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 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

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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> references = connection.allocations;
for (int i = 0; i < references.size(); ) {
Reference 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()) {
connection.idleAtNanos = now - keepAliveDurationNs;
return 0;
}
}
return references.size();
}
总结

链接池的使用重点为三个方法，用通俗的语言来讲： put为将用完的链接放入到连接池中去，在放入的过程中检查连接池中是否有过期的待被清理。 get为从连接池中取出链接复用，通俗的来讲，其需要满足线路起点重点相同，线路不能过度拥挤，否则不允许从连接池中复用 cleanup 为从链接池中找过期了的道路，寻找的方法为看那一条道路最长时间没有被使用。在寻找的过程中，可能有些道路上的干道记录错误（内存泄露），于是便在寻找的过程中将其修复。