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...