Java 从 HttpUrlConnection 对象获取标头
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Get header from HttpUrlConnection object
提问by MyTitle
I want to send request to servlet and read headers from response. So I try it using this:
我想向 servlet 发送请求并从响应中读取标头。所以我尝试使用这个：
URL url = new URL(contextPath + "file_operations");
HttpURLConnection conn = null;
try {
conn = (HttpURLConnection) url.openConnection();
conn.setDoOutput(true);
conn.setDoInput(true);
conn.setRequestMethod("POST");
conn.setRequestProperty("charset", "utf-8");
conn.setUseCaches(false);
conn.setConnectTimeout(1000 * 5);
conn.connect();
conn.getHeaderField("MyHeader")
.....
But received headers are always null. Servlet works fine (i tried work with servlet using standalone HTTP client)
但是收到的标头总是null. Servlet 工作正常（我尝试使用独立 HTTP 客户端使用 servlet）
回答by Juned Ahsan
Make sure you are getting the successful response before you try to fetch the headers. This is how you can check for your response:
在尝试获取标头之前，请确保获得成功的响应。您可以通过以下方式检查您的回复：
int status = conn.getResponseCode();
if (status == HttpURLConnection.HTTP_OK) {
String header = conn.getHeaderField("MyHeader");
}
Also make sure the Servlet response is not a redirect response, if redirected all the session information, including headers will be lost.
还要确保 Servlet 响应不是重定向响应，如果重定向了所有会话信息，包括头信息都会丢失。
回答by A.J.Bauer
Before the connect (right after setRquestPropert, setDoOutput a.s.o):
在连接之前（在 setRquestPropert 之后，setDoOutput aso）：
for (Map.Entry<String, List<String>> entries : conn.getRequestProperties().entrySet()) {
String values = "";
for (String value : entries.getValue()) {
values += value + ",";
}
Log.d("Request", entries.getKey() + " - " + values );
}
Before disconnect (after reading response a.s.o):
断开连接前（在阅读响应后）：
for (Map.Entry<String, List<String>> entries : conn.getHeaderFields().entrySet()) {
String values = "";
for (String value : entries.getValue()) {
values += value + ",";
}
Log.d("Response", entries.getKey() + " - " + values );
}
