首先来看看OkHttp的基础，异步get请求
OkHttpClient client = new OkHttpClient();//1
Request request = new Request.Builder()
.url("http://xxxxxx")
.build();//2
client.newCall(request).enqueue(new Callback() {//3
@Override
public void onFailure(Call call, IOException e) {
}
@Override
public void onResponse(Call call, Response response) throws IOException {
if(response.isSuccessful()){//回调的方法执行在子线程。
}
}
});
都是主要三个步骤：1.创建OkHttpClient实例；2.使用构造器创建请求；3.提交请求。
OkHttpClient构造方法
public OkHttpClient() {
this(new Builder());
}
public Builder() {
this.dispatcher = builder.dispatcher;
this.proxy = builder.proxy;//代理
this.protocols = builder.protocols;//协议
this.connectionSpecs = builder.connectionSpecs;//策略
this.interceptors = Util.immutableList(builder.interceptors);//拦截器
this.networkInterceptors = Util.immutableList(builder.networkInterceptors);//网络拦截器
this.eventListenerFactory = builder.eventListenerFactory;
this.proxySelector = builder.proxySelector;
this.cookieJar = builder.cookieJar;
this.cache = builder.cache;
this.internalCache = builder.internalCache;
this.socketFactory = builder.socketFactory;
boolean isTLS = false;
for (ConnectionSpec spec : connectionSpecs) {
isTLS = isTLS || spec.isTls();
}
if (builder.sslSocketFactory != null || !isTLS) {
this.sslSocketFactory = builder.sslSocketFactory;
this.certificateChainCleaner = builder.certificateChainCleaner;
} else {
X509TrustManager trustManager = systemDefaultTrustManager();
this.sslSocketFactory = systemDefaultSslSocketFactory(trustManager);
this.certificateChainCleaner = CertificateChainCleaner.get(trustManager);
}
this.hostnameVerifier = builder.hostnameVerifier;
this.certificatePinner = builder.certificatePinner.withCertificateChainCleaner(
certificateChainCleaner);
this.proxyAuthenticator = builder.proxyAuthenticator;
this.authenticator = builder.authenticator;
this.connectionPool = builder.connectionPool;
this.dns = builder.dns;
this.followSslRedirects = builder.followSslRedirects;
this.followRedirects = builder.followRedirects;
this.retryOnConnectionFailure = builder.retryOnConnectionFailure;
this.connectTimeout = builder.connectTimeout;
this.readTimeout = builder.readTimeout;
this.writeTimeout = builder.writeTimeout;
this.pingInterval = builder.pingInterval;
if (interceptors.contains(null)) {
throw new IllegalStateException("Null interceptor: " + interceptors);
}
if (networkInterceptors.contains(null)) {
throw new IllegalStateException("Null network interceptor: " + networkInterceptors);
}
}
可以看到，创建的过程主要是调用了构造器的构造方法，初始化了一些请求相关的参数，这里面几乎都是跟HTTP请求相关的参数，就不深入去追究，主要是要注意这里有三个东西：就是interceptors和networkInterceptors（这两个类就是这篇文章的主题，后面再细说），还有一个Dispatcher，我们需要进去看看，在这里我们可以知道OkHttp请求的任务调度原理：
public final class Dispatcher {
private int maxRequests = 64;//最大请求任务个数
private int maxRequestsPerHost = 5;//最大允许同一host的请求任务个数
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;
}
...
}
可以看出来，Dispatcher就像是一个调度中心，这里负责所有的任务执行，值得注意的是我们看到executorService这个对象是个线程池，而且是个不限制大小的线程池，这里用到了SynchronousQueue（没有缓存大小的阻塞队列），这个线程池跟Executors默认的newCachedThreadPool原理相同，当任务加入时如果有空闲的线程就复用，没有的话就创建新的线程，每个线程空闲后60秒被销毁。
创建请求
Request request = new Request.Builder()
.url("http://xxxxxx")
.build();
发起请求
Response response = client.newCall(request).execute();
RealCall.execute
@Override public Response execute() throws IOException {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
captureCallStackTrace();
eventListener.callStart(this);
try {
client.dispatcher().executed(this);//1
Response result = getResponseWithInterceptorChain();//2
if (result == null) throw new IOException("Canceled");
return result;
} catch (IOException e) {
eventListener.callFailed(this, e);
throw e;
} finally {
client.dispatcher().finished(this);
}
}
Response result = getResponseWithInterceptorChain();
拦截器原理开始
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
List<Interceptor> interceptors = new ArrayList<>();
interceptors.addAll(client.interceptors());//自定义的拦截器
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);
}
}
其实可以看出来，这个方法主要就是添加各种interceptor，然后文章最开始提到的第一个interceptors其实是自定义的拦截器，添加到了最前面，然后依次添加了四种拦截器过后，第五个是networkInterceptors，最后一共加了七种拦截器
拦截器责任链
public RealInterceptorChain(List<Interceptor> interceptors, StreamAllocation streamAllocation,
HttpCodec httpCodec, RealConnection connection, int index, Request request, Call call,
EventListener eventListener, int connectTimeout, int readTimeout, int writeTimeout) {
this.interceptors = interceptors;
this.connection = connection;
this.streamAllocation = streamAllocation;
this.httpCodec = httpCodec;
this.index = index;
this.request = request;
this.call = call;
this.eventListener = eventListener;
this.connectTimeout = connectTimeout;
this.readTimeout = readTimeout;
this.writeTimeout = writeTimeout;
}
这里主要是初始化了各种属性，这里面的参数大多数和OkHttp的属性无异，但需要注意的是这里的interceptors不同于OkHttp构造里面的那个interceptors，这里是表示刚才组成的所有拦截器的数组，包含interceptors和networkInterceptors在内的七种拦截器，然后index这个属性初始化为“0”。
接下来在getResponseWithInterceptorChain最后一句执行了它的proceed方法，在这个方法里有很多抛出异常的语句，这里不去深究，主要看看中间主要的一两句：
RealInterceptorChain.proceed
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
if (index >= interceptors.size()) throw new AssertionError();
calls++;
... ...
// 重点！！！主要就看看这几句
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;
}
划重点！在这里就到了最核心的几句了
首先：又实例化了一个RealInterceptorChain，取名为next（从名字真的可以看出很多蹊跷），构造参数没变化，只是把index+1；
然后：从interceptors数组中用index来依次取出Interceptor（每次执行index都会+1，所以是依次取出来）；
最后：调用interceptor.intercept(next)，把next传入调用intercept方法
intercept(Chain chain)
@Override public Response intercept(Chain chain) throws IOException {
RealInterceptorChain realChain = (RealInterceptorChain) chain;
response = realChain.proceed(request, streamAllocation, null, null);
}
这个方法里面会对一些参数做修改，又调用了proceed方法，在proceed方法里我们知道index又会+1，然后取出下一个拦截器并且传入修改后的参数。这就完成了这个链式结构的运转，一步一步的往下传递，然后到了最后又依次返回Response
各种拦截器
RetryAndFollowUpInterceptor
用来实现连接失败的重试和重定向
BridgeInterceptor
用来修改请求和响应的 header 信息
CacheInterceptor
用来实现响应缓存。比如获取到的 Response 带有 Date，Expires，Last-Modified，Etag 等 header，表示该 Response 可以缓存一定的时间，下次请求就可以不需要发往服务端，直接拿缓存的
ConnectInterceptor
用来打开到服务端的连接。其实是调用了 StreamAllocation 的newStream 方法来打开连接的。建联的 TCP 握手，TLS 握手都发生该阶段。过了这个阶段，和服务端的 socket 连接打通
interceptors和networkInterceptors分别位于拦截器链的第一个和第六个，从每个拦截器的作用大致猜到一些。interceptors是肯定每次都会执行的，但是，在networkInterceptors之前有个ConnectInterceptor，这个拦截器的作用是用于建立跟服务器的连接。那如果我们现在设备离线，直接读取缓存呢？对，那这样的话networkInterceptors就不会执行了。
网友评论