淺析okHttp3的網絡請求流程

時間  2019-11-16
標籤 淺析 okhttp3 okhttp 網絡 請求 流程 欄目 系統網絡 简体版
原文   原文鏈接

寫在開頭

okHttp目前能夠稱的上是Android主流網絡框架,甚至連谷歌官方也將網絡請求的實現替換成okHttp.java

網上也有不少人對okHttp的源碼進行了分析,不過基於每一個人的分析思路都不盡相同,讀者看起來的收穫也各不相同,因此我仍是整理了下思路,寫了點本身的分析感悟。算法

本文基於okhttp3.11.0版本分析json

基本用法

String url = "http://www.baidu.com";
//'1. 生成OkHttpClient實例對象'
OkHttpClient okHttpClient = new OkHttpClient();
//'2. 生成Request對象'
Request request = new Request.Builder()
    .url(url)
    .post(RequestBody.create(MediaType.parse("application/json; charset=utf-8"),"test content"))
    .build();
Call call = okHttpClient.newCall(request);

call.enqueue(new Callback() {
    @Override
    public void onFailure(@NonNull Call call, @NonNull IOException e) {
    }

    @Override
    public void onResponse(@NonNull Call call, @NonNull Response response) {

    }
});
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總體流程

借用別人的一張流程圖來歸納一下okHttp的請求走向 原圖出處c#

okHttp的總體流程大體分爲如下幾個階段

  1. 建立請求對象 (url, method,body)-->request-->Callapi

  2. 請求事件隊列,線程池分發 enqueue-->Runnable-->ThreadPoolExecutor緩存

  3. 遞歸Interceptor攔截器,發送請求。 InterceptorChain安全

  4. 請求回調,數據解析。 Respose-->(code,message,requestBody)bash

建立請求對象

其中 Request維護請求對象的屬性服務器

public final class Request {
    final HttpUrl url;  
    final String method;
    final Headers headers;
    final @Nullable RequestBody body;
    //請求的標記,在okHttp2.x的時候，okHttpClint提供Cancel(tag)的方法來批量取消請求
    //不過在3.x上批量請求的api被刪除了,要取消請求只能在Callback中調用 call.cancel()
    //所以這個tags參數只能由開發者本身編寫函數來實現批量取消請求的操做
    final Map<Class<?>, Object> tags;  
}
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請求響應的包裝接口Callcookie

public interface Call extends Cloneable {
	Request request();
	Response execute() throws IOException;
	void enqueue(Callback responseCallback);
	void cancel();
}
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請求事件隊列,線程池分發

Call的實現類RealCall和AsyncCall

@Override 
  public void enqueue(Callback responseCallback) {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
  }

//其中AsyncCall是RealCall的一個內部類,繼承自Runnable,這樣就能經過線程池來回調AsyncCall的execute函數

final class AsyncCall extends NamedRunnable {
    @Override 
    protected void execute() {
        boolean signalledCallback = false;
        try {
            //getResponseWithInterceptorChain 攔截鏈的邏輯,也是發起請求的真正入口
            Response response = getResponseWithInterceptorChain();
            if (retryAndFollowUpInterceptor.isCanceled()) {
                signalledCallback = true;
                responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
            } else {
                signalledCallback = true;
                responseCallback.onResponse(RealCall.this, response);
            }
        } catch (IOException e) {
            ...
        } 
        ...
    }
}
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遞歸Interceptor攔截器,發送請求

Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
 	//用戶自定義的攔截器(注意addAll 因此能夠添加多個自定義的攔截器)
    interceptors.addAll(client.interceptors());
    //重試與重定向攔截器
    interceptors.add(retryAndFollowUpInterceptor);
    //內容攔截器
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    //緩存攔截器
    interceptors.add(new CacheInterceptor(client.internalCache()));
    //網絡鏈接攔截器
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
      //用戶自定義的網絡攔截器
      interceptors.addAll(client.networkInterceptors());
    }
    //服務請求的攔截器
    interceptors.add(new CallServerInterceptor(forWebSocket));

    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
        originalRequest, this, eventListener, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());

    return chain.proceed(originalRequest);
  }
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okHttp的核心部分就是這個Interceptor攔截鏈,每一個Interceptor各自負責一部分功能,內部經過遞歸的方式遍歷每個Interceptor攔截器。遞歸邏輯在RealInterceptorChain類下

public final class RealInterceptorChain implements Interceptor.Chain {
    
    //攔截器遞歸的入口
    public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec, RealConnection connection) throws IOException {
     ...
    //攔截器遞歸的核心代碼,根據interceptors列表執行每個攔截器的intercept函數
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);
	
    ....
    return response;
  }
}
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遞歸結束後會得到請求響應,那麼說明咱們的request行爲就在這個攔截鏈中,接下來咱們先看看負責網絡請求的那部分攔截器,從類名上就能比較容易的看出 ConnectInterceptor和 CallServerInterceptor這兩個攔截器的主要工做。

網絡鏈接攔截器ConnectInterceptor
public final class ConnectInterceptor implements Interceptor {
  
    @Override 
    public Response intercept(Chain chain) throws IOException {
        RealInterceptorChain realChain = (RealInterceptorChain) chain;
        Request request = realChain.request();
        StreamAllocation streamAllocation = realChain.streamAllocation();

        // We need the network to satisfy this request. Possibly for validating a conditional GET.
        boolean doExtensiveHealthChecks = !request.method().equals("GET");
        HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
        RealConnection connection = streamAllocation.connection();

        return realChain.proceed(request, streamAllocation, httpCodec, connection);
    }
}
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其中有幾個對象說明一下

  • **StreamAllocation:**內存流的存儲空間,這個對象能夠直接從realChain中直接獲取,說明在以前的攔截鏈中就已經賦值過

  • HttpCodec(Encodes HTTP requests and decodes HTTP responses): 對請求的編碼以及對響應數據的解碼

  • **realChain.proceed():**通知下一個攔截器執行

接下來看建立HttpCodec對象的newStream函數中作了些什麼

//HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);

public HttpCodec newStream( OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
   	
    ...
    try {
      //findHealthyConnection內部經過一個死循環查找一個可用的鏈接,優先使用存在的可用鏈接,不然就經過 //線程池來生成,其中多處使用 synchronized關鍵字,防止由於多併發致使問題
      RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
          writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);
      HttpCodec resultCodec = resultConnection.newCodec(client, chain, this);

      synchronized (connectionPool) {
        codec = resultCodec;
        return resultCodec;
      }
    } catch (IOException e) {
      throw new RouteException(e);
    }
  }
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沿着代碼往下走,你會發現實際上負責網絡鏈接功能的類是一個叫RealConnection的類,該類中有一個connect的函數

RealConnection#connect

public void connect(int connectTimeout, int readTimeout, int writeTimeout, int pingIntervalMillis, boolean connectionRetryEnabled, Call call, EventListener eventListener) {
    ...
        
    while (true) {
      try {
        if (route.requiresTunnel()) {
          //這個函數最終仍是會走到connectSocket()函數中
          connectTunnel(connectTimeout, readTimeout, writeTimeout, call, eventListener);
          if (rawSocket == null) {
            // We were unable to connect the tunnel but properly closed down our resources.
            break;
          }
        } else {
          connectSocket(connectTimeout, readTimeout, call, eventListener);
        }
      }
      ...
  }
  
  //最終調用的仍是Socket對象來建立網絡鏈接,包括connectTimeout,readTimeout等參數也是這個時候真正設置的。
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網絡請求攔截器 CallServerInterceptor

This is the last interceptor in the chain. It makes a network call to the server.

直接看CallServerInterceptor的intercept函數

@Override
public Response intercept(Chain chain) throws IOException{
    //下面的各參數都是以前幾個攔截器所生成的
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    HttpCodec httpCodec = realChain.httpStream();
    StreamAllocation streamAllocation = realChain.streamAllocation();
    RealConnection connection = (RealConnection) realChain.connection();
    Request request = realChain.request();
    
    //發送請求頭,也是網絡請求的開始
    httpCodec.writeRequestHeaders(request);
    
    Response.Builder responseBuilder = null;
    //請求不是get,而且有添加了請求體,寫入請求體信息
    if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
      //若是請求頭中有Expect:100-continue這麼一個屬性
      //會先發送一個header部分給服務器,並詢問服務器是否支持Expect:100-continue 這麼一個擴展域
      //okhttp3提供這麼個判斷是爲了兼容http2的鏈接複用行爲的
      if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
        //刷新緩存區,能夠理解爲向服務端寫入數據
        httpCodec.flushRequest();
        realChain.eventListener().responseHeadersStart(realChain.call());
        responseBuilder = httpCodec.readResponseHeaders(true);
      }
		
      //寫入請求body
      if (responseBuilder == null) {
        realChain.eventListener().requestBodyStart(realChain.call());
        long contentLength = request.body().contentLength();
        CountingSink requestBodyOut =
            new CountingSink(httpCodec.createRequestBody(request, contentLength));
        BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);

        request.body().writeTo(bufferedRequestBody);
        bufferedRequestBody.close();
        realChain.eventListener()
            .requestBodyEnd(realChain.call(), requestBodyOut.successfulCount);
      } 
      ...
    httpCodec.finishRequest();
    
    //響應相關的代碼
    ...
}
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寫入請求body的核心代碼

//將請求體寫入到BufferedSink中,而BufferedSink是另一個類庫Okio中的類
CountingSink requestBodyOut =
            new CountingSink(httpCodec.createRequestBody(request, contentLength));
BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
request.body().writeTo(bufferedRequestBody);


//httpCodec.finishRequest 最終會調用 sink.flush(),sink是BufferedSink的對象,BufferedSink在底層
//會將其內的數據推給服務端,至關因而一個刷新緩衝區的功能
httpCodec.finishRequest();
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響應相關的代碼

if (responseBuilder == null) {
    realChain.eventListener().responseHeadersStart(realChain.call());
  	//讀取響應頭,實際的返回流存放位置在okio庫下的buffer對象中,讀取過程當中作了判斷,只有當code==100時纔會 //有返回,否則拋出異常並攔截,因此下面這段代碼確定有響應頭返回,否則直到超時也不會回調
    responseBuilder = httpCodec.readResponseHeaders(false);
}

Response response = responseBuilder
            .request(request)
    .handshake(streamAllocation.connection().handshake())
    .sentRequestAtMillis(sentRequestMillis)
    .receivedResponseAtMillis(System.currentTimeMillis())
    .build();

int code = response.code();
if (code == 100) {
    //若是服務端響應碼爲100,須要咱們再次請求,注意這裏的100是響應碼和以前的100不一樣
    //以前的100是headerLine的標識碼
    responseBuilder = httpCodec.readResponseHeaders(false);

    response = responseBuilder
        .request(request)
        .handshake(streamAllocation.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();

    code = response.code();
}

if (forWebSocket && code == 101) {
    //Connection is upgrading, but we need to ensure interceptors see a 
    //non-null response body.
    response = response.newBuilder()
        .body(Util.EMPTY_RESPONSE)
        .build();
} else {
    //讀取響應body
    response = response.newBuilder()
        .body(httpCodec.openResponseBody(response))
        .build();
}
return response;
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讀取響應body HttpCodec#openResponseBody

public ResponseBody openResponseBody(Response response) throws IOException {
    ...
    Source source = newFixedLengthSource(contentLength);
	return new RealResponseBody(contentType, contentLength, Okio.buffer(source));
    ...
}

//openResponseBody將Socket的輸入流InputStream對象交給OkIo的Source對象，而後封裝成RealResponseBody（該類是ResponseBody的子類）做爲Response的body.

//具體讀取是在RealResponseBody父類ResponseBody中,其中有個string()函數

//響應主體存放在內存中,而後調用source.readString來讀取服務器的數據。須要注意的是該方法最後調用closeQuietly來關閉了當前請求的InputStream輸入流，因此string()方法只能調用一次,再次調用的話會報錯
public final String string() throws IOException {
    BufferedSource source = source();
    try {
        Charset charset = Util.bomAwareCharset(source, charset());
        return source.readString(charset);
    } finally {
        Util.closeQuietly(source);
    }
}
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請求回調,數據解析

拿到請求回調的Response以後,再回到咱們最開始調用的代碼,

String url = "http://www.baidu.com";
//'1. 生成OkHttpClient實例對象'
OkHttpClient okHttpClient = new OkHttpClient();
//'2. 生成Request對象'
Request request = new Request.Builder()
    .url(url)
    .post(RequestBody.create(MediaType.parse("application/json; charset=utf-8"),"test content"))
    .build();
Call call = okHttpClient.newCall(request);

call.enqueue(new Callback() {
    @Override
    public void onFailure(@NonNull Call call, @NonNull IOException e) {
    }

    @Override
    public void onResponse(@NonNull Call call, @NonNull Response response) {
        Headers responseHeaders = response.headers();
        for (int i = 0, size = responseHeaders.size(); i < size; i++) {
            System.out.println(responseHeaders.name(i) + ": " + responseHeaders.value(i));
        }
        System.out.println(response.body().string());
    }
});
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咱們能夠從Response對象中獲取全部咱們所須要的數據,包括header,body.至此,okHttp的網絡請求的大體流程已經分析完成,至於還有部分沒有講到的攔截器就再也不本文綴述了.有興趣的能夠看下文末的參考鏈接或者自行谷歌。

參考文章

Okhttp之CallServerInterceptor簡單分析

okHttp各攔截器解析

Android技能樹 — 網絡小結之 OkHttp超超超超超超超詳細解析

OkHttp3.0解析 —— 從源碼的角度談談發起網絡請求時作的操做

擴展閱讀

關於Http的請求頭 Expect:100-Continue

Expect請求頭部域，用於指出客戶端要求的特殊服務器行爲。若服務器不能理解或者知足
Expect域中的任何指望值，則必須返回417(Expectation Failed)狀態，或者若是請求
有其餘問題，返回4xx狀態。

Expect:100-Continue握手的目的，是爲了容許客戶端在發送請求內容以前，判斷源服務器是否願意接受
請求（基於請求頭部）。
Expect:100-Continue握手需謹慎使用，由於遇到不支持HTTP/1.1協議的服務器或者代理時會引發問題。
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http2比起http1.x的有點主要體如今如下幾點

  • 新的數據格式, http基於文件協議解析,http2基於二進制協議解析,
  • 鏈接共享,多路複用（MultiPlexing）
  • header壓縮,減少header的體積,使得請求更快
  • 壓縮算法從gzip改爲HPACK的算法,防破解
  • 重置鏈接表現更好,http1.x取消請求的是直接斷開鏈接，http2則是斷開某個鏈接的stream流
  • 更安全的SSL

參考資料

http1.x與http2的區別

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