okhttp简单使用
- 创建Request
- 创建Call，将Request添加到Call中
- 使用异步enqueue，或者同步的execute方法获得结果
okhttp网络请求过程分析
Call
同步请求
@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(false);
if (result == null) throw new IOException("Canceled");
return result;
} finally {
client.dispatcher().finished(this);
}
}
首先加锁置标志位，接着使用分配器的executed方法将call加入到同步队列中，然后调用getResponseWithInterceptorChain方法（稍后分析）执行http请求，最后调用finishied方法将call从同步队列中删除
异步请求
void enqueue(Callback responseCallback, boolean forWebSocket) {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
client.dispatcher().enqueue(new AsyncCall(responseCallback, forWebSocket));
}
同样先置标志位，然后将封装的一个执行体放到异步执行队列中。这里面引入了一个新的类AsyncCall，这个类继承于NamedRunnable，实现了Runnable接口。NamedRunnable可以给当前的线程设置名字，并且用模板方法将线程的执行体放到了execute方法中，所以我们分析AsyncCall只需要看execute方法
@Override protected void execute() {
boolean signalledCallback = false;
try {
Response response = getResponseWithInterceptorChain(forWebSocket);
if (canceled) {
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!
logger.log(Level.INFO, "Callback failure for " + toLoggableString(), e);
} else {
responseCallback.onFailure(RealCall.this, e);
}
} finally {
client.dispatcher().finished(this);
}
}
通过getResponseWithInterceptorChain方法来执行http请求，这个方法是不是很熟悉，在同步请求中也是用的这个方法来执行http请求。紧接着判断call是否被cancel来执行不同的回调，最后使用finished方法将call从异步执行队列中移除。这里有个需要注意的地方，onResponse回调被执行的条件是本次http请求是完整的，也就是说即使服务器返回的是错误信息，依然会走onResponse回调，我们在应用层使用的时候，可以自己再封装一次。
OK，以上就是okhttp可以同时支持同步和异步请求的分析过程，而在getResponseWithInterceptorChain方法中我们将会分析okhttp的另一个重要模块：拦截器
拦截器
这是我最喜欢okhttp的地方，你可以拦截当前正在发出的请求。我们可以使用拦截器做很多事情，例如：添加log方便调试，在服务器还没有ready的情况下模拟一个网络应答等。在getResponseWithInterceptorChain方法中处理了拦截器的相关逻辑
private Response getResponseWithInterceptorChain(boolean forWebSocket) throws IOException {
Interceptor.Chain chain = new ApplicationInterceptorChain(0, originalRequest, forWebSocket);
return chain.proceed(originalRequest);
}
这里ApplicationInterceptorChain实现了Interceptor.Chain接口，然后在preceed方法中处理相应的逻辑，preceed代码如下
@Override public Response proceed(Request request) throws IOException {
// If there's another interceptor in the chain, call that.
if (index < client.interceptors().size()) {
Interceptor.Chain chain = new ApplicationInterceptorChain(index + 1, request, forWebSocket);
Interceptor interceptor = client.interceptors().get(index);
Response interceptedResponse = interceptor.intercept(chain);
if (interceptedResponse == null) {
throw new NullPointerException("application interceptor " + interceptor
+ " returned null");
}
return interceptedResponse;
}
// No more interceptors. Do HTTP.
return getResponse(request, forWebSocket);
}
在index在getResponseWithInterceptorChain方法中被初始化为0，当我们添加了拦截器之后index < client.interceptors().size()就会走到true的代码段，之后会从client.interceptors()中拿出一个拦截器，执行我们的拦截回调。这里也可以看到在拦截回调中是必须要有个Response返回的，否则会出现异常。如果没有自定义拦截器的话，将会调用getResponse方法执行真正的网络请求逻辑（相对于拦截器模块来说是执行了真正的网络请求，其实后面还有缓存模块）
有意思的是我们可以定义多个拦截器，这就对应了ApplicationInterceptorChain类的名称应用拦截链。只要我们在自定义的拦截器回调方法中调用chan.proceed，拦截器就会链式的调用下去。如果我们不希望okhttp执行真正的网络请求，只需要在拦截器中虚拟一个response即可。需要注意的是，如果某个拦截器内部没有调用chan.proceed方法，那么在它之后添加的拦截器都不会再被执行
getResponse方法将会把网络请求交给Engine处理
Response getResponse(Request request, boolean forWebSocket) throws IOException {
// Copy body metadata to the appropriate request headers.
RequestBody body = request.body();
if (body != null) {
Request.Builder requestBuilder = request.newBuilder();
MediaType contentType = body.contentType();
if (contentType != null) {
requestBuilder.header("Content-Type", contentType.toString());
}
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");
}
request = requestBuilder.build();
}
// Create the initial HTTP engine. Retries and redirects need new engine for each attempt.
engine = new HttpEngine(client, request, false, false, forWebSocket, null, null, null);
int followUpCount = 0;
while (true) {
if (canceled) {
engine.releaseStreamAllocation();
throw new IOException("Canceled");
}
boolean releaseConnection = true;
try {
engine.sendRequest();
engine.readResponse();
releaseConnection = false;
} catch (RequestException e) {
// The attempt to interpret the request failed. Give up.
throw e.getCause();
} catch (RouteException e) {
// The attempt to connect via a route failed. The request will not have been sent.
HttpEngine retryEngine = engine.recover(e.getLastConnectException(), null);
if (retryEngine != null) {
releaseConnection = false;
engine = retryEngine;
continue;
}
// Give up; recovery is not possible.
throw e.getLastConnectException();
} catch (IOException e) {
// An attempt to communicate with a server failed. The request may have been sent.
HttpEngine retryEngine = engine.recover(e, null);
if (retryEngine != null) {
releaseConnection = false;
engine = retryEngine;
continue;
}
// Give up; recovery is not possible.
throw e;
} finally {
// We're throwing an unchecked exception. Release any resources.
if (releaseConnection) {
StreamAllocation streamAllocation = engine.close();
streamAllocation.release();
}
}
Response response = engine.getResponse();
Request followUp = engine.followUpRequest();
if (followUp == null) {
if (!forWebSocket) {
engine.releaseStreamAllocation();
}
return response;
}
StreamAllocation streamAllocation = engine.close();
if (++followUpCount > MAX_FOLLOW_UPS) {
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
if (!engine.sameConnection(followUp.url())) {
streamAllocation.release();
streamAllocation = null;
}
request = followUp;
engine = new HttpEngine(client, request, false, false, forWebSocket, streamAllocation, null,
response);
}
}
}
getResponse会先根据request body的contentType来设置相应的header，Content-Length相信都比较熟悉，在http header中Transfer-Encoding: chunked表示的是内容长度不定，这里比较奇怪的是header的属性分别在不同的设置，不清楚为何不放在一起设置。接着会创建一个HttpEngine对象，设置追加发送的请求次数，在HttpEngine中处理网络请求代码如下
engine.sendRequest();
engine.readResponse();
Response response = engine.getResponse();
紧接着是处理各种异常和发送追加请求，获取发送追加请求是在HttpEngine的followUpRequest方法中处理，在三种情况下okhttp会发送追加请求，通过MAX_FOLLOW_UPS = 20控制最大追加请求
- 未授权（401）：调用
okhttpclient授权方法重新授权 - 重定向（3xx）
- 请求超时（408）：重复发送原请求
未完待续...
