一、OkHttp的基本流程
1. OkHttp的基本流程
OkHttpClient、Request -> RealCall -> Dispatcher -> interceptors -> RetryAndFollowInterceptor -> BirdgeInterceptor -> CacheInterceptor -> ConnectInterceptor -> CallServerInterceptor
2. 相关概念
2.1 OkHttpClient
OKHTTP使用OkHttpClient来发送和读取请求的响应，OkHttpClient在使用时，应该被设置为单例，只实例化一次，应该每个OkHttpClient都包含一个连接池和线程池。重用OkHttpClient可以减少延迟并节省内存
2.2 Request
OkHttp的请求
2.3 Dispatcher
Dispatch主要用于控制并发的请求，请求执行的策略
2.4 interceptors
拦截器是OkHttp中提供一种强大机制，它可以实现网络监听、请求以及响应重写，请求失败重试等功能
2.5 RetryAndFollowUpInterceptor
重试重定向拦截器
2.6 BridgeInterceptor
桥接拦截器，负责将用户构建的request转化为能够进行网络访问的请求，然后发起请求，最后将这个网络请求回来的响应Response转化为用户的Response。
2.7 CacheInterceptor
缓存拦截器，负责写入和读取缓存
2.8 ConnectInterceptor
链接拦截器，与服务器创建连接，并将其传给下一个拦截器
2.9 CallServerInterceptor
发起请求拦截器，这是最后一个拦截器，会对服务器进行调用
二、OkHttp源码解析
1 从请求处理开始分析
1.1 请求步骤
- 创建一个OkHttpClient对象
- 构建一个Request对象，通过OkHttpClient和Request对象，构建出Call对象
- 执行Call的execute（同步）/enqueue（异步）方法
1.2 请求源码分析
请求网络需要用OkHttpClient.newCall(request)进行execute或者enqueue操作：当调用newCall方法是，会调用如下代码：
/**
* Prepares the {@code request} to be executed at some point in the future.
*/
@Override public Call newCall(Request request) {
return new RealCall(this, request);
}
其实际返回的是一个RealCall类。调用enqueue异步请求网络实际上是调用了RealCall的enqueue方法。查看RealCall的enqueue方法
@Override public void enqueue(Callback responseCallback) {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
client.dispatcher().enqueue(new AsyncCall(responseCallback));
}
最终的请求是dispatcher来完成的。
2 Dispatcher任务调度
2.1 Dispatcher源码分析
Dispatch主要用于控制并发的请求，它主要维护了以下变量：
/** 最大并发请求数 */
private int maxRequests = 64;
/** 最大主机请求数 */
private int maxRequestsPerHost = 5;
private Runnable idleCallback;
/** 消费者线程池 */
private ExecutorService executorService;
/** 准备运行的异步请求队列 */
private final Deque<RealCall.AsyncCall> readyAsyncCalls = new ArrayDeque<>();
/** 正在运行的异步请求队列 */
private final Deque<RealCall.AsyncCall> runningAsyncCalls = new ArrayDeque<>();
/** 正在运行的同步请求队列 */
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
Dispatcher的构造方法：
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;
}
默认线程池，核心线程数为0，最大线程数为整型最大值，空闲时间为60s。
当调用RealCall的enqueue方法时，实际上是调用Dispatch的enqueue方法：
synchronized void enqueue(AsyncCall call) {
if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
runningAsyncCalls.add(call);
executorService().execute(call);
} else {
readyAsyncCalls.add(call);
}
}
当正在运行的异步请求队列中的数量小于64并且正在运行的请求主机数小于5时，把请求加载到runningAsynCalls中并在线程池中执行，否则就加入到readyAsyncCalls中进行缓存等待。线程池中传进来的参数是AsyncCall，它是RealCall内部类，其内部也实现了execute方法
@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);
}
}
在上面代码的finally中，可以看出无论这个请求的结果如何，都会执行client.dispatcher().finished(this)，finished方法如下：
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();
}
}
finished方法将此次请求从runningAsyncCalls移除后还执行了promoteCalls方法：
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.
}
}
最关键的一点就是会从readyAsyncCalls取出下一个请求，加入runningAsyncCalls中并交由线程池处理。
回到AsyncCall的execute方法：
@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);
}
}
从上面的代码看出，getResponseWithInterceptorChain方法返回了Response，这里是真正请求网络的地方。
2.2 回答一些问题
- OkHttp如何实现同步异步请求的？
发送的同步/异步请求都会在dispatcher中管理器状态 - 到底什么是Dispatcher
Dispatcher的作用是维护请求的状态，并维护一个线程池，用于执行请求 - 异步请求为什么需要两个队列
Dispatcher：生产者
ExecutorService：消费者池 Deque:缓存 Deque:正在运行的任务 - Call执行完肯定需要在runningAsyncCalls队列中移除这个线程，那么readyAsyncCalls队列中的线程在什么时候才会被执行呢
从上面的源码分析中可以看到，无论网络请求是否成功，都会在finally 代码块中执行dispatcher的finished方法，然后再finished方法中判断，是否满足最大并发数小于等于64，最大主机请求数小于等于5的条件，如果满足，则从等待队列移除，加入到运行队列中，并执行这个请求。
3 Interceptor拦截器
拦截器是一种能够监控、重写、重试调用的机制。通常情况下，拦截器用来添加、移除、转换请求和响应的头部信息。比如将域名替换为IP地址，在请求头中添加host属性。
3.1 getResponseWithInterceptorChain
getResponseWithInterceptorChain的代码：
private Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
List<Interceptor> interceptors = new ArrayList<>();
interceptors.addAll(client.interceptors());
interceptors.add(retryAndFollowUpInterceptor);
interceptors.add(new BridgeInterceptor(client.cookieJar()));
interceptors.add(new CacheInterceptor(client.internalCache()));
interceptors.add(new ConnectInterceptor(client));
if (!retryAndFollowUpInterceptor.isForWebSocket()) {
interceptors.addAll(client.networkInterceptors());
}
interceptors.add(new CallServerInterceptor(
retryAndFollowUpInterceptor.isForWebSocket()));
Interceptor.Chain chain = new RealInterceptorChain(
interceptors, null, null, null, 0, originalRequest);
return chain.proceed(originalRequest);
}
从这里可以看出，构建里一个拦截器的列表，并且执行了RealInterceptorChain.proceed方法，chain.proceed方法代码：
public Response proceed(Request request, StreamAllocation streamAllocation, HttpStream httpStream,
Connection connection) throws IOException {
...
// Call the next interceptor in the chain.
RealInterceptorChain next = new RealInterceptorChain(
interceptors, streamAllocation, httpStream, connection, index + 1, request);
Interceptor interceptor = interceptors.get(index);
Response response = interceptor.intercept(next);
...
return response;
}
proceed方法每次从拦截器列表中取出拦截器。当存在多个拦截器时，会等待下一个拦截器的调用返回。
proceed方法会依次执行拦截器的intercept方法，以下是各个拦截器的源码分析。
3.2 RetryAndFollowUpInterceptor
RetryAndFollowUpInterceptor的intercept方法：
@Override public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
streamAllocation = new StreamAllocation(
client.connectionPool(), createAddress(request.url()));
int followUpCount = 0;
Response priorResponse = null;
while (true) {
...
response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);
releaseConnection = false;
...
// 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 = followUpRequest(response);
...
if (++followUpCount > MAX_FOLLOW_UPS) {
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
...
request = followUp;
priorResponse = response;
}
}
RetryAndFollowUpInterceptor主要做了以下几件事：
- 创建StreamAllocation对象
- 调用((RealInterceptorChain) chain).proceed方法，执行下一个拦截器的操作
- 根据异常结果或者响应结果判断是否进行重新请求，重新请求最大次数为20次。
- 获取到下一次拦截器的response后，对response进行处理，并返回给上一个拦截器
3.3 BridgeInterceptor
BridgeInterceptor的intercept方法：
@Override public Response intercept(Chain chain) throws IOException {
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();
RequestBody body = userRequest.body();
if (body != null) {
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");
}
}
...
if (userRequest.header("User-Agent") == null) {
requestBuilder.header("User-Agent", Version.userAgent());
}
Response networkResponse = chain.proceed(requestBuilder.build());
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);
responseBuilder.body(new RealResponseBody(strippedHeaders, Okio.buffer(responseBody)));
}
return responseBuilder.build();
}
BridgeInterceptor主要做了以下几件事：
- 负责将用户构建的一个Request请求转化为能够进行网络访问的请求，如添加请求头等相关信息
- 将这个符合网络请求的Request提交给下一个拦截器进行网络请求
- 将网络请求回来的响应Response转化为用户可用的Response，如Gzip解压等
3.3 CacheInterceptor
CacheInterceptor的intercept方法：
@Override public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
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.
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(EMPTY_BODY)
.sentRequestAtMillis(-1L)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
}
// If we don't need the network, we're done.
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
Response networkResponse = null;
try {
networkResponse = chain.proceed(networkRequest);
} 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 (validate(cacheResponse, networkResponse)) {
Response response = cacheResponse.newBuilder()
.headers(combine(cacheResponse.headers(), networkResponse.headers()))
.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());
}
}
CacheInterceptor主要做了以下几件事：
- 通过Cache获取缓存，Cache类是进行缓存的读取和存取操作的，Cache以url的MD5为key，只缓存GET请求，通过DiskLruCache类，进行缓存。
- 根据CacheStrategy处理得到的networkRequest和cacheResponse进行判断处理，如果两个都为null，也就是不进行网络请求并且缓存不存在networkRequest或者过期，则返回504错误；当只有networkRequest为null时也就是不进行网络请求，如果有有效缓存则直接返回缓存；其他情况则请求网络
- 在判断为需要请求网络的时候，将请求传递给下一个拦截器进行网络请求
- 解析HTTP响应报头，如果有缓存并且可用，则用缓存的数据并更新缓存，否则就用网络请求返回的数据。
如何判断缓存是否有效？如果缓存有效的，服务器则返回304 Not Modified，否则直接返回body。如果缓存过期或者强制放弃缓存，则缓存策略全部交给服务器判断，客户端只需要发送条件GET请求即可。条件GET请求有两种方式：一种是Last-Modified-Date，另一种是ETag。这里采用了Last-Modified-Date，通过缓存和网络请求响应中的Last-Modified来计算是否是最新数据。如果是，则缓存有效。
3.4 ConnectInterceptor
ConnectInterceptor的intercept方法：
@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");
HttpStream httpStream = streamAllocation.newStream(client, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();
return realChain.proceed(request, streamAllocation, httpStream, connection);
}
ConnectInterceptor主要做了两件事：
- ConnectInterceptor获取Interceptor传过来的StreamAllocation，通过streamAllocation.newStream()方法获取到HttpStream,HttpStream是用来处理request和response的
- 将刚才创建的用于网络IO的RealConnection对象，以及对于与服务器交互最为关键的HttpStream等对象传递给后面的拦截器
接下来看streamAllocation.newStream方法：
public HttpStream newStream(OkHttpClient client, boolean doExtensiveHealthChecks) {
...
RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
writeTimeout, connectionRetryEnabled, doExtensiveHealthChecks);
HttpStream resultStream;
...
resultStream = new Http2xStream(client, this, resultConnection.framedConnection);
...
resultStream = new Http1xStream(
client, this, resultConnection.source, resultConnection.sink);
...
return resultStream;
...
}
newStream方法就是通过findHealthyConnection方法获取到健康的连接RealConnection，和根据Http1.0还是Http2.0协议创建不同的HttpStream。
下面是findHealthyConnection方法：
/**
* Finds a connection and returns it if it is healthy. If it is unhealthy the process is repeated
* until a healthy connection is found.
*/
private RealConnection findHealthyConnection(int connectTimeout, int readTimeout,
int writeTimeout, boolean connectionRetryEnabled, boolean doExtensiveHealthChecks)
throws IOException {
while (true) {
RealConnection candidate = findConnection(connectTimeout, readTimeout, writeTimeout,
connectionRetryEnabled);
// If this is a brand new connection, we can skip the extensive health checks.
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.
if (!candidate.isHealthy(doExtensiveHealthChecks)) {
noNewStreams();
continue;
}
return candidate;
}
}
findHealthyConnection通过findConnection方法循环遍历连接池，如果是新的连接，则直接使用此连接，如果是健康的连接，比如socket被关闭，或者输入输出流被关闭，则释放资源，继续遍历连接池。
下面是findConnection方法：
/**
* Returns a connection to host a new stream. This prefers the existing connection if it exists,
* then the pool, finally building a new connection.
*/
private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
boolean connectionRetryEnabled) throws IOException {
...
// Attempt to get a connection from the pool.
RealConnection pooledConnection = Internal.instance.get(connectionPool, address, this);
if (pooledConnection != null) {
this.connection = pooledConnection;
return pooledConnection;
}
...
RealConnection newConnection = new RealConnection(selectedRoute);
acquire(newConnection);
synchronized (connectionPool) {
Internal.instance.put(connectionPool, newConnection);
this.connection = newConnection;
if (canceled) throw new IOException("Canceled");
}
newConnection.connect(connectTimeout, readTimeout, writeTimeout, address.connectionSpecs(),
connectionRetryEnabled);
routeDatabase().connected(newConnection.route());
return newConnection;
}
findConnection首先会去尝试从连接此中，根据主机地址，尝试复用连接，如果不存在能复用的连接，则创建新的连接，放入连接池中，然后新的连接进行connect操作，创建Tunneled连接或者Socket连接，通过Okio进行流的操作。也就是说，OkHttp为了解决TCP3次握手4次挥手的效率问题，通过HTTP的keepalive connections的机制，使用了连接池复用连接。
3.5 OkHttp复用连接池
前面connectionPool复用连接池，现在看下connectionPool的具体代码
/**
* Background threads are used to cleanup expired connections. There will be at most a single
* thread running per connection pool. The thread pool executor permits the pool itself to be
* garbage collected.
*/
private static final Executor executor = new ThreadPoolExecutor(0 /* corePoolSize */,
Integer.MAX_VALUE /* maximumPoolSize */, 60L /* keepAliveTime */, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp ConnectionPool", true));
/** The maximum number of idle connections for each address. */
private final int maxIdleConnections;
private final long keepAliveDurationNs;
private final Deque<RealConnection> connections = new ArrayDeque<>();
final RouteDatabase routeDatabase = new RouteDatabase();
boolean cleanupRunning;
连接池的主要变量：
- executor线程池，用于连接池的清理回收
- Deque，双向队列，双端队列同时具有队列和栈性质，经常在缓存中被使用，里面维护了RealConnection也就是socket物理连接的包装
- RouteDatabase，用来记录连接失败的路线名单，当连接失败的时候就会把失败的线路加进去。
以下是ConnectionPool的构造方法：
/**
* Create a new connection pool with tuning parameters appropriate for a single-user application.
* The tuning parameters in this pool are subject to change in future OkHttp releases. Currently
* this pool holds up to 5 idle connections which will be evicted after 5 minutes of inactivity.
*/
public ConnectionPool() {
this(5, 5, TimeUnit.MINUTES);
}
public ConnectionPool(int maxIdleConnections, long keepAliveDuration, TimeUnit timeUnit) {
this.maxIdleConnections = maxIdleConnections;
this.keepAliveDurationNs = timeUnit.toNanos(keepAliveDuration);
// Put a floor on the keep alive duration, otherwise cleanup will spin loop.
if (keepAliveDuration <= 0) {
throw new IllegalArgumentException("keepAliveDuration <= 0: " + keepAliveDuration);
}
}
通过构造方法可以看出ConnectionPool默认空闲的socket最大连接数为5个，socket的keepAlive时间为5分钟。ConnectionPool是在OkHttpClient实例化时创建。
3.5.1 ConnectionPool的put方法：
void put(RealConnection connection) {
assert (Thread.holdsLock(this));
if (!cleanupRunning) {
cleanupRunning = true;
executor.execute(cleanupRunnable);
}
connections.add(connection);
}
先从连接池中清理回收空闲连接，再添加连接。
3.5.2 ConnectionPool的get方法：
/** Returns a recycled connection to {@code address}, or null if no such connection exists. */
RealConnection get(Address address, StreamAllocation streamAllocation) {
assert (Thread.holdsLock(this));
for (RealConnection connection : connections) {
if (connection.allocations.size() < connection.allocationLimit
&& address.equals(connection.route().address)
&& !connection.noNewStreams) {
streamAllocation.acquire(connection);
return connection;
}
}
return null;
}
遍历connections缓存列表。当某个连接计数的次数小于限制的大小，并且request的地址和缓存列表中此连接的地址完全匹配时，则直接复用缓存列表中的connection作为request的连接。
3.5.3 ConnectionPool的cleanupRunnable：
private final Runnable cleanupRunnable = new Runnable() {
@Override public void run() {
while (true) {
long waitNanos = cleanup(System.nanoTime());
if (waitNanos == -1) return;
if (waitNanos > 0) {
long waitMillis = waitNanos / 1000000L;
waitNanos -= (waitMillis * 1000000L);
synchronized (ConnectionPool.this) {
try {
ConnectionPool.this.wait(waitMillis, (int) waitNanos);
} catch (InterruptedException ignored) {
}
}
}
}
}
};
线程不断地调用cleanup方法来进行清理，并返回下次需要清理的间隔时间，然后调用wait方法进行等待一释放锁与时间片。当等待时间到了后，再次进行清理，并返回下次要清理的间隔时间。如此循环下去。
3.5.4 ConnectionPool的cleanup方法：
/**
* Performs maintenance on this pool, evicting the connection that has been idle the longest if
* either it has exceeded the keep alive limit or the idle connections limit.
*
* <p>Returns the duration in nanos to sleep until the next scheduled call to this method. Returns
* -1 if no further cleanups are required.
*/
long cleanup(long now) {
int inUseConnectionCount = 0;
int idleConnectionCount = 0;
RealConnection longestIdleConnection = null;
long longestIdleDurationNs = Long.MIN_VALUE;
// Find either a connection to evict, or the time that the next eviction is due.
synchronized (this) {
for (Iterator<RealConnection> i = connections.iterator(); i.hasNext(); ) {
RealConnection connection = i.next();
// If the connection is in use, keep searching.
if (pruneAndGetAllocationCount(connection, now) > 0) {
inUseConnectionCount++;
continue;
}
idleConnectionCount++;
// If the connection is ready to be evicted, we're done.
long idleDurationNs = now - connection.idleAtNanos;
if (idleDurationNs > longestIdleDurationNs) {
longestIdleDurationNs = idleDurationNs;
longestIdleConnection = connection;
}
}
if (longestIdleDurationNs >= this.keepAliveDurationNs
|| idleConnectionCount > this.maxIdleConnections) {
// We've found a connection to evict. Remove it from the list, then close it below (outside
// of the synchronized block).
connections.remove(longestIdleConnection);
} else if (idleConnectionCount > 0) {
// A connection will be ready to evict soon.
return keepAliveDurationNs - longestIdleDurationNs;
} else if (inUseConnectionCount > 0) {
// All connections are in use. It'll be at least the keep alive duration 'til we run again.
return keepAliveDurationNs;
} else {
// No connections, idle or in use.
cleanupRunning = false;
return -1;
}
}
cleanup方法主要做了以下几件事：
- 根据连接中的引用计数来计算空闲连接和活跃连接数，然后标记出空闲的连接
- 如果空闲连接keepAlive时间超过5分钟，或者空闲连接数超过5个，则从Deque中移除此连接
- 根据空闲连接或者活跃连接来返回下次需要清理的时间数：如果空闲连接大于0，则返回此连接即将到期的时间；如果都是活跃连接并且大于0，则返回默认的keepAlive时间5分钟；如果没有任何连接，则跳出循环并返回-1.
3.5.5 ConnectionPool的pruneAndGetAllocationCount方法：
/**
* Prunes any leaked allocations and then returns the number of remaining live allocations on
* {@code connection}. Allocations are leaked if the connection is tracking them but the
* application code has abandoned them. Leak detection is imprecise and relies on garbage
* collection.
*/
private int pruneAndGetAllocationCount(RealConnection connection, long now) {
List<Reference<StreamAllocation>> references = connection.allocations;
for (int i = 0; i < references.size(); ) {
Reference<StreamAllocation> reference = references.get(i);
if (reference.get() != null) {
i++;
continue;
}
// We've discovered a leaked allocation. This is an application bug.
Platform.get().log(WARN, "A connection to " + connection.route().address().url()
+ " was leaked. Did you forget to close a response body?", null);
references.remove(i);
connection.noNewStreams = true;
// If this was the last allocation, the connection is eligible for immediate eviction.
if (references.isEmpty()) {
connection.idleAtNanos = now - keepAliveDurationNs;
return 0;
}
}
return references.size();
}
pruneAndGetAllocationCount方法首先遍历传进来的RealConnection的StreamAllocation列表。如果StreamAllocation = null，则从列表中移除。如果列比为空，则说明连接没有引用了，则返回0，表示此连接是空闲连接；否则返回列表的数量，表示连接的活跃数量。
3.5.6 引用计数
/**
* Use this allocation to hold {@code connection}. Each call to this must be paired with a call to
* {@link #release} on the same connection.
*/
public void acquire(RealConnection connection) {
connection.allocations.add(new WeakReference<>(this));
}
/** Remove this allocation from the connection's list of allocations. */
private void release(RealConnection connection) {
for (int i = 0, size = connection.allocations.size(); i < size; i++) {
Reference<StreamAllocation> reference = connection.allocations.get(i);
if (reference.get() == this) {
connection.allocations.remove(i);
return;
}
}
throw new IllegalStateException();
}
OkHttp回收连接使用了类似GC的引用计数算法，跟踪socket流的调用。这里的计数对象是StreamAllocation，它被反复执行acquire与release操作，这两个方法其实是在改变RealConnection中的List<Reference>的大小。
RealConnection是socket物理连接的包装，里面维护了List<Reference>的引用。List中StreamAllocation的数量也就是socket被引用的计数。如果计数为0，则表示连接没有被使用，空闲状态，需要回收；如果不为0，则表示仍在引用，不能关闭连接。
3.5.7 小结
连接池复用的核心就是用Deque来存储连接，通过put、get、connectionBecameIdle和evictAll几个操作来对Deque进行操作，另外通过判断连接中的计数对象StreamAllocation来进行自动回收连接。
3.6 CallServerInterceptor
CallServerInterceptor的intercept方法：
@Override public Response intercept(Chain chain) throws IOException {
HttpStream httpStream = ((RealInterceptorChain) chain).httpStream();
StreamAllocation streamAllocation = ((RealInterceptorChain) chain).streamAllocation();
Request request = chain.request();
long sentRequestMillis = System.currentTimeMillis();
httpStream.writeRequestHeaders(request);
if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
Sink requestBodyOut = httpStream.createRequestBody(request, request.body().contentLength());
BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
request.body().writeTo(bufferedRequestBody);
bufferedRequestBody.close();
}
httpStream.finishRequest();
Response response = httpStream.readResponseHeaders()
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
if (!forWebSocket || response.code() != 101) {
response = response.newBuilder()
.body(httpStream.openResponseBody(response))
.build();
}
if ("close".equalsIgnoreCase(response.request().header("Connection"))
|| "close".equalsIgnoreCase(response.header("Connection"))) {
streamAllocation.noNewStreams();
}
int code = response.code();
if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
throw new ProtocolException(
"HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
}
return response;
}
主要是根据前面拦截器处理的HttpStream、StreamAllocation以及Request进行实际的网络请求，以及获取response。
4. OkHttp网络请求过程的总结
- Call对象对请求进行封装
- dispatcher对请求进行分发
- getResponseWithInterceptors()方法，创建拦截器链
- 执行RetryAndFollowUpInterceptor，对请求进行重试重定向
- 执行BridgeInterceptor，对我们使用的OkHttp的request和response与HTTP协议中的request和response进行转换
- 执行CacheInterceptor，对请求进行判断，是否需要使用缓存
- 执行ConnectInterceptor，负责建立连接和流对象
- 执行CallServerInterceptor，负责最终实际的网络请求，主要进行发送请求和读取响应的操作。
参考文献：
Android进阶之光
