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socket keepalive理解

参考URL: https://www.cnblogs.com/xiao-tao/p/9718017.html

java socket编程中有个keepalive选项，看到这个选项经常会误解为长连接，不设置则为短连接，实则不然。

socket连接建立之后，只要双方均未主动关闭连接，那这个连接就是会一直保持的，就是持久的连接。keepalive只是为了防止连接的双方发生意外而通知不到对方，导致一方还持有连接，占用资源。

其实这个选项的意思是TCP连接空闲时是否需要向对方发送探测包，实际上是依赖于底层的TCP模块实现的，java中只能设置是否开启，不能设置其详细参数，只能依赖于系统配置。

SocketOptions.java 部分源码

/**     * When the keepalive option is set for a TCP socket and no data     * has been exchanged across the socket in either direction for     * 2 hours (NOTE: the actual value is implementation dependent),     * TCP automatically sends a keepalive probe to the peer. This probe is a     * TCP segment to which the peer must respond.     * One of three responses is expected:     * 1. The peer responds with the expected ACK. The application is not     *    notified (since everything is OK). TCP will send another probe     *    following another 2 hours of inactivity.     * 2. The peer responds with an RST, which tells the local TCP that     *    the peer host has crashed and rebooted. The socket is closed.     * 3. There is no response from the peer. The socket is closed.     *     * The purpose of this option is to detect if the peer host crashes.     *     * Valid only for TCP socket: SocketImpl     *     * @see Socket#setKeepAlive     * @see Socket#getKeepAlive     */    @Native public final static int SO_KEEPALIVE = 0x0008;

源码的意思是，如果这个连接上双方任意方向在2小时之内没有发送过数据，那么tcp会自动发送一个探测探测包给对方，这种探测包对方是必须回应的，回应结果有三种：

1.正常ack，继续保持连接；

这里说的两小时，其实是依赖于系统配置

在linux系统中（windows在注册表中，可以自行查询资料），tcp的keepalive参数

linux下查看相关命令

[root@VM_0_14_centos ~]# echo /proc/sys/net/ipv4/tcp_keepalive* | xargs   -n 1  strings -1 -f/proc/sys/net/ipv4/tcp_keepalive_intvl: 75/proc/sys/net/ipv4/tcp_keepalive_probes: 9/proc/sys/net/ipv4/tcp_keepalive_time: 7200[root@VM_0_14_centos ~]#

/proc/sys/net/ipv4/tcp_keepalive_time 当keepalive起用的时候，TCP发送keepalive消息的频度。默认是2小时。

总结：net.ipv4.tcp_keepalive_intvl = 75 （发送探测包的周期，前提是当前连接一直没有数据交互，才会以该频率进行发送探测包，如果中途有数据交互，则会重新计时tcp_keepalive_time，到达规定时间没有数据交互，才会重新以该频率发送探测包）

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