为什么 URL up ​​时会出现“java.net.ConnectException: Connection timed out”异常？

我得到了一个ConnectException: Connection timed out我的代码中有一定频率。我试图访问的 URL 已打开。相同的代码适用于某些用户，但不适用于其他用户。似乎一旦一个用户开始收到此异常，他们就会继续收到该异常。

这是堆栈跟踪：

java.net.ConnectException: Connection timed out
Caused by: java.net.ConnectException: Connection timed out
    at java.net.PlainSocketImpl.socketConnect(Native Method)
    at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:333)
    at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:195)
    at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:182)
    at java.net.Socket.connect(Socket.java:516)
    at java.net.Socket.connect(Socket.java:466)
    at sun.net.NetworkClient.doConnect(NetworkClient.java:157)
    at sun.net.www.http.HttpClient.openServer(HttpClient.java:365)
    at sun.net.www.http.HttpClient.openServer(HttpClient.java:477)
    at sun.net.www.http.HttpClient.<init>(HttpClient.java:214)
    at sun.net.www.http.HttpClient.New(HttpClient.java:287)
    at sun.net.www.http.HttpClient.New(HttpClient.java:299)
    at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:796)
    at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:748)
    at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:673)
    at sun.net.www.protocol.http.HttpURLConnection.getOutputStream(HttpURLConnection.java:840)

这是我的代码片段：

URLConnection urlConnection = null;
OutputStream outputStream = null;
OutputStreamWriter outputStreamWriter = null;
InputStream inputStream = null;

try {
    URL url = new URL(urlBase);
    urlConnection = url.openConnection();
    urlConnection.setDoOutput(true);

    outputStream = urlConnection.getOutputStream(); // exception occurs on this line
    outputStreamWriter = new OutputStreamWriter(outputStream);
    outputStreamWriter.write(urlString);
    outputStreamWriter.flush();
    inputStream = urlConnection.getInputStream();
    String response = IOUtils.toString(inputStream);
    return processResponse(urlString, urlBase, response);
} catch (IOException e) {
    throw new Exception("Error querying url: " + urlString, e);
} finally {
    IoUtil.close(inputStream);
    IoUtil.close(outputStreamWriter);
    IoUtil.close(outputStream);
}

连接超时（假设有本地网络和多台客户端计算机）通常是由于

a) 某种防火墙，它只是简单地吃掉数据包，而不告诉发送者诸如“没有到主机的路由”之类的信息

b) 由于网络配置错误或线路过载而导致的丢包

c) 过多的请求使服务器超载

d) 服务器上有少量同时可用的线程/进程，这导致它们全部被占用。这种情况尤其发生在需要很长时间运行并且可能与 c) 结合的请求时。