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    1   /*
    2    * Copyright 2009 Red Hat, Inc.
    3    *
    4    * Red Hat licenses this file to you under the Apache License, version 2.0
    5    * (the "License"); you may not use this file except in compliance with the
    6    * License.  You may obtain a copy of the License at:
    7    *
    8    *    http://www.apache.org/licenses/LICENSE-2.0
    9    *
    10   * Unless required by applicable law or agreed to in writing, software
    11   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
    12   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
    13   * License for the specific language governing permissions and limitations
    14   * under the License.
    15   */
    16  package org.jboss.netty.handler.codec.frame;
    17
    18  import org.jboss.netty.buffer.ChannelBuffer;
    19  import org.jboss.netty.buffer.ChannelBufferFactory;
    20  import org.jboss.netty.channel.Channel;
    21  import org.jboss.netty.channel.ChannelHandlerContext;
    22  import org.jboss.netty.channel.Channels;
    23  import org.jboss.netty.handler.codec.serialization.ObjectDecoder;
    24
    25  /**
    26   * A decoder that splits the received {@link ChannelBuffer}s dynamically by the
    27   * value of the length field in the message.  It is particularly useful when you
    28   * decode a binary message which has an integer header field that represents the
    29   * length of the message body or the whole message.
    30   * <p>
    31   * {@link LengthFieldBasedFrameDecoder} has many configuration parameters so
    32   * that it can decode any message with a length field, which is often seen in
    33   * proprietary client-server protocols. Here are some example that will give
    34   * you the basic idea on which option does what.
    35   *
    36   * <h3>2 bytes length field at offset 0, do not strip header</h3>
    37   *
    38   * The value of the length field in this example is <tt>12 (0x0C)</tt> which
    39   * represents the length of "HELLO, WORLD".  By default, the decoder assumes
    40   * that the length field represents the number of the bytes that follows the
    41   * length field.  Therefore, it can be decoded with the simplistic parameter
    42   * combination.
    43   * <pre>
    44   * <b>lengthFieldOffset</b>   = <b>0</b>
    45   * <b>lengthFieldLength</b>   = <b>2</b>
    46   * lengthAdjustment    = 0
    47   * initialBytesToStrip = 0 (= do not strip header)
    48   *
    49   * BEFORE DECODE (14 bytes)         AFTER DECODE (14 bytes)
    50   * +--------+----------------+      +--------+----------------+
    51   * | Length | Actual Content |----->| Length | Actual Content |
    52   * | 0x000C | "HELLO, WORLD" |      | 0x000C | "HELLO, WORLD" |
    53   * +--------+----------------+      +--------+----------------+
    54   * </pre>
    55   *
    56   * <h3>2 bytes length field at offset 0, strip header</h3>
    57   *
    58   * Because we can get the length of the content by calling
    59   * {@link ChannelBuffer#readableBytes()}, you might want to strip the length
    60   * field by specifying <tt>initialBytesToStrip</tt>.  In this example, we
    61   * specified <tt>2</tt>, that is same with the length of the length field, to
    62   * strip the first two bytes.
    63   * <pre>
    64   * lengthFieldOffset   = 0
    65   * lengthFieldLength   = 2
    66   * lengthAdjustment    = 0
    67   * <b>initialBytesToStrip</b> = <b>2</b> (= the length of the Length field)
    68   *
    69   * BEFORE DECODE (14 bytes)         AFTER DECODE (12 bytes)
    70   * +--------+----------------+      +----------------+
    71   * | Length | Actual Content |----->| Actual Content |
    72   * | 0x000C | "HELLO, WORLD" |      | "HELLO, WORLD" |
    73   * +--------+----------------+      +----------------+
    74   * </pre>
    75   *
    76   * <h3>2 bytes length field at offset 0, do not strip header, the length field
    77   *     represents the length of the whole message</h3>
    78   *
    79   * In most cases, the length field represents the length of the message body
    80   * only, as shown in the previous examples.  However, in some protocols, the
    81   * length field represents the length of the whole message, including the
    82   * message header.  In such a case, we specify a non-zero
    83   * <tt>lengthAdjustment</tt>.  Because the length value in this example message
    84   * is always greater than the body length by <tt>2</tt>, we specify <tt>-2</tt>
    85   * as <tt>lengthAdjustment</tt> for compensation.
    86   * <pre>
    87   * lengthFieldOffset   =  0
    88   * lengthFieldLength   =  2
    89   * <b>lengthAdjustment</b>    = <b>-2</b> (= the length of the Length field)
    90   * initialBytesToStrip =  0
    91   *
    92   * BEFORE DECODE (14 bytes)         AFTER DECODE (14 bytes)
    93   * +--------+----------------+      +--------+----------------+
    94   * | Length | Actual Content |----->| Length | Actual Content |
    95   * | 0x000E | "HELLO, WORLD" |      | 0x000E | "HELLO, WORLD" |
    96   * +--------+----------------+      +--------+----------------+
    97   * </pre>
    98   *
    99   * <h3>3 bytes length field at the end of 5 bytes header, do not strip header</h3>
    100  *
    101  * The following message is a simple variation of the first example.  An extra
    102  * header value is prepended to the message.  <tt>lengthAdjustment</tt> is zero
    103  * again because the decoder always takes the length of the prepended data into
    104  * account during frame length calculation.
    105  * <pre>
    106  * <b>lengthFieldOffset</b>   = <b>2</b> (= the length of Header 1)
    107  * <b>lengthFieldLength</b>   = <b>3</b>
    108  * lengthAdjustment    = 0
    109  * initialBytesToStrip = 0
    110  *
    111  * BEFORE DECODE (17 bytes)                      AFTER DECODE (17 bytes)
    112  * +----------+----------+----------------+      +----------+----------+----------------+
    113  * | Header 1 |  Length  | Actual Content |----->| Header 1 |  Length  | Actual Content |
    114  * |  0xCAFE  | 0x00000C | "HELLO, WORLD" |      |  0xCAFE  | 0x00000C | "HELLO, WORLD" |
    115  * +----------+----------+----------------+      +----------+----------+----------------+
    116  * </pre>
    117  *
    118  * <h3>3 bytes length field at the beginning of 5 bytes header, do not strip header</h3>
    119  *
    120  * This is an advanced example that shows the case where there is an extra
    121  * header between the length field and the message body.  You have to specify a
    122  * positive <tt>lengthAdjustment</tt> so that the decoder counts the extra
    123  * header into the frame length calculation.
    124  * <pre>
    125  * lengthFieldOffset   = 0
    126  * lengthFieldLength   = 3
    127  * <b>lengthAdjustment</b>    = <b>2</b> (= the length of Header 1)
    128  * initialBytesToStrip = 0
    129  *
    130  * BEFORE DECODE (17 bytes)                      AFTER DECODE (17 bytes)
    131  * +----------+----------+----------------+      +----------+----------+----------------+
    132  * |  Length  | Header 1 | Actual Content |----->|  Length  | Header 1 | Actual Content |
    133  * | 0x00000C |  0xCAFE  | "HELLO, WORLD" |      | 0x00000C |  0xCAFE  | "HELLO, WORLD" |
    134  * +----------+----------+----------------+      +----------+----------+----------------+
    135  * </pre>
    136  *
    137  * <h3>2 bytes length field at offset 1 in the middle of 4 bytes header,
    138  *     strip the first header field and the length field</h3>
    139  *
    140  * This is a combination of all the examples above.  There are the prepended
    141  * header before the length field and the extra header after the length field.
    142  * The prepended header affects the <tt>lengthFieldOffset</tt> and the extra
    143  * header affects the <tt>lengthAdjustment</tt>.  We also specified a non-zero
    144  * <tt>initialBytesToStrip</tt> to strip the length field and the prepended
    145  * header from the frame.  If you don't want to strip the prepended header, you
    146  * could specify <tt>0</tt> for <tt>initialBytesToSkip</tt>.
    147  * <pre>
    148  * lengthFieldOffset</b>   = 1 (= the length of HDR1)
    149  * lengthFieldLength</b>   = 2
    150  * <b>lengthAdjustment</b>    = <b>1</b> (= the length of HDR2)
    151  * <b>initialBytesToStrip</b> = <b>3</b> (= the length of HDR1 + LEN)
    152  *
    153  * BEFORE DECODE (16 bytes)                       AFTER DECODE (13 bytes)
    154  * +------+--------+------+----------------+      +------+----------------+
    155  * | HDR1 | Length | HDR2 | Actual Content |----->| HDR2 | Actual Content |
    156  * | 0xCA | 0x000C | 0xFE | "HELLO, WORLD" |      | 0xFE | "HELLO, WORLD" |
    157  * +------+--------+------+----------------+      +------+----------------+
    158  * </pre>
    159  *
    160  * <h3>2 bytes length field at offset 1 in the middle of 4 bytes header,
    161  *     strip the first header field and the length field, the length field
    162  *     represents the length of the whole message</h3>
    163  *
    164  * Let's give another twist to the previous example.  The only difference from
    165  * the previous example is that the length field represents the length of the
    166  * whole message instead of the message body, just like the third example.
    167  * We have to count the length of HDR1 and Length into <tt>lengthAdjustment</tt>.
    168  * Please note that we don't need to take the length of HDR2 into account
    169  * because the length field already includes the whole header length.
    170  * <pre>
    171  * lengthFieldOffset   =  1
    172  * lengthFieldLength   =  2
    173  * <b>lengthAdjustment</b>    = <b>-3</b> (= the length of HDR1 + LEN, negative)
    174  * <b>initialBytesToStrip</b> = <b> 3</b>
    175  *
    176  * BEFORE DECODE (16 bytes)                       AFTER DECODE (13 bytes)
    177  * +------+--------+------+----------------+      +------+----------------+
    178  * | HDR1 | Length | HDR2 | Actual Content |----->| HDR2 | Actual Content |
    179  * | 0xCA | 0x0010 | 0xFE | "HELLO, WORLD" |      | 0xFE | "HELLO, WORLD" |
    180  * +------+--------+------+----------------+      +------+----------------+
    181  * </pre>
    182  *
    183  * @author <a href="http://www.jboss.org/netty/">The Netty Project</a>
    184  * @author <a href="http://gleamynode.net/">Trustin Lee</a>
    185  *
    186  * @version $Rev:231 $, $Date:2008-06-12 16:44:50 +0900 (목, 12 6월 2008) $
    187  *
    188  * @see LengthFieldPrepender
    189  */
    190 public class LengthFieldBasedFrameDecoder extends FrameDecoder {
    191
    192     private final int maxFrameLength;
    193     private final int lengthFieldOffset;
    194     private final int lengthFieldLength;
    195     private final int lengthFieldEndOffset;
    196     private final int lengthAdjustment;
    197     private final int initialBytesToStrip;
    198     private boolean discardingTooLongFrame;
    199     private long tooLongFrameLength;
    200     private long bytesToDiscard;
    201
    202     /**
    203      * Creates a new instance.
    204      *
    205      * @param maxFrameLength
    206      *        the maximum length of the frame.  If the length of the frame is
    207      *        greater than this value, {@link TooLongFrameException} will be
    208      *        thrown.
    209      * @param lengthFieldOffset
    210      *        the offset of the length field
    211      * @param lengthFieldLength
    212      *        the length of the length field
    213      *
    214      */
    215     public LengthFieldBasedFrameDecoder(
    216             int maxFrameLength,
    217             int lengthFieldOffset, int lengthFieldLength) {
    218         this(maxFrameLength, lengthFieldOffset, lengthFieldLength, 0, 0);
    219     }
    220
    221     /**
    222      * Creates a new instance.
    223      *
    224      * @param maxFrameLength
    225      *        the maximum length of the frame.  If the length of the frame is
    226      *        greater than this value, {@link TooLongFrameException} will be
    227      *        thrown.
    228      * @param lengthFieldOffset
    229      *        the offset of the length field
    230      * @param lengthFieldLength
    231      *        the length of the length field
    232      * @param lengthAdjustment
    233      *        the compensation value to add to the value of the length field
    234      * @param initialBytesToStrip
    235      *        the number of first bytes to strip out from the decoded frame
    236      */
    237     public LengthFieldBasedFrameDecoder(
    238             int maxFrameLength,
    239             int lengthFieldOffset, int lengthFieldLength,
    240             int lengthAdjustment, int initialBytesToStrip) {
    241         if (maxFrameLength <= 0) {
    242             throw new IllegalArgumentException(
    243                     "maxFrameLength must be a positive integer: " +
    244                     maxFrameLength);
    245         }
    246
    247         if (lengthFieldOffset < 0) {
    248             throw new IllegalArgumentException(
    249                     "lengthFieldOffset must be a non-negative integer: " +
    250                     lengthFieldOffset);
    251         }
    252
    253         if (initialBytesToStrip < 0) {
    254             throw new IllegalArgumentException(
    255                     "initialBytesToStrip must be a non-negative integer: " +
    256                     initialBytesToStrip);
    257         }
    258
    259         if (lengthFieldLength != 1 && lengthFieldLength != 2 &&
    260             lengthFieldLength != 3 && lengthFieldLength != 4 &&
    261             lengthFieldLength != 8) {
    262             throw new IllegalArgumentException(
    263                     "lengthFieldLength must be either 1, 2, 3, 4, or 8: " +
    264                     lengthFieldLength);
    265         }
    266
    267         if (lengthFieldOffset > maxFrameLength - lengthFieldLength) {
    268             throw new IllegalArgumentException(
    269                     "maxFrameLength (" + maxFrameLength + ") " +
    270                     "must be equal to or greater than " +
    271                     "lengthFieldOffset (" + lengthFieldOffset + ") + " +
    272                     "lengthFieldLength (" + lengthFieldLength + ").");
    273         }
    274
    275         this.maxFrameLength = maxFrameLength;
    276         this.lengthFieldOffset = lengthFieldOffset;
    277         this.lengthFieldLength = lengthFieldLength;
    278         this.lengthAdjustment = lengthAdjustment;
    279         lengthFieldEndOffset = lengthFieldOffset + lengthFieldLength;
    280         this.initialBytesToStrip = initialBytesToStrip;
    281     }
    282
    283     @Override
    284     protected Object decode(
    285             ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception {
    286
    287         if (discardingTooLongFrame) {
    288             long bytesToDiscard = this.bytesToDiscard;
    289             int localBytesToDiscard = (int) Math.min(bytesToDiscard, buffer.readableBytes());
    290             buffer.skipBytes(localBytesToDiscard);
    291             bytesToDiscard -= localBytesToDiscard;
    292             this.bytesToDiscard = bytesToDiscard;
    293             if (bytesToDiscard == 0) {
    294                 // Reset to the initial state and tell the handlers that
    295                 // the frame was too large.
    296                 // TODO Let user choose when the exception should be raised - early or late?
    297                 //      If early, fail() should be called when discardingTooLongFrame is set to true.
    298                 long tooLongFrameLength = this.tooLongFrameLength;
    299                 this.tooLongFrameLength = 0;
    300                 fail(ctx, tooLongFrameLength);
    301             } else {
    302                 // Keep discarding.
    303             }
    304             return null;
    305         }
    306
    307         if (buffer.readableBytes() < lengthFieldEndOffset) {
    308             return null;
    309         }
    310
    311         int actualLengthFieldOffset = buffer.readerIndex() + lengthFieldOffset;
    312         long frameLength;
    313         switch (lengthFieldLength) {
    314         case 1:
    315             frameLength = buffer.getUnsignedByte(actualLengthFieldOffset);
    316             break;
    317         case 2:
    318             frameLength = buffer.getUnsignedShort(actualLengthFieldOffset);
    319             break;
    320         case 3:
    321             frameLength = buffer.getUnsignedMedium(actualLengthFieldOffset);
    322             break;
    323         case 4:
    324             frameLength = buffer.getUnsignedInt(actualLengthFieldOffset);
    325             break;
    326         case 8:
    327             frameLength = buffer.getLong(actualLengthFieldOffset);
    328             break;
    329         default:
    330             throw new Error("should not reach here");
    331         }
    332
    333         if (frameLength < 0) {
    334             buffer.skipBytes(lengthFieldEndOffset);
    335             throw new CorruptedFrameException(
    336                     "negative pre-adjustment length field: " + frameLength);
    337         }
    338
    339         frameLength += lengthAdjustment + lengthFieldEndOffset;
    340         if (frameLength < lengthFieldEndOffset) {
    341             buffer.skipBytes(lengthFieldEndOffset);
    342             throw new CorruptedFrameException(
    343                     "Adjusted frame length (" + frameLength + ") is less " +
    344                     "than lengthFieldEndOffset: " + lengthFieldEndOffset);
    345         }
    346
    347         if (frameLength > maxFrameLength) {
    348             // Enter the discard mode and discard everything received so far.
    349             discardingTooLongFrame = true;
    350             tooLongFrameLength = frameLength;
    351             bytesToDiscard = frameLength - buffer.readableBytes();
    352             buffer.skipBytes(buffer.readableBytes());
    353             return null;
    354         }
    355
    356         // never overflows because it's less than maxFrameLength
    357         int frameLengthInt = (int) frameLength;
    358         if (buffer.readableBytes() < frameLengthInt) {
    359             return null;
    360         }
    361
    362         if (initialBytesToStrip > frameLengthInt) {
    363             buffer.skipBytes(frameLengthInt);
    364             throw new CorruptedFrameException(
    365                     "Adjusted frame length (" + frameLength + ") is less " +
    366                     "than initialBytesToStrip: " + initialBytesToStrip);
    367         }
    368         buffer.skipBytes(initialBytesToStrip);
    369
    370         // extract frame
    371         int readerIndex = buffer.readerIndex();
    372         int actualFrameLength = frameLengthInt - initialBytesToStrip;
    373         ChannelBuffer frame = extractFrame(buffer, readerIndex, actualFrameLength);
    374         buffer.readerIndex(readerIndex + actualFrameLength);
    375         return frame;
    376     }
    377
    378     /**
    379      * Extract the sub-region of the specified buffer. This method is called by
    380      * {@link #decode(ChannelHandlerContext, Channel, ChannelBuffer)} for each
    381      * frame.  The default implementation returns a copy of the sub-region.
    382      * For example, you could override this method to use an alternative
    383      * {@link ChannelBufferFactory}.
    384      * <p>
    385      * If you are sure that the frame and its content are not accessed after
    386      * the current {@link #decode(ChannelHandlerContext, Channel, ChannelBuffer)}
    387      * call returns, you can even avoid memory copy by returning the sliced
    388      * sub-region (i.e. <tt>return buffer.slice(index, length)</tt>).
    389      * It's often useful when you convert the extracted frame into an object.
    390      * Refer to the source code of {@link ObjectDecoder} to see how this method
    391      * is overridden to avoid memory copy.
    392      */
    393     protected ChannelBuffer extractFrame(ChannelBuffer buffer, int index, int length) {
    394         ChannelBuffer frame = buffer.factory().getBuffer(length);
    395         frame.writeBytes(buffer, index, length);
    396         return frame;
    397     }
    398
    399     private void fail(ChannelHandlerContext ctx, long frameLength) {
    400         if (frameLength > 0) {
    401             Channels.fireExceptionCaught(
    402                     ctx.getChannel(),
    403                     new TooLongFrameException(
    404                             "Adjusted frame length exceeds " + maxFrameLength +
    405                             ": " + frameLength + " - discarded"));
    406         } else {
    407             Channels.fireExceptionCaught(
    408                     ctx.getChannel(),
    409                     new TooLongFrameException(
    410                             "Adjusted frame length exceeds " + maxFrameLength +
    411                             " - discarding"));
    412         }
    413     }
    414 }