prefixedstringdecoder.java

mina是以Java实现的一个开源的网络程序框架

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 *  Licensed to the Apache Software Foundation (ASF) under one
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 *  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
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 *  KIND, either express or implied.  See the License for the
 *  specific language governing permissions and limitations
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package org.apache.mina.filter.codec.prefixedstring;

import org.apache.mina.core.buffer.BufferDataException;
import org.apache.mina.core.buffer.IoBuffer;
import org.apache.mina.core.session.IoSession;
import org.apache.mina.filter.codec.CumulativeProtocolDecoder;
import org.apache.mina.filter.codec.ProtocolDecoder;
import org.apache.mina.filter.codec.ProtocolDecoderOutput;

import java.nio.charset.Charset;

/**
 * A {@link ProtocolDecoder} which decodes a String using a fixed-length length prefix.
 *
 * @author The Apache MINA Project ([e-mail])
 * @version $Rev: 591182 $, $Date: 2007-11-02 01:59:27 +0100 (vr, 02 nov 2007) $,
 */
public class PrefixedStringDecoder extends CumulativeProtocolDecoder {

    public final static int DEFAULT_PREFIX_LENGTH = 4;

    public final static int DEFAULT_MAX_DATA_LENGTH = 2048;

    private final Charset charset;

    private int prefixLength = DEFAULT_PREFIX_LENGTH;

    private int maxDataLength = DEFAULT_MAX_DATA_LENGTH;

    /**
     * @param charset       the charset to use for encoding
     * @param prefixLength  the length of the prefix
     * @param maxDataLength maximum number of bytes allowed for a single String
     */
    public PrefixedStringDecoder(Charset charset, int prefixLength, int maxDataLength) {
        this.charset = charset;
        this.prefixLength = prefixLength;
        this.maxDataLength = maxDataLength;
    }

    public PrefixedStringDecoder(Charset charset, int prefixLength) {
        this(charset, prefixLength, DEFAULT_MAX_DATA_LENGTH);
    }

    public PrefixedStringDecoder(Charset charset) {
        this(charset, DEFAULT_PREFIX_LENGTH);
    }

    /**
     * Sets the number of bytes used by the length prefix
     *
     * @param prefixLength the length of the length prefix (1, 2, or 4)
     */
    public void setPrefixLength(int prefixLength) {
        this.prefixLength = prefixLength;
    }

    /**
     * Gets the length of the length prefix (1, 2, or 4)
     *
     * @return length of the length prefix
     */
    public int getPrefixLength() {
        return prefixLength;
    }

    /**
     * Sets the maximum allowed value specified as data length in the incoming data
     * <p>
     * Useful for preventing an OutOfMemory attack by the peer.
     * The decoder will throw a {@link BufferDataException} when data length
     * specified in the incoming data is greater than maxDataLength
     * The default value is {@link PrefixedStringDecoder#DEFAULT_MAX_DATA_LENGTH}.
     * </p>
     *
     * @param maxDataLength maximum allowed value specified as data length in the incoming data
     */
    public void setMaxDataLength(int maxDataLength) {
        this.maxDataLength = maxDataLength;
    }

    /**
     * Gets the maximum number of bytes allowed for a single String
     *
     * @return maximum number of bytes allowed for a single String
     */
    public int getMaxDataLength() {
        return maxDataLength;
    }

    protected boolean doDecode(IoSession session, IoBuffer in, ProtocolDecoderOutput out) throws Exception {
        if (in.prefixedDataAvailable(prefixLength, maxDataLength)) {
            String msg = in.getPrefixedString(prefixLength, charset.newDecoder());
            out.write(msg);
            return true;
        } else {
            return false;
        }
    }
}