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 * i-net software provides programming examples for illustration only, without warranty
 * either expressed or implied, including, but not limited to, the implied warranties
 * of merchantability and/or fitness for a particular purpose. This programming example
 * assumes that you are familiar with the programming language being demonstrated and
 * the tools used to create and debug procedures. i-net software support professionals
 * can help explain the functionality of a particular procedure, but they will not modify
 * these examples to provide added functionality or construct procedures to meet your
 * specific needs.
 *
 * Copyright © 1999-2025 i-net software GmbH, Berlin, Germany.
**/
package proxyservlet;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Enumeration;

import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

/**
 * A proxy implementation for using in a frontend web server to forward the requests to a standalone backend server.
 */
public class ProxyServlet extends HttpServlet {

    /** Setup the URL of the report server here. You can do it statically or dynamically in init(). */
    private static String REPORTING_URL = "http://localhost:9000";

    /**
     * Forward the request to the reporting server {@inheritDoc}
     */
    @Override
    protected void service( HttpServletRequest request, HttpServletResponse response ) throws ServletException, IOException {

        String pathInfo = request.getPathInfo();
        String context = request.getRequestURI();
        String backendUrl;
        if( pathInfo == null ) {
            backendUrl = "/";
        } else {
            backendUrl = pathInfo;
            context = context.substring( 0, context.length() - pathInfo.length() );
        }
        if( request.getQueryString() != null ) {
            backendUrl += '?' + request.getQueryString();
        }
        URL url = new URL( REPORTING_URL + backendUrl );
        HttpURLConnection http = (HttpURLConnection)url.openConnection();
        http.setAllowUserInteraction( false );

        //disable NTLM support, we does not want login the frontend server in the backend server
        //you does not need it if you are sure that the backend server will never request a NTLM authentication
        try {
            java.lang.reflect.Field field = http.getClass().getDeclaredField( "tryTransparentNTLMServer" );
            field.setAccessible( true );
            field.set( http, Boolean.FALSE );
        } catch( Exception e1 ) {
            e1.printStackTrace();
        }

        String method = request.getMethod();
        http.setRequestMethod( method );
        http.setInstanceFollowRedirects( false );

        // copy the header from client to server
        Enumeration<String> headers = request.getHeaderNames();
        while( headers.hasMoreElements() ) {
            String header = headers.nextElement();

            Enumeration values = request.getHeaders( header );
            while( values.hasMoreElements() ) {
                String value = (String)values.nextElement();
                if( value != null ) {
                    http.addRequestProperty( header, value );
                }
            }
        }

        // set some additional header
        String host = request.getHeader( "Host" );
        if( host == null ) {
            host = request.getServerName() + ":" + request.getServerPort();
        }
        http.addRequestProperty( "X-Forwarded-Host", host );
        http.addRequestProperty( "X-Forwarded-For", request.getRemoteAddr() );
        http.addRequestProperty( "X-Forwarded-Proto", request.getScheme() );
        http.addRequestProperty( "X-Forwarded-Context", context );
        http.setUseCaches( false );

        boolean noData = "GET".equalsIgnoreCase( method ) || "HEAD".equalsIgnoreCase( method );

        InputStream result;
        try {
            if( !noData ) {
                // copy the data
                http.setDoOutput( true );
                OutputStream out = http.getOutputStream();
                copy( request.getInputStream(), out );
            }

            result = http.getInputStream();
        } catch( Exception e ) {
            result = http.getErrorStream();
        }

        int resposeCode = http.getResponseCode();
        response.setStatus( resposeCode );

        // copy the header from server to the client
        for( int h = 0;; h++ ) {
            String header = http.getHeaderFieldKey( h );
            String value = http.getHeaderField( h );
            if( header == null && value == null ) {
                break;
            }
            if( header != null && value != null ) {
                if( "Transfer-Encoding".equalsIgnoreCase( header ) ) {
                    continue;
                }
                if( "WWW-Authenticate".equalsIgnoreCase( header ) && "NTLM".equalsIgnoreCase( value ) ) {
                    //NTLM does not work with a proxy
                    continue;
                }
                response.addHeader( header, value );
            }
        }

        // copy the data from server to the client
        if( result != null ) {
            copy( result, response.getOutputStream() );
        }
    }

    /**
     * Copy the data from the input to the output
     * @param input the input data
     * @param output the target
     * @throws IOException if IO issues
     */
    private void copy( InputStream input, OutputStream output ) throws IOException {
        byte[] buf = new byte[4096];
        int len;
        while( (len = input.read( buf )) > 0 ) {
            output.write( buf, 0, len );
        }
    }
}