一、okhttp使用簡介

1.創(chuàng)建一個OkHttpClient對象

2.創(chuàng)建一個request對象，通過內(nèi)部類Builder調(diào)用生成Request對象

3.創(chuàng)建一個Call對象，調(diào)用execute(同步)/enqueue(異步)

4.返回Response對象，對Response對象進行處理

    //第一步
    private final OkHttpClient client = new OkHttpClient();

    //第二步
    Request request = new Request.Builder()
            .url("http://www.baidu.com").build();

    //同步請求
    public void okHttpAsycGet() throws IOException {
        //第三步
        Response response = client.newCall(request).execute();

        if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);
        System.out.println(response.body().string());
    }

    //異步請求
    public void okHttpSyncGet() throws IOException {
        //第三步
        client.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                e.printStackTrace();
            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);
                System.out.println(response.body().string());
            }
        });
    }

二、okhttp代碼分析

同步方法代碼分析

在client.newCall(request).execute();代碼中的newCall方法實現(xiàn)如下：

@Override 
public Call newCall(Request request) {
  return new RealCall(this, request, false /* for web socket */);
}

newCall()方法會返回一個RealCall對象，RealCall中的execute()方法實現(xiàn)如下：

@Override 
public Response execute() throws IOException {
    synchronized (this) { //方法檢測，execute()只會執(zhí)行一次
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    try {
      client.dispatcher().executed(this); //將RealCall添加到Dispatcher中的同步請求隊列中
      Response result = getResponseWithInterceptorChain(); //獲取Response
      if (result == null) throw new IOException("Canceled");
      return result; //返回Response
    } finally {
      client.dispatcher().finished(this); //通知Dispatcher該RealCall已執(zhí)行完
    }
  }

其中的獲取Response的關(guān)鍵方法getResponseWithInterceptorChain()實現(xiàn)如下：

Response getResponseWithInterceptorChain() throws IOException {
  // Build a full stack of interceptors.
  List<Interceptor> interceptors = new ArrayList<>(); //攔截器棧
  interceptors.addAll(client.interceptors()); //添加client中的所有攔截器
  interceptors.add(retryAndFollowUpInterceptor); //失敗重試攔截器
  interceptors.add(new BridgeInterceptor(client.cookieJar())); //橋接攔截器
  interceptors.add(new CacheInterceptor(client.internalCache())); //緩存攔截器
  interceptors.add(new ConnectInterceptor(client)); //與服務(wù)器進行連接的攔截器
  if (!forWebSocket) {
    interceptors.addAll(client.networkInterceptors());  //網(wǎng)絡(luò)攔截器
  }
  interceptors.add(new CallServerInterceptor(forWebSocket)); //網(wǎng)絡(luò)服務(wù)端連接攔截器

  Interceptor.Chain chain = new RealInterceptorChain(
      interceptors, null, null, null, 0, originalRequest); //將攔截器棧連成鏈
  return chain.proceed(originalRequest);
}

異步方法代碼分析

newCall()方法會返回一個RealCall對象，RealCall中的enqueue()方法實現(xiàn)如下：

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

可以看到，方法的最后調(diào)用了Dispatcher的enqueue()方法，該方法的具體實現(xiàn)如下：

/** Ready async calls in the order they'll be run. */
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>(); //準備中的異步隊列

/** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>(); //運行中的異步隊列

/** Running synchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>(); //運行中的同步隊列

synchronized void enqueue(AsyncCall call) {
  if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) { //若當前正在運行的異步隊列可容納新的任務(wù)
    runningAsyncCalls.add(call); //添加回調(diào)方法到正在運行中的異步隊列
    executorService().execute(call); //使用線程池執(zhí)行該回調(diào)方法
  } else {
    readyAsyncCalls.add(call); //添加回調(diào)方法到準備中的異步方法
  }
}