OKHttp3源碼解析系列

• OkHttp3源碼解析(一)之請求流程
• OkHttp3源碼解析(二)——攔截器鏈和緩存策略

本文基于OkHttp3的3.11.0版本

implementation 'com.squareup.okhttp3:okhttp:3.11.0' 復制代碼

我們已經分析了OkHttp3的攔截器鏈和緩存策略，今天我們再來看看OkHttp3的連接池復用。

客戶端和服務器建立socket連接需要經歷TCP的三次握手和四次揮手，是一種比較消耗資源的動作。Http中有一種keepAlive connections的機制，在和客戶端通信結束以后可以保持連接指定的時間。OkHttp3支持5個并發socket連接，默認的keepAlive時間為5分鐘。下面我們來看看OkHttp3是怎么實現連接池復用的。

OkHttp3的連接池--ConnectionPool

public final class ConnectionPool {//線程池，用于執行清理空閑連接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));//最大的空閑socket連接數private final int maxIdleConnections;//socket的keepAlive時間private final long keepAliveDurationNs;private final Deque<RealConnection> connections = new ArrayDeque<>();final RouteDatabase routeDatabase = new RouteDatabase();boolean cleanupRunning; } 復制代碼

ConnectionPool里的幾個重要變量：

（1）executor線程池，類似于CachedThreadPool，用于執行清理空閑連接的任務。

（2）Deque雙向隊列，同時具有隊列和棧的性質，經常在緩存中被使用，里面維護的RealConnection是socket物理連接的包裝

（3）RouteDatabase，用來記錄連接失敗的路線名單

下面看看ConnectionPool的構造函數

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);} } 復制代碼

從構造函數中可以看出，ConnectionPool的默認空閑連接數為5個，keepAlive時間為5分鐘。ConnectionPool是什么時候被創建的呢？是在OkHttpClient的builder中：

public static final class Builder {...ConnectionPool connectionPool;...public Builder() {...connectionPool = new ConnectionPool();...}//我們也可以定制連接池public Builder connectionPool(ConnectionPool connectionPool) {if (connectionPool == null) throw new NullPointerException("connectionPool == null");this.connectionPool = connectionPool;return this;} } 復制代碼

緩存操作：添加、獲取、回收連接

（1）從緩存中獲取連接

//ConnectionPool.class @Nullable RealConnection get(Address address, StreamAllocation streamAllocation, Route route) {assert (Thread.holdsLock(this));for (RealConnection connection : connections) {if (connection.isEligible(address, route)) {streamAllocation.acquire(connection, true);return connection;}}return null; } 復制代碼

獲取連接的邏輯比較簡單，就遍歷連接池里的連接connections，然后用RealConnection的isEligible方法找到符合條件的連接，如果有符合條件的連接則復用。需要注意的是，這里還調用了streamAllocation的acquire方法。acquire方法的作用是對RealConnection引用的streamAllocation進行計數，OkHttp3是通過RealConnection的StreamAllocation的引用計數是否為0來實現自動回收連接的。

//StreamAllocation.class public void acquire(RealConnection connection, boolean reportedAcquired) {assert (Thread.holdsLock(connectionPool));if (this.connection != null) throw new IllegalStateException();this.connection = connection;this.reportedAcquired = reportedAcquired;connection.allocations.add(new StreamAllocationReference(this, callStackTrace)); }public static final class StreamAllocationReference extends WeakReference<StreamAllocation> {public final Object callStackTrace;StreamAllocationReference(StreamAllocation referent, Object callStackTrace) {super(referent);this.callStackTrace = callStackTrace;} } 復制代碼//RealConnection.class public final List<Reference<StreamAllocation>> allocations = new ArrayList<>(); 復制代碼

每一個RealConnection中都有一個allocations變量，用于記錄對于StreamAllocation的引用。StreamAllocation中包裝有HttpCodec，而HttpCodec里面封裝有Request和Response讀寫Socket的抽象。每一個請求Request通過Http來請求數據時都需要通過StreamAllocation來獲取HttpCodec，從而讀取響應結果，而每一個StreamAllocation都是和一個RealConnection綁定的，因為只有通過RealConnection才能建立socket連接。所以StreamAllocation可以說是RealConnection、HttpCodec和請求之間的橋梁。

當然同樣的StreamAllocation還有一個release方法，用于移除計數，也就是將當前的StreamAllocation的引用從對應的RealConnection的引用列表中移除。

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(); } 復制代碼

（2）向緩存中添加連接

//ConnectionPool.class void put(RealConnection connection) {assert (Thread.holdsLock(this));if (!cleanupRunning) {cleanupRunning = true;executor.execute(cleanupRunnable);}connections.add(connection);} 復制代碼

添加連接之前會先調用線程池執行清理空閑連接的任務，也就是回收空閑的連接。

（3）空閑連接的回收

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) {}}}}} }; 復制代碼

cleanupRunnable中執行清理任務是通過cleanup方法來完成，cleanup方法會返回下次需要清理的間隔時間，然后會調用wait方法釋放鎖和時間片。等時間到了就再次進行清理。下面看看具體的清理邏輯：

long cleanup(long now) {//記錄活躍的連接數int inUseConnectionCount = 0;//記錄空閑的連接數int idleConnectionCount = 0;//空閑時間最長的連接RealConnection longestIdleConnection = null;long longestIdleDurationNs = Long.MIN_VALUE;synchronized (this) {for (Iterator<RealConnection> i = connections.iterator(); i.hasNext(); ) {RealConnection connection = i.next();//判斷連接是否在使用，也就是通過StreamAllocation的引用計數來判斷//返回值大于0說明正在被使用if (pruneAndGetAllocationCount(connection, now) > 0) {//活躍的連接數+1inUseConnectionCount++;continue;}//說明是空閑連接，所以空閑連接數+1idleConnectionCount++;//找出了空閑時間最長的連接，準備移除long idleDurationNs = now - connection.idleAtNanos;if (idleDurationNs > longestIdleDurationNs) {longestIdleDurationNs = idleDurationNs;longestIdleConnection = connection;}}if (longestIdleDurationNs >= this.keepAliveDurationNs|| idleConnectionCount > this.maxIdleConnections) {//如果空閑時間最長的連接的空閑時間超過了5分鐘//或是空閑的連接數超過了限制，就移除connections.remove(longestIdleConnection);} else if (idleConnectionCount > 0) {//如果存在空閑連接但是還沒有超過5分鐘//就返回剩下的時間，便于下次進行清理return keepAliveDurationNs - longestIdleDurationNs;} else if (inUseConnectionCount > 0) {//如果沒有空閑的連接，那就等5分鐘后再嘗試清理return keepAliveDurationNs;} else {//當前沒有任何連接，就返回-1，跳出循環cleanupRunning = false;return -1;}}closeQuietly(longestIdleConnection.socket());// Cleanup again immediately.return 0; } 復制代碼

下面我們看看判斷連接是否是活躍連接的pruneAndGetAllocationCount方法

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);//如果存在引用，就說明是活躍連接，就繼續看下一個StreamAllocationif (reference.get() != null) {i++;continue;}// We've discovered a leaked allocation. This is an application bug.//發現泄漏的引用，會打印日志StreamAllocation.StreamAllocationReference streamAllocRef =(StreamAllocation.StreamAllocationReference) reference;String message = "A connection to " + connection.route().address().url()+ " was leaked. Did you forget to close a response body?";Platform.get().logCloseableLeak(message, streamAllocRef.callStackTrace);//如果沒有引用，就移除references.remove(i);connection.noNewStreams = true;//如果列表為空，就說明此連接上沒有StreamAllocation引用了，就返回0，表示是空閑的連接if (references.isEmpty()) {connection.idleAtNanos = now - keepAliveDurationNs;return 0;}}//遍歷結束后，返回引用的數量，說明當前連接是活躍連接return references.size(); } 復制代碼

至此我們就分析完OkHttp3的連接池復用了。

總結

（1）OkHttp3中支持5個并發socket連接，默認的keepAlive時間為5分鐘，當然我們可以在構建OkHttpClient時設置不同的值。

（2）OkHttp3通過Deque來存儲連接，通過put、get等操作來管理連接。

（3）OkHttp3通過每個連接的引用計數對象StreamAllocation的計數來回收空閑的連接，向連接池添加新的連接時會觸發執行清理空閑連接的任務。清理空閑連接的任務通過線程池來執行。

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