public class MainClientExec implements ClientExecChain {

 @Override
 public CloseableHttpResponse execute(
  final HttpRoute route,
  final HttpRequestWrapper request,
  final HttpClientContext context,
  final HttpExecutionAware execAware) throws IOException, HttpException {
　　　　　//從連接管理器HttpClientConnectionManager中獲取一個(gè)連接請(qǐng)求ConnectionRequest
 final ConnectionRequest connRequest = connManager.requestConnection(route, userToken);final HttpClientConnection managedConn;
 final int timeout = config.getConnectionRequestTimeout(); //從連接請(qǐng)求ConnectionRequest中獲取一個(gè)被管理的連接HttpClientConnection
 managedConn = connRequest.get(timeout > 0 ? timeout : 0, TimeUnit.MILLISECONDS);
　　　　 //將連接管理器HttpClientConnectionManager與被管理的連接HttpClientConnection交給一個(gè)ConnectionHolder持有
 final ConnectionHolder connHolder = new ConnectionHolder(this.log, this.connManager, managedConn);
 try {
  HttpResponse response;
  if (!managedConn.isOpen()) {　　　　　　　　　　//如果當(dāng)前被管理的連接不是出于打開狀態(tài)，需要重新建立連接
  establishRoute(proxyAuthState, managedConn, route, request, context);
  }
　　　　　　　//通過連接HttpClientConnection發(fā)送請(qǐng)求
  response = requestExecutor.execute(request, managedConn, context);
　　　　　　　//通過連接重用策略判斷是否連接可重用  
  if (reuseStrategy.keepAlive(response, context)) {
  //獲得連接有效期
  final long duration = keepAliveStrategy.getKeepAliveDuration(response, context);
  //設(shè)置連接有效期
  connHolder.setValidFor(duration, TimeUnit.MILLISECONDS);　　　　　　　　　 //將當(dāng)前連接標(biāo)記為可重用狀態(tài)
  connHolder.markReusable();
  } else {
  connHolder.markNonReusable();
  }
 }
 final HttpEntity entity = response.getEntity();
 if (entity == null || !entity.isStreaming()) {
  //將當(dāng)前連接釋放到池中，供下次調(diào)用
  connHolder.releaseConnection();
  return new HttpResponseProxy(response, null);
 } else {
  return new HttpResponseProxy(response, connHolder);
 }
}

public ConnectionRequest requestConnection(
  final HttpRoute route,
  final Object state) {final Future<CPoolEntry> future = this.pool.lease(route, state, null);
 return new ConnectionRequest() {
  @Override
  public boolean cancel() {
  return future.cancel(true);
  }
  @Override
  public HttpClientConnection get(
   final long timeout,
   final TimeUnit tunit) throws InterruptedException, ExecutionException, ConnectionPoolTimeoutException {
  final HttpClientConnection conn = leaseConnection(future, timeout, tunit);
  if (conn.isOpen()) {
   final HttpHost host;
   if (route.getProxyHost() != null) {
   host = route.getProxyHost();
   } else {
   host = route.getTargetHost();
   }
   final SocketConfig socketConfig = resolveSocketConfig(host);
   conn.setSocketTimeout(socketConfig.getSoTimeout());
  }
  return conn;
  }
 };
 }

private E getPoolEntryBlocking(
  final T route, final Object state,
  final long timeout, final TimeUnit tunit,
  final Future<E> future) throws IOException, InterruptedException, TimeoutException {
　　　　　//首先對(duì)當(dāng)前連接池加鎖，當(dāng)前鎖是可重入鎖ReentrantLockthis.lock.lock();
 try {　　　　　　　 //獲得一個(gè)當(dāng)前HttpRoute對(duì)應(yīng)的連接池，對(duì)于HttpClient的連接池而言，總池有個(gè)大小，每個(gè)route對(duì)應(yīng)的連接也是個(gè)池，所以是“池中池”
  final RouteSpecificPool<T, C, E> pool = getPool(route);
  E entry;
  for (;;) {
  Asserts.check(!this.isShutDown, "Connection pool shut down");　　　　　　　　　　//死循環(huán)獲得連接
  for (;;) {　　　　　　　　　　　　//從route對(duì)應(yīng)的池中拿連接，可能是null，也可能是有效連接
   entry = pool.getFree(state);　　　　　　　　　　　　//如果拿到null，就退出循環(huán)
   if (entry == null) {
   break;
   }　　　　　　　　　　　　//如果拿到過期連接或者已關(guān)閉連接，就釋放資源，繼續(xù)循環(huán)獲取
   if (entry.isExpired(System.currentTimeMillis())) {
   entry.close();
   }
   if (entry.isClosed()) {
   this.available.remove(entry);
   pool.free(entry, false);
   } else {　　　　　　　　　　　　　　//如果拿到有效連接就退出循環(huán)
   break;
   }
  }　　　　　　　　　　//拿到有效連接就退出
  if (entry != null) {
   this.available.remove(entry);
   this.leased.add(entry);
   onReuse(entry);
   return entry;
  }
　　　　　　　　　 //到這里證明沒有拿到有效連接，需要自己生成一個(gè)  
  final int maxPerRoute = getMax(route);
  //每個(gè)route對(duì)應(yīng)的連接最大數(shù)量是可配置的，如果超過了，就需要通過LRU清理掉一些連接
  final int excess = Math.max(0, pool.getAllocatedCount() + 1 - maxPerRoute);
  if (excess > 0) {
   for (int i = 0; i < excess; i++) {
   final E lastUsed = pool.getLastUsed();
   if (lastUsed == null) {
    break;
   }
   lastUsed.close();
   this.available.remove(lastUsed);
   pool.remove(lastUsed);
   }
  }
　　　　　　　　　 //當(dāng)前route池中的連接數(shù)，沒有達(dá)到上線
  if (pool.getAllocatedCount() < maxPerRoute) {
   final int totalUsed = this.leased.size();
   final int freeCapacity = Math.max(this.maxTotal - totalUsed, 0);　　　　　　　　　　　　//判斷連接池是否超過上線，如果超過了，需要通過LRU清理掉一些連接
   if (freeCapacity > 0) {
   final int totalAvailable = this.available.size();　　　　　　　　　　　　　　 //如果空閑連接數(shù)已經(jīng)大于剩余可用空間，則需要清理下空閑連接
   if (totalAvailable > freeCapacity - 1) {
    if (!this.available.isEmpty()) {
    final E lastUsed = this.available.removeLast();
    lastUsed.close();
    final RouteSpecificPool<T, C, E> otherpool = getPool(lastUsed.getRoute());
    otherpool.remove(lastUsed);
    }
   }　　　　　　　　　　　　　　//根據(jù)route建立一個(gè)連接
   final C conn = this.connFactory.create(route);　　　　　　　　　　　　　　//將這個(gè)連接放入route對(duì)應(yīng)的“小池”中
   entry = pool.add(conn);　　　　　　　　　　　　　　//將這個(gè)連接放入“大池”中
   this.leased.add(entry);
   return entry;
   }
  }
　　　　　　　　　//到這里證明沒有從獲得route池中獲得有效連接，并且想要自己建立連接時(shí)當(dāng)前route連接池已經(jīng)到達(dá)最大值，即已經(jīng)有連接在使用，但是對(duì)當(dāng)前線程不可用
  boolean success = false;
  try {
   if (future.isCancelled()) {
   throw new InterruptedException("Operation interrupted");
   }　　　　　　　　　　　　//將future放入route池中等待
   pool.queue(future);　　　　　　　　　　　　//將future放入大連接池中等待
   this.pending.add(future);　　　　　　　　　　　　//如果等待到了信號(hào)量的通知,success為true
   if (deadline != null) {
   success = this.condition.awaitUntil(deadline);
   } else {
   this.condition.await();
   success = true;
   }
   if (future.isCancelled()) {
   throw new InterruptedException("Operation interrupted");
   }
  } finally {
   //從等待隊(duì)列中移除
   pool.unqueue(future);
   this.pending.remove(future);
  }
  //如果沒有等到信號(hào)量通知并且當(dāng)前時(shí)間已經(jīng)超時(shí)，則退出循環(huán)
  if (!success && (deadline != null && deadline.getTime() <= System.currentTimeMillis())) {
   break;
  }
  }　　　　　　　//最終也沒有等到信號(hào)量通知，沒有拿到可用連接，則拋異常
  throw new TimeoutException("Timeout waiting for connection");
 } finally {　　　　　　　//釋放對(duì)大連接池的鎖
  this.lock.unlock();
 }
 }

protected HttpClientConnection leaseConnection(
  final Future<CPoolEntry> future,
  final long timeout,
  final TimeUnit tunit) throws InterruptedException, ExecutionException, ConnectionPoolTimeoutException {
 final CPoolEntry entry;
 try {　　　　　　　//從異步操作Future<CPoolEntry>中獲得CPoolEntry
  entry = future.get(timeout, tunit);
  if (entry == null || future.isCancelled()) {
  throw new InterruptedException();
  }
  Asserts.check(entry.getConnection() != null, "Pool entry with no connection");
  if (this.log.isDebugEnabled()) {
  this.log.debug("Connection leased: " + format(entry) + formatStats(entry.getRoute()));
  }　　　　　　　//獲得一個(gè)CPoolEntry的代理對(duì)象，對(duì)其操作都是使用同一個(gè)底層的HttpClientConnection
  return CPoolProxy.newProxy(entry);
 } catch (final TimeoutException ex) {
  throw new ConnectionPoolTimeoutException("Timeout waiting for connection from pool");
 }
 }

//發(fā)送Http連接  response = requestExecutor.execute(request, managedConn, context);
  //根據(jù)重用策略判斷當(dāng)前連接是否要復(fù)用
  if (reuseStrategy.keepAlive(response, context)) {
   //需要復(fù)用的連接，獲取連接超時(shí)時(shí)間，以response中的timeout為準(zhǔn)
   final long duration = keepAliveStrategy.getKeepAliveDuration(response, context);
   if (this.log.isDebugEnabled()) {
   final String s;　　　　　　　　　　　　　　 //timeout的是毫秒數(shù)，如果沒有設(shè)置則為-1，即沒有超時(shí)時(shí)間
   if (duration > 0) {
    s = "for " + duration + " " + TimeUnit.MILLISECONDS;
   } else {
    s = "indefinitely";
   }
   this.log.debug("Connection can be kept alive " + s);
   }　　　　　　　　　　　　//設(shè)置超時(shí)時(shí)間，當(dāng)請(qǐng)求結(jié)束時(shí)連接管理器會(huì)根據(jù)超時(shí)時(shí)間決定是關(guān)閉還是放回到池中
   connHolder.setValidFor(duration, TimeUnit.MILLISECONDS);
   //將連接標(biāo)記為可重用　　　　　　　　　　　　connHolder.markReusable();
  } else {　　　　　　　　　　　　//將連接標(biāo)記為不可重用
   connHolder.markNonReusable();
  }

public class DefaultClientConnectionReuseStrategy extends DefaultConnectionReuseStrategy {

 public static final DefaultClientConnectionReuseStrategy INSTANCE = new DefaultClientConnectionReuseStrategy();

 @Override
 public boolean keepAlive(final HttpResponse response, final HttpContext context) {
　　　　　//從上下文中拿到request
  final HttpRequest request = (HttpRequest) context.getAttribute(HttpCoreContext.HTTP_REQUEST);
  if (request != null) {　　　　　　　//獲得Connection的Header
   final Header[] connHeaders = request.getHeaders(HttpHeaders.CONNECTION);
   if (connHeaders.length != 0) {
    final TokenIterator ti = new BasicTokenIterator(new BasicHeaderIterator(connHeaders, null));
    while (ti.hasNext()) {
     final String token = ti.nextToken();　　　　　　　　　　　　//如果包含Connection:Close首部，則代表請(qǐng)求不打算保持連接，會(huì)忽略response的意愿，該頭部這是HTTP/1.1的規(guī)范
     if (HTTP.CONN_CLOSE.equalsIgnoreCase(token)) {
      return false;
     }
    }
   }
  }　　　　 //使用父類的的復(fù)用策略
  return super.keepAlive(response, context);
 }
}

if (canResponseHaveBody(request, response)) {
    final Header[] clhs = response.getHeaders(HTTP.CONTENT_LEN);
    //如果reponse的Content-Length沒有正確設(shè)置，則不復(fù)用連接　　　　　　　　　 //因?yàn)閷?duì)于持久化連接，兩次傳輸之間不需要重新建立連接，則需要根據(jù)Content-Length確認(rèn)內(nèi)容屬于哪次請(qǐng)求，以正確處理“粘包”現(xiàn)象    //所以，沒有正確設(shè)置Content-Length的response連接不能復(fù)用
    if (clhs.length == 1) {
     final Header clh = clhs[0];
     try {
      final int contentLen = Integer.parseInt(clh.getValue());
      if (contentLen < 0) {
       return false;
      }
     } catch (final NumberFormatException ex) {
      return false;
     }
    } else {
     return false;
    }
   }
  if (headerIterator.hasNext()) {
   try {
    final TokenIterator ti = new BasicTokenIterator(headerIterator);
    boolean keepalive = false;
    while (ti.hasNext()) {
     final String token = ti.nextToken();　　　　　　　　　　　　//如果response有Connection:Close首部，則明確表示要關(guān)閉，則不復(fù)用
     if (HTTP.CONN_CLOSE.equalsIgnoreCase(token)) {
      return false;　　　　　　　　　　　　//如果response有Connection:Keep-Alive首部，則明確表示要持久化，則復(fù)用
     } else if (HTTP.CONN_KEEP_ALIVE.equalsIgnoreCase(token)) {
      keepalive = true;
     }
    }
    if (keepalive) {
     return true;
    }
   } catch (final ParseException px) {
    return false;
   }
  }
　　　　　//如果response中沒有相關(guān)的Connection首部說明，則高于HTTP/1.0版本的都復(fù)用連接 
  return !ver.lessEquals(HttpVersion.HTTP_1_0);

public CloseableHttpClient build() {   //如果指定了要清理過期連接與空閑連接，才會(huì)啟動(dòng)清理線程，默認(rèn)是不啟動(dòng)的
   if (evictExpiredConnections || evictIdleConnections) {　　　　　　　　　　//創(chuàng)造一個(gè)連接池的清理線程
    final IdleConnectionEvictor connectionEvictor = new IdleConnectionEvictor(cm,
      maxIdleTime > 0 ? maxIdleTime : 10, maxIdleTimeUnit != null ? maxIdleTimeUnit : TimeUnit.SECONDS,
      maxIdleTime, maxIdleTimeUnit);
    closeablesCopy.add(new Closeable() {
     @Override
     public void close() throws IOException {
      connectionEvictor.shutdown();
      try {
       connectionEvictor.awaitTermination(1L, TimeUnit.SECONDS);
      } catch (final InterruptedException interrupted) {
       Thread.currentThread().interrupt();
      }
     }

    });　　　　　　　　　　//執(zhí)行該清理線程
    connectionEvictor.start();
}

public IdleConnectionEvictor(
   final HttpClientConnectionManager connectionManager,
   final ThreadFactory threadFactory,
   final long sleepTime, final TimeUnit sleepTimeUnit,
   final long maxIdleTime, final TimeUnit maxIdleTimeUnit) {
  this.connectionManager = Args.notNull(connectionManager, "Connection manager");
  this.threadFactory = threadFactory != null ? threadFactory : new DefaultThreadFactory();
  this.sleepTimeMs = sleepTimeUnit != null ? sleepTimeUnit.toMillis(sleepTime) : sleepTime;
  this.maxIdleTimeMs = maxIdleTimeUnit != null ? maxIdleTimeUnit.toMillis(maxIdleTime) : maxIdleTime;
  this.thread = this.threadFactory.newThread(new Runnable() {
   @Override
   public void run() {
    try {　　　　　　　　　　　　//死循環(huán)，線程一直執(zhí)行
     while (!Thread.currentThread().isInterrupted()) {　　　　　　　　　　　　　　//休息若干秒后執(zhí)行，默認(rèn)10秒
      Thread.sleep(sleepTimeMs);　　　　　　　　　　　　　　 //清理過期連接
      connectionManager.closeExpiredConnections();　　　　　　　　　　　　　　 //如果指定了最大空閑時(shí)間，則清理空閑連接
      if (maxIdleTimeMs > 0) {
       connectionManager.closeIdleConnections(maxIdleTimeMs, TimeUnit.MILLISECONDS);
      }
     }
    } catch (final Exception ex) {
     exception = ex;
    }

   }
  });
 }