LengthFieldPrepender.java

/*
 * Copyright 2012 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:
 *
 *   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 io.netty.handler.codec;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandler.Sharable;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelOutboundHandler;
import io.netty.channel.ChannelOutboundHandlerAdapter;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.ChannelPromise;
import io.netty.util.internal.ObjectUtil;

import java.nio.ByteOrder;

/**
 * An encoder that prepends the length of the message.  The length value is
 * prepended as a binary form.
 * <p>
 * For example, <tt>{@link LengthFieldPrepender}(2)</tt> will encode the
 * following 12-bytes string:
 * <pre>
 * +----------------+
 * | "HELLO, WORLD" |
 * +----------------+
 * </pre>
 * into the following:
 * <pre>
 * +--------+----------------+
 * + 0x000C | "HELLO, WORLD" |
 * +--------+----------------+
 * </pre>
 * If you turned on the {@code lengthIncludesLengthFieldLength} flag in the
 * constructor, the encoded data would look like the following
 * (12 (original data) + 2 (prepended data) = 14 (0xE)):
 * <pre>
 * +--------+----------------+
 * + 0x000E | "HELLO, WORLD" |
 * +--------+----------------+
 * </pre>
 */
@Sharable
public class LengthFieldPrepender extends ChannelOutboundHandlerAdapter {

    private final ByteOrder byteOrder;
    private final int lengthFieldLength;
    private final boolean lengthIncludesLengthFieldLength;
    private final int lengthAdjustment;

    /**
     * Creates a new instance.
     *
     * @param lengthFieldLength the length of the prepended length field.
     *                          Only 1, 2, 3, 4, and 8 are allowed.
     *
     * @throws IllegalArgumentException
     *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
     */
    public LengthFieldPrepender(int lengthFieldLength) {
        this(lengthFieldLength, false);
    }

    /**
     * Creates a new instance.
     *
     * @param lengthFieldLength the length of the prepended length field.
     *                          Only 1, 2, 3, 4, and 8 are allowed.
     * @param lengthIncludesLengthFieldLength
     *                          if {@code true}, the length of the prepended
     *                          length field is added to the value of the
     *                          prepended length field.
     *
     * @throws IllegalArgumentException
     *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
     */
    public LengthFieldPrepender(int lengthFieldLength, boolean lengthIncludesLengthFieldLength) {
        this(lengthFieldLength, 0, lengthIncludesLengthFieldLength);
    }

    /**
     * Creates a new instance.
     *
     * @param lengthFieldLength the length of the prepended length field.
     *                          Only 1, 2, 3, 4, and 8 are allowed.
     * @param lengthAdjustment  the compensation value to add to the value
     *                          of the length field
     *
     * @throws IllegalArgumentException
     *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
     */
    public LengthFieldPrepender(int lengthFieldLength, int lengthAdjustment) {
        this(lengthFieldLength, lengthAdjustment, false);
    }

    /**
     * Creates a new instance.
     *
     * @param lengthFieldLength the length of the prepended length field.
     *                          Only 1, 2, 3, 4, and 8 are allowed.
     * @param lengthAdjustment  the compensation value to add to the value
     *                          of the length field
     * @param lengthIncludesLengthFieldLength
     *                          if {@code true}, the length of the prepended
     *                          length field is added to the value of the
     *                          prepended length field.
     *
     * @throws IllegalArgumentException
     *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
     */
    public LengthFieldPrepender(int lengthFieldLength, int lengthAdjustment, boolean lengthIncludesLengthFieldLength) {
        this(ByteOrder.BIG_ENDIAN, lengthFieldLength, lengthAdjustment, lengthIncludesLengthFieldLength);
    }

    /**
     * Creates a new instance.
     *
     * @param byteOrder         the {@link ByteOrder} of the length field
     * @param lengthFieldLength the length of the prepended length field.
     *                          Only 1, 2, 3, 4, and 8 are allowed.
     * @param lengthAdjustment  the compensation value to add to the value
     *                          of the length field
     * @param lengthIncludesLengthFieldLength
     *                          if {@code true}, the length of the prepended
     *                          length field is added to the value of the
     *                          prepended length field.
     *
     * @throws IllegalArgumentException
     *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
     */
    public LengthFieldPrepender(
            ByteOrder byteOrder, int lengthFieldLength,
            int lengthAdjustment, boolean lengthIncludesLengthFieldLength) {
        if (lengthFieldLength != 1 && lengthFieldLength != 2 &&
            lengthFieldLength != 3 && lengthFieldLength != 4 &&
            lengthFieldLength != 8) {
            throw new IllegalArgumentException(
                    "lengthFieldLength must be either 1, 2, 3, 4, or 8: " +
                    lengthFieldLength);
        }
        ObjectUtil.checkNotNull(byteOrder, "byteOrder");

        this.byteOrder = byteOrder;
        this.lengthFieldLength = lengthFieldLength;
        this.lengthIncludesLengthFieldLength = lengthIncludesLengthFieldLength;
        this.lengthAdjustment = lengthAdjustment;
    }

    /**
     * Returns {@code true} if the given message should be handled. If {@code false} it will be passed to the next
     * {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     */
    private static boolean acceptOutboundMessage(Object msg) {
        return msg instanceof ByteBuf;
    }

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        try {
            if (acceptOutboundMessage(msg)) {
                @SuppressWarnings("unchecked")
                ByteBuf cast = (ByteBuf) msg;

                int length = cast.readableBytes() + lengthAdjustment;
                if (lengthIncludesLengthFieldLength) {
                    length += lengthFieldLength;
                }

                if (length < 0) {
                    throw new IllegalArgumentException(
                            "Adjusted frame length (" + length + ") is less than zero");
                }

                switch (lengthFieldLength) {
                    case 1:
                        if (length >= 256) {
                            throw new IllegalArgumentException(
                                    "length does not fit into a byte: " + length);
                        }
                        ctx.write(ctx.alloc().buffer(1).order(byteOrder).writeByte((byte) length), ctx.voidPromise());
                        break;
                    case 2:
                        if (length >= 65536) {
                            throw new IllegalArgumentException(
                                    "length does not fit into a short integer: " + length);
                        }
                        ctx.write(ctx.alloc().buffer(2).order(byteOrder).writeShort((short) length), ctx.voidPromise());
                        break;
                    case 3:
                        if (length >= 16777216) {
                            throw new IllegalArgumentException(
                                    "length does not fit into a medium integer: " + length);
                        }
                        ctx.write(ctx.alloc().buffer(3).order(byteOrder).writeMedium(length), ctx.voidPromise());
                        break;
                    case 4:
                        ctx.write(ctx.alloc().buffer(4).order(byteOrder).writeInt(length), ctx.voidPromise());
                        break;
                    case 8:
                        ctx.write(ctx.alloc().buffer(8).order(byteOrder).writeLong(length), ctx.voidPromise());
                        break;
                    default:
                        throw new Error("should not reach here");
                }
            }
            ctx.write(msg, promise);
        } catch (EncoderException e) {
            throw e;
        } catch (Exception e) {
            throw new EncoderException(e);
        }
    }
}