okhttp源码解析--dispatcher
Okhttpclient流程图.png
1. dispatcher到底是什么、做了什么？
public final class Dispatcher {
private int maxRequests = 64;
private int maxRequestsPerHost = 5;
private @Nullable Runnable idleCallback;
/** Executes calls. Created lazily. */
private @Nullable ExecutorService executorService;
/** Ready async calls in the order they'll be run. */
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
/** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
/** Running synchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
通过上一篇的学习了解到不论是同步还是异步请求，最终调用的都是dispatcher中的方法，并分别将请求线程AsyncCall添加到异步队列(readyAsyncCalls、runningAsyncCalls)和同步队列(runningSyncCalls)中.
synchronized void enqueue(AsyncCall call) {
if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
runningAsyncCalls.add(call);
executorService().execute(call);
} else {
readyAsyncCalls.add(call);
}
}
/** Used by {@code Call#execute} to signal it is in-flight. */
synchronized void executed(RealCall call) {
runningSyncCalls.add(call);
}
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;
}
executorService()创建了一个线程池，在通过上面两张代码，不难看出Dispatcher主要工作就是维护请求队列和通过线程池来执行网络请求
1.1 Dispatcher的readyAsyncCalls、runningAsyncCalls等队列是怎样进行控制的？
从上一段代码中可以看到同步方法executed很简单直接将请求添加到同步队列中（runningSyncCalls）。异步请求也只有几行代码,判断了下是否小于最大请求数（maxRequests）和小于正在进行中的网络请求最大值（maxRequestsPerHost），是--就添加到进行中异步队列（runningAsyncCalls）并执行线程池，否者就添加到异步等待队列(readyAsyncCalls)。
看到这里我就产生了一个疑问，这里只是对消息队列进行了简单的添加，并没有和我们的线程池产生联系啊！不着急，我们继续。上一段代码我们以异步请求enqueue 方法继续分析 ：当请求满足条件时执行了
runningAsyncCalls.add(call);
executorService().execute(call);
这两步操作：第一步添加到执行中的异步请求队列（runningAsyncCalls），第二步线程池直接执行当前网络请求（注意executorService()返回线程池、AsyncCall（继承自NamedRunnable）就是一个线程哦，这俩在上一篇中已介绍过就不多说了）
/**
* Runnable implementation which always sets its thread name.
*/
public abstract class NamedRunnable implements Runnable {
protected final String name;
public NamedRunnable(String format, Object... args) {
this.name = Util.format(format, args);
}
@Override public final void run() {
String oldName = Thread.currentThread().getName();
Thread.currentThread().setName(name);
try {
execute();
} finally {
Thread.currentThread().setName(oldName);
}
}
protected abstract void execute();
}
当线程池执行网络请求时，实际就是执行了子线程的run（）方法，AsyncCall继承的NamedRunnable就是一个Runnable，在它的run（）方法中执行了它的抽象方法execute()。
我们继续到AsyncCall中去看看execute()的具体实现：
final class AsyncCall extends NamedRunnable {
private final Callback responseCallback;
AsyncCall(Callback responseCallback) {
super("OkHttp %s", redactedUrl());
this.responseCallback = responseCallback;
}
String host() {
return originalRequest.url().host();
}
Request request() {
return originalRequest;
}
RealCall get() {
return RealCall.this;
}
@Override protected void execute() {
boolean signalledCallback = false;
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) {
if (signalledCallback) {
// Do not signal the callback twice!
Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
} else {
responseCallback.onFailure(RealCall.this, e);
}
} finally {
client.dispatcher().finished(this);
}
}
}
retryAndFollowUpInterceptor.isCanceled()首先判断请求是否取消，
取消就调用 responseCallback的onFailure(...)失败方法，
否则responseCallback的onResponse(...)返回数据。这样一个异步请求就完成了。
最终在finlly看到了这段代码client.dispatcher().finished(this)调用了dispatcher的finished方法
/** Used by {@code AsyncCall#run} to signal completion. */
void finished(AsyncCall call) {
finished(runningAsyncCalls, call, true);
}
/** Used by {@code Call#execute} to signal completion. */
void finished(RealCall call) {
finished(runningSyncCalls, call, false);
}
private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
int runningCallsCount;
Runnable idleCallback;
synchronized (this) {
if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
if (promoteCalls) promoteCalls();
runningCallsCount = runningCallsCount();
idleCallback = this.idleCallback;
}
if (runningCallsCount == 0 && idleCallback != null) {
idleCallback.run();
}
}
首先将执行完成的网络请求从runningSyncCalls中移除，第二步通过promoteCalls判断是否是异步请求，是就执行promoteCalls()：
private void promoteCalls() {
if (runningAsyncCalls.size() >= maxRequests) return; // Already running max capacity.
if (readyAsyncCalls.isEmpty()) return; // No ready calls to promote.
for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
AsyncCall call = i.next();
if (runningCallsForHost(call) < maxRequestsPerHost) {
i.remove();
runningAsyncCalls.add(call);
executorService().execute(call);
}
if (runningAsyncCalls.size() >= maxRequests) return; // Reached max capacity.
}
}
/** Returns the number of running calls that share a host with {@code call}. */
private int runningCallsForHost(AsyncCall call) {
int result = 0;
for (AsyncCall c : runningAsyncCalls) {
if (c.host().equals(call.host())) result++;
}
return result;
}
首先判断请求进行中的异步队列（runningAsyncCalls）达到最大数量就return，异步等待队列（readyAsyncCalls）是为空就return。
然后遍历异步等待队列（readyAsyncCalls）：
-
判断现在最大请求数是否达到，没到就将请求从异步等待队列
（readyAsyncCalls）移除，并添加到异步队列（runningAsyncCalls）执行线程池，就又开始了下一次网络请求。 -
在此判断请求是否到达了最大值，因为在上面可能刚刚添加了一个网络请求
这样队列中的请求就完成了一个循环。队列也和线程池联系了起来
简单的总结一下：
executorService().execute(call)-->AsyncCall执行execute()-->dispatcher().finished(this)--> executorService().execute(call)
