okhttp实现连接池原理

为啥需要

对于tcp/ip的网络请求，是短连接请求，每次理论上是需要三次握手和四次挥手的。频繁的进行建立Sokcet连接和断开Socket是非常消耗网络资源和浪费时间的。

然后HTTP的keep-alive也是用来优化的连接的。

普通的HTTP请求是客户端连接到服务端了，请求结束后关闭连接。流程是反复的。
带keep-alive头HTTP请求，服务端接受到这个字段，在一定的时间内，会维持这次连接，这期间的请求不会再次连接，而是直接复用了。

由此可见网络请求的优化是至关重要的。而okhttp则是采用连接池进行复用，减少连接的创建和关闭，增加系统负载能力。

如何实现

先说结论：OkHttp里面使用ConnectionPool实现连接池，而且默认支持5个并发KeepAlive，默认链路生命为5分钟。

先看初始化位置，okhttp采用的构建者模式创建的，其中build方法有如下：

 public Builder() {dispatcher = new Dispatcher();protocols = DEFAULT_PROTOCOLS;connectionSpecs = DEFAULT_CONNECTION_SPECS;eventListenerFactory = EventListener.factory(EventListener.NONE);proxySelector = ProxySelector.getDefault();if (proxySelector == null) {proxySelector = new NullProxySelector();}cookieJar = CookieJar.NO_COOKIES;socketFactory = SocketFactory.getDefault();hostnameVerifier = OkHostnameVerifier.INSTANCE;certificatePinner = CertificatePinner.DEFAULT;proxyAuthenticator = Authenticator.NONE;authenticator = Authenticator.NONE;//创建默认连接池，也可以指定其他参数connectionPool = new ConnectionPool();dns = Dns.SYSTEM;followSslRedirects = true;followRedirects = true;retryOnConnectionFailure = true;callTimeout = 0;connectTimeout = 10_000;readTimeout = 10_000;writeTimeout = 10_000;pingInterval = 0;}

跟进看ConnectionPool构造方法：

  /*** 使用适合于单用户应用程序的调整参数创建一个新的连接池。 *此池中的调整参数可能会在将来的OkHttp版本中更改。当前*该池最多可容纳5个空闲连接，这些空闲连接在闲置5分钟后将被驱逐。**/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);}}

构造中默认设置了并发数量为5个，默认存活时间为5分钟。

再看下类中静态的变量，随着类创建一起创建的，如下：

/*** 后台线程用于清除过期的连接。每个连接池最多只能运行一个*线程。线程池执行程序允许对池本身进行*垃圾收集。*/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));

这里就是创建了一个线程池，只会执行一种线程，是用来执行连接池清理任务的。任务执行内容等会再看。
类中还有个关键的全局变量，双端队列Deque，双端都能进出，用来存储连接的：

  private final Deque<RealConnection> connections = new ArrayDeque<>();

再来说下是如何实现的：

put函数：

void put(RealConnection connection) {assert (Thread.holdsLock(this));if (!cleanupRunning) {//没有连接的时候调用cleanupRunning = true;executor.execute(cleanupRunnable);}connections.add(connection);}

RealConnection是双端队列connections存放的连接，先判断是否在锁状态后，继续判断是否在清理状态，cleanupRunning首先初始化是false值，也是无连接时候。执行cleanupRunnable只会在无连接、空闲或者使用时才会把cleanupRunning置成false，因为放入的连接后，连接池可能超出最大规定容量或者存在存活时间超时情况，所以要执行cleanupRunnable。最后会将这次连接放入双端队列中。

get函数

 @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;}

遍历队列集合，获取匹配且可用的连接并返回，没有则返回null。

cleanup函数
前面提到的线程池来执行的任务，就是不停的调用cleanupRunnable来清楚线程池。
先看Runnable执行代码块：

private final Runnable cleanupRunnable = new Runnable() {@Override public void run() {while (true) {//等待时间long waitNanos = cleanup(System.nanoTime());if (waitNanos == -1) return;if (waitNanos > 0) {long waitMillis = waitNanos / 1000000L;waitNanos -= (waitMillis * 1000000L);synchronized (ConnectionPool.this) {try {//线程开始等待休眠，下一次调用会唤醒ConnectionPool.this.wait(waitMillis, (int) waitNanos);} catch (InterruptedException ignored) {}}}}}};

cleanup方法会返回休眠时间，让出cpu节省资源。
看cleanup方法：

  /*** 在此池上执行维护，如果超过了“保持活动”限制或“空闲连接”限制，则驱逐最长的空闲连接。  <p>以nanos为单位返回睡眠时间，直到下一次计划调用此方法为止。如果不需要进一步清理，则返回-1* -1 不需要进一步处理*/long cleanup(long now) {int inUseConnectionCount = 0;int idleConnectionCount = 0;RealConnection longestIdleConnection = null;long longestIdleDurationNs = Long.MIN_VALUE;// 找到驱逐的连接，或下一次驱逐的时间synchronized (this) {for (Iterator<RealConnection> i = connections.iterator(); i.hasNext(); ) {RealConnection connection = i.next();// 如果连接在使用，则跳过继续寻找，inUseConnectionCount++if (pruneAndGetAllocationCount(connection, now) > 0) {//使用计数inUseConnectionCount++;continue;}//空闲计数idleConnectionCount++;// 如果准备好退出连接，那么我们就完成了。long idleDurationNs = now - connection.idleAtNanos;if (idleDurationNs > longestIdleDurationNs) {longestIdleDurationNs = idleDurationNs;longestIdleConnection = connection;}}//上面是找到了最长空闲时间连接//keepAliveDurationNs  是构造定义的最大维持存活时间//maxIdleConnections  是构造定义的最大存活数量if (longestIdleDurationNs >= this.keepAliveDurationNs|| idleConnectionCount > this.maxIdleConnections) {// 符合条件，就从队列移除connections.remove(longestIdleConnection);} else if (idleConnectionCount > 0) {// 如果还有连接未清除，那么现在的线程的等待休眠时间return keepAliveDurationNs - longestIdleDurationNs;} else if (inUseConnectionCount > 0) {// 所有连接都在使用中。直到我们再次运行，这至少是保持生命的持续时间。return keepAliveDurationNs;} else {// 没有连接在使用cleanupRunning = false;return -1;}}//真正的关闭socket连接closeQuietly(longestIdleConnection.socket());// Cleanup again immediately.return 0;}

那如何判断是闲置连接了？

再来看下get函数中的一句代码：streamAllocation.acquire(connection, true);

//使用此分配来保存connection。每个对此的调用必须与同一连接上对{@link #release}的调用配对。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));}

acquire函数，做的就是add操作。
对比看下release函数，其实是allocations移除StreamAllocationReference操作

 /** 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();}

其中StreamAllocationReference是弱引用，RealConnection中，有一个StreamAllocation引用列表allocations。每创建一个连接，就会把连接对应的StreamAllocationReference添加进该列表中，如果连接关闭以后就将该对象移除。这样可以根据弱引用的堆栈回收信息来计数。
再看cleanup函数中，确定计数的函数pruneAndGetAllocationCount

 /***整理所有泄漏的分配，然后返回* {@code connection}上剩余的实时分配数。如果连接正在跟踪分配，但是应用程序代码已放弃分配，则分配将泄漏。泄漏检测不精确，并且依赖于垃圾收集。*/private int pruneAndGetAllocationCount(RealConnection connection, long now) {//拿到了上文提到的引用列表 allocationsList<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.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);//被回收了或者一些其他情况，导致StreamAllocation为null，然后从集合移除此引用references.remove(i);connection.noNewStreams = true;// 如果这是最后一次分配，则该连接可以立即驱逐。if (references.isEmpty()) {connection.idleAtNanos = now - keepAliveDurationNs;return 0;}}//返回计数的sizereturn references.size();}

这里配合前面的弱引用，完成计数。
其实可以这样理解，在上层反复调用acquire和release函数，来增加或减少connection.allocations所维持的集合的大小，到最后如果size大于0，则代表RealConnection还在使用连接，如果size等于0，那就说明已经处于空闲状态了。

怎么获取？

get函数怎么调用的，其实很简单，往上找代码就行了，简单说说。

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text{fill:#9370db;stroke:none;font-size:10px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-gwgrrvtRb7RPdgau g.stateGroup text{fill:#9370db;fill:#333;stroke:none;font-size:10px}#mermaid-svg-gwgrrvtRb7RPdgau g.statediagram-cluster .cluster-label text{fill:#333}#mermaid-svg-gwgrrvtRb7RPdgau g.stateGroup .state-title{font-weight:bolder;fill:#000}#mermaid-svg-gwgrrvtRb7RPdgau g.stateGroup rect{fill:#ECECFF;stroke:#9370db}#mermaid-svg-gwgrrvtRb7RPdgau g.stateGroup line{stroke:#9370db;stroke-width:1}#mermaid-svg-gwgrrvtRb7RPdgau .transition{stroke:#9370db;stroke-width:1;fill:none}#mermaid-svg-gwgrrvtRb7RPdgau .stateGroup .composit{fill:white;border-bottom:1px}#mermaid-svg-gwgrrvtRb7RPdgau .stateGroup .alt-composit{fill:#e0e0e0;border-bottom:1px}#mermaid-svg-gwgrrvtRb7RPdgau .state-note{stroke:#aa3;fill:#fff5ad}#mermaid-svg-gwgrrvtRb7RPdgau .state-note text{fill:black;stroke:none;font-size:10px}#mermaid-svg-gwgrrvtRb7RPdgau .stateLabel .box{stroke:none;stroke-width:0;fill:#ECECFF;opacity:0.7}#mermaid-svg-gwgrrvtRb7RPdgau .edgeLabel text{fill:#333}#mermaid-svg-gwgrrvtRb7RPdgau .stateLabel text{fill:#000;font-size:10px;font-weight:bold;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-gwgrrvtRb7RPdgau .node circle.state-start{fill:black;stroke:black}#mermaid-svg-gwgrrvtRb7RPdgau .node circle.state-end{fill:black;stroke:white;stroke-width:1.5}#mermaid-svg-gwgrrvtRb7RPdgau #statediagram-barbEnd{fill:#9370db}#mermaid-svg-gwgrrvtRb7RPdgau .statediagram-cluster rect{fill:#ECECFF;stroke:#9370db;stroke-width:1px}#mermaid-svg-gwgrrvtRb7RPdgau .statediagram-cluster rect.outer{rx:5px;ry:5px}#mermaid-svg-gwgrrvtRb7RPdgau .statediagram-state .divider{stroke:#9370db}#mermaid-svg-gwgrrvtRb7RPdgau .statediagram-state .title-state{rx:5px;ry:5px}#mermaid-svg-gwgrrvtRb7RPdgau .statediagram-cluster.statediagram-cluster .inner{fill:white}#mermaid-svg-gwgrrvtRb7RPdgau .statediagram-cluster.statediagram-cluster-alt .inner{fill:#e0e0e0}#mermaid-svg-gwgrrvtRb7RPdgau .statediagram-cluster .inner{rx:0;ry:0}#mermaid-svg-gwgrrvtRb7RPdgau .statediagram-state rect.basic{rx:5px;ry:5px}#mermaid-svg-gwgrrvtRb7RPdgau .statediagram-state rect.divider{stroke-dasharray:10,10;fill:#efefef}#mermaid-svg-gwgrrvtRb7RPdgau .note-edge{stroke-dasharray:5}#mermaid-svg-gwgrrvtRb7RPdgau .statediagram-note rect{fill:#fff5ad;stroke:#aa3;stroke-width:1px;rx:0;ry:0}:root{--mermaid-font-family: '"trebuchet ms", verdana, arial';--mermaid-font-family: "Comic Sans MS", "Comic Sans", cursive}#mermaid-svg-gwgrrvtRb7RPdgau .error-icon{fill:#522}#mermaid-svg-gwgrrvtRb7RPdgau .error-text{fill:#522;stroke:#522}#mermaid-svg-gwgrrvtRb7RPdgau .edge-thickness-normal{stroke-width:2px}#mermaid-svg-gwgrrvtRb7RPdgau .edge-thickness-thick{stroke-width:3.5px}#mermaid-svg-gwgrrvtRb7RPdgau .edge-pattern-solid{stroke-dasharray:0}#mermaid-svg-gwgrrvtRb7RPdgau .edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-gwgrrvtRb7RPdgau .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-gwgrrvtRb7RPdgau .marker{fill:#333}#mermaid-svg-gwgrrvtRb7RPdgau .marker.cross{stroke:#333}:root { --mermaid-font-family: "trebuchet ms", verdana, arial;}#mermaid-svg-gwgrrvtRb7RPdgau {color: rgba(0, 0, 0, 0.75);font: ;}

发送call请求

Dispatcher分发

拦截器ConnectInterceptor处理

streamAllocation.newStream

findHealthyConnection

findConnection

Internal.instance.get

pool.get