OkHttp源码阅读（一）-——初识OkHttp
2021-04-10 本文已影响0人
Sherlock丶Aza
OkHttp是由square公司研发一款开源的轻量级网络请求框架，一直备受Android端青睐，OkHttp的使用率远比Volley要高，同事square公司开发的Retrofit的使用也越来越多，Retrofit实际上对OKhttp有强制依赖性，其实质就是对OkHttp的封装，所以重点还是OkHttp，以下是个人的OKHttp深入学习，以此记录.
OKhttp的导入方式不再赘述，直接干活!
一个简单网络请求的执行流程
/**
* 同步请求
*/
OkHttpClient client = new OkHttpClient.Builder().readTimeout(10000, TimeUnit.MILLISECONDS).build();
Request request = new Request.Builder().url("www.sherlockaza.com").get().build();
Call call = client.newCall(request);
try {
Response response = call.execute();
String json = response.body().string();
//解析
} catch (IOException e) {
e.printStackTrace();
}
/**
* 异步请求
*/
OkHttpClient client = new OkHttpClient.Builder().readTimeout(10000, TimeUnit.MILLISECONDS).build();
Request request = new Request.Builder().url("www.sherlockaza.com").get().build();
Call call = client.newCall(request);
call.enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
}
@Override
public void onResponse(Call call, Response response) throws IOException {
String json = response.body().toString();
}
});
由上边两端代码可以看出，无论是同步请求还是异步请求，都需要几个重要的角色：
- OkHttpClient
- Request
- Call
详细如下:
OkHttpClient
OkHttpClient主要作用是封装了一个网络请求所需要的一些配置信息，通过下边源码可以看到，OkHttpClient拥有大量的成员变量，这些成员变量都是一个网络请求所需要携带的属性
final Dispatcher dispatcher;
final @Nullable Proxy proxy;
final List<Protocol> protocols;
final List<ConnectionSpec> connectionSpecs;
final List<Interceptor> interceptors;
final List<Interceptor> networkInterceptors;
final EventListener.Factory eventListenerFactory;
final ProxySelector proxySelector;
final CookieJar cookieJar;
final @Nullable Cache cache;
final @Nullable InternalCache internalCache;
final SocketFactory socketFactory;
final @Nullable SSLSocketFactory sslSocketFactory;
final @Nullable CertificateChainCleaner certificateChainCleaner;
final HostnameVerifier hostnameVerifier;
final CertificatePinner certificatePinner;
final Authenticator proxyAuthenticator;
final Authenticator authenticator;
final ConnectionPool connectionPool;
final Dns dns;
final boolean followSslRedirects;
final boolean followRedirects;
final boolean retryOnConnectionFailure;
final int connectTimeout;
final int readTimeout;
final int writeTimeout;
final int pingInterval;
譬如说请求超时时间，重试次数，连接池，分发器，拦截器等等，这里OkHttpClient使用了Builder构造者模式进行了OkHttpClient的初始化工作，
public Builder() {
dispatcher = new Dispatcher();
protocols = DEFAULT_PROTOCOLS;
connectionSpecs = DEFAULT_CONNECTION_SPECS;
eventListenerFactory = EventListener.factory(EventListener.NONE);
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;
pingInterval = 0;
}
什么是Builder构造者模式可以参考本人的这边博客设计模式之Builder模式,初始化的这些属性中比较重要的属性有Dispatcher 请求分发器,ConnectionPool连接池，Interceptor拦截器等等，这里先不赘述，后面一一介绍。
Request
Request 顾名思义是一个网络请求，他的封装很简单
final HttpUrl url;
final String method;
final Headers headers;
final @Nullable RequestBody body;
final Object tag;
private volatile CacheControl cacheControl; // Lazily initialized.
请求URL、请求方法、请求头、还有Post请求需要的请求body,还有一个后面会用到取消请求的tag标记，不过有一个特殊成员变量是CacheControl,官方的注释是Lazily initialized.，并不是一开始就初始化，CacheControl的作用是缓存控制器，后边的网络缓存策略会使用到，先不赘述，同样Request的初始化方法也采用的是Builder模式.
Call
Call是一个接口，他是链接Request和Response之间的桥梁，Call的接口定义很简单
public interface Call extends Cloneable {
/** 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.
*
* <p>To avoid leaking resources callers should close the {@link Response} which in turn will
* close the underlying {@link ResponseBody}.
*
* <pre>@{code
*
* // ensure the response (and underlying response body) is closed
* try (Response response = client.newCall(request).execute()) {
* ...
* }
*
* }</pre>
*
* <p>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} or the
* Response.
*
* <p>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.
*
* <p>The {@link OkHttpClient#dispatcher dispatcher} defines when the request will run: usually
* immediately unless there are several other requests currently being executed.
*
* <p>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();
/**
* Create a new, identical call to this one which can be enqueued or executed even if this call
* has already been.
*/
Call clone();
interface Factory {
Call newCall(Request request);
}
}
- execute()方法，同步网络请求调用的是execute方法
- enqueue方法，异步请求调用的方法
- cancel 取消请求方法
- 还有个内部接口返回一个Call对象
RealCall
上边说到Call是一个接口，实际上的网络请求执行的方法都是交给他的子类RealCall。
public class OkHttpClient implements Cloneable, Call.Factory, WebSocket.Factory {
/**
*
*
*
*/
@Override public Call newCall(Request request) {
return RealCall.newRealCall(this, request, false /* for web socket */);
}
}
上边我们看出OkHttpClient实现了Call.Factory接口，并且该接口的实现内容是返回的Call的实现类RealCall。接下来调用excute或者enqueue方法进行网络请求
@Override public Response execute() throws IOException {
//同步标记位，判断当前的请求是否已经执行过
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
captureCallStackTrace();//一些log信息
eventListener.callStart(this);//网络时间监听的回调
//--------------重要代码块start----------------//
try {
/**同步请求交给Dispatcher执行*/
client.dispatcher().executed(this);
/**最终得到的response要通过拦截器链一步一步的回调才能最终得到*/
Response result = getResponseWithInterceptorChain();
if (result == null) throw new IOException("Canceled");
return result;
} catch (IOException e) {
eventListener.callFailed(this, e);
throw e;
} finally {
/**执行完请求要关闭资源*/
client.dispatcher().finished(this);
}
//--------------重要代码块end----------------//
}
@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));
}
同步和异步请求分别调用execute和enqueue方法，两个方法的实现截然不同，但都拥有一个重要的角色Dispatcher，无论是同步还是异步请求，实际上都是由Dispatcher调度分配的，源码中的注释比较粗略，下一篇会着重的分析Dispatcher调度原理。这篇内容主要是熟悉下一个网络请求的大致流程。
总结
根据以上的分析，简单的了解了一个Http请求的流程, 如图:
1.jpeg
接下来分析OKHttp的核心之一 Dispatcher
