本文深入解析Retrofit如何封装OkHttp，提升网络请求的便利性，以及OkHttp的链式调用机制，详细分析了各Interceptor的功能，包括重试、缓存、连接管理和数据传输。
调用流程分析
上篇文章分析到了调用enqueue之前，接着往下走。
repos.enqueue(new Callback<List<Repo>>() {
@Override
public void onResponse(Call<List<Repo>> call, Response<List<Repo>> response) {
Log.d("MainActivity", response.toString());
}
@Override
public void onFailure(Call<List<Repo>> call, Throwable t) {
Log.d("MainActivity", t.toString());
}
这里的enqueue实际调用的是ExecutorCallbackCall的enqueue方法。
@Override public void enqueue(final Callback<T> callback) {
checkNotNull(callback, "callback == null");
delegate.enqueue(new Callback<T>() {
@Override public void onResponse(Call<T> call, final Response<T> response) {
callbackExecutor.execute(new Runnable() {
@Override public void run() {
if (delegate.isCanceled()) {
// Emulate OkHttp's behavior of throwing/delivering an IOException on cancellation.
callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
} else {
callback.onResponse(ExecutorCallbackCall.this, response);
}
}
});
}
@Override public void onFailure(Call<T> call, final Throwable t) {
callbackExecutor.execute(new Runnable() {
@Override public void run() {
callback.onFailure(ExecutorCallbackCall.this, t);
}
});
}
});
}
这个delegate就是 new ExecutorCallbackCall时，传入的OKHttpCall。从这里我们就知道了，Retrofit实际还是用的OkHttp来网络请求。接下来就到了OKHttpCall的enqueue方法。
@Override public void enqueue(final Callback<T> callback) {
checkNotNull(callback, "callback == null");
okhttp3.Call call;
Throwable failure;
synchronized (this) {
if (executed) throw new IllegalStateException("Already executed.");
executed = true;
call = rawCall;
failure = creationFailure;
if (call == null && failure == null) {
try {
call = rawCall = createRawCall();
} catch (Throwable t) {
throwIfFatal(t);
failure = creationFailure = t;
}
}
}
if (failure != null) {
callback.onFailure(this, failure);
return;
}
if (canceled) {
call.cancel();
}
call.enqueue(new okhttp3.Callback() {
@Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse) {
Response<T> response;
try {
response = parseResponse(rawResponse);
} catch (Throwable e) {
throwIfFatal(e);
callFailure(e);
return;
}
try {
callback.onResponse(OkHttpCall.this, response);
} catch (Throwable t) {
t.printStackTrace();
}
}
@Override public void onFailure(okhttp3.Call call, IOException e) {
callFailure(e);
}
private void callFailure(Throwable e) {
try {
callback.onFailure(OkHttpCall.this, e);
} catch (Throwable t) {
t.printStackTrace();
}
}
});
}
这个方法先判断了一下请求是否已经执行过了。如果没有则创建okhttp3.Call对象，叫做RealCall，然后调用其enqueue方法。
@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));
}
这个client是RetrofitBuilder.build时创建的默认的OkHttpClient对象。client.dispatcher()返回的就是OkHttpClient的Dispatcher对象。Dispatcher主要用来管理请求。每一个Dispatcher内部有一个不限数量的线程池（核心线程为0，加快回收），以及三个Call队列，readyAsyncCalls、runningAsyncCalls、runningSyncCalls，队列用ArrayQueue实现（ArrayQueue的内部是一个动态扩展的循环数组，通过head和tail维护数组的开始和结尾，其插入删除的效率非常高，都是O(N)）。Dispatcher还规定了异步请求的最大请求数为64以及单个主机的最大请求数为5。
接下来就是执行Dispatcher的enqueue方法。
void enqueue(AsyncCall call) {
synchronized (this) {
readyAsyncCalls.add(call);
}
promoteAndExecute();
}
这个方法传入的一个AsyncCall，首先把这个AsyncCall加入到readyAsyncCalls中，然后执行promoteAndExecute。
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.最大请求数64
if (runningCallsForHost(asyncCall) >= maxRequestsPerHost) continue; // Host max capacity.5
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;
}
promoteAndExecute方法遍历readyAsyncCalls，将其内的Call对象先取出移除，然后添加到executableCalls和runningAsyncCalls中。接着遍历executableCalls中的对象，放到线程池中执行。
AsyscCall继承了NamedRunnable。NamedRunnable实现了Runnable接口。在NamedRunnable是一个抽象类。在其run方法中调用了抽象方法protected abstract void execute();。也就是到线程池后，执行的是每个AsyncCall自己的execute方法。
@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);
}
}
关键就一个方法getResponseWithInterceptorChain，其它都是请求完成之后的回调。
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
List<Interceptor> interceptors = new ArrayList<>();
interceptors.addAll(client.interceptors());//添加自定义的interceptor
interceptors.add(retryAndFollowUpInterceptor);//重试interceptor
interceptors.add(new BridgeInterceptor(client.cookieJar()));//桥接interceptor
interceptors.add(new CacheInterceptor(client.internalCache()));//缓存的Interceptor
interceptors.add(new ConnectInterceptor(client));//创建Socket连接的Interceptor
if (!forWebSocket) {
interceptors.addAll(client.networkInterceptors());//如果不是WebSocket，添加网络Interceptor
}
interceptors.add(new CallServerInterceptor(forWebSocket));//最后添加向服务器发起请求的Interceptor
Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
originalRequest, this, eventListener, client.connectTimeoutMillis(),
client.readTimeoutMillis(), client.writeTimeoutMillis());
return chain.proceed(originalRequest);
}
这个方法可以说是OkHttp的核心方法了。这个方法将所有的interceptors添加到一个List中，组成一个调用链。我们先弄清楚调用链的执行流程，再回过来分析每个Interceptor的作用。
调用链的执行流程
先看下创建调用链的构造函数。
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) {
...
}
可以看到，传入了interceptors列表。streamAllocation、httpCodec、connection都是null。index为0，表示从interceptors的第0个元素开始（后面会看到这个index的作用）。connectTimeout、readTimeout、writeTimeout都是10000ms。
然后调用了chain.proceed方法。
@Override public Response proceed(Request request) throws IOException {
return proceed(request, streamAllocation, httpCodec, connection);
}
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
if (index >= interceptors.size()) throw new AssertionError();
calls++;
//第一次调用进来httpCodec=null
// If we already have a stream, confirm that the incoming request will use it.
if (this.httpCodec != null && !this.connection.supportsUrl(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.httpCodec != 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, httpCodec,
connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
writeTimeout);
Interceptor interceptor = interceptors.get(index);//得到当前的interceptor
Response response = interceptor.intercept(next);//调用当前interceptor的方法，并把下一个interceptor的调用链作为参数传入。
// Confirm that the next interceptor made its required call to chain.proceed().
//确保下一个interceptor存在且没有被执行过
if (httpCodec != 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");
}
if (response.body() == null) {
throw new IllegalStateException(
"interceptor " + interceptor + " returned a response with no body");
}
return response;
}
这个proceed方法主要做了两件事，第一件是创建了下一个interceptor的RealInterceptorChain，第二是执行当前interceptor的intercept方法并把下一个interceptor的调用链RealInterceptorChain作为参数传入。在每个当前interceptor的intercept方法中都会调用下一个调用链的chain.proceed方法，直到最后一个interceptor执行完。这样就形成了链式调用。
也就是说，程序会按顺序遍历interceptors列表中的对象，执行每个interceptor的intercept方法。最后依次返回结果。
各个Interceptor的作用
- RetryAndFollowUpInterceptor
@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");
}
Response response;
boolean releaseConnection = true;
try {
response = realChain.proceed(request, streamAllocation, null, null);
releaseConnection = false;
} catch (RouteException e) {
// The attempt to connect via a route failed. The request will not have been sent.
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.
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.
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 {
followUp = followUpRequest(response, streamAllocation.route());
} catch (IOException e) {
streamAllocation.release();
throw e;
}
if (followUp == null) {
streamAllocation.release();
return response;
}
closeQuietly(response.body());
if (++followUpCount > MAX_FOLLOW_UPS) {//最大重试次数，默认为20
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);
//如果不是相同的连接，释放调之前的，重新创建一个StreamAllocation。
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;//将下一次的请求赋值给request，用于下一次循环。
priorResponse = response;
}
}
RetryAndFollowUpInterceptor主要负责失败后的重试以及处理重定向的工作。我们可以看到，RetryAndFollowUpInterceptor里面有一个while(true)的死循环。如果请求失败了或者需要重定向，从followUpRequest方法中拿到下一次的request，重新进入循环。
- BridgeInterceptor
@Override public Response intercept(Chain chain) throws IOException {
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();
RequestBody body = userRequest.body();//上层发来的用户请求
if (body != null) {//请求不为空
MediaType contentType = body.contentType();
if (contentType != null) {
requestBuilder.header("Content-Type", contentType.toString());//设置Content-Type
}
long contentLength = body.contentLength();
if (contentLength != -1) {
requestBuilder.header("Content-Length", Long.toString(contentLength));
requestBuilder.removeHeader("Transfer-Encoding");
} else {
requestBuilder.header("Transfer-Encoding", "chunked");
requestBuilder.removeHeader("Content-Length");
}
}
if (userRequest.header("Host") == null) {
requestBuilder.header("Host", hostHeader(userRequest.url(), false));
}
if (userRequest.header("Connection") == null) {
requestBuilder.header("Connection", "Keep-Alive");
}
// If we add an "Accept-Encoding: gzip" header field we're responsible for also decompressing
// the transfer stream.
boolean transparentGzip = false;
if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
transparentGzip = true;
requestBuilder.header("Accept-Encoding", "gzip");
}
List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
if (!cookies.isEmpty()) {
requestBuilder.header("Cookie", cookieHeader(cookies));
}
if (userRequest.header("User-Agent") == null) {
requestBuilder.header("User-Agent", Version.userAgent());
}
Response networkResponse = chain.proceed(requestBuilder.build());//网络请求返回的response
HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
Response.Builder responseBuilder = networkResponse.newBuilder()
.request(userRequest);
if (transparentGzip
&& "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))
&& HttpHeaders.hasBody(networkResponse)) {
GzipSource responseBody = new GzipSource(networkResponse.body().source());
Headers strippedHeaders = networkResponse.headers().newBuilder()
.removeAll("Content-Encoding")
.removeAll("Content-Length")
.build();
responseBuilder.headers(strippedHeaders);
String contentType = networkResponse.header("Content-Type");
responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
}
return responseBuilder.build();
}
BridgeInterceptor主要负责将应用层编写的代码转换成能用于网络请求的代码，网络请求结束之后再把返回的response数据转换成应用层的response。
- CacheInterceptor
@Override public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;//先从缓存中查找是否已经有同一个request请求的返回数据。
long now = System.currentTimeMillis();
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
//CacheStrategy主要用来确定是用缓存中的response还是进行网络请求
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.
//如果根据BridgeInterceptor生成的networkRequest为空，缓存中也没有相应的返回，则直接返回504.
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networkRequest为null，但是cacheResponse不为null。直接从缓存中返回就行。
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
Response networkResponse = null;
try {
networkResponse = chain.proceed(networkRequest);//执行下一个Interceptor
} 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) {
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.
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;
}
CacheInterceptor主要负责Request缓存的存取工作。根据BridgeInterceptor生成的请求，去缓存里面查找是否存在相同请求的返回数据，如果存在则直接从缓存里面取。如果不存在，就执行下一个interceptor进行网络请求，拿到返回的数据后根据条件写入缓存中。
4.ConnectInterceptor
@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");
HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();
return realChain.proceed(request, streamAllocation, httpCodec, connection);
}
ConnectInterceptor主要负责与服务器建立socket连接的工作。虽然这个类的intercept方法很短，但是很重要。这个方法一共就做了两件事，通过streamAllocation.newStream方法得到了httpCodec对象，调用streamAllocation.connection创建连接，返回了RealConnection。接着就沿着调用链往下执行了。
先来看下streamAllocation.newStream方法。
public HttpCodec newStream(
OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
int connectTimeout = chain.connectTimeoutMillis();
int readTimeout = chain.readTimeoutMillis();
int writeTimeout = chain.writeTimeoutMillis();
int pingIntervalMillis = client.pingIntervalMillis();
boolean connectionRetryEnabled = client.retryOnConnectionFailure();
try {
RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);//查找HealthyConnection
//创建HttpCodec对象，
HttpCodec resultCodec = resultConnection.newCodec(client, chain, this);
synchronized (connectionPool) {
codec = resultCodec;
return resultCodec;
}
} catch (IOException e) {
throw new RouteException(e);
}
}
private RealConnection findHealthyConnection(int connectTimeout, int readTimeout,
int writeTimeout, int pingIntervalMillis, boolean connectionRetryEnabled,
boolean doExtensiveHealthChecks) throws IOException {
while (true) {
//查找连接
RealConnection candidate = findConnection(connectTimeout, readTimeout, writeTimeout,
pingIntervalMillis, connectionRetryEnabled);
// If this is a brand new connection, we can skip the extensive health checks.
//如果是全新的连接，可以跳过是否health的检查
synchronized (connectionPool) {
if (candidate.successCount == 0) {
return candidate;
}
}
// Do a (potentially slow) check to confirm that the pooled connection is still good. If it
// isn't, take it out of the pool and start again.
//判断当前的连接是否可用的，如果不可用则从连接池中取出，重新启动。
if (!candidate.isHealthy(doExtensiveHealthChecks)) {
noNewStreams();
continue;
}
return candidate;
}
}
findConnection方法是一个核心方法。下面重点分析。
private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
int pingIntervalMillis, boolean connectionRetryEnabled) throws IOException {
boolean foundPooledConnection = false;
RealConnection result = null;
Route selectedRoute = null;
Connection releasedConnection;
Socket toClose;
// connectionPool的操作加锁
synchronized (connectionPool) {
if (released) throw new IllegalStateException("released");
if (codec != null) throw new IllegalStateException("codec != null");
if (canceled) throw new IOException("Canceled");
// Attempt to use an already-allocated connection. We need to be careful here because our
// already-allocated connection may have been restricted from creating new streams.
releasedConnection = this.connection;
toClose = releaseIfNoNewStreams();
if (this.connection != null) {
// 我们已经有了一个分配好了的可用的连接，把这个连接赋值给result，后续直接返回。
// We had an already-allocated connection and it's good.
result = this.connection;
releasedConnection = null;
}
if (!reportedAcquired) {
// 如果这个连接不需要通知被取出，释放的时候也不要通知给监听者。
// If the connection was never reported acquired, don't report it as released!
releasedConnection = null;
}
if (result == null) {
//尝试从连接池中取出一个连接，这里没有直接从连接池connectionPool中取
//而是先通过Internal转发了一下。
// Attempt to get a connection from the pool.
Internal.instance.get(connectionPool, address, this, null);
if (connection != null) {
foundPooledConnection = true;
result = connection;
} else {
selectedRoute = route;
}
}
}
closeQuietly(toClose);
if (releasedConnection != null) {
eventListener.connectionReleased(call, releasedConnection);
}
if (foundPooledConnection) {
eventListener.connectionAcquired(call, result);
}
if (result != null) {
//如果找到了一个已经分配好的可用的连接，直接返回
// If we found an already-allocated or pooled connection, we're done.
return result;
}
//如果需要选择线路，接着往下走
// If we need a route selection, make one. This is a blocking operation.
boolean newRouteSelection = false;
if (selectedRoute == null && (routeSelection == null || !routeSelection.hasNext())) {
newRouteSelection = true;//代表需要选择线路
routeSelection = routeSelector.next();//拿到线路集合
}
synchronized (connectionPool) {
if (canceled) throw new IOException("Canceled");
if (newRouteSelection) {
//我们拿到了一组线路的集合，根据这组线路继续在连接池中找到是否存在可用的连接
// Now that we have a set of IP addresses, make another attempt at getting a connection from
// the pool. This could match due to connection coalescing.
List<Route> routes = routeSelection.getAll();
for (int i = 0, size = routes.size(); i < size; i++) {
Route route = routes.get(i);
Internal.instance.get(connectionPool, address, this, route);
if (connection != null) {
foundPooledConnection = true;
result = connection;
this.route = route;
break;
}
}
}
if (!foundPooledConnection) {
if (selectedRoute == null) {
selectedRoute = routeSelection.next();
}
//如果还是没找到，直接创建一个新的连接
// Create a connection and assign it to this allocation immediately. This makes it possible
// for an asynchronous cancel() to interrupt the handshake we're about to do.
route = selectedRoute;
refusedStreamCount = 0;
result = new RealConnection(connectionPool, selectedRoute);
acquire(result, false);
}
}
//如果在第二轮查找中找到了一个可用连接，返回结果。
// If we found a pooled connection on the 2nd time around, we're done.
if (foundPooledConnection) {
eventListener.connectionAcquired(call, result);
return result;
}
//如果没有找到可用的连接，是新创建的连接。建立连接的关键方法就是这个connect方法。
// Do TCP + TLS handshakes. This is a blocking operation.
result.connect(connectTimeout, readTimeout, writeTimeout, pingIntervalMillis,
connectionRetryEnabled, call, eventListener);
routeDatabase().connected(result.route());
Socket socket = null;
synchronized (connectionPool) {
reportedAcquired = true;
//将新建立的连接加入连接池
// Pool the connection.
Internal.instance.put(connectionPool, result);
//如果是否多路复用的连接，则释放当前连接，使用复用的连接。
// If another multiplexed connection to the same address was created concurrently, then
// release this connection and acquire that one.
if (result.isMultiplexed()) {
socket = Internal.instance.deduplicate(connectionPool, address, this);
result = connection;
}
}
closeQuietly(socket);
eventListener.connectionAcquired(call, result);
return result;
}
看一下connect方法
public void connect(int connectTimeout, int readTimeout, int writeTimeout,
int pingIntervalMillis, boolean connectionRetryEnabled, Call call,
EventListener eventListener) {
if (protocol != null) throw new IllegalStateException("already connected");
RouteException routeException = null;
List<ConnectionSpec> connectionSpecs = route.address().connectionSpecs();
ConnectionSpecSelector connectionSpecSelector = new ConnectionSpecSelector(connectionSpecs);
if (route.address().sslSocketFactory() == null) {
if (!connectionSpecs.contains(ConnectionSpec.CLEARTEXT)) {
throw new RouteException(new UnknownServiceException(
"CLEARTEXT communication not enabled for client"));
}
String host = route.address().url().host();
if (!Platform.get().isCleartextTrafficPermitted(host)) {
throw new RouteException(new UnknownServiceException(
"CLEARTEXT communication to " + host + " not permitted by network security policy"));
}
} else {
if (route.address().protocols().contains(Protocol.H2_PRIOR_KNOWLEDGE)) {
throw new RouteException(new UnknownServiceException(
"H2_PRIOR_KNOWLEDGE cannot be used with HTTPS"));
}
}
while (true) {
try {
//如果是HTTPS请求，需要建立通道
if (route.requiresTunnel()) {
connectTunnel(connectTimeout, readTimeout, writeTimeout, call, eventListener);
if (rawSocket == null) {
// We were unable to connect the tunnel but properly closed down our resources.
break;
}
} else {
//如果是HTTP请求，直接创建Socket连接就可以了。
connectSocket(connectTimeout, readTimeout, call, eventListener);
}
establishProtocol(connectionSpecSelector, pingIntervalMillis, call, eventListener);
eventListener.connectEnd(call, route.socketAddress(), route.proxy(), protocol);
break;
} catch (IOException e) {
closeQuietly(socket);
closeQuietly(rawSocket);
socket = null;
rawSocket = null;
source = null;
sink = null;
handshake = null;
protocol = null;
http2Connection = null;
eventListener.connectFailed(call, route.socketAddress(), route.proxy(), null, e);
if (routeException == null) {
routeException = new RouteException(e);
} else {
routeException.addConnectException(e);
}
if (!connectionRetryEnabled || !connectionSpecSelector.connectionFailed(e)) {
throw routeException;
}
}
}
if (route.requiresTunnel() && rawSocket == null) {
//最大通道数为21，超过则抛出异常
ProtocolException exception = new ProtocolException("Too many tunnel connections attempted: "
+ MAX_TUNNEL_ATTEMPTS);
throw new RouteException(exception);
}
if (http2Connection != null) {
synchronized (connectionPool) {
allocationLimit = http2Connection.maxConcurrentStreams();
}
}
}
先来看下connectSocket的代码。
/** Does all the work necessary to build a full HTTP or HTTPS connection on a raw socket. */
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 {
//这里是关键操作，建立当前平台的Socket连接。
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 {
//OkHttp的底层是用Okio来做IO操作的，source负责读，sink负责写。
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);
}
}
}
再看下connectTunnel方法。
private void connectTunnel(int connectTimeout, int readTimeout, int writeTimeout, Call call,
EventListener eventListener) throws IOException {
Request tunnelRequest = createTunnelRequest();
HttpUrl url = tunnelRequest.url();
for (int i = 0; i < MAX_TUNNEL_ATTEMPTS; i++) {
//首先建立Socket连接
connectSocket(connectTimeout, readTimeout, call, eventListener);
//创建HTTPS请求的通道，这个方法会对Https请求做校验。
tunnelRequest = createTunnel(readTimeout, writeTimeout, tunnelRequest, url);
//如果tunnelRequest==null，说明已经通道创建成功了。
if (tunnelRequest == null) break; // Tunnel successfully created.
//如果tunnelRequest！=null，将关闭当前连接，创建一个带证书的连接。
// The proxy decided to close the connection after an auth challenge. We need to create a new
// connection, but this time with the auth credentials.
closeQuietly(rawSocket);
rawSocket = null;
sink = null;
source = null;
eventListener.connectEnd(call, route.socketAddress(), route.proxy(), null);
}
}
重点看下createTunnel方法
private Request createTunnel(int readTimeout, int writeTimeout, Request tunnelRequest,
HttpUrl url) throws IOException {
// Make an SSL Tunnel on the first message pair of each SSL + proxy connection.
String requestLine = "CONNECT " + Util.hostHeader(url, true) + " HTTP/1.1";
while (true) {
//新建一个Http1.1的代理请求类。先向服务器发起一次请求。
Http1Codec tunnelConnection = new Http1Codec(null, null, source, sink);
source.timeout().timeout(readTimeout, MILLISECONDS);
sink.timeout().timeout(writeTimeout, MILLISECONDS);
tunnelConnection.writeRequest(tunnelRequest.headers(), requestLine);
tunnelConnection.finishRequest();
Response response = tunnelConnection.readResponseHeaders(false)
.request(tunnelRequest)
.build();
//如果能够通过认证，response body应该是空的。否则说明没有通过校验，我们需要另外处理。
// The response body from a CONNECT should be empty, but if it is not then we should consume
// it before proceeding.
long contentLength = HttpHeaders.contentLength(response);
if (contentLength == -1L) {
contentLength = 0L;
}
Source body = tunnelConnection.newFixedLengthSource(contentLength);
Util.skipAll(body, Integer.MAX_VALUE, TimeUnit.MILLISECONDS);
body.close();
switch (response.code()) {
case HTTP_OK:
// Assume the server won't send a TLS ServerHello until we send a TLS ClientHello. If
// that happens, then we will have buffered bytes that are needed by the SSLSocket!
// This check is imperfect: it doesn't tell us whether a handshake will succeed, just
// that it will almost certainly fail because the proxy has sent unexpected data.
if (!source.buffer().exhausted() || !sink.buffer().exhausted()) {
throw new IOException("TLS tunnel buffered too many bytes!");
}
return null;
case HTTP_PROXY_AUTH:
tunnelRequest = route.address().proxyAuthenticator().authenticate(route, response);
if (tunnelRequest == null) throw new IOException("Failed to authenticate with proxy");
if ("close".equalsIgnoreCase(response.header("Connection"))) {
return tunnelRequest;
}
break;
default:
throw new IOException(
"Unexpected response code for CONNECT: " + response.code());
}
}
}
执行到这里就算建立了Socket连接了。接下来将会执行调用链的最后一个Interceptor，CallServerInterceptor。
@Override public Response intercept(Chain chain) throws IOException {
//前面这一大段代码都是拿到请求的参数
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());
//这句代码会向通道中写入请求头的数据，如果没有请求体，到这里服务器会直接返回给我们数据了。
httpCodec.writeRequestHeaders(request);
realChain.eventListener().requestHeadersEnd(realChain.call(), request);
Response.Builder responseBuilder = null;
if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
//如果需要有请求，且请求体也有内容。
// If there's a "Expect: 100-continue" header on the request, wait for a "HTTP/1.1 100
// Continue" response before transmitting the request body. If we don't get that, return
// what we did get (such as a 4xx response) without ever transmitting the request body.
if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
httpCodec.flushRequest();
realChain.eventListener().responseHeadersStart(realChain.call());
responseBuilder = httpCodec.readResponseHeaders(true);
}
if (responseBuilder == null) {
// Write the request body if the "Expect: 100-continue" expectation was met.
realChain.eventListener().requestBodyStart(realChain.call());
long contentLength = request.body().contentLength();
CountingSink requestBodyOut =
new CountingSink(httpCodec.createRequestBody(request, contentLength));
BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
//将请求体的数据发送给服务器，到这里数据全部发送到了服务器，后面获取返回结果就可以了。
request.body().writeTo(bufferedRequestBody);
bufferedRequestBody.close();
realChain.eventListener()
.requestBodyEnd(realChain.call(), requestBodyOut.successfulCount);
} else if (!connection.isMultiplexed()) {
// If the "Expect: 100-continue" expectation wasn't met, prevent the HTTP/1 connection
// from being reused. Otherwise we're still obligated to transmit the request body to
// leave the connection in a consistent state.
streamAllocation.noNewStreams();
}
}
httpCodec.finishRequest();
//下面的代码都是解析服务器返回结果的。
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();
int code = response.code();
if (code == 100) {
// server sent a 100-continue even though we did not request one.
// try again to read the actual response
responseBuilder = httpCodec.readResponseHeaders(false);
response = responseBuilder
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
code = response.code();
}
realChain.eventListener()
.responseHeadersEnd(realChain.call(), response);
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 {
response = response.newBuilder()
.body(httpCodec.openResponseBody(response))
.build();
}
if ("close".equalsIgnoreCase(response.request().header("Connection"))
|| "close".equalsIgnoreCase(response.header("Connection"))) {
streamAllocation.noNewStreams();
}
if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
throw new ProtocolException(
"HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
}
return response;
}
这个intercept方法return之后，response数据会沿着调用链从后往前，依次处理返回数据，最终回调下面这个Callback中。
repos.enqueue(new Callback<List<Repo>>() {
@Override
public void onResponse(Call<List<Repo>> call, Response<List<Repo>> response) {
Log.d("MainActivity", response.toString());
}
@Override
public void onFailure(Call<List<Repo>> call, Throwable t) {
Log.d("MainActivity", t.toString());
}
});
整个Retrofit源码的调用链分析到这里，总结一下：
- Retrofit是对OkHttp的封装，提升了OkHttp接口的可用性，方便我们定义网络请求、添加CallApdater、自定义Interceptor等操作。
- OkHttp的请求设计成了链式调用，每个Interceptor都可以对发送的请求添加自己的内容，也可以对返回的数据封装自己的操作。
- OkHttp规定了异步请求的最大请求数为64，单个主机的最大请求数为5
扫一扫
704
被折叠的 条评论
为什么被折叠?
到【灌水乐园】发言
