Debug a HttpURLConnection problem

Take this example.

import java.io.InputStreamReader;
import java.io.Reader;
import java.net.URL;
import java.net.URLConnection;

public class HttpConnect {
    public static void main(String[] args) throws Exception {
      URL url = new URL("http://www.rgagnon.com/howto.html");
      URLConnection con = url.openConnection();
      Reader reader = new InputStreamReader(con.getInputStream());
      while (true) {
        int ch = reader.read();
        if (ch==-1) {
          break;
        }
        System.out.print((char)ch);
      }
    }
}

The above snippet reads the HTML page and dumps it to the console.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML><HEAD>
<!-- developpe par  Real Gagnon, Quebec Canada -->
<LINK REL="SHORTCUT ICON" HREF="http://www.maixuanviet.com/favicon.ico">
<META NAME="description"
      Content="Real's JAVA JAVASCRIPT WSH and PowerBuilder How-to pages with useful code snippets">
<META NAME="keywords"
      Content="java,javascript,wsh,vbscript,how-to,powerbuilder">
<LINK title="mystyle" href="howto.css" type="text/css" rel="stylesheet">
...

To trace what is going on at the HTTP protocol level, we can switch the HttpURLConnection/UrlConnection in debug mode.

You must enable the java-logging mechanism by setting a special property when starting the JVM :

java.exe -Djava.util.logging.config.file=logging.properties HttpConnect

and put in logging.properties file (by default in JRE_HOME\lib) the following property

sun.net.www.protocol.http.HttpURLConnection.level = ALL

Note :
If running from Eclipse, you put the -Djava.util.logging… via the Run configuration dialog -> Arguments Tab -> VM arguments textarea.

The result is :

2010-08-07 00:00:31 sun.net.www.protocol.http.HttpURLConnection writeRequests
FIN: sun.net.www.MessageHeader@16caf435 pairs: {GET /howto.html HTTP/1.1: null}{User-Agent: Java/1.6.0_20}{Host: www.maixuanviet.com}{Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2}{Connection: keep-alive}
2010-08-07 00:00:31 sun.net.www.protocol.http.HttpURLConnection getInputStream
FIN: sun.net.www.MessageHeader@5ac0728 pairs: {null: HTTP/1.1 200 OK}{Date: Sat, 07 Aug 2010 04:00:33 GMT}{Server: Apache}{Accept-Ranges: bytes}{Content-Length: 17912}{Keep-Alive: timeout=5, max=64}{Connection: Keep-Alive}{Content-Type: text/html}
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML><HEAD>
<!-- developpe par  Real Gagnon, Quebec Canada -->
<LINK REL="SHORTCUT ICON" HREF="http://www.maixuanviet.com/favicon.ico">
<META NAME="description"
      Content="Real's JAVA JAVASCRIPT WSH and PowerBuilder How-to pages with useful code snippets">
...

Done! Happy Coding!

Spring Security Remember Me

Convert Hex to ASCII in Java

Hamcrest Collections Cookbook

Java Program to Find the Number of Ways to Write a Number as the Sum of Numbers Smaller than Itself

Java Program to Implement Johnson’s Algorithm

Java Program to Implement Ternary Search Tree

Guide to Escaping Characters in Java RegExps

Giới thiệu Aspect Oriented Programming (AOP)

Rest Web service: Filter và Interceptor với Jersey 2.x (P1)

Java 8 Streams peek() API

Các nguyên lý thiết kế hướng đối tượng – SOLID