/*
    * Copyright 2015 The Netty Project
    *
    * The Netty Project 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:
    *
    *   https://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 io.netty.handler.codec.http2;
  
  import io.netty.channel.Channel;
  import io.netty.handler.codec.http2.Http2HeadersEncoder.SensitivityDetector;
  import io.netty.util.internal.UnstableApi;
  
  import static io.netty.handler.codec.http2.Http2CodecUtil.DEFAULT_HEADER_LIST_SIZE;
  import static io.netty.handler.codec.http2.Http2CodecUtil.DEFAULT_MAX_RESERVED_STREAMS;
  import static io.netty.handler.codec.http2.Http2PromisedRequestVerifier.ALWAYS_VERIFY;
  import static io.netty.util.internal.ObjectUtil.checkNotNull;
  import static io.netty.util.internal.ObjectUtil.checkPositiveOrZero;
  
  /**
   * Abstract base class which defines commonly used features required to build {@link Http2ConnectionHandler} instances.
   *
   * <h3>Three ways to build a {@link Http2ConnectionHandler}</h3>
   * <h4>Let the builder create a {@link Http2ConnectionHandler}</h4>
   * Simply call all the necessary setter methods, and then use {@link #build()} to build a new
   * {@link Http2ConnectionHandler}. Setting the following properties are prohibited because they are used for
   * other ways of building a {@link Http2ConnectionHandler}.
   * conflicts with this option:
   * <ul>
   *   <li>{@link #connection(Http2Connection)}</li>
   *   <li>{@link #codec(Http2ConnectionDecoder, Http2ConnectionEncoder)}</li>
   * </ul>
   *
   *
   * <h4>Let the builder use the {@link Http2ConnectionHandler} you specified</h4>
   * Call {@link #connection(Http2Connection)} to tell the builder that you want to build the handler from the
   * {@link Http2Connection} you specified. Setting the following properties are prohibited and thus will trigger
   * an {@link IllegalStateException} because they conflict with this option.
   * <ul>
   *   <li>{@link #server(boolean)}</li>
   *   <li>{@link #codec(Http2ConnectionDecoder, Http2ConnectionEncoder)}</li>
   * </ul>
   *
   * <h4>Let the builder use the {@link Http2ConnectionDecoder} and {@link Http2ConnectionEncoder} you specified</h4>
   * Call {@link #codec(Http2ConnectionDecoder, Http2ConnectionEncoder)} to tell the builder that you want to built the
   * handler from the {@link Http2ConnectionDecoder} and {@link Http2ConnectionEncoder} you specified. Setting the
   * following properties are prohibited and thus will trigger an {@link IllegalStateException} because they conflict
   * with this option:
   * <ul>
   *   <li>{@link #server(boolean)}</li>
   *   <li>{@link #connection(Http2Connection)}</li>
   *   <li>{@link #frameLogger(Http2FrameLogger)}</li>
   *   <li>{@link #headerSensitivityDetector(SensitivityDetector)}</li>
   *   <li>{@link #encoderEnforceMaxConcurrentStreams(boolean)}</li>
   *   <li>{@link #encoderIgnoreMaxHeaderListSize(boolean)}</li>
   * </ul>
   *
   * <h3>Exposing necessary methods in a subclass</h3>
   * {@link #build()} method and all property access methods are {@code protected}. Choose the methods to expose to the
   * users of your builder implementation and make them {@code public}.
   *
   * @param <T> The type of handler created by this builder.
   * @param <B> The concrete type of this builder.
   */
  @UnstableApi
  public abstract class AbstractHttp2ConnectionHandlerBuilder<T extends Http2ConnectionHandler,
                                                              B extends AbstractHttp2ConnectionHandlerBuilder<T, B>> {
  
      private static final SensitivityDetector DEFAULT_HEADER_SENSITIVITY_DETECTOR = Http2HeadersEncoder.NEVER_SENSITIVE;
  
      private static final int DEFAULT_MAX_RST_FRAMES_PER_CONNECTION_FOR_SERVER = 200;
  
      // The properties that can always be set.
      private Http2Settings initialSettings = Http2Settings.defaultSettings();
      private Http2FrameListener frameListener;
      private long gracefulShutdownTimeoutMillis = Http2CodecUtil.DEFAULT_GRACEFUL_SHUTDOWN_TIMEOUT_MILLIS;
      private boolean decoupleCloseAndGoAway;
      private boolean flushPreface = true;
  
      // The property that will prohibit connection() and codec() if set by server(),
      // because this property is used only when this builder creates an Http2Connection.
      private Boolean isServer;
      private Integer maxReservedStreams;
  
      // The property that will prohibit server() and codec() if set by connection().
      private Http2Connection connection;
  
      // The properties that will prohibit server() and connection() if set by codec().
      private Http2ConnectionDecoder decoder;
      private Http2ConnectionEncoder encoder;
 
     // The properties that are:
     // * mutually exclusive against codec() and
     // * OK to use with server() and connection()
     private Boolean validateHeaders;
     private Http2FrameLogger frameLogger;
     private SensitivityDetector headerSensitivityDetector;
     private Boolean encoderEnforceMaxConcurrentStreams;
     private Boolean encoderIgnoreMaxHeaderListSize;
     private Http2PromisedRequestVerifier promisedRequestVerifier = ALWAYS_VERIFY;
     private boolean autoAckSettingsFrame = true;
     private boolean autoAckPingFrame = true;
     private int maxQueuedControlFrames = Http2CodecUtil.DEFAULT_MAX_QUEUED_CONTROL_FRAMES;
     private int maxConsecutiveEmptyFrames = 2;
     private Integer maxRstFramesPerWindow;
     private int secondsPerWindow = 30;
 
     /**
      * Sets the {@link Http2Settings} to use for the initial connection settings exchange.
      */
     protected Http2Settings initialSettings() {
         return initialSettings;
     }
 
     /**
      * Sets the {@link Http2Settings} to use for the initial connection settings exchange.
      */
     protected B initialSettings(Http2Settings settings) {
         initialSettings = checkNotNull(settings, "settings");
         return self();
     }
 
     /**
      * Returns the listener of inbound frames.
      *
      * @return {@link Http2FrameListener} if set, or {@code null} if not set.
      */
     protected Http2FrameListener frameListener() {
         return frameListener;
     }
 
     /**
      * Sets the listener of inbound frames.
      * This listener will only be set if the decoder's listener is {@code null}.
      */
     protected B frameListener(Http2FrameListener frameListener) {
         this.frameListener = checkNotNull(frameListener, "frameListener");
         return self();
     }
 
     /**
      * Returns the graceful shutdown timeout of the {@link Http2Connection} in milliseconds. Returns -1 if the
      * timeout is indefinite.
      */
     protected long gracefulShutdownTimeoutMillis() {
         return gracefulShutdownTimeoutMillis;
     }
 
     /**
      * Sets the graceful shutdown timeout of the {@link Http2Connection} in milliseconds.
      */
     protected B gracefulShutdownTimeoutMillis(long gracefulShutdownTimeoutMillis) {
         if (gracefulShutdownTimeoutMillis < -1) {
             throw new IllegalArgumentException("gracefulShutdownTimeoutMillis: " + gracefulShutdownTimeoutMillis +
                                                " (expected: -1 for indefinite or >= 0)");
         }
         this.gracefulShutdownTimeoutMillis = gracefulShutdownTimeoutMillis;
         return self();
     }
 
     /**
      * Returns if {@link #build()} will to create a {@link Http2Connection} in server mode ({@code true})
      * or client mode ({@code false}).
      */
     protected boolean isServer() {
         return isServer != null ? isServer : true;
     }
 
     /**
      * Sets if {@link #build()} will to create a {@link Http2Connection} in server mode ({@code true})
      * or client mode ({@code false}).
      */
     protected B server(boolean isServer) {
         enforceConstraint("server", "connection", connection);
         enforceConstraint("server", "codec", decoder);
         enforceConstraint("server", "codec", encoder);
 
         this.isServer = isServer;
         return self();
     }
 
     /**
      * Get the maximum number of streams which can be in the reserved state at any given time.
      * <p>
      * By default this value will be ignored on the server for local endpoint. This is because the RFC provides
      * no way to explicitly communicate a limit to how many states can be in the reserved state, and instead relies
      * on the peer to send RST_STREAM frames when they will be rejected.
      */
     protected int maxReservedStreams() {
         return maxReservedStreams != null ? maxReservedStreams : DEFAULT_MAX_RESERVED_STREAMS;
     }
 
     /**
      * Set the maximum number of streams which can be in the reserved state at any given time.
      */
     protected B maxReservedStreams(int maxReservedStreams) {
         enforceConstraint("server", "connection", connection);
         enforceConstraint("server", "codec", decoder);
         enforceConstraint("server", "codec", encoder);
 
         this.maxReservedStreams = checkPositiveOrZero(maxReservedStreams, "maxReservedStreams");
         return self();
     }
 
     /**
      * Returns the {@link Http2Connection} to use.
      *
      * @return {@link Http2Connection} if set, or {@code null} if not set.
      */
     protected Http2Connection connection() {
         return connection;
     }
 
     /**
      * Sets the {@link Http2Connection} to use.
      */
     protected B connection(Http2Connection connection) {
         enforceConstraint("connection", "maxReservedStreams", maxReservedStreams);
         enforceConstraint("connection", "server", isServer);
         enforceConstraint("connection", "codec", decoder);
         enforceConstraint("connection", "codec", encoder);
 
         this.connection = checkNotNull(connection, "connection");
 
         return self();
     }
 
     /**
      * Returns the {@link Http2ConnectionDecoder} to use.
      *
      * @return {@link Http2ConnectionDecoder} if set, or {@code null} if not set.
      */
     protected Http2ConnectionDecoder decoder() {
         return decoder;
     }
 
     /**
      * Returns the {@link Http2ConnectionEncoder} to use.
      *
      * @return {@link Http2ConnectionEncoder} if set, or {@code null} if not set.
      */
     protected Http2ConnectionEncoder encoder() {
         return encoder;
     }
 
     /**
      * Sets the {@link Http2ConnectionDecoder} and {@link Http2ConnectionEncoder} to use.
      */
     protected B codec(Http2ConnectionDecoder decoder, Http2ConnectionEncoder encoder) {
         enforceConstraint("codec", "server", isServer);
         enforceConstraint("codec", "maxReservedStreams", maxReservedStreams);
         enforceConstraint("codec", "connection", connection);
         enforceConstraint("codec", "frameLogger", frameLogger);
         enforceConstraint("codec", "validateHeaders", validateHeaders);
         enforceConstraint("codec", "headerSensitivityDetector", headerSensitivityDetector);
         enforceConstraint("codec", "encoderEnforceMaxConcurrentStreams", encoderEnforceMaxConcurrentStreams);
 
         checkNotNull(decoder, "decoder");
         checkNotNull(encoder, "encoder");
 
         if (decoder.connection() != encoder.connection()) {
             throw new IllegalArgumentException("The specified encoder and decoder have different connections.");
         }
 
         this.decoder = decoder;
         this.encoder = encoder;
 
         return self();
     }
 
     /**
      * Returns if HTTP headers should be validated according to
      * <a href="https://tools.ietf.org/html/rfc7540#section-8.1.2.6">RFC 7540, 8.1.2.6</a>.
      */
     protected boolean isValidateHeaders() {
         return validateHeaders != null ? validateHeaders : true;
     }
 
     /**
      * Sets if HTTP headers should be validated according to
      * <a href="https://tools.ietf.org/html/rfc7540#section-8.1.2.6">RFC 7540, 8.1.2.6</a>.
      */
     protected B validateHeaders(boolean validateHeaders) {
         enforceNonCodecConstraints("validateHeaders");
         this.validateHeaders = validateHeaders;
         return self();
     }
 
     /**
      * Returns the logger that is used for the encoder and decoder.
      *
      * @return {@link Http2FrameLogger} if set, or {@code null} if not set.
      */
     protected Http2FrameLogger frameLogger() {
         return frameLogger;
     }
 
     /**
      * Sets the logger that is used for the encoder and decoder.
      */
     protected B frameLogger(Http2FrameLogger frameLogger) {
         enforceNonCodecConstraints("frameLogger");
         this.frameLogger = checkNotNull(frameLogger, "frameLogger");
         return self();
     }
 
     /**
      * Returns if the encoder should queue frames if the maximum number of concurrent streams
      * would otherwise be exceeded.
      */
     protected boolean encoderEnforceMaxConcurrentStreams() {
         return encoderEnforceMaxConcurrentStreams != null ? encoderEnforceMaxConcurrentStreams : false;
     }
 
     /**
      * Sets if the encoder should queue frames if the maximum number of concurrent streams
      * would otherwise be exceeded.
      */
     protected B encoderEnforceMaxConcurrentStreams(boolean encoderEnforceMaxConcurrentStreams) {
         enforceNonCodecConstraints("encoderEnforceMaxConcurrentStreams");
         this.encoderEnforceMaxConcurrentStreams = encoderEnforceMaxConcurrentStreams;
         return self();
     }
 
     /**
      * Returns the maximum number of queued control frames that are allowed before the connection is closed.
      * This allows to protected against various attacks that can lead to high CPU / memory usage if the remote-peer
      * floods us with frames that would have us produce control frames, but stops to read from the underlying socket.
      *
      * {@code 0} means no protection is in place.
      */
     protected int encoderEnforceMaxQueuedControlFrames() {
         return maxQueuedControlFrames;
     }
 
     /**
      * Sets the maximum number of queued control frames that are allowed before the connection is closed.
      * This allows to protected against various attacks that can lead to high CPU / memory usage if the remote-peer
      * floods us with frames that would have us produce control frames, but stops to read from the underlying socket.
      *
      * {@code 0} means no protection should be applied.
      */
     protected B encoderEnforceMaxQueuedControlFrames(int maxQueuedControlFrames) {
         enforceNonCodecConstraints("encoderEnforceMaxQueuedControlFrames");
         this.maxQueuedControlFrames = checkPositiveOrZero(maxQueuedControlFrames, "maxQueuedControlFrames");
         return self();
     }
 
     /**
      * Returns the {@link SensitivityDetector} to use.
      */
     protected SensitivityDetector headerSensitivityDetector() {
         return headerSensitivityDetector != null ? headerSensitivityDetector : DEFAULT_HEADER_SENSITIVITY_DETECTOR;
     }
 
     /**
      * Sets the {@link SensitivityDetector} to use.
      */
     protected B headerSensitivityDetector(SensitivityDetector headerSensitivityDetector) {
         enforceNonCodecConstraints("headerSensitivityDetector");
         this.headerSensitivityDetector = checkNotNull(headerSensitivityDetector, "headerSensitivityDetector");
         return self();
     }
 
     /**
      * Sets if the <a href="https://tools.ietf.org/html/rfc7540#section-6.5.2">SETTINGS_MAX_HEADER_LIST_SIZE</a>
      * should be ignored when encoding headers.
      * @param ignoreMaxHeaderListSize {@code true} to ignore
      * <a href="https://tools.ietf.org/html/rfc7540#section-6.5.2">SETTINGS_MAX_HEADER_LIST_SIZE</a>.
      * @return this.
      */
     protected B encoderIgnoreMaxHeaderListSize(boolean ignoreMaxHeaderListSize) {
         enforceNonCodecConstraints("encoderIgnoreMaxHeaderListSize");
         encoderIgnoreMaxHeaderListSize = ignoreMaxHeaderListSize;
         return self();
     }
 
     /**
      * Does nothing, do not call.
      *
      * @deprecated Huffman decoding no longer depends on having a decode capacity.
      */
     @Deprecated
     protected B initialHuffmanDecodeCapacity(int initialHuffmanDecodeCapacity) {
         return self();
     }
 
     /**
      * Set the {@link Http2PromisedRequestVerifier} to use.
      * @return this.
      */
     protected B promisedRequestVerifier(Http2PromisedRequestVerifier promisedRequestVerifier) {
         enforceNonCodecConstraints("promisedRequestVerifier");
         this.promisedRequestVerifier = checkNotNull(promisedRequestVerifier, "promisedRequestVerifier");
         return self();
     }
 
     /**
      * Get the {@link Http2PromisedRequestVerifier} to use.
      * @return the {@link Http2PromisedRequestVerifier} to use.
      */
     protected Http2PromisedRequestVerifier promisedRequestVerifier() {
         return promisedRequestVerifier;
     }
 
     /**
      * Returns the maximum number of consecutive empty DATA frames (without end_of_stream flag) that are allowed before
      * the connection is closed. This allows to protect against the remote peer flooding us with such frames and
      * so use up a lot of CPU. There is no valid use-case for empty DATA frames without end_of_stream flag.
      *
      * {@code 0} means no protection is in place.
      */
     protected int decoderEnforceMaxConsecutiveEmptyDataFrames() {
         return maxConsecutiveEmptyFrames;
     }
 
     /**
      * Sets the maximum number of consecutive empty DATA frames (without end_of_stream flag) that are allowed before
      * the connection is closed. This allows to protect against the remote peer flooding us with such frames and
      * so use up a lot of CPU. There is no valid use-case for empty DATA frames without end_of_stream flag.
      *
      * {@code 0} means no protection should be applied.
      */
     protected B decoderEnforceMaxConsecutiveEmptyDataFrames(int maxConsecutiveEmptyFrames) {
         enforceNonCodecConstraints("maxConsecutiveEmptyFrames");
         this.maxConsecutiveEmptyFrames = checkPositiveOrZero(
                 maxConsecutiveEmptyFrames, "maxConsecutiveEmptyFrames");
         return self();
     }
 
     /**
      * Sets the maximum number RST frames that are allowed per window before
      * the connection is closed. This allows to protect against the remote peer flooding us with such frames and
      * so use up a lot of CPU.
      *
      * {@code 0} for any of the parameters means no protection should be applied.
      */
     protected B decoderEnforceMaxRstFramesPerWindow(int maxRstFramesPerWindow, int secondsPerWindow) {
         enforceNonCodecConstraints("decoderEnforceMaxRstFramesPerWindow");
         this.maxRstFramesPerWindow = checkPositiveOrZero(
                 maxRstFramesPerWindow, "maxRstFramesPerWindow");
         this.secondsPerWindow = checkPositiveOrZero(secondsPerWindow, "secondsPerWindow");
         return self();
     }
 
     /**
      * Determine if settings frame should automatically be acknowledged and applied.
      * @return this.
      */
     protected B autoAckSettingsFrame(boolean autoAckSettings) {
         enforceNonCodecConstraints("autoAckSettingsFrame");
         autoAckSettingsFrame = autoAckSettings;
         return self();
     }
 
     /**
      * Determine if the SETTINGS frames should be automatically acknowledged and applied.
      * @return {@code true} if the SETTINGS frames should be automatically acknowledged and applied.
      */
     protected boolean isAutoAckSettingsFrame() {
         return autoAckSettingsFrame;
     }
 
     /**
      * Determine if PING frame should automatically be acknowledged or not.
      * @return this.
      */
     protected B autoAckPingFrame(boolean autoAckPingFrame) {
         enforceNonCodecConstraints("autoAckPingFrame");
         this.autoAckPingFrame = autoAckPingFrame;
         return self();
     }
 
     /**
      * Determine if the PING frames should be automatically acknowledged or not.
      * @return {@code true} if the PING frames should be automatically acknowledged.
      */
     protected boolean isAutoAckPingFrame() {
         return autoAckPingFrame;
     }
 
     /**
      * Determine if the {@link Channel#close()} should be coupled with goaway and graceful close.
      * @param decoupleCloseAndGoAway {@code true} to make {@link Channel#close()} directly close the underlying
      *   transport, and not attempt graceful closure via GOAWAY.
      * @return {@code this}.
      */
     protected B decoupleCloseAndGoAway(boolean decoupleCloseAndGoAway) {
         this.decoupleCloseAndGoAway = decoupleCloseAndGoAway;
         return self();
     }
 
     /**
      * Determine if the {@link Channel#close()} should be coupled with goaway and graceful close.
      */
     protected boolean decoupleCloseAndGoAway() {
         return decoupleCloseAndGoAway;
     }
 
     /**
      * Determine if the <a href="https://datatracker.ietf.org/doc/html/rfc7540#section-3.5">Preface</a>
      * should be automatically flushed when the {@link Channel} becomes active or not.
      * <p>
      * Client may choose to opt-out from this automatic behavior and manage flush manually if it's ready to send
      * request frames immediately after the preface. It may help to avoid unnecessary latency.
      *
      * @param flushPreface {@code true} to automatically flush, {@code false otherwise}.
      * @return {@code this}.
      * @see <a href="https://datatracker.ietf.org/doc/html/rfc7540#section-3.5">HTTP/2 Connection Preface</a>
      */
     protected B flushPreface(boolean flushPreface) {
         this.flushPreface = flushPreface;
         return self();
     }
 
     /**
      * Determine if the <a href="https://datatracker.ietf.org/doc/html/rfc7540#section-3.5">Preface</a>
      * should be automatically flushed when the {@link Channel} becomes active or not.
      * <p>
      * Client may choose to opt-out from this automatic behavior and manage flush manually if it's ready to send
      * request frames immediately after the preface. It may help to avoid unnecessary latency.
      *
      * @return {@code true} if automatically flushed.
      * @see <a href="https://datatracker.ietf.org/doc/html/rfc7540#section-3.5">HTTP/2 Connection Preface</a>
      */
     protected boolean flushPreface() {
         return flushPreface;
     }
 
     /**
      * Create a new {@link Http2ConnectionHandler}.
      */
     protected T build() {
         if (encoder != null) {
             assert decoder != null;
             return buildFromCodec(decoder, encoder);
         }
 
         Http2Connection connection = this.connection;
         if (connection == null) {
             connection = new DefaultHttp2Connection(isServer(), maxReservedStreams());
         }
 
         return buildFromConnection(connection);
     }
 
     private T buildFromConnection(Http2Connection connection) {
         Long maxHeaderListSize = initialSettings.maxHeaderListSize();
         Http2FrameReader reader = new DefaultHttp2FrameReader(new DefaultHttp2HeadersDecoder(isValidateHeaders(),
                 maxHeaderListSize == null ? DEFAULT_HEADER_LIST_SIZE : maxHeaderListSize,
                 /* initialHuffmanDecodeCapacity= */ -1));
         Http2FrameWriter writer = encoderIgnoreMaxHeaderListSize == null ?
                 new DefaultHttp2FrameWriter(headerSensitivityDetector()) :
                 new DefaultHttp2FrameWriter(headerSensitivityDetector(), encoderIgnoreMaxHeaderListSize);
 
         if (frameLogger != null) {
             reader = new Http2InboundFrameLogger(reader, frameLogger);
             writer = new Http2OutboundFrameLogger(writer, frameLogger);
         }
 
         Http2ConnectionEncoder encoder = new DefaultHttp2ConnectionEncoder(connection, writer);
         boolean encoderEnforceMaxConcurrentStreams = encoderEnforceMaxConcurrentStreams();
 
         if (maxQueuedControlFrames != 0) {
             encoder = new Http2ControlFrameLimitEncoder(encoder, maxQueuedControlFrames);
         }
         if (encoderEnforceMaxConcurrentStreams) {
             if (connection.isServer()) {
                 encoder.close();
                 reader.close();
                 throw new IllegalArgumentException(
                         "encoderEnforceMaxConcurrentStreams: " + encoderEnforceMaxConcurrentStreams +
                         " not supported for server");
             }
             encoder = new StreamBufferingEncoder(encoder);
         }
 
         DefaultHttp2ConnectionDecoder decoder = new DefaultHttp2ConnectionDecoder(connection, encoder, reader,
             promisedRequestVerifier(), isAutoAckSettingsFrame(), isAutoAckPingFrame(), isValidateHeaders());
         return buildFromCodec(decoder, encoder);
     }
 
     private T buildFromCodec(Http2ConnectionDecoder decoder, Http2ConnectionEncoder encoder) {
         int maxConsecutiveEmptyDataFrames = decoderEnforceMaxConsecutiveEmptyDataFrames();
         if (maxConsecutiveEmptyDataFrames > 0) {
             decoder = new Http2EmptyDataFrameConnectionDecoder(decoder, maxConsecutiveEmptyDataFrames);
         }
         final int maxRstFrames;
         if (maxRstFramesPerWindow == null) {
             // Only enable by default on the server.
             if (isServer()) {
                 maxRstFrames = DEFAULT_MAX_RST_FRAMES_PER_CONNECTION_FOR_SERVER;
             } else {
                 maxRstFrames = 0;
             }
         } else {
             maxRstFrames = maxRstFramesPerWindow;
         }
         if (maxRstFrames > 0 && secondsPerWindow > 0) {
             decoder = new Http2MaxRstFrameDecoder(decoder, maxRstFrames, secondsPerWindow);
         }
         final T handler;
         try {
             // Call the abstract build method
             handler = build(decoder, encoder, initialSettings);
         } catch (Throwable t) {
             encoder.close();
             decoder.close();
             throw new IllegalStateException("failed to build an Http2ConnectionHandler", t);
         }
 
         // Setup post build options
         handler.gracefulShutdownTimeoutMillis(gracefulShutdownTimeoutMillis);
         if (handler.decoder().frameListener() == null) {
             handler.decoder().frameListener(frameListener);
         }
         return handler;
     }
 
     /**
      * Implement this method to create a new {@link Http2ConnectionHandler} or its subtype instance.
      * <p>
      * The return of this method will be subject to the following:
      * <ul>
      *   <li>{@link #frameListener(Http2FrameListener)} will be set if not already set in the decoder</li>
      *   <li>{@link #gracefulShutdownTimeoutMillis(long)} will always be set</li>
      * </ul>
      */
     protected abstract T build(Http2ConnectionDecoder decoder, Http2ConnectionEncoder encoder,
                                Http2Settings initialSettings) throws Exception;
 
     /**
      * Returns {@code this}.
      */
     @SuppressWarnings("unchecked")
     protected final B self() {
         return (B) this;
     }
 
     private void enforceNonCodecConstraints(String rejected) {
         enforceConstraint(rejected, "server/connection", decoder);
         enforceConstraint(rejected, "server/connection", encoder);
     }
 
     private static void enforceConstraint(String methodName, String rejectorName, Object value) {
         if (value != null) {
             throw new IllegalStateException(
                     methodName + "() cannot be called because " + rejectorName + "() has been called already.");
         }
     }
 }