[网络协议]Okhttp链接池的使用，看完这一篇你就懂了

 并向connection持有的allocations中增加了一条新的流的弱引用
也就是往这条连接中增加了一条流。 
```

而其中的条件为isEligible，跟踪进去可以看到，具体的条件为

1.  当前这次连接的最大并发数没有达到上限
2.  两个address的其他参数相同
3.  两个address的url的host相同

若满足以上条件，则说明host是相同的可以直接复用，如果不满足以上条件的话，仍旧有机会使用连接（将连接合并）：

1.  首先这个连接需要使用HTTP/2
2.  要与复用前的Ip address相同，且不能使用代理
3.  这个连接的服务器证书授权中，必须包括新的主机。
4.  锁定证书（certificatePinner）必须匹配主机

```
public boolean isEligible(Address address, @Nullable Route route) {
  // If this connection is not accepting new streams, we're done.
  if (allocations.size() >= allocationLimit || noNewStreams) return false;

  // If the non-host fields of the address don't overlap, we're done.
  if (!Internal.instance.equalsNonHost(this.route.address(), address)) return false;

  // If the host exactly matches, we're done: this connection can carry the address.
  if (address.url().host().equals(this.route().address().url().host())) {
    return true; // This connection is a perfect match.
  }

  // At this point we don't have a hostname match. But we still be able to carry the request if
  // our connection coalescing requirements are met. See also:
  // https://hpbn.co/optimizing-application-delivery/#eliminate-domain-sharding
  // https://daniel.haxx.se/blog/2016/08/18/http2-connection-coalescing/

  // 1. This connection must be HTTP/2.
  if (http2Connection == null) return false;

  // 2. The routes must share an IP address. This requires us to have a DNS address for both
  // hosts, which only happens after route planning. We can't coalesce connections that use a
  // proxy, since proxies don't tell us the origin server's IP address.
  if (route == null) return false;
  if (route.proxy().type() != Proxy.Type.DIRECT) return false;
  if (this.route.proxy().type() != Proxy.Type.DIRECT) return false;
  if (!this.route.socketAddress().equals(route.socketAddress())) return false;

  // 3. This connection's server certificate's must cover the new host.
  if (route.address().hostnameVerifier() != OkHostnameVerifier.INSTANCE) return false;
  if (!supportsUrl(address.url())) return false;

  // 4. Certificate pinning must match the host.
  try {
    address.certificatePinner().check(address.url().host(), handshake().peerCertificates());
  } catch (SSLPeerUnverifiedException e) {
    return false;
  }

  return true; // The caller's address can be carried by this connection.
} 
```

3. 连接池的清理和回收

   在put方法中已经用过了连接池的清理和回收 executor.execute(cleanupRunnable);现在变来详细看下其所做的事情：跟踪进入cleanupRunnable，发现其逻辑为定时执行cleanup，其中的定时等待是加入了同步锁，不允许打断。

   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) {
             }
           }
         }
       }
     }
   }; 
   

   下面聚焦的重点clean up之中。总的来说他的作用是找到限制的链接并清理，具体分析可以发现：

   • 其通过inUseConnectionCount记录正在使用的链接数目，利用idleConnectionCount记录闲置的链接数。这两个链接数目的改变，都是通过pruneAndGetAllocationCount()方法控制的，起作用也就自然而然为判断传入的链接是闲置的还是运行的。
   • 程序又根据闲置时间对connection 选择了一个限制时间最长的链接，如果其大于keep_alive的极限时间（keepAliveDurationNs 5分钟），或者空闲链接个数大于连接池的最大值（maxIdleConnections5个），则移除该connection

   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;
       }
     }
   
     closeQuietly(longestIdleConnection.socket());
   
     // Cleanup again immediately.
     return 0;
   } 
   

   跟踪进去，发现其是通过维护Reference类型的链表（references）达到效果的。起作用为记录connection 活跃情况的（>0 表示活跃=0 表示空闲)

   整体逻辑的核心为：如果 StreamAlloction 引用空闲,但是connection的引用列表中仍旧存在该项，那么便发生了内存泄露

   //用于清理可能泄露的 StreamAllocation并返回正在使用此连接的StreamA1location的数量,
   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;
       } //表示该流活跃
   
       // 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);
   
       references.remove(i);
       connection.noNewStreams = true;
   
       // If this was the last allocation, the connection is eligible for immediate eviction.
       if (references.isEmpty()) {