OKHttp3源码解析系列

• OkHttp3源码解析(一)之请求流程
• OkHttp3源码解析(二)——拦截器链和缓存策略

本文基于OkHttp3的3.11.0版本

implementation 'com.squareup.okhttp3:okhttp:3.11.0'

我们已经分析了OkHttp3的拦截器链和缓存策略，今天我们再来看看OkHttp3的连接池复用。

客户端和服务器建立socket连接需要经历TCP的三次握手和四次挥手，是一种比较消耗资源的动作。Http中有一种keepAlive connections的机制，在和客户端通信结束以后可以保持连接指定的时间。OkHttp3支持5个并发socket连接，默认的keepAlive时间为5分钟。下面我们来看看OkHttp3是怎么实现连接池复用的。

OkHttp3的连接池--ConnectionPool

public final class ConnectionPool {
    
    //线程池，用于执行清理空闲连接
    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));
    //最大的空闲socket连接数
    private final int maxIdleConnections;
    //socket的keepAlive时间
    private final long keepAliveDurationNs;
    
    private final Deque<RealConnection> connections = new ArrayDeque<>();
    final RouteDatabase routeDatabase = new RouteDatabase();
    boolean cleanupRunning;
}

ConnectionPool里的几个重要变量：

（1）executor线程池，类似于CachedThreadPool，用于执行清理空闲连接的任务。

（2）Deque双向队列，同时具有队列和栈的性质，经常在缓存中被使用，里面维护的RealConnection是socket物理连接的包装

（3）RouteDatabase，用来记录连接失败的路线名单

下面看看ConnectionPool的构造函数

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);
    }
}

从构造函数中可以看出，ConnectionPool的默认空闲连接数为5个，keepAlive时间为5分钟。ConnectionPool是什么时候被创建的呢？是在OkHttpClient的builder中：

public static final class Builder {
    ...
    ConnectionPool connectionPool;
    ...
    public Builder() {
        ...
        connectionPool = new ConnectionPool();
        ...
    }
    
    //我们也可以定制连接池
    public Builder connectionPool(ConnectionPool connectionPool) {
        if (connectionPool == null) throw new NullPointerException("connectionPool == null");
        this.connectionPool = connectionPool;
        return this;
    }
}

缓存操作：添加、获取、回收连接

（1）从缓存中获取连接

//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;
}

获取连接的逻辑比较简单，就遍历连接池里的连接connections，然后用RealConnection的isEligible方法找到符合条件的连接，如果有符合条件的连接则复用。需要注意的是，这里还调用了streamAllocation的acquire方法。acquire方法的作用是对RealConnection引用的streamAllocation进行计数，OkHttp3是通过RealConnection的StreamAllocation的引用计数是否为0来实现自动回收连接的。

//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;
    }
}

//RealConnection.class
public final List<Reference<StreamAllocation>> allocations = new ArrayList<>();

每一个RealConnection中都有一个allocations变量，用于记录对于StreamAllocation的引用。StreamAllocation中包装有HttpCodec，而HttpCodec里面封装有Request和Response读写Socket的抽象。每一个请求Request通过Http来请求数据时都需要通过StreamAllocation来获取HttpCodec，从而读取响应结果，而每一个StreamAllocation都是和一个RealConnection绑定的，因为只有通过RealConnection才能建立socket连接。所以StreamAllocation可以说是RealConnection、HttpCodec和请求之间的桥梁。

当然同样的StreamAllocation还有一个release方法，用于移除计数，也就是将当前的StreamAllocation的引用从对应的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();
}

（2）向缓存中添加连接

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

添加连接之前会先调用线程池执行清理空闲连接的任务，也就是回收空闲的连接。

（3）空闲连接的回收

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) {
            }
          }
        }
      }
    }
};

cleanupRunnable中执行清理任务是通过cleanup方法来完成，cleanup方法会返回下次需要清理的间隔时间，然后会调用wait方法释放锁和时间片。等时间到了就再次进行清理。下面看看具体的清理逻辑：

long cleanup(long now) {
    //记录活跃的连接数
    int inUseConnectionCount = 0;
    //记录空闲的连接数
    int idleConnectionCount = 0;
    //空闲时间最长的连接
    RealConnection longestIdleConnection = null;
    long longestIdleDurationNs = Long.MIN_VALUE;

    synchronized (this) {
      for (Iterator<RealConnection> i = connections.iterator(); i.hasNext(); ) {
        RealConnection connection = i.next();

        //判断连接是否在使用，也就是通过StreamAllocation的引用计数来判断
        //返回值大于0说明正在被使用
        if (pruneAndGetAllocationCount(connection, now) > 0) {
            //活跃的连接数+1
            inUseConnectionCount++;
            continue;
        }
        //说明是空闲连接，所以空闲连接数+1
        idleConnectionCount++;

        //找出了空闲时间最长的连接，准备移除
        long idleDurationNs = now - connection.idleAtNanos;
        if (idleDurationNs > longestIdleDurationNs) {
          longestIdleDurationNs = idleDurationNs;
          longestIdleConnection = connection;
        }
      }

      if (longestIdleDurationNs >= this.keepAliveDurationNs
          || idleConnectionCount > this.maxIdleConnections) {
        //如果空闲时间最长的连接的空闲时间超过了5分钟
        //或是空闲的连接数超过了限制，就移除
        connections.remove(longestIdleConnection);
      } else if (idleConnectionCount > 0) {
        //如果存在空闲连接但是还没有超过5分钟
        //就返回剩下的时间，便于下次进行清理
        return keepAliveDurationNs - longestIdleDurationNs;
      } else if (inUseConnectionCount > 0) {
        //如果没有空闲的连接，那就等5分钟后再尝试清理
        return keepAliveDurationNs;
      } else {
        //当前没有任何连接，就返回-1，跳出循环
        cleanupRunning = false;
        return -1;
      }
    }

    closeQuietly(longestIdleConnection.socket());

    // Cleanup again immediately.
    return 0;
}

下面我们看看判断连接是否是活跃连接的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，就说明是活跃连接，就继续看下一个StreamAllocation
        if (reference.get() != null) {
            i++;
            continue;
        }

      // We've discovered a leaked allocation. This is an application bug.
      //发现泄漏的引用，会打印日志
        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);
        
        //如果没有引用，就移除
        references.remove(i);
        connection.noNewStreams = true;

        //如果列表为空，就说明此连接上没有StreamAllocation引用了，就返回0，表示是空闲的连接
        if (references.isEmpty()) {
            connection.idleAtNanos = now - keepAliveDurationNs;
            return 0;
        }
    }
    //遍历结束后，返回引用的数量，说明当前连接是活跃连接
    return references.size();
}

至此我们就分析完OkHttp3的连接池复用了。

总结

（1）OkHttp3中支持5个并发socket连接，默认的keepAlive时间为5分钟，当然我们可以在构建OkHttpClient时设置不同的值。

（2）OkHttp3通过Deque来存储连接，通过put、get等操作来管理连接。

（3）OkHttp3通过每个连接的引用计数对象StreamAllocation的计数来回收空闲的连接，向连接池添加新的连接时会触发执行清理空闲连接的任务。清理空闲连接的任务通过线程池来执行。

OKHttp3源码解析系列

• OkHttp3源码解析(一)之请求流程
• OkHttp3源码解析(二)——拦截器链和缓存策略