code: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You 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 org.apache.skywalking.oap.server.telemetry.prometheus.httpserver;
  
  import io.netty.buffer.Unpooled;
  import io.netty.channel.ChannelFuture;
  import io.netty.channel.ChannelFutureListener;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.channel.SimpleChannelInboundHandler;
  import io.netty.handler.codec.http.DefaultFullHttpResponse;
  import io.netty.handler.codec.http.FullHttpResponse;
  import io.netty.handler.codec.http.HttpObject;
  import io.netty.handler.codec.http.HttpRequest;
  import io.netty.handler.codec.http.HttpUtil;
  import io.netty.handler.codec.http.HttpVersion;
  import io.netty.util.CharsetUtil;
  import io.prometheus.client.CollectorRegistry;
  import io.prometheus.client.exporter.common.TextFormat;
  import java.io.IOException;
  import lombok.extern.slf4j.Slf4j;
  import org.apache.logging.log4j.core.util.StringBuilderWriter;
  
  import static io.netty.channel.ChannelFutureListener.CLOSE;
  import static io.netty.handler.codec.http.HttpHeaderNames.CONNECTION;
  import static io.netty.handler.codec.http.HttpHeaderNames.CONTENT_LENGTH;
  import static io.netty.handler.codec.http.HttpHeaderNames.CONTENT_TYPE;
  import static io.netty.handler.codec.http.HttpHeaderValues.KEEP_ALIVE;
  import static io.netty.handler.codec.http.HttpHeaderValues.TEXT_PLAIN;
  import static io.netty.handler.codec.http.HttpResponseStatus.INTERNAL_SERVER_ERROR;
  import static io.netty.handler.codec.http.HttpResponseStatus.OK;
  
  @Slf4j
  public class HttpServerHandler  extends SimpleChannelInboundHandler<HttpObject> {
  
      private final CollectorRegistry registry = CollectorRegistry.defaultRegistry;
      private final StringBuilderWriter buf = new StringBuilderWriter();
  
      @Override
      public void channelReadComplete(ChannelHandlerContext ctx) {
          ctx.flush();
      }
  
      @Override
      public void channelRead0(ChannelHandlerContext ctx, HttpObject msg) {
          if (msg instanceof HttpRequest) {
              HttpRequest req = (HttpRequest) msg;
  
              boolean keepAlive = HttpUtil.isKeepAlive(req);
  
              buf.getBuilder().setLength(0);
              try {
                  TextFormat.write004(buf, registry.metricFamilySamples());
              } catch (IOException e) {
                  ctx.fireExceptionCaught(e);
                  return;
              }
              FullHttpResponse response = new DefaultFullHttpResponse(req.protocolVersion(), OK,
                  Unpooled.copiedBuffer(buf.getBuilder().toString(), CharsetUtil.UTF_8));
              response.headers()
                  .set(CONTENT_TYPE, TEXT_PLAIN)
                  .setInt(CONTENT_LENGTH, response.content().readableBytes());
  
              if (keepAlive) {
                  if (!req.protocolVersion().isKeepAliveDefault()) {
                      response.headers().set(CONNECTION, KEEP_ALIVE);
                  }
              } else {
                  // Tell the client we're going to close the connection.
                  response.headers().set(CONNECTION, CLOSE);
              }
  
              ChannelFuture f = ctx.write(response);
  
              if (!keepAlive) {
                  f.addListener(CLOSE);
              }
          }
      }
  
      @Override
      public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
          log.error("Prometheus exporter error", cause);
          FullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1,
              INTERNAL_SERVER_ERROR,
              Unpooled.wrappedBuffer(cause.getMessage().getBytes()));
          ctx.writeAndFlush(response).addListener(ChannelFutureListener.CLOSE);
          ctx.close();
      }
  }