引言

最近在做网络访问功能，用到了失败重传，效果却不理想。其问题点在于，对同一个地址连续访问多次，如果第一次失败花了几秒钟的时间，后续访问失败根本不用花几秒（甚至到了几毫秒）就结束。一度让我怀疑，我应不应该做失败重传，好在经过我的实验，找到了问题所在，并得出了相应结论以及解决办法。但更为深层次的原因，还需要继续探究。所以现在跟我一起看看是怎么回事儿吧！

既然是失败重传，那么就得创造网络访问失败的环境，这个环境是这样：手机正常连接路由器WiFi，将路由器WAN口的网线拔掉，即让路由断网。

环境准备好了，就开始上代码运行，代码详情如下

println("lr------ programming launch")
for (i in 0 until 5) {
    val start = System.currentTimeMillis()
    var connection: HttpURLConnection? = null
    try {
        connection = URL("https://www.baidu.com").openConnection() as HttpURLConnection
        connection.connectTimeout = 2000
        connection.readTimeout = 2000
        connection.connect()
    } catch (e: Exception) {
        println("lr------ err:$e")
    } finally {
        connection?.disconnect()
    }
    println("lr------ running time: " + (System.currentTimeMillis() - start) / 1_000.0 + "s")
}
println("lr------ programming end")

上述代码进行了5次同一个地址的网络连接请求，我们看看在我们创造网络访问失败的环境的情况下的打印输出

lr------ programming launch
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 16.043s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 0.005s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 0.005s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 0.004s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 0.004s
lr------ programming end

可以看到，第一网络连接请求花了16s的时间，后续4次都只是花了几毫秒，反复测试也是如此。后几次的访问看上去就是简单复制了前一次的错误信息，根本没有去真实的访问网络，或者说，第一次的连接或许没有断开过，即便调用了connection?.disconnect()

如何解决这个问题呢，有了上面的猜想，我决定在每次网络访连接请求后都延时1200毫秒，于是得到如下代码

println("lr------ programming launch")
for (i in 0 until 5) {
    val start = System.currentTimeMillis()
    var connection: HttpURLConnection? = null
    try {
        connection = URL("https://www.baidu.com").openConnection() as HttpURLConnection
        connection.connectTimeout = 2000
        connection.readTimeout = 2000
        connection.connect()
    } catch (e: Exception) {
        println("lr------ err:$e")
    } finally {
        connection?.disconnect()
    }
    println("lr------ running time: " + (System.currentTimeMillis() - start) / 1_000.0 + "s")
    TimeUnit.MILLISECONDS.sleep(1200)
}
println("lr------ programming end")

运行后，打印输出如下

lr------ programming launch
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 16.037s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 0.002s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 16.029s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 0.006s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 16.029s
lr------ programming end

通过这次打印输出，我发现其规律，无论我尝试多少次，花费时间是间隔的，即前一次花了16秒，后一次就花几毫秒。看上去情况有所改善，猜想是延时时间设置小了，于是我改成了2000毫秒，再次运行后，打印输出如下

lr------ programming launch
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 16.195s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 16.029s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 16.03s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 16.033s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu.com": No address associated with hostname
lr------ running time: 16.031s
lr------ programming end

可以看到，几次网络连接请求花费的时间都是16秒左右，看上去这个问题解决了。但前面提到，网络连接请求同一个网络地址的时候，会出现这个问题，那连续网络连接请求的是不同的网络地址会是一个怎样的情况，我们执行下面的代码来看情况

lr------ programming launch
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu0.com": No address associated with hostname
lr------ running time: 16.044s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu1.com": No address associated with hostname
lr------ running time: 16.032s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu2.com": No address associated with hostname
lr------ running time: 16.029s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu3.com": No address associated with hostname
lr------ running time: 16.031s
lr------ err:java.net.UnknownHostException: Unable to resolve host "www.baidu4.com": No address associated with hostname
lr------ running time: 16.031s
lr------ programming end

看上去问题也得到解决了，此外，像Unable to resolve host "xxx"这类错误，整个网络连接请求流程没有完全建立，为什么这么说呢，因为抓包是抓不到滴！！！

最后

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