Access Windows Registry using 'pure' Java

(rehash of my old post since all those wonderful blog post drafts I was planning to publish are unfortunately out-dated or irrelevant)

One can use the private code of Sun's Preferences API to access values of type REG_SZ in the Windows Registry. Stupid hack. Maybe I should make a library out of this. I have used this soooo many times.

The java.util.prefs.WindowsPreferences is the concrete implementation of AbstractPreferences in the Windows platform. This class provides methods like WindowsRegQueryValueEx, etc. Using Reflection, one can use the methods in this class to query string values under HKEY_LOCAL_MACHINE and HKEY_CURRENT_USER


Given below is a code-snippet that demonstrates how to get the ProxyServer setting on Windows (this is what IE uses/sets) and the Internet Explorer version

import java.lang.reflect.Method;
import java.util.prefs.Preferences;

public class JavaRegistryHack {

  private static final int HKEY_CURRENT_USER = 0x80000001;
  private static final int KEY_QUERY_VALUE = 1;
  private static final int KEY_SET_VALUE = 2;
  private static final int KEY_READ = 0x20019;

  public static void main(String args[]) {
      final Preferences userRoot = Preferences.userRoot();
      final Preferences systemRoot = Preferences.systemRoot();
      final Class clz = userRoot.getClass();
      try {
          final Method openKey = clz.getDeclaredMethod("openKey",
                  byte[].class, int.class, int.class);
          openKey.setAccessible(true);

          final Method closeKey = clz.getDeclaredMethod("closeKey",
                  int.class);
          closeKey.setAccessible(true);

          final Method winRegQueryValue = clz.getDeclaredMethod(
                  "WindowsRegQueryValueEx", int.class, byte[].class);
          winRegQueryValue.setAccessible(true);
          final Method winRegEnumValue = clz.getDeclaredMethod(
                  "WindowsRegEnumValue1", int.class, int.class, int.class);
          winRegEnumValue.setAccessible(true);
          final Method winRegQueryInfo = clz.getDeclaredMethod(
                  "WindowsRegQueryInfoKey1", int.class);
          winRegQueryInfo.setAccessible(true);


          byte[] valb = null;
          String vals = null;
          String key = null;
          Integer handle = -1;

          //Query Internet Settings for Proxy
          key = "Software\\Microsoft\\Windows\\CurrentVersion\\Internet Settings";
          handle = (Integer) openKey.invoke(userRoot, toCstr(key), KEY_READ, KEY_READ);
          valb = (byte[]) winRegQueryValue.invoke(userRoot, handle.intValue(),
                  toCstr("ProxyServer"));
          vals = (valb != null ? new String(valb).trim() : null);
          System.out.println("Proxy Server = " + vals);
          closeKey.invoke(Preferences.userRoot(), handle);

          // Query for IE version
          key = "SOFTWARE\\Microsoft\\Internet Explorer";
          handle = (Integer) openKey.invoke(systemRoot, toCstr(key), KEY_READ, KEY_READ);
          valb = (byte[]) winRegQueryValue.invoke(systemRoot, handle, toCstr("Version"));
          vals = (valb != null ? new String(valb).trim() : null);
          System.out.println("Internet Explorer Version = " + vals);
          closeKey.invoke(Preferences.systemRoot(), handle);

      } catch (Exception e) {
          e.printStackTrace();
      }
  }

 
   private static byte[] toCstr(String str) {
       byte[] result = new byte[str.length() + 1];
       for (int i = 0; i < str.length(); i++) {
           result[i] = (byte) str.charAt(i);
       }
       result[str.length()] = 0;
       return result;
   }