AdminClient 类提供了创建、删除 topic 的 api。
在项目中创建了一个 AdminClient 对象，每次创建 topic 时，调用
org.apache.kafka.clients.admin.AdminClient#createTopics
如果长时间不使用这个对象，客户端与 broker 之间的连接会被关掉，相关的参数：
connections.max.idle.ms
这个最大空闲参数在 broker 和 客户端都可以配置，即 broker 和客户端都会关闭空闲太久的连接。
org.apache.kafka.common.network.Selector#maybeCloseOldestConnection
private void maybeCloseOldestConnection(long currentTimeNanos) {
if (idleExpiryManager == null)
return;
Map.Entry<String, Long> expiredConnection = idleExpiryManager.pollExpiredConnection(currentTimeNanos);
if (expiredConnection != null) {
String connectionId = expiredConnection.getKey();
KafkaChannel channel = this.channels.get(connectionId);
if (channel != null) {
if (log.isTraceEnabled())
log.trace("About to close the idle connection from {} due to being idle for {} millis",
connectionId, (currentTimeNanos - expiredConnection.getValue()) / 1000 / 1000);
channel.state(ChannelState.EXPIRED);
close(channel, CloseMode.GRACEFUL);
}
}
}
org.apache.kafka.common.network.Selector.IdleExpiryManager#pollExpiredConnection
lruConnections 是 LinkedHashMap 类型，可以按照插入和访问顺序进行排序，这里是按访问顺序进行排序，访问过的顺序放到双向链表的结尾。
public Map.Entry<String, Long> pollExpiredConnection(long currentTimeNanos) {
if (currentTimeNanos <= nextIdleCloseCheckTime)
return null;
if (lruConnections.isEmpty()) {
nextIdleCloseCheckTime = currentTimeNanos + connectionsMaxIdleNanos;
return null;
}
Map.Entry<String, Long> oldestConnectionEntry = lruConnections.entrySet().iterator().next();
Long connectionLastActiveTime = oldestConnectionEntry.getValue();
nextIdleCloseCheckTime = connectionLastActiveTime + connectionsMaxIdleNanos;
if (currentTimeNanos > nextIdleCloseCheckTime)
return oldestConnectionEntry;
else
return null;
}