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apache连接池的连接有效性
server一般会配置keep-alive超时时间，过了这个时间还没新请求到来，则关闭连接。客户端从连接池里拿出连接时，会检查一下连接是否已关闭，如已关闭，会丢弃掉该连接，并尝试从连接池再拿一个新的连接，代码机制在AbstractConnPool.lease方法里：
public Future<E> lease(final T route, final Object state, final FutureCallback<E> callback) {Args.notNull(route, "Route");Asserts.check(!this.isShutDown, "Connection pool shut down");return new Future<E>() {...public E get(long timeout, TimeUnit timeUnit) throws InterruptedException, ExecutionException, TimeoutException {while(true) {synchronized(this) {PoolEntry var10000;try {E entry = (PoolEntry)this.entryRef.get();if (entry != null) {var10000 = entry;} else {...// 从连接池租借一个连接E leasedEntry = AbstractConnPool.this.getPoolEntryBlocking(route, state, timeout, timeUnit, this);// 如果租借的连接未关闭，就用该连接返回之if (AbstractConnPool.this.validateAfterInactivity <= 0 || leasedEntry.getUpdated() + (long)AbstractConnPool.this.validateAfterInactivity > System.currentTimeMillis() || AbstractConnPool.this.validate(leasedEntry)) {if (!this.done.compareAndSet(false, true)) {AbstractConnPool.this.release(leasedEntry, true);throw new ExecutionException(AbstractConnPool.operationAborted());}this.entryRef.set(leasedEntry);this.done.set(true);AbstractConnPool.this.onLease(leasedEntry);if (callback != null) {callback.completed(leasedEntry);}var10000 = leasedEntry;return var10000;}// 租借的连接已关闭，关闭该连接，并回到while循环开始，继续调用getPoolEntryBlocking获得新的连接，若池子里没有连接，创建一个新连接。leasedEntry.close();AbstractConnPool.this.release(leasedEntry, false);continue;}} catch (IOException var8) {...}return var10000;}}}};}
AbstractConnPool.this.validate会调用connection的isStale方法：
//CPool.java
protected boolean validate(CPoolEntry entry) {return !((ManagedHttpClientConnection)entry.getConnection()).isStale();}
那么一个连接是如何判定不新鲜（stale）的呢？逻辑如下：
//BHttpConnectionBase.java
public boolean isStale() {if (!this.isOpen()) {return true;} else {try {int bytesRead = this.fillInputBuffer(1);return bytesRead < 0;} catch (SocketTimeoutException var2) {return false;} catch (IOException var3) {return true;}}}
fillInputBuffer方法会尝试从socket里读取字节，返回值为读取的字节数，若返回-1，说明连接已关闭。
顺带说一下，实测发现，isStale的判定对于server端正常或异常关闭连接的情况，都能检测到。
各web服务器的keep-alive策略配置
很显然，一个用于生产的web服务器是要配置keep-alive超时的，毕竟机器的IO连接资源有限，万一大量的长连接被占用，新来的请求将得不到服务。
fastAPI可以在启动时指定keepalive的超时时间，像这样：
app = FastAPI()@app.get("/test")
async def root():return "Hello fastapi"if __name__ == "__main__":import uvicornuvicorn.run(app, host="0.0.0.0", port=8002, timeout_keep_alive=600)
这里我们指定600s，默认keepalive超时是5s，即5s没有请求则关闭连接
tomcat的keep-alive策略配置在server.xml里，除了keepAliveTimeout之外，还有maxKeepAliveRequests选项，意思是服务多少个请求后就关闭连接，例如下面的例子，在服务5个请求后关闭连接：
<Connector port="8080" protocol="HTTP/1.1"connectionTimeout="20000"maxThreads="1000"acceptCount="100"redirectPort="8443"URIEncoding="UTF-8"maxKeepAliveRequests="5"/>
两个参数的含义如下：
keepAliveTimeout:
The number of milliseconds Tomcat will wait for a subsequent request before closing the connectionmaxKeepAliveRequests:
Maximum number of Keep-Alive requests to honor per connection