是否可用的判斷邏輯,也就是下面這行程式碼的執行邏輯。
cacheStrategy = new CacheStrategy.Factory(now, request, cacheCandidate).get();
public final class CacheStrategy {
// 構造方法,nowMillis為傳入的系統此刻時間
public Factory(long nowMillis, Request request, Response cacheResponse) {
this.nowMillis = nowMillis;
this.request = request;
this.cacheResponse = cacheResponse;
if (cacheResponse != null) {
// 取出response中的headers
Headers headers = cacheResponse.headers();
// 遍歷所有headers,解析出與cache過期有關的headers,並給相應成員變數賦值
for (int i = 0, size = headers.size(); i < size; i++) {
String fieldName = headers.name(i);
String value = headers.value(i);
if ("Date".equalsIgnoreCase(fieldName)) {
// Date header處理
servedDate = HttpDate.parse(value);
servedDateString = value;
} else if ("Expires".equalsIgnoreCase(fieldName)) {
// Expires header處理
expires = HttpDate.parse(value);
} else if ("Last-Modified".equalsIgnoreCase(fieldName)) {
// Last-Modified header處理
lastModified = HttpDate.parse(value);
lastModifiedString = value;
} else if ("ETag".equalsIgnoreCase(fieldName)) {
// ETag header處理
etag = value;
} else if ("Age".equalsIgnoreCase(fieldName)) {
// Age header處理
ageSeconds = HeaderParser.parseSeconds(value, -1);
} else if (OkHeaders.SENT_MILLIS.equalsIgnoreCase(fieldName)) {
sentRequestMillis = Long.parseLong(value);
} else if (OkHeaders.RECEIVED_MILLIS.equalsIgnoreCase(fieldName)) {
receivedResponseMillis = Long.parseLong(value);
}
}
}
}
public CacheStrategy get() {
// CacheStrategy生成的主要方法
CacheStrategy candidate = getCandidate();
if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) {
// 網路被使用者禁止使用,並且cache不可用,此時networkRequest和cacheResponse都為null
return new CacheStrategy(null, null);
}
return candidate;
}
private CacheStrategy getCandidate() {
// 沒有此request的cache response
if (cacheResponse == null) {
return new CacheStrategy(request, null);
}
// 對於HTTPS,必須有handshake欄位,否則認為此cache不可用
if (request.isHttps() && cacheResponse.handshake() == null) {
return new CacheStrategy(request, null);
}
// 此response不能使用cache,比如金融類資料,一般追求實時性,不適合使用cache
if (!isCacheable(cacheResponse, request)) {
return new CacheStrategy(request, null);
}
// 使用cache前需要先驗證一下儲存的response,或者request中有條件GET的headers
// noCache()方法命名不好,有歧義。它不是表示不能使用cache或者沒有cache,而是表示使用前要先驗證。
CacheControl requestCaching = request.cacheControl();
if (requestCaching.noCache() || hasConditions(request)) {
return new CacheStrategy(request, null);
}
// 計算cache是否過期
long ageMillis = cacheResponseAge(); // 目前response生成時的絕對時間
long freshMillis = computeFreshnessLifetime(); // 到什麼時候為止仍然是新鮮的,絕對時間
if (requestCaching.maxAgeSeconds() != -1) {
freshMillis = Math.min(freshMillis, SECONDS.toMillis(requestCaching.maxAgeSeconds()));
}
long minFreshMillis = 0; // 剩餘的最低的保鮮期,相對時間
if (requestCaching.minFreshSeconds() != -1) {
minFreshMillis = SECONDS.toMillis(requestCaching.minFreshSeconds());
}
long maxStaleMillis = 0; // 最大過期時間,相對時間
CacheControl responseCaching = cacheResponse.cacheControl();
if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
}
if (!responseCaching.noCache() && ageMillis + minFreshMillis < freshMillis + maxStaleMillis) {
Response.Builder builder = cacheResponse.newBuilder();
if (ageMillis + minFreshMillis >= freshMillis) {
builder.addHeader("Warning", "110 HttpURLConnection \"Response is stale\"");
}
long oneDayMillis = 24 * 60 * 60 * 1000L;
if (ageMillis > oneDayMillis && isFreshnessLifetimeHeuristic()) {
builder.addHeader("Warning", "113 HttpURLConnection \"Heuristic expiration\"");
}
// cache可用時,將networkRequest賦值為null,可以看出OKHttp是優先使用cache的
return new CacheStrategy(null, builder.build());
}
// 條件GET的處理。
// 條件GET一般是cache過期了,需要傳送驗證request給server,以判斷cache response是否修改了。如果沒有修改,還是可以接著使用cache的。
Request.Builder conditionalRequestBuilder = request.newBuilder();
if (etag != null) {
conditionalRequestBuilder.header("If-None-Match", etag);
} else if (lastModified != null) {
conditionalRequestBuilder.header("If-Modified-Since", lastModifiedString);
} else if (servedDate != null) {
conditionalRequestBuilder.header("If-Modified-Since", servedDateString);
}
Request conditionalRequest = conditionalRequestBuilder.build();
return hasConditions(conditionalRequest)
? new CacheStrategy(conditionalRequest, cacheResponse)
: new CacheStrategy(conditionalRequest, null);
}
}
writeRequestHeaders()向HttpConnection的buffer中以UTF-8的編碼格式寫入start line和headers,合適的時機會發送到socket中傳輸出去
public final class HttpTransport implements Transport {
// 這個名字起的不好,這個方法不僅寫入了headers,還寫入了start line
public void writeRequestHeaders(Request request) throws IOException {
// 傳送request之前必須立刻呼叫,它記錄了傳送request的系統時間
httpEngine.writingRequestHeaders();
// 生成start line,後面有詳細分析
String requestLine = RequestLine.get(
request, httpEngine.getConnection().getRoute().getProxy().type());
// 將start line和headers寫入到buffer中,UTF-8格式,合適的時機再將buffer中資料通過socket傳輸出去
httpConnection.writeRequest(request.headers(), requestLine);
}
}
public final class RequestLine {
// 生成request的start line,Http協議中它的格式為 method url version
static String get(Request request, Proxy.Type proxyType) {
StringBuilder result = new StringBuilder();
// 寫入method
result.append(request.method());
result.append(' ');
// 寫入url
if (includeAuthorityInRequestLine(request, proxyType)) {
result.append(request.httpUrl());
} else {
result.append(requestPath(request.httpUrl()));
}
// 寫入version,可以看到OKHttp支援的是HTTP/1.1版本
result.append(" HTTP/1.1");
return result.toString();
}
}
3 readResponse()原始碼分析
public void readResponse() throws IOException {
if (userResponse != null) {
// response已經有了,這可能是利用cache生成的response或其他情況,
// 此時我們就不用去接收server端的response了,其實一般此時也沒有server端的response讓我們去接收,哈哈~
return;
}
if (networkRequest == null && cacheResponse == null) {
throw new IllegalStateException("call sendRequest() first!");
}
if (networkRequest == null) {
return; // No network response to read.
}
Response networkResponse;
if (forWebSocket) {
// 先將start line和header寫入socket中
transport.writeRequestHeaders(networkRequest);
// 傳送request,並讀取response,後面詳細分析
networkResponse = readNetworkResponse();
} else if (!callerWritesRequestBody) {
// 先執行攔截器,再寫入request到HttpConnection的buffer中,最後傳送buffer,並讀取response
// 和上面情況比較像,這裡就不展開分析了
networkResponse = new NetworkInterceptorChain(0, networkRequest).proceed(networkRequest);
} else {
// 將request body的buffer發出去,這樣requestBodyOut中就有了body
if (bufferedRequestBody != null && bufferedRequestBody.buffer().size() > 0) {
bufferedRequestBody.emit();
}
// 處理request headers,並將start line和header寫入socket中
if (sentRequestMillis == -1) {
if (OkHeaders.contentLength(networkRequest) == -1
&& requestBodyOut instanceof RetryableSink) {
// 如果之前content-length值不清楚,此時在body已經ready的情況下,可以計算出content-length,並將它新增到header中
long contentLength = ((RetryableSink) requestBodyOut).contentLength();
networkRequest = networkRequest.newBuilder()
.header("Content-Length", Long.toString(contentLength))
.build();
}
// 將start line和header寫入socket中
transport.writeRequestHeaders(networkRequest);
}
// 將body寫入socket中
if (requestBodyOut != null) {
if (bufferedRequestBody != null) {
// This also closes the wrapped requestBodyOut.
bufferedRequestBody.close();
} else {
requestBodyOut.close();
}
if (requestBodyOut instanceof RetryableSink) {
// body 寫入socket中
transport.writeRequestBody((RetryableSink) requestBodyOut);
}
}
// 傳送request,並讀取response,後面會詳細分析
networkResponse = readNetworkResponse();
}
// 開始處理獲取到的response
// 讀取並處理response的headers
receiveHeaders(networkResponse.headers());
// cache response存在的情況下,應該是cache過期了,做了一次條件GET來驗證cache的內容是否有變更。
// 根據Http協議,如果cache未變,回覆304,not modified。且response中不會包含body,
// 如果cache改變,回覆200, OK。response中包含body
if (cacheResponse != null) {
if (validate(cacheResponse, networkResponse)) {
// 再驗證通過,cache內容未變,使用cache構造response
userResponse = cacheResponse.newBuilder()
.request(userRequest)
.priorResponse(stripBody(priorResponse))
.headers(combine(cacheResponse.headers(), networkResponse.headers()))
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
networkResponse.body().close();
releaseConnection();
// 更新cache
InternalCache responseCache = Internal.instance.internalCache(client);
responseCache.trackConditionalCacheHit();
responseCache.update(cacheResponse, stripBody(userResponse));
userResponse = unzip(userResponse);
return;
} else {
// cache未命中,response中會包含我們想要的body的。關閉cache body流
closeQuietly(cacheResponse.body());
}
}
// cache未命中,利用server返回的response string構造client使用的Response物件
// 此時會將response快取起來,以便下次使用
userResponse = networkResponse.newBuilder()
.request(userRequest)
.priorResponse(stripBody(priorResponse))
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
if (hasBody(userResponse)) {
maybeCache();
userResponse = unzip(cacheWritingResponse(storeRequest, userResponse));
}
}
下面詳細分析readNetworkResponse(),它會通過socket流讀取response string的start line,headers和body。
private Response readNetworkResponse() throws IOException {
// 將HttpTransport中的buffer flush出去
transport.finishRequest();
// 讀取server的response string,並構造出Response物件
Response networkResponse = transport.readResponseHeaders()
.request(networkRequest)
.handshake(connection.getHandshake())
.header(OkHeaders.SENT_MILLIS, Long.toString(sentRequestMillis))
.header(OkHeaders.RECEIVED_MILLIS, Long.toString(System.currentTimeMillis()))
.build();
if (!forWebSocket) {
networkResponse = networkResponse.newBuilder()
.body(transport.openResponseBody(networkResponse))
.build();
}
return networkResponse;
}
public final class HttpTransport implements Transport {
@Override public Response.Builder readResponseHeaders() throws IOException {
return httpConnection.readResponse();
}
}
public final class HttpConnection {
public Response.Builder readResponse() throws IOException {
if (state != STATE_OPEN_REQUEST_BODY && state != STATE_READ_RESPONSE_HEADERS) {
throw new IllegalStateException("state: " + state);
}
try {
while (true) {
// 解析start line,response的start line格式為 protocol,code, message
StatusLine statusLine = StatusLine.parse(source.readUtf8LineStrict());
// 將解析出的protocol, code, message分別放入構造的Response物件中
Response.Builder responseBuilder = new Response.Builder()
.protocol(statusLine.protocol)
.code(statusLine.code)
.message(statusLine.message);
// 解析response string的headers
Headers.Builder headersBuilder = new Headers.Builder();
// 一行行讀取headers, 直到遇到空行結束
readHeaders(headersBuilder);
headersBuilder.add(OkHeaders.SELECTED_PROTOCOL, statusLine.protocol.toString());
// 將headers新增到Response物件中
responseBuilder.headers(headersBuilder.build());
// 如果返回code不是100, continue,則可以直接將Response物件返回
// 對於100,continue,server還會繼續返回response string,我們需要在while迴圈中繼續接收並解析
if (statusLine.code != HTTP_CONTINUE) {
state = STATE_OPEN_RESPONSE_BODY;
return responseBuilder;
}
}
} catch (EOFException e) {
// Provide more context if the server ends the stream before sending a response.
IOException exception = new IOException("unexpected end of stream on " + connection
+ " (recycle count=" + Internal.instance.recycleCount(connection) + ")");
exception.initCause(e);
throw exception;
}
}
}
4 followUpRequest()原始碼分析
client傳送一個request之後,server可能回覆一個重定向的response,並在這個response中告知client需要重新訪問的server的IP。此時,client需要重新向新的server傳送request,並等待新server的回覆。所以我們需要單獨判斷重定向response,併發送多次request。有了OKHttp,這一切你都不用管,它會自動幫你完成所有這一切。OKHttp中followUpRequest()方法就是完成這個功能的。是不是瞬間感覺OKHttp強大到不要不要的啊~。這個方法流程比較簡單,就不給出流程圖了。親們如果對Http協議不熟悉,可以先看我的這篇文章Http協議簡介
public Request followUpRequest() throws IOException {
if (userResponse == null) throw new IllegalStateException();
Proxy selectedProxy = getRoute() != null
? getRoute().getProxy()
: client.getProxy();
int responseCode = userResponse.code();
// 利用responseCode來分析是否需要自動傳送後續request
switch (responseCode) {
// 未認證使用者,不能訪問server或代理,故需要傳送認證的request
case HTTP_PROXY_AUTH:
if (selectedProxy.type() != Proxy.Type.HTTP) {
throw new ProtocolException("Received HTTP_PROXY_AUTH (407) code while not using proxy");
}
case HTTP_UNAUTHORIZED:
return OkHeaders.processAuthHeader(client.getAuthenticator(), userResponse, selectedProxy);
// 永久重定向,暫時重定向,永久移動了等和重定向相關的response,response code中以3打頭的都是
// 它們需要重新發送request給新的server,新sever的ip在response中會給出
case HTTP_PERM_REDIRECT:
case HTTP_TEMP_REDIRECT:
if (!userRequest.method().equals("GET") && !userRequest.method().equals("HEAD")) {
return null;
}
case HTTP_MULT_CHOICE:
case HTTP_MOVED_PERM:
case HTTP_MOVED_TEMP:
case HTTP_SEE_OTHER:
// Does the client allow redirects?
if (!client.getFollowRedirects()) return null;
// 新的server的IP地址在response的Location header中給出
String location = userResponse.header("Location");
if (location == null) return null;
HttpUrl url = userRequest.httpUrl().resolve(location);
// Don't follow redirects to unsupported protocols.
if (url == null) return null;
// If configured, don't follow redirects between SSL and non-SSL.
boolean sameScheme = url.scheme().equals(userRequest.httpUrl().scheme());
if (!sameScheme && !client.getFollowSslRedirects()) return null;
// Redirects don't include a request body.
Request.Builder requestBuilder = userRequest.newBuilder();
if (HttpMethod.permitsRequestBody(userRequest.method())) {
requestBuilder.method("GET", null);
requestBuilder.removeHeader("Transfer-Encoding");
requestBuilder.removeHeader("Content-Length");
requestBuilder.removeHeader("Content-Type");
}
// 刪掉使用者認證資訊
if (!sameConnection(url)) {
requestBuilder.removeHeader("Authorization");
}
return requestBuilder.url(url).build();
default:
return null;
}
}
5 總結
OKHttp底層原始碼還是相當複雜的,畢竟它的功能如此之強大嘛。OKHttp預設採用了Keep-Alive持久連線技術,可支援gzip編碼的response。在cache的處理上,如果cache可用,則直接使用cache,否則使用網路資料。OKHttp會做cache過期的判斷和過期後的再驗證。有了OKHttp,這一切你都不用管,它幫你cover掉了!
當需要做使用者驗證和重定向時,我們一般需要傳送認證request,或向新server傳送request,也就是要重新再生成新request併發送出去。有了OKHttp,這一切你都不用管,它又幫你cover掉了!
OKHttp功能實在是強大到爆表,掌握好了它的實現原理和底層流程之後,你就可以在專案中游刃有餘的放心使用它了!另外,小編可是花了整個週末才完成了這幾篇文章,走過路過的朋友幫忙寫寫評論吧,謝謝!