服务器返回URL的HTTP响应代码：400：java.io.IOException

Question

What am I doing wrong? Please have a look at code.

HttpURLConnection conn = (HttpURLConnection) new URL(http_url).openConnection();
conn.setRequestMethod("GET");
BufferedReader rd = new BufferedReader(new InputStreamReader(conn.getInputStream()));
/*
java.io.IOException: Server returned HTTP response code: 400 for URL: http_url
  at sun.net.www.protocol.http.HttpURLConnection.getInputStream
*/

If I open the http_url in browser, it's working.

Answer 1

You are not connected (next code from http://alvinalexander.com/blog/post/java/how-open-url-read-contents-httpurl-connection-java ).

 HttpURLConnection connection = (HttpURLConnection) url.openConnection();

      // just want to do an HTTP GET here
      connection.setRequestMethod("GET");

      // uncomment this if you want to write output to this url
      //connection.setDoOutput(true);

      // give it 15 seconds to respond
      connection.setReadTimeout(15*1000);
      connection.connect();

      // read the output from the server
      reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));

Answer 2

I was confused since I was getting the Java error instead of the error payload. Later when I checked the http_url in postman, it showed the error payload as well as response code 400. Browser doesn't show response code if payload is available. Actually I should have used conn.getErrorStream() instead of conn.getInputStream() in case of response code >= 400.

Answer 3

You have to follow HTTP protocol, and set all HTTP headers server expects to get before reading a response. Definitely you missed one or more headers, like content-type, accept etc. also maybe server does not like Java agent header and likes agents from IE, Chrome, Firefox etc. So server is right, you request is bad :-)