從源碼角度深入理解OKHttp3

在日常開發中網絡請求是很常見的功能。OkHttp作爲Android開發中最常用的網絡請求框架，在Android開發中我們經常結合retrofit一起使用，俗話說得好：“知其然知其所以然”，所以這篇文章我們通過源碼來深入理解OKHttp3（基於3.12版本）

常規使用

在瞭解源碼前，我們先了解如何使用OKHttp這個框架（框架地址）

//框架引入項目
implementation("com.squareup.okhttp3:okhttp:3.12.0")

//引用官方Demo的例子
@Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      //主線程不能進行耗時操作
       new Thread(){
          @Override
          public void run() {
              super.run();
              /**
               * 同步請求
               */
              GetExample getexample = new GetExample();
              String syncresponse = null;
              try {
                  syncresponse = getexample.run("https://raw.github.com/square/okhttp/master/README.md");
                  Log.i("maoqitian","異步請求返回參數"+syncresponse);
              } catch (IOException e) {
                  e.printStackTrace();
              }
          }
      }.start();
      /**
       * 異步請求
       */
      PostExample postexample = new PostExample();
      String json = postexample.bowlingJson("Jesse", "Jake");
      try {
          postexample.post("http://www.roundsapp.com/post", json);
      } catch (IOException e) {
          e.printStackTrace();
      }
  }
   /**
    * 異步請求
    */
   class PostExample {
       final MediaType JSON = MediaType.get("application/json; charset=utf-8");
       //獲取 OkHttpClient 對象
       OkHttpClient client = new OkHttpClient();

       void post(String url, String json) throws IOException {
           RequestBody body = RequestBody.create(JSON, json);
           Request request = new Request.Builder()
                   .url(url)
                   .post(body)
                   .build();
           client.newCall(request).enqueue(new Callback() {
              @Override
              public void onFailure(Call call, IOException e) {
                Log.i("maoqitian","異步請求返回參數"+e.toString());
              }

              @Override
              public void onResponse(Call call, Response response) throws IOException {
                  String asynresponse= response.body().string();
                  Log.i("maoqitian","異步請求返回參數"+asynresponse);
              }
          });
       }

       String bowlingJson(String player1, String player2) {
           return "{'winCondition':'HIGH_SCORE',"
                   + "'name':'Bowling',"
                   + "'round':4,"
                   + "'lastSaved':1367702411696,"
                   + "'dateStarted':1367702378785,"
                   + "'players':["
                   + "{'name':'" + player1 + "','history':[10,8,6,7,8],'color':-13388315,'total':39},"
                   + "{'name':'" + player2 + "','history':[6,10,5,10,10],'color':-48060,'total':41}"
                   + "]}";
       }
   }
  /**
   * 同步請求
   */
  class GetExample {
      OkHttpClient client = new OkHttpClient();
      String run(String url) throws IOException {
          Request request = new Request.Builder()
                  .url(url)
                  .build();
          try (Response response = client.newCall(request).execute()) {
              return response.body().string();
          }
      }
  }

由例子我們可以看到，client.newCall(request).execute()執行的是異步請求，我們可以加入Callback來異步獲取返回值，Response response = client.newCall(request).execute()執行的是同步請求，更多post請求方式例子可以查看官方sample項目

源碼分析

首先看一個流程圖，對於接下來的源碼分析有個大體印象

通過上面的例子可以看到，不管是同步請求還是異步請求，首先調用的OkHttpClient的newCall(request)方法，先來看看這個方法

 /**
  * Prepares the {@code request} to be executed at some point in the future.
  */
  @Override public Call newCall(Request request) {
  return RealCall.newRealCall(this, request, false /* for web socket */);
 }

通過newCall方法的源碼可以看到該方法返回值是Call，Call是一個接口，他的實現類是RealCall，所以我們執行的同步execute()方法或者異步enqueue()方法都是RealCall的方法。newCall方法接收了的網絡請求參數，接下來我們看看execute()和enqueue()方法

/**
 * 同步請求
 */
 @Override public Response execute() throws IOException {
  synchronized (this) {
  if (executed) throw new IllegalStateException("Already Executed");
  executed = true;
  }
  captureCallStackTrace();
  timeout.enter();
  eventListener.callStart(this);
  try {
    client.dispatcher().executed(this);
    Response result = getResponseWithInterceptorChain();
    if (result == null) throw new IOException("Canceled");
    return result;
    } catch (IOException e) {
    e = timeoutExit(e);
    eventListener.callFailed(this, e);
    throw e;
    } finally {
    client.dispatcher().finished(this);
    }
   }
 /**
  * 異步請求
  */
 @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));
 }

這裏先看異步的enqueue方法，很直觀可以看到真正執行網絡請求的是最後一句代碼，而它是怎麼做的呢，我們還得先弄明白dispatcher，Dispatcher的本質是異步請求的調度器，它內部持有一個線程池，結合線程池調配併發請求。官方文檔描述也說了這一點。

 /**最大併發請求數*/
 private int maxRequests = 64;
 /**每個主機最大請求數*/
 private int maxRequestsPerHost = 5;
 
 /** Ready async calls in the order they'll be run. 準備要執行的異步請求隊列*/
 private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();

 /** Running asynchronous calls. Includes canceled calls that haven't finished yet.
 正在執行的異步請求隊列*/
 private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();

 /** Running synchronous calls. Includes canceled calls that haven't finished yet. 
 正在執行的同步請求隊列*/
 private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
 
 /** Dispatcher 構造方法 */
 public Dispatcher(ExecutorService executorService) {
 this.executorService = executorService;
  }

  public Dispatcher() {
  }

 public synchronized ExecutorService  executorService() {
  if (executorService == null) {
  executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
      new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false));
  }
  return executorService;
 }

通過Dispatcher的構造方法我們知道我們可以使用自己的線程池，也可以使用Dispatcher默認的線程池，默認的線程池相當於CachedThreadPool線程池，這個線程池比較適合執行大量的耗時較少的任務（線程池介紹）。
瞭解了Dispatcher之後，我們繼續探究Dispatcher的enqueue方法

void enqueue(AsyncCall call) {
  synchronized (this) {
  readyAsyncCalls.add(call);
 }
 promoteAndExecute();
 }
 /**
  * Promotes eligible calls from {@link #readyAsyncCalls} to {@link #runningAsyncCalls} and runs
  * them on the executor service. Must not be called with synchronization because executing calls
  * can call into user code.
  *
  * @return true if the dispatcher is currently running calls.
  */
  private boolean promoteAndExecute() {
  assert (!Thread.holdsLock(this));

  List<AsyncCall> executableCalls = new ArrayList<>();
  boolean isRunning;
  synchronized (this) {
  for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
    AsyncCall asyncCall = i.next();

    if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.
    if (runningCallsForHost(asyncCall) >= maxRequestsPerHost) continue; // Host max capacity.

    i.remove();
    executableCalls.add(asyncCall);
    runningAsyncCalls.add(asyncCall);
    }
    isRunning = runningCallsCount() > 0;
    }

    for (int i = 0, size = executableCalls.size(); i < size; i++) {
    AsyncCall asyncCall = executableCalls.get(i);
    asyncCall.executeOn(executorService());
    }
    return isRunning;
 }

這裏分三步走，首先將傳入的AsyncCall（這其實是一個Runnable對象）加入準備要執行的異步請求隊列，其次調用promoteAndExecute()方法，變量準備要執行的異步請求隊列，如果隊列任務數超過最大併發請求數，則直接退出遍歷，則不會進行下面的操作；如果超過每個主機最大請求數，則跳過這次循環，繼續下一次遍歷，否則將異步任務加入到正在執行的異步請求隊列，最後遍歷保存異步任務的隊列，執行AsyncCall.executeOn(executorService())方法，並且傳入了Dispatcher的默認線程池。

/**
 * Attempt to enqueue this async call on {@code executorService}. This will attempt to clean up
 * if the executor has been shut down by reporting the call as failed.
 */
  void executeOn(ExecutorService executorService) {
  assert (!Thread.holdsLock(client.dispatcher()));
  boolean success = false;
  try {
    executorService.execute(this);
    success = true;
  } catch (RejectedExecutionException e) {
    InterruptedIOException ioException = new InterruptedIOException("executor rejected");
    ioException.initCause(e);
    eventListener.callFailed(RealCall.this, ioException);
    responseCallback.onFailure(RealCall.this, ioException);
  } finally {
    if (!success) {
      client.dispatcher().finished(this); // This call is no longer running!
    }
  }
}

通過執行AsyncCall.executeOn()方法的源碼，我們看到Dispatcher的線程池執行了execute(this)方法，執行異步任務，並且指向的是this，也就是當前的AsyncCall對象（RealCall的內部類），而AsyncCall實現了抽象類NamedRunnable

/**
 * Runnable implementation which always sets its thread name.
 */
public abstract class NamedRunnable implements Runnable {
protected final String name;

public NamedRunnable(String format, Object... args) {
this.name = Util.format(format, args);
}

 @Override public final void run() {
  String oldName = Thread.currentThread().getName();
  Thread.currentThread().setName(name);
 try {
  execute();
 } finally {
  Thread.currentThread().setName(oldName);
   }
 }
 protected abstract void execute();
 }

可以看到NamedRunnable中run()方法調用了抽象方法execute()，也就說明execute()的實現在AsyncCall對象中，而上面線程池執行的異步任務也是調用這個execute()方法，我們看看AsyncCall對象中execute()方法的實現

 @Override protected void execute() {
  boolean signalledCallback = false;
  timeout.enter();
  try {
    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) {
    e = timeoutExit(e);
    if (signalledCallback) {
      // Do not signal the callback twice!
      Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
    } else {
      eventListener.callFailed(RealCall.this, e);
      responseCallback.onFailure(RealCall.this, e);
    }
   } finally {
    client.dispatcher().finished(this);
   }
   }
   
    /**Dispatcher的finished方法*/
    /** Used by {@code AsyncCall#run} to signal completion. */
    void finished(AsyncCall call) {
    finished(runningAsyncCalls, call);
    }

    /** Used by {@code Call#execute} to signal completion. */
     void finished(RealCall call) {
     finished(runningSyncCalls, call);
    }

    private <T> void finished(Deque<T> calls, T call) {
      Runnable idleCallback;
     synchronized (this) {
     if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
     idleCallback = this.idleCallback;
    }

    boolean isRunning = promoteAndExecute();

    if (!isRunning && idleCallback != null) {
     idleCallback.run();
    }
  }

我們可以先關注最後一行，不管前面請求如何，最後finally代碼塊中都執行了Dispatcher的finished方法，要是要將當前任務從runningAsyncCalls或runningSyncCalls 中移除，同時把readyAsyncCalls的任務調度到runningAsyncCalls中並執行而finished方法中執行了promoteAndExecute()方法，經過前面對該方法分析，說明不管當前執行的任務如何，都會OkHttp都會去readyAsyncCalls（準備要執行的異步請求隊列）取出下一個請求繼續執行。接下來我們繼續回到AsyncCall對象中的execute()方法，可以發現getResponseWithInterceptorChain()的方法返回了Response，說明在該方法中執行了我們的網絡請求。而不管同步還是異步請求，都是通過getResponseWithInterceptorChain()完成網絡請求。

   Response getResponseWithInterceptorChain() throws IOException {
   // Build a full stack of interceptors.
   List<Interceptor> interceptors = new ArrayList<>();
   interceptors.addAll(client.interceptors());
   interceptors.add(retryAndFollowUpInterceptor);
   interceptors.add(new BridgeInterceptor(client.cookieJar()));
   interceptors.add(new CacheInterceptor(client.internalCache()));
   interceptors.add(new ConnectInterceptor(client));
   if (!forWebSocket) {
    interceptors.addAll(client.networkInterceptors());
    }
   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);
   }

由getResponseWithInterceptorChain()方法我們看到添加了很多Interceptor（攔截器），首先要了解每個Interceptor的作用，也能大致瞭解OKHttp完成網絡請求的過程。
- 1.首先加入我們自定義的interceptors
- 2.通過RetryAndFollowUpInterceptor處理網絡請求重試
- 3.通過BridgeInterceptor處理請求對象轉換，應用層到網絡層
- 4.通過CacheInterceptor處理緩存
- 5.通過ConnectInterceptor處理網絡請求鏈接
- 6.通過CallServerInterceptor處理讀寫，和服務器通信，進行真正的網絡請求
責任鏈模式

在閱讀接下來源碼之前，我們先要了解責任鏈模式。通俗化的講在責任鏈模式中有很多對象，這些對象可以理解爲上面列出的攔截器，而每個對象之間都通過一條鏈子連接，網絡請求在這條鏈子上傳遞，直到某一個對象處理了這個網絡請求，也就是完成了網絡請求。使用這個模式的好處就是不管你用多少攔截器處理什麼操作，最終都不會影響我們的發出請求的目的，就是完成網絡請求，攔截過程你可以任意添加分配責任。

接着繼續看Interceptor.Chain，他是Interceptor的內部接口，前面添加的每一個攔截器都實現了Interceptor接口，而RealInterceptorChain是Interceptor.Chain接口的實現類。先看RealInterceptorChain的proceed方法源碼

public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
if (index >= interceptors.size()) throw new AssertionError();

 calls++;
 ......
 // Call the next interceptor in the chain.
  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);

  // Confirm that the next interceptor made its required call to chain.proceed().
  if (httpCodec != null && index + 1 < interceptors.size() && next.calls != 1) {
  throw new IllegalStateException("network interceptor " + interceptor
    + " must call proceed() exactly once");
  }
  .....
  return response;
 }

通過源碼可以注意到interceptor.intercept(next)，RetryAndFollowUpInterceptor作爲默認攔截器的第一個攔截器，也就是執行了它的intercept方法
RetryAndFollowUpInterceptor
前面說過RetryAndFollowUpInterceptor攔截器執行OKHttp網絡重試，先看它的intercept方法

  /**RetryAndFollowUpInterceptor的intercept方法 **/
  @Override public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Call call = realChain.call();
EventListener eventListener = realChain.eventListener();

StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
   createAddress(request.url()), call, eventListener, callStackTrace);
this.streamAllocation = streamAllocation;

int followUpCount = 0;
Response priorResponse = null;
while (true) {
 if (canceled) {
   streamAllocation.release();
   throw new IOException("Canceled");
 }
  //將請求通過鏈子chain傳遞到下一個攔截器
 Response response;
 boolean releaseConnection = true;
 try {
   response = realChain.proceed(request, streamAllocation, null, null);
   releaseConnection = false;
 } catch (RouteException e) {
   // 線路異常，連接失敗，檢查是否可以重新連接
   if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
     throw e.getFirstConnectException();
   }
   releaseConnection = false;
   continue;
 } catch (IOException e) {
   // An attempt to communicate with a server failed. The request may have been sent.
   // IO異常，連接失敗，檢查是否可以重新連接
   boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
   if (!recover(e, streamAllocation, requestSendStarted, request)) throw e;
   releaseConnection = false;
   continue;
 } finally {
   // We're throwing an unchecked exception. Release any resources. 釋放資源
   if (releaseConnection) {
     streamAllocation.streamFailed(null);
     streamAllocation.release();
   }
 }

 // Attach the prior response if it exists. Such responses never have a body.
 if (priorResponse != null) {
   response = response.newBuilder()
       .priorResponse(priorResponse.newBuilder()
               .body(null)
               .build())
       .build();
 }

 Request followUp;
 try {
   //效驗狀態碼、身份驗證頭信息、跟蹤重定向或處理客戶端請求超時
   followUp = followUpRequest(response, streamAllocation.route());
 } catch (IOException e) {
   streamAllocation.release();
   throw e;
 }

 if (followUp == null) {
   streamAllocation.release();
    // 不需要重定向，正常返回結果
   return response;
 }

 closeQuietly(response.body());
 //超過重試次數
 if (++followUpCount > MAX_FOLLOW_UPS) {
   streamAllocation.release();
   throw new ProtocolException("Too many follow-up requests: " + followUpCount);
 }

 if (followUp.body() instanceof UnrepeatableRequestBody) {
   streamAllocation.release();
   throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
 }
 if (!sameConnection(response, followUp.url())) {
   streamAllocation.release();
   streamAllocation = new StreamAllocation(client.connectionPool(),
       createAddress(followUp.url()), call, eventListener, callStackTrace);
   this.streamAllocation = streamAllocation;
 } else if (streamAllocation.codec() != null) {
   throw new IllegalStateException("Closing the body of " + response
       + " didn't close its backing stream. Bad interceptor?");
 }
 request = followUp;
 priorResponse = response;
 }
}

通過RetryAndFollowUpInterceptor攔截器intercept方法源碼，能夠理解到OKHttp的重試機制
- 1.首先創建StreamAllocation對象（稍後分析），在一個死循環中將通過鏈子chain傳遞到下一個攔截器，如果捕獲異常，則判斷異常是否恢復連接，不能連接則拋出異常，跳出循環並是否創建的連接池資源
- 2.第一步沒有異常，還要返回值效驗狀態碼、頭部信息、是否需要重定向、連接超時等信息，捕獲異常則拋出並退出循環
- 3.如果如果重定向，循環超出RetryAndFollowUpInterceptor攔截器的最大重試次數，也拋出異常，退出循環
通過攔截器RetryAndFollowUpInterceptor調用(RealInterceptorChain) chain.proceed()方法，又再次回到了我們剛剛分析proceed()方法，而該方法繼續調用下一個攔截器的intercept()方法，這個攔截器就是默認的第二個攔截器BridgeInterceptor

/**
 * Bridges from application code to network code. First it builds a network request from a user
 * request. Then it proceeds to call the network. Finally it builds a user response from the network
 * response.
 * BridgeInterceptor的intercept方法
 */
@Override public Response intercept(Chain chain) throws IOException {
 Request userRequest = chain.request();
 Request.Builder requestBuilder = userRequest.newBuilder();

 RequestBody body = userRequest.body();
 ......
 Response networkResponse = chain.proceed(requestBuilder.build());
 ......
}

該攔截器主要實現了網絡請求中應用層到網絡層之間的數據編碼橋樑。根據用戶請求建立網絡連接，根據網絡響應建立網絡響應，也可以看到該方法繼續調用了chain.proceed()方法，同理，根據前面分析會調用第三個默認攔截器CacheInterceptor的intercept方法。
CacheInterceptor
前面我們說過這個攔截器是處理緩存的，接下來看看源碼是如何實現的

/**
 * 攔截器CacheInterceptor的intercept方法
 */
@Override public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
   ? cache.get(chain.request())
   : null;

long now = System.currentTimeMillis();
//獲取策略，假設當前可以使用網絡
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;

if (cache != null) {
 cache.trackResponse(strategy);
}

if (cacheCandidate != null && cacheResponse == null) {
 closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
}

 // If we're forbidden from using the network and the cache is insufficient, fail. 如果網絡被禁止使用並且沒有緩存，則請求失敗
 if (networkRequest == null && cacheResponse == null) {
     return new Response.Builder()
     .request(chain.request())
     .protocol(Protocol.HTTP_1_1)
     .code(504)
     .message("Unsatisfiable Request (only-if-cached)")
     .body(Util.EMPTY_RESPONSE)
     .sentRequestAtMillis(-1L)
     .receivedResponseAtMillis(System.currentTimeMillis())
     .build();
}

// If we don't need the network, we're done.如果有緩存，則返回響應緩存，請求完成
if (networkRequest == null) {
 return cacheResponse.newBuilder()
     .cacheResponse(stripBody(cacheResponse))
     .build();
}

Response networkResponse = null;
try {
//沒有緩存，則進行網絡請求，執行下一個攔截器
 networkResponse = chain.proceed(networkRequest);
 } finally {
 // If we're crashing on I/O or otherwise, don't leak the cache body.
 if (networkResponse == null && cacheCandidate != null) {
   closeQuietly(cacheCandidate.body());
 }
}

 // If we have a cache response too, then we're doing a conditional get.
 if (cacheResponse != null) {
  //狀態碼 304 
  if (networkResponse.code() == HTTP_NOT_MODIFIED) {
   Response response = cacheResponse.newBuilder()
       .headers(combine(cacheResponse.headers(), networkResponse.headers()))
       .sentRequestAtMillis(networkResponse.sentRequestAtMillis())
       .receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
       .cacheResponse(stripBody(cacheResponse))
       .networkResponse(stripBody(networkResponse))
       .build();
   networkResponse.body().close();

   // Update the cache after combining headers but before stripping the
   // Content-Encoding header (as performed by initContentStream()).
   cache.trackConditionalCacheHit();
   cache.update(cacheResponse, response);
   return response;
  } else {
   closeQuietly(cacheResponse.body());
  }
 }

 Response response = networkResponse.newBuilder()
   .cacheResponse(stripBody(cacheResponse))
   .networkResponse(stripBody(networkResponse))
   .build();

 if (cache != null) {
 if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
   // Offer this request to the cache.
   //保存緩存
   CacheRequest cacheRequest = cache.put(response);
   return cacheWritingResponse(cacheRequest, response);
 }

  if (HttpMethod.invalidatesCache(networkRequest.method())) {
   try {
     cache.remove(networkRequest);
   } catch (IOException ignored) {
     // The cache cannot be written.
   }
  }
 }

 return response;
}

先看看intercept方法的大致邏輯
- 1.首先通過CacheStrategy.Factory().get()獲取緩存策略
- 2.如果網絡被禁止使用並且沒有緩存，則請求失敗，返回504
- 3.如果有響應緩存，則返回響應緩存，請求完成
- 4.沒有緩存，則進行網絡請求，執行下一個攔截器
- 5.進行網絡請求，如果響應狀態碼爲304，說明客戶端緩存了目標資源但不確定該緩存資源是否是最新版本，服務端數據沒變化，繼續使用緩存
- 6.最後保存緩存
緩存的場景也符合設計模式中的策略模式，需要CacheStrategy提供策略在不同場景下讀緩存還是請求網絡。

瞭解了緩存邏輯，繼續深入瞭解OKHttp的緩存是如何做的。首先我們應該回到最初的緩存攔截器設置代碼

/**RealCall 設置緩存攔截器*/
interceptors.add(new CacheInterceptor(client.internalCache()));

/**OkHttpClient 設置緩存*/
Cache cache;
@Override public void setCache(OkHttpClient.Builder builder, InternalCache internalCache) {
  builder.setInternalCache(internalCache);
}
void setInternalCache(@Nullable InternalCache internalCache) {
this.internalCache = internalCache;
this.cache = null;
}
InternalCache internalCache() {
return cache != null ? cache.internalCache : internalCache;
}

/**Cache類中 內部持有 InternalCache */
 final DiskLruCache cache;
 final InternalCache internalCache = new InternalCache() {
 @Override public Response get(Request request) throws IOException {
 return Cache.this.get(request);
 }

 @Override public CacheRequest put(Response response) throws IOException {
 return Cache.this.put(response);
 }

 @Override public void remove(Request request) throws IOException {
 Cache.this.remove(request);
 }

 @Override public void update(Response cached, Response network) {
 Cache.this.update(cached, network);
 }

 @Override public void trackConditionalCacheHit() {
 Cache.this.trackConditionalCacheHit();
 }

 @Override public void trackResponse(CacheStrategy cacheStrategy) {
 Cache.this.trackResponse(cacheStrategy);
 }
};

上面我們分別截取了 RealCall類、OkHttpClient類和Cache類的源碼，可以瞭解到攔截器使用的緩存類是DiskLruCache，設置緩存緩存只能通過OkHttpClient的builder來設置，緩存操作實現是在Cache類中，但是Cache沒有實現InternalCache接口，而是持有InternalCache接口的內部類對象來實現緩存的操作方法，這樣就使得緩存的操作實現只在Cache內部，外部用戶是無法實現緩存操作的，方便框架內部使用，接口擴展也不影響外部。
ConnectInterceptor
根據前面的分析，緩存攔截器中也會調用chain.proceed方法，所以這時候執行到了第四個默認攔截器ConnectInterceptor，接着看它的intercept方法

/**
  * 攔截器ConnectInterceptor的intercept方法
  */
@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");
//打開連接
HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();
//交由下一個攔截器處理
return realChain.proceed(request, streamAllocation, httpCodec, connection);
}

我們看到intercept源碼非常簡單，通過StreamAllocation打開連接，然後就交由下一個攔截器處理請求。如何連接呢？我們需要搞懂StreamAllocation。
StreamAllocation對象負責協調請求和連接池之間的聯繫。每一個OKHttpClient有它對應的一個連接池，經過前面的分析我們知道StreamAllocation對象的創建在RetryAndFollowUpInterceptor攔截器的intercept方法中創建，而StreamAllocation打開了連接，則連接池在哪創建呢，答案就在OKHttpClient的Builder類構造方法中

public Builder() {
  .......
  connectionPool = new ConnectionPool();
  .......
}

瞭解了StreamAllocation對象和ConnectionPool對象的創建，下面來分析StreamAllocation是如何打開連接的。首先是streamAllocation.newStream()方法

public HttpCodec newStream(
  OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
  ........
  try {
  RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
      writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);
  .......
    }
  } catch (IOException e) {
  throw new RouteException(e);
  }
 }

/**
* Finds a connection and returns it if it is healthy. If it is unhealthy the process is repeated
* until a healthy connection is found.
*/
  private RealConnection findHealthyConnection(int connectTimeout, int readTimeout,
  int writeTimeout, int pingIntervalMillis, boolean connectionRetryEnabled,
  boolean doExtensiveHealthChecks) throws IOException {
  while (true) {
  RealConnection candidate = findConnection(connectTimeout, readTimeout, writeTimeout,
      pingIntervalMillis, connectionRetryEnabled);
  ........      
  return candidate;
  }
} 
/**
* Returns a connection to host a new stream. This prefers the existing connection if it exists,
* then the pool, finally building a new connection.
*/
private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
  int pingIntervalMillis, boolean connectionRetryEnabled) throws IOException {
  ............
  //
  if (result == null) {
    // Attempt to get a connection from the pool.
    Internal.instance.get(connectionPool, address, this, null);
    if (connection != null) {
      //連接複用
      foundPooledConnection = true;
      result = connection;
    } else {
      selectedRoute = route;
    }
  }
 
 ..........

  if (!foundPooledConnection) {
    ........
    result = new RealConnection(connectionPool, selectedRoute);
    //記錄每個連接的引用，每個調用必須與同一連接上的調用配對。
    acquire(result, false);
   }
  }
  .........
  synchronized (connectionPool) {
   .......
  // Pool the connection. 將連接放入連接池
  Internal.instance.put(connectionPool, result);
  ......
  }
 }
.......
return result;
}

根據上面的源碼，我們可以知道findHealthyConnection在循環找健康的連接，直到找到連接，說明findConnection方法是尋找連接的核心方法，該方法中存在可以複用的連接則複用，否則創建新的連接，並且記錄連接引用，我們可以明白StreamAllocation主要是爲攔截器提供一個連接，如果連接池中有複用的連接則複用連接，如果沒有則創建新的連接。
ConnectionPool連接池實現
明白StreamAllocation是如何創建和複用連接池，我們還要明白連接池（ConnectionPool）的是如何實現的。
理解ConnectionPool之前，我們需要明白TCP連接的知識，Tcp建立連接三次握手和斷開連接四次握手過程是需要消耗時間的，在http/1.0每一次請求只能打開一次連接，而在http/1.1是支持持續連接（persistent connection），使得一次連接打開之後會保持一段時間，如果還是同一個請求並且使同一個服務器則在這段時間內繼續請求連接是可以複用的。而ConnectionPool也實現了這個機制，在它內部持有一個線程池和一個緩存連接的雙向列表，連接中最多隻能存在5個空閒連接，空閒連接最多隻能存活5分鐘，空閒連接到期之後定時清理。

 public final class ConnectionPool {
 /**
  * Background threads are used to cleanup expired connections. There will be at most a single
  * thread running per connection pool. The thread pool executor permits the pool itself to be
  * garbage collected.
  */
  //線程池
  private static final Executor executor = new ThreadPoolExecutor(0 /* corePoolSize */,
  Integer.MAX_VALUE /* maximumPoolSize */, 60L /* keepAliveTime */, TimeUnit.SECONDS,
  new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp ConnectionPool", true));

  /** The maximum number of idle connections for each address. */
  private final int maxIdleConnections;
  private final long  keepAliveDurationNs;
  private final Runnable cleanupRunnable = new Runnable() {
  @Override public void run() {
  // 後臺定期清理連接的線程
  while (true) {
    long waitNanos = cleanup(System.nanoTime());
    if (waitNanos == -1) return;
    if (waitNanos > 0) {
      long waitMillis = waitNanos / 1000000L;
      waitNanos -= (waitMillis * 1000000L);
      synchronized (ConnectionPool.this) {
        try {
          ConnectionPool.this.wait(waitMillis, (int) waitNanos);
        } catch (InterruptedException ignored) {
        }
        }
      }
    }
   }
  };
  //緩存連接的雙向隊列
  private final Deque<RealConnection> connections = new ArrayDeque<>();
  ............
   /**
   * Create a new connection pool with tuning parameters appropriate for a single-user  application.
   * The tuning parameters in this pool are subject to change in future OkHttp releases. Currently
   * this pool holds up to 5 idle connections which will be evicted after 5 minutes of inactivity.
   */
   public ConnectionPool() {
   this(5, 5, TimeUnit.MINUTES);
  }
  ............
 }

這裏還要說的一點是streamAllocation.newStream()返回的HttpCodec對象就是我們編碼HTTP請求並解碼HTTP響應的接口，他的實現類Http2Codec和Http1Codec對應https和http的解析request與響應response對socket讀寫過程實現，並最終放到RealConnection對象newCodec類中創建。

/** 
RealConnection類newCodec方法
*/
 public HttpCodec newCodec(OkHttpClient client, Interceptor.Chain chain,
  StreamAllocation streamAllocation) throws SocketException {
 if (http2Connection != null) {
  return new Http2Codec(client, chain, streamAllocation, http2Connection);
 } else {
  socket.setSoTimeout(chain.readTimeoutMillis());
  source.timeout().timeout(chain.readTimeoutMillis(), MILLISECONDS);
  sink.timeout().timeout(chain.writeTimeoutMillis(), MILLISECONDS);
  return new Http1Codec(client, streamAllocation, source, sink);
  }
}

streamAllocation得到連接對象，也就是RealConnection對象，它封裝了套接字socket連接，也就是該類的connectSocket方法。並且使用OKio來對數據讀寫。OKio封裝了Java的I/O操作，這裏就不細說了。最後返回的ConnectInterceptor攔截器的intercept方法同樣調用了Chain.proceed，將拿到的連接交由CallServerInterceptor做處理。

/** Does all the work necessary to build a full HTTP or HTTPS connection on a raw socket. 
RealConnection類connectSocket方法
*/
private void connectSocket(int connectTimeout, int readTimeout, Call call,
  EventListener eventListener) throws IOException {
Proxy proxy = route.proxy();
Address address = route.address();

rawSocket = proxy.type() == Proxy.Type.DIRECT || proxy.type() == Proxy.Type.HTTP
    ? address.socketFactory().createSocket()
    : new Socket(proxy);

eventListener.connectStart(call, route.socketAddress(), proxy);
rawSocket.setSoTimeout(readTimeout);
try {
  //打開 socket 連接
  Platform.get().connectSocket(rawSocket, route.socketAddress(), connectTimeout);
} catch (ConnectException e) {
  ConnectException ce = new ConnectException("Failed to connect to " + route.socketAddress());
  ce.initCause(e);
  throw ce;
}

// The following try/catch block is a pseudo hacky way to get around a crash on Android 7.0
// More details:
// https://github.com/square/okhttp/issues/3245
// https://android-review.googlesource.com/#/c/271775/
try {
  //使用OKio來對數據讀寫
  source = Okio.buffer(Okio.source(rawSocket));
  sink = Okio.buffer(Okio.sink(rawSocket));
 } catch (NullPointerException npe) {
  if (NPE_THROW_WITH_NULL.equals(npe.getMessage())) {
    throw new IOException(npe);
  }
 }
}

最後返回的ConnectInterceptor攔截器的intercept方法同樣調用了Chain.proceed，將拿到的連接交由CallServerInterceptor做處理。
CallServerInterceptor
在經過前面一系列攔截器之後，OKHttp最終把拿到網絡請求連接給到CallServerInterceptor攔截器進行網絡請求和服務器通信。

/**CallServerInterceptor的intercept方法*/ 
@Override public Response intercept(Chain chain) throws IOException {
RealInterceptorChain realChain = (RealInterceptorChain) chain;
HttpCodec httpCodec = realChain.httpStream();
StreamAllocation streamAllocation = realChain.streamAllocation();
RealConnection connection = (RealConnection) realChain.connection();
Request request = realChain.request();
long sentRequestMillis = System.currentTimeMillis();
realChain.eventListener().requestHeadersStart(realChain.call());
//按照HTTP協議，依次寫入請求體
httpCodec.writeRequestHeaders(request);
.................
 if (responseBuilder == null) {
  realChain.eventListener().responseHeadersStart(realChain.call());
  //
  responseBuilder = httpCodec.readResponseHeaders(false);
 }

 Response response = responseBuilder
    .request(request)
    .handshake(streamAllocation.connection().handshake())
    .sentRequestAtMillis(sentRequestMillis)
    .receivedResponseAtMillis(System.currentTimeMillis())
    .build();
  ...............
  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 {
  //響應數據OKio寫入
  response = response.newBuilder()
      .body(httpCodec.openResponseBody(response))
      .build();
   }
  }
  return response;
}
/**Http1Codec方法**/
//OKio 讀寫對象
final BufferedSource source;
final BufferedSink sink;
@Override public void writeRequestHeaders(Request request) throws IOException {
//構造好請求頭
String requestLine = RequestLine.get(
    request, streamAllocation.connection().route().proxy().type());
 writeRequest(request.headers(), requestLine);
}
 /** Returns bytes of a request header for sending on an HTTP transport.
 將請求信息寫入sink
 */
public void writeRequest(Headers headers, String requestLine) throws IOException {
if (state != STATE_IDLE) throw new IllegalStateException("state: " + state);
sink.writeUtf8(requestLine).writeUtf8("\r\n");
for (int i = 0, size = headers.size(); i < size; i++) {
  sink.writeUtf8(headers.name(i))
      .writeUtf8(": ")
      .writeUtf8(headers.value(i))
      .writeUtf8("\r\n");
 }
sink.writeUtf8("\r\n");
state = STATE_OPEN_REQUEST_BODY;
}

可以看到在CallServerInterceptor攔截器的方法中首先通過HttpCodec（上面貼的是Http1Codec的方法）writeRequestHeaders和writeRequest方法寫入請求體，並將請求體寫入OKio的寫入對象sink中

/**Http1Codec方法**/
/**
 讀取響應頭信息
*/ 
@Override public Response.Builder readResponseHeaders(boolean expectContinue) throws IOException {
if (state != STATE_OPEN_REQUEST_BODY && state != STATE_READ_RESPONSE_HEADERS) {
  throw new IllegalStateException("state: " + state);
}

 try {
  StatusLine statusLine = StatusLine.parse(readHeaderLine());

  Response.Builder responseBuilder = new Response.Builder()
      .protocol(statusLine.protocol)
      .code(statusLine.code)
      .message(statusLine.message)
      .headers(readHeaders());

  if (expectContinue && statusLine.code == HTTP_CONTINUE) {
    return null;
  } else if (statusLine.code == HTTP_CONTINUE) {
    state = STATE_READ_RESPONSE_HEADERS;
    return responseBuilder;
  }

  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 " + streamAllocation);
  exception.initCause(e);
  throw exception;
  }
}
 /**
 寫入響應輸入到ResponseBody
 */
 @Override public ResponseBody openResponseBody(Response response) throws IOException {
 streamAllocation.eventListener.responseBodyStart(streamAllocation.call);
 String contentType = response.header("Content-Type");

 if (!HttpHeaders.hasBody(response)) {
  Source source = newFixedLengthSource(0);
  return new RealResponseBody(contentType, 0, Okio.buffer(source));
 }

 if ("chunked".equalsIgnoreCase(response.header("Transfer-Encoding"))) {
  Source source = newChunkedSource(response.request().url());
  return new RealResponseBody(contentType, -1L, Okio.buffer(source));
 }

 long contentLength = HttpHeaders.contentLength(response);
 if (contentLength != -1) {
  Source source = newFixedLengthSource(contentLength);
  return new RealResponseBody(contentType, contentLength, Okio.buffer(source));
 }

  return new RealResponseBody(contentType, -1L, Okio.buffer(newUnknownLengthSource()));
}

通過readResponseHeaders方法讀取響應頭信息，openResponseBody得到響應體信息。最終將網絡請求的響應信息通過Callback()回調方法異步傳遞出去，同步請求則直接返回。到此OKHttp源碼理解到此爲止。

最後，通過OKHttp這個框架源碼閱讀，也是對自己的一個提升，不僅瞭解了框架原理，設計模式在適宜場景的運用，同時也是對自己耐心的一次考驗，源碼的閱讀是枯燥的，但是隻要靜下心來，也能發現閱讀源碼的樂趣。由於本人水平有限，文章中如果有錯誤，請大家給我提出來，大家一起學習進步，如果覺得我的文章給予你幫助，也請給我一個喜歡和關注。

參考鏈接：
- 深入解析OkHttp3
- OkHttp3源碼和設計模式-1
參考書籍：
- 《計算機網絡》第六版

從源碼角度深入理解OKHttp3

常規使用

源碼分析

責任鏈模式

RetryAndFollowUpInterceptor

CacheInterceptor

ConnectInterceptor

ConnectionPool連接池實現

CallServerInterceptor

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