CloseableHttpClient httpClient = HttpClients.custom()
  .setConnectionManager(connectionManager)
  .setMaxConnTotal(400)
  .setMaxConnPerRoute(150)
  .evictExpiredConnections()
  .build();

evictExpiredConnections 這個配置作用：

設定一個定時執行緒，定時清理閒置連線,可以將這個定時時間設定為 keep alive timeout 時間的一半以保證超時前回收

每個httpClient 物件都會有自己獨立的定時執行緒

這樣如果應用中httpClient物件很多，就會導致上圖中執行緒太多

原始碼中，如果設定了evictExpiredConnections 會有下面一段邏輯

 List<Closeable> closeablesCopy = closeables != null ? new ArrayList<Closeable>(closeables) : null;
  if (!this.connManagerShared) {
   if (closeablesCopy == null) {
    closeablesCopy = new ArrayList<Closeable>(1);
   }
   final HttpClientConnectionManager cm = connManagerCopy;
 
   if (evictExpiredConnections || evictIdleConnections) {
    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();
      }
     }
 
    });
    connectionEvictor.start();
   }
   closeablesCopy.add(new Closeable() {
 
    @Override
    public void close() throws IOException {
     cm.shutdown();
    }
 
   });
  }

IdleConnectionEvictor 物件是

public final class IdleConnectionEvictor {
 
 private final HttpClientConnectionManager connectionManager;
 private final ThreadFactory threadFactory;
 private final Thread thread;
 private final long sleepTimeMs;
 private final long maxIdleTimeMs;
 
 private volatile Exception exception;
 
 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 {
     while (!Thread.currentThread().isInterrupted()) {
      Thread.sleep(sleepTimeMs);
      connectionManager.closeExpiredConnections();
      if (maxIdleTimeMs > 0) {
       connectionManager.closeIdleConnections(maxIdleTimeMs,TimeUnit.MILLISECONDS);
      }
     }
    } catch (final Exception ex) {
     exception = ex;
    }
 
   }
  });
 }

會出現一個執行緒，這個執行緒裡面就是去關閉超時不用的閒置連結

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