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REST Controller Framework for java back-end applications
There is a newer version: 0.1.0-20170604
/*
 * Copyright 2016-2017 Agapsys Tecnologia Ltda-ME.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.agapsys.rcf.util;

import com.agapsys.rcf.util.UriBuilder.QueryParameters;
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 java.util.LinkedList;
import java.util.List;
import java.util.Map;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public abstract class ProxyServlet extends HttpServlet {

    // 
    // =========================================================================
    public static final int DEFAULT_BUFFER_SIZE = 512;

    private static class Header {

        public final String name;
        public final String value;

        public Header(String name, String value) {
            if (name == null || name.trim().isEmpty()) {
                throw new IllegalArgumentException("Null/empty name");
            }

            this.name = name;

            if (value == null) {
                throw new IllegalArgumentException("Null value");
            }

            this.value = value;
        }

        @Override
        public String toString() {
            return String.format("%s: %s", name, value);
        }
    }

    private static List __getHeaders(HttpServletRequest req) {
        List headerList = new LinkedList<>();

        Enumeration names = req.getHeaderNames();
        while (names.hasMoreElements()) {
            String name = names.nextElement();
            Enumeration values = req.getHeaders(name);
            while (values.hasMoreElements()) {
                headerList.add(new Header(name, values.nextElement()));
            }
        }

        return headerList;
    }

    private static void __redirect(InputStream is, OutputStream os, int bufferSize) throws IOException {
        byte[] buffer = new byte[bufferSize];
        int bytesRead;
        while ((bytesRead = is.read(buffer)) != -1) {
            os.write(buffer, 0, bytesRead);
        }
    }
    // =========================================================================
    // 

    /**
     * Returns the forwarding location related to given path
     *
     * @param path path associated to forwarding location
     * @return forwarding location
     */
    protected abstract String getForwardLocation(String path);

    /**
     * Returns the size of internal buffer (in bytes) used to fetch data
     *
     * @return internal buffer size. Default implementation returns {@linkplain ProxyServlet#DEFAULT_BUFFER_SIZE}.
     */
    protected int getBufferSize() {
        return DEFAULT_BUFFER_SIZE;
    }

    @Override
    protected final void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String path = req.getPathInfo();
        QueryParameters queryParams = UriBuilder.getQueryParameters(req.getQueryString());

        UriBuilder uriBuilder = new UriBuilder(getForwardLocation(path));
        for (Map.Entry entry : queryParams.entrySet()) {
            uriBuilder.addParameter(entry.getKey(), entry.getValue());
        }

        URL url = new URL(uriBuilder.toString());
        HttpURLConnection con = (HttpURLConnection) url.openConnection();
        con.setRequestMethod(req.getMethod());
        for (Header header : __getHeaders(req)) {
            con.setRequestProperty(header.name, header.value);
        }

        int bufferSize = getBufferSize();

        switch (req.getMethod()) {
            case "POST":
            case "PATCH":
            case "PUT":
                InputStream is = req.getInputStream();
                OutputStream os = con.getOutputStream();
                __redirect(is, os, bufferSize);
                os.flush();
                os.close();
                break;
        }

        resp.setStatus(con.getResponseCode());
        for (Map.Entry> entry : con.getHeaderFields().entrySet()) {
            String name = entry.getKey();
            List values = entry.getValue();
            for (String value : values) {
                resp.addHeader(name, value);
            }
        }

        InputStream is = con.getInputStream();
        try (OutputStream os = resp.getOutputStream()) {
            __redirect(is, os, bufferSize);
            os.flush();
        }
    }

}