Android OkHttp（三）源码解析

- 日理万妓 2022-07-15 15:07 276阅读 0赞

前面两篇文章，我们介绍了OKHttp的基本用法，今天这篇文章将从OkHttp源码的角度来分析OkHttp的整个工作流程。

初始化一个OkHttpClient对象，

OkHttpClient mOkHttpClient = new OkHttpClient();

OkHttpClient的构造函数中又初始化了一个Builder对象，

public OkHttpClient() {
        this(new Builder());
      }

Builder的构造函数中，默认会创建对象以及一些属性，Builder对象是用来构造网络请求的。这种设计模式和Android的对话框中的Builder几乎是一个意思。

public Builder() {
          dispatcher = new Dispatcher();
          protocols = DEFAULT_PROTOCOLS;
          connectionSpecs = DEFAULT_CONNECTION_SPECS;
          proxySelector = ProxySelector.getDefault();
          cookieJar = CookieJar.NO_COOKIES;
          socketFactory = SocketFactory.getDefault();
          hostnameVerifier = OkHostnameVerifier.INSTANCE;
          certificatePinner = CertificatePinner.DEFAULT;
          proxyAuthenticator = Authenticator.NONE;
          authenticator = Authenticator.NONE;
          connectionPool = new ConnectionPool();
          dns = Dns.SYSTEM;
          followSslRedirects = true;
          followRedirects = true;
          retryOnConnectionFailure = true;
          connectTimeout = 10_000;
          readTimeout = 10_000;
          writeTimeout = 10_000;
        }

接着，创建一个请求，

Request request = new Request.Builder()  
          .url(url)  
          .build();

Request类是HTTP请求，它携带了请求地址、请求方法、请求头部、请求体以及其他信息。它也是通过Builder模式创建的。Request.Builder().build()源码，

public Request build() {
          if (url == null) throw new IllegalStateException("url == null");
          return new Request(this);
        }

Request的构造方法，

private Request(Builder builder) {
        this.url = builder.url;
        this.method = builder.method;
        this.headers = builder.headers.build();
        this.body = builder.body;
        this.tag = builder.tag != null ? builder.tag : this;
      }

发起请求有两种方式，一种是同步，一种是异步，下面分别看看如何实现，

同步请求，

Response  response = mOkHttpClient.newCall(request).execute();

Response是HTTP响应，它继承自Closeable（Closeable继承自AutoCloseable，AutoCloseable资源自动关闭的接口），它携带了请求、网络请求协议、返回状态码、请求头、响应体等等，其中，网络请求协议是Protocol，Protocol是一个枚举类型，包含4中类型，

public enum Protocol {
 /**
 * An obsolete plaintext framing that does not use persistent sockets by default.
 */
 HTTP_1_0("http/1.0"),
 
 /**
 * A plaintext framing that includes persistent connections.
 *
 * This version of OkHttp implements <a href="http://www.ietf.org/rfc/rfc2616.txt">RFC
 * 2616</a>, and tracks revisions to that spec.
 */
 HTTP_1_1("http/1.1"),
 
 /**
 * Chromium's binary-framed protocol that includes header compression, multiplexing multiple
 * requests on the same socket, and server-push. HTTP/1.1 semantics are layered on SPDY/3.
 *
 * This version of OkHttp implements SPDY 3 <a
 * href="http://dev.chromium.org/spdy/spdy-protocol/spdy-protocol-draft3-1">draft 3.1</a>. Future
 * releases of OkHttp may use this identifier for a newer draft of the SPDY spec.
 */
 SPDY_3("spdy/3.1"),
 
 /**
 * The IETF's binary-framed protocol that includes header compression, multiplexing multiple
 * requests on the same socket, and server-push. HTTP/1.1 semantics are layered on HTTP/2.
 *
 * HTTP/2 requires deployments of HTTP/2 that use TLS 1.2 support {@linkplain
 * CipherSuite#TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256} , present in Java 8+ and Android 5+. Servers
 * that enforce this may send an exception message including the string {@code
 * INADEQUATE_SECURITY}.
 */
 HTTP_2("h2");
 
 }

接着看

OkHttpClient.newCall(request);

**Call类是一个准备执行的请求，它可以被取消，它代表着一个单一的请求/响应流，不能被执行两次**。Call类是一个接口，

public interface Call {
 /** Returns the original request that initiated this call. */
 Request request();
 
 /**
 * Invokes the request immediately, and blocks until the response can be processed or is in
 * error.
 *
 * The caller may read the response body with the response's {@link Response#body} method. To
 * avoid leaking resources callers must {@linkplain ResponseBody close the response body}.
 *
 * Note that transport-layer success (receiving a HTTP response code, headers and body) does
 * not necessarily indicate application-layer success: {@code response} may still indicate an
 * unhappy HTTP response code like 404 or 500.
 *
 * @throws IOException if the request could not be executed due to cancellation, a connectivity
 * problem or timeout. Because networks can fail during an exchange, it is possible that the
 * remote server accepted the request before the failure.
 * @throws IllegalStateException when the call has already been executed.
 */
 Response execute() throws IOException;
 
 /**
 * Schedules the request to be executed at some point in the future.
 *
 * The {@link OkHttpClient#dispatcher dispatcher} defines when the request will run: usually
 * immediately unless there are several other requests currently being executed.
 *
 * This client will later call back {@code responseCallback} with either an HTTP response or a
 * failure exception.
 *
 * @throws IllegalStateException when the call has already been executed.
 */
 void enqueue(Callback responseCallback);
 
 /** Cancels the request, if possible. Requests that are already complete cannot be canceled. */
 void cancel();
 
 /**
 * Returns true if this call has been either {@linkplain #execute() executed} or {@linkplain
 * #enqueue(Callback) enqueued}. It is an error to execute a call more than once.
 */
 boolean isExecuted();
 
 boolean isCanceled();
 
 interface Factory {
 Call newCall(Request request);
 }
 }

newCall()方法是创建一个Call对象，

@Override 
        public Call newCall(Request request) {
        return new RealCall(this, request);
      }

RealCall类实现了Call类，下面展示RealCall()方法的代码，

protected RealCall(OkHttpClient client, Request originalRequest) {
        this.client = client;
        this.originalRequest = originalRequest;
        this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client);
      }

接着是RealCall的execute()方法。execute()方法是同步方法，即一直等待http请求, 直到返回了响应. 在这之间会阻塞进程, 所以通过同步方法不能在Android的主线程中执行, 否则会报错。

OKHttp提供了execute()方法（同步方法）和enqueue()方法（异步方法），下面我们先看看execute()方法（同步方法），

@Override public Response execute() throws IOException {
        synchronized (this) {
          if (executed) throw new IllegalStateException("Already Executed");
          executed = true;
        }
        try {
          client.dispatcher().executed(this);
          Response result = getResponseWithInterceptorChain();
          if (result == null) throw new IOException("Canceled");
          return result;
        } finally {
          client.dispatcher().finished(this);
        }
      }

**execute()方法，首先判断是否已执行过，如果已经执行过，则抛出异常信息，也就是说一次Call实例只能调用一次execute()方法，和我们之前说的一样。**

如果未执行，则调用Dispatcher类的executed()方法将该Call加入到一个双端队列中，

...
 private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();//双端队列
 ...
 /** Used by {@code Call#execute} to signal it is in-flight. */
 synchronized void executed(RealCall call) {
 runningSyncCalls.add(call);
 }

接着调用getResponseWithInterceptorChain()方法返回Response对象，最后finally中，调用Dispatcher的finished()方法，在从已经执行的双端队列中移除本次Call。

void finished(RealCall call) {
 finished(runningSyncCalls, call, false);
 }
 
 private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
 int runningCallsCount;
 Runnable idleCallback;
 synchronized (this) {
 if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
 if (promoteCalls) promoteCalls();
 runningCallsCount = runningCallsCount();
 idleCallback = this.idleCallback;
 }
 
 if (runningCallsCount == 0 && idleCallback != null) {
 idleCallback.run();
 }
 }

通过上面分析，真正发出网络请求，返回结果应该是**getResponseWithInterceptorChain()**方法，那么接下来，主要看看这个方法

private 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 (!retryAndFollowUpInterceptor.isForWebSocket()) {
 interceptors.addAll(client.networkInterceptors());
 }
 interceptors.add(new CallServerInterceptor(
 retryAndFollowUpInterceptor.isForWebSocket()));
 
 Interceptor.Chain chain = new RealInterceptorChain(
 interceptors, null, null, null, 0, originalRequest);
 return chain.proceed(originalRequest);
 }

仔细查看便会发现，该方法是组装各种拦截器为一个拦截器链，最后调用RealInterceptorChain的proceed()方法，来处理这个请求，

@Override public Response proceed(Request request) throws IOException {
 return proceed(request, streamAllocation, httpStream, connection);
 }
 
 public Response proceed(Request request, StreamAllocation streamAllocation, HttpStream httpStream,
 Connection connection) throws IOException {
 if (index >= interceptors.size()) throw new AssertionError();
 
 calls++;
 
 // If we already have a stream, confirm that the incoming request will use it.
 if (this.httpStream != null && !sameConnection(request.url())) {
 throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
 + " must retain the same host and port");
 }
 
 // If we already have a stream, confirm that this is the only call to chain.proceed().
 if (this.httpStream != null && calls > 1) {
 throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
 + " must call proceed() exactly once");
 }
 
 // Call the next interceptor in the chain.
 RealInterceptorChain next = new RealInterceptorChain(
 interceptors, streamAllocation, httpStream, connection, index + 1, request);
 Interceptor interceptor = interceptors.get(index);
 Response response = interceptor.intercept(next);
 
 // Confirm that the next interceptor made its required call to chain.proceed().
 if (httpStream != null && index + 1 < interceptors.size() && next.calls != 1) {
 throw new IllegalStateException("network interceptor " + interceptor
 + " must call proceed() exactly once");
 }
 
 // Confirm that the intercepted response isn't null.
 if (response == null) {
 throw new NullPointerException("interceptor " + interceptor + " returned null");
 }
 
 return response;
 }

拦截器Interceptor和拦截器链Chain都是接口，

public interface Interceptor {
      Response intercept(Chain chain) throws IOException;
    
      interface Chain {
        Request request();
    
        Response proceed(Request request) throws IOException;
    
        Connection connection();
      }
    }

下面用一张流程图来说明拦截器链递归从拦截器中返回Response（这个过程很类似Android中的事件处理机制）

![Center][]

下面再看看enqueue()方法（异步方法），指在另外的工作线程中执行http请求, 请求时不会阻塞当前的线程, 所以可以在Android主线程中使用。

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

该方法和同步方法一样，首先都校验这个Call是否已经被执行，如果执行过，就报异常。如果为执行，则调用Dispatcher分发器的enqueue()方法。首先，我们看看AsyncCall这个类，

final class AsyncCall extends NamedRunnable {
        private final Callback responseCallback;
    
        private AsyncCall(Callback responseCallback) {
          super("OkHttp %s", redactedUrl().toString());
          this.responseCallback = responseCallback;
        }
    
        String host() {
          return originalRequest.url().host();
        }
    
        Request request() {
          return originalRequest;
        }
    
        RealCall get() {
          return RealCall.this;
        }
    
        @Override protected void execute() {
          boolean signalledCallback = false;
          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) {
            if (signalledCallback) {
              // Do not signal the callback twice!
              Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
            } else {
              responseCallback.onFailure(RealCall.this, e);
            }
          } finally {
            client.dispatcher().finished(this);
          }
        }
      }

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();
    }

AsyncCall–实际上是一个Runnable，在run()方法中调用了execute()方法，在该方法中，我们又看到既熟悉又陌生的代码（getResponseWithInterceptorChain()），

Response response = getResponseWithInterceptorChain();

执行该方法后，根据响应结果设置回调方法的结果。AsyncCall执行完调用Dispatcher的finished(this)方法，

void finished(AsyncCall call) {
 finished(runningAsyncCalls, call, true);
 }
 
 private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
 int runningCallsCount;
 Runnable idleCallback;
 synchronized (this) {
 if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
 if (promoteCalls) promoteCalls();
 runningCallsCount = runningCallsCount();
 idleCallback = this.idleCallback;
 }
 
 if (runningCallsCount == 0 && idleCallback != null) {
 idleCallback.run();
 }
 }

finished()方法先从runningAsyncCalls(异步请求队列)删除已经执行的异步请求，然后接着调用了promoteCalls()方法，

private void promoteCalls() {
 if (runningAsyncCalls.size() >= maxRequests) return; // Already running max capacity.
 if (readyAsyncCalls.isEmpty()) return; // No ready calls to promote.
 
 for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
 AsyncCall call = i.next();
 
 if (runningCallsForHost(call) < maxRequestsPerHost) {
 i.remove();
 runningAsyncCalls.add(call);
 executorService().execute(call);
 }
 
 if (runningAsyncCalls.size() >= maxRequests) return; // Reached max capacity.
 }
 }

首先判断runningAsyncCalls(异步请求队列)是否还有请求，如果有，则返回；否则readyAsyncCalls（异步调用准备任务）是否为空，如果为空，则返回；否则，循环readyAsyncCalls（异步调用准备任务），将call加入到runningAsyncCalls(异步请求队列)中，并在readyAsyncCalls（异步调用准备任务）删除掉该call，接着线程池执行call。

下面接着看Dispatcher分发器的enqueue()方法，

synchronized void enqueue(AsyncCall call) {
 if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
 runningAsyncCalls.add(call);
 executorService().execute(call);
 } else {
 readyAsyncCalls.add(call);
 }
 }

如果runningAsyncCalls的大小小于最大请求数量（最大线程数量、并发数量）并且call小于最大主机请求限制，那么将call 加入到runningAsyncCalls中，接着线程池执行call；否则，将call加入到readyAsyncCalls（异步调用准备任务）。

PS: runningCallsForHost()方法，循环判断cal的hostl和runningAsyncCalls的中的call的host相同的数量。同一请求是否超过想过请求同时存在的最大值。

private int runningCallsForHost(AsyncCall call) {
        int result = 0;
        for (AsyncCall c : runningAsyncCalls) {
          if (c.host().equals(call.host())) result++;
        }
        return result;
      }

可以看到请求的最核心代码，与Dispatcher类分不开，下面就看看Dispatcher类，Dispatcher是异步请求的策略，

public final class Dispatcher {
 private int maxRequests = 64;
 private int maxRequestsPerHost = 5;
 private Runnable idleCallback;
 
 /** Executes calls. Created lazily. */
 private ExecutorService executorService;
 
 /** Ready async calls in the order they'll be run. */
 private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
 
 /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
 private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
 
 /** Running synchronous calls. Includes canceled calls that haven't finished yet. */
 private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
 ...
 }

Dispatcher维护了如下变量， 
    int maxRequests = 64: 最大并发请求数为64 
    int maxRequestsPerHost = 5: 每个主机最大请求数为5 
    Runnable idleCallback：Runnable对象，在删除任务时执行 
    ExecutorService executorService：消费者池（也就是线程池） 
    Deque<AsyncCall> readyAsyncCalls：缓存，异步调用准备执行任务 
    Deque<AsyncCall> runningAsyncCalls：正在运行的异步任务，包含取消尚未完成的调用

Deque<RealCall> runningSyncCalls： 正在运行的同步任务，包含取消尚未完成的调用

至此，OKHttp的源码分析就结束了！希望本篇文章对读者有所收获！![微笑][smile.gif]

下面结合本篇文章，总结OKHttp的整体流程，流程图如下所示，

![Center 1][]

[Center]: /images/20220715/e1cdc3551fd745fca310c6f51e289abd.png
[smile.gif]: /images/20220715/7c69e9936ea44ddab1fbdf50a127a486.png
[Center 1]: /images/20220715/a247de572bb74c669f2c7a16e5bc8816.png