最近发现一个问题，在服务器上通过netstat命令发现有大量的Close_Wait长时间存在，甚至有时候数量接近1000：
查看服务器参数(etc/sysctl.conf):
net.ipv4.tcp_keepalive_time 网管已经修改成1200。
参数值还可以改小，但似乎是治标不治本，出现这种问题，肯定是某个地方的程序本身存在问题。
根据ip及端口信息，不难发现是什么地方除问题了，项目中有涉及到图片上传，于是找到图片上传的代码，结果发现代码非常简单，一行上传权限初始化代码，一行CDN官方提供的一个静态方法，之后就是处理响应结果的代码了。代码少且简单，上传调用代码没什么问题，那么问题可能出在CDN官方提供的jar包了，好在CDN有提供源码，于是查看源码，源码中使用apache 的是httpClient包，调用代码大致如下：
String response = "";
HttpPost httpPost = null;
CloseableHttpResponse ht = null;
String startTime = formatter.format(new Date());//请求时间
String endTime = "-";
String statusCode = "-";
String contentLength = "-";
String contentType = "-";
try {
httpPost = new HttpPost(url);
List<NameValuePair> paramsList = new ArrayList<NameValuePair>();
if (file != null) {
MultipartEntityBuilder mEntityBuilder = MultipartEntityBuilder.create();
BandwithLimiterFileBody fileBody = new BandwithLimiterFileBody(file, null, "application/octet-stream", null, BaseBlockUtil.maxRate, progressNotifier);
mEntityBuilder.addPart("file", fileBody);
mEntityBuilder.addTextBody("desc", file.getName());
if (params != null && params.size() > 0) {
for (String name : params.keySet()) {
mEntityBuilder.addTextBody(name, params.get(name), ContentType.create("text/plain", Charset.forName("UTF-8")));
}
}
httpPost.setEntity(mEntityBuilder.build());
} else if (params != null && params.size() > 0) {
for (String name : params.keySet()) {
paramsList.add(new BasicNameValuePair(name, params.get(name)));
}
HttpEntity he = new UrlEncodedFormEntity(paramsList, "utf-8");
httpPost.setEntity(he);
}
if (headMap != null && headMap.size() > 0) {
for (String name : headMap.keySet()) {
httpPost.addHeader(name, headMap.get(name));
}
}
if(!httpPost.containsHeader("User-Agent"))
httpPost.addHeader("User-Agent", Config.VERSION_NO);
CloseableHttpClient hc = HttpClients.createDefault();
RequestConfig requestConfig = RequestConfig.custom().setSocketTimeout(30000).setConnectTimeout(30000).build();//设置请求和传输超时时间
httpPost.setConfig(requestConfig);
ht = hc.execute(httpPost);
endTime = formatter.format(new Date());
Header[] headerArr = ht.getAllHeaders();
for (Header header : headerArr) {
BufferedHeader bh = (BufferedHeader) header;
if (bh.getBuffer().toString().contains("Content-Length")) {
contentLength = bh.getValue();
} else if (bh.getBuffer().toString().contains("Content-Type")) {
contentType = bh.getValue();
}
}
HttpEntity het = ht.getEntity();
InputStream is = het.getContent();
BufferedReader br = new BufferedReader(new InputStreamReader(is, "utf8"));
String readLine;
while ((readLine = br.readLine()) != null) {
response = response + readLine;
}
is.close();
br.close();
int status = ht.getStatusLine().getStatusCode();
statusCode = String.valueOf(status);
if (status == 200) {
if (!new JsonValidator().validate(response)) {
response = EncodeUtils.urlsafeDecodeString(response);
}
}
return new HttpClientResult(status, response);
} catch (Exception e) {
statusCode = "500";
endTime = formatter.format(new Date());
throw new HttpClientException(e);
} finally {
if (httpPost != null) {
httpPost.releaseConnection();
}
if (ht != null) {
try {
ht.close();
} catch (IOException ignored) {
}
}
writeHttpLog(startTime, url, "-", (null != params ? params.get("token") : "-"), (null != file ? file.getName() : "-"), "-", "-", endTime, statusCode, contentType, contentLength, response);
}
查看TCP协议端口状态说明 , 如果一直保持在CLOSE_WAIT状态，那么只有一种情况，就是在对方关闭连接之后服务器程序自己没有进一步发出ack信号。因此要解决这个问题大致有以下几种方案：
a、实例化httpClient 时，使用alwaysClose 的SimpleHttpConnectionManager
通常默认情况实例化httpClient 的时候使用的是无参构造的SimpleHttpConnectionManager，因此可替换为带参构造：
new HttpClient(new SimpleHttpConnectionManager(true));
b、在method.releaseConnection() 之后 通过获取HttpConnectionManager，进行关闭(getConnectionManager方法在httpclient 4.3之后已经标记为过期，后期可能会移除该方法)：
hc.getConnectionManager().shutdown();
c、在method.releaseConnection() 之后 通过获取HttpConnectionManager 调用closeIdleConnections方法进行关闭，(getConnectionManager方法在httpclient 4.3之后已经标记为过期，后期可能会移除该方法)：
hc.getConnectionManager().releaseConnection(conn, validDuration, timeUnit);
d、通过设置header由服务端自动关闭.
method.setHeader("Connection", "close");
HTTP协议中有关于这个属性的定义：
HTTP/1.1 defines the "close" connection option for the sender to signal that the connection will be closed after completion of the response. For example：
Connection: close
e、使用MultiThreadedHttpConnectionManager 进行复用，但同时也必须在合适的地方进行关闭处理；
f、请求之后未收到响应信息时，调用method.abort()进行处理
可参考：http://wiki.apache.org/HttpComponents/FrequentlyAskedConnectionManagementQuestions
TCP/IP详解卷一