http://netty.io/netty-all/: Netty is an asynchronous event-driven network application framework for rapid development of maintainable high performance protocol servers and clients. (The Netty Project) 
/*
 * 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.compression;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelPromise;
import io.netty.channel.ChannelPromiseNotifier;
import io.netty.util.concurrent.EventExecutor;

import java.util.concurrent.TimeUnit;
import java.util.zip.CRC32;
import java.util.zip.Deflater;


Compresses a ByteBuf using the deflate algorithm. 
/**
 * Compresses a {@link ByteBuf} using the deflate algorithm.
 */
public class JdkZlibEncoder extends ZlibEncoder {

    private final ZlibWrapper wrapper;
    private final Deflater deflater;
    private volatile boolean finished;
    private volatile ChannelHandlerContext ctx;

    /*
     * GZIP support
     */
    private final CRC32 crc = new CRC32();
    private static final byte[] gzipHeader = {0x1f, (byte) 0x8b, Deflater.DEFLATED, 0, 0, 0, 0, 0, 0, 0};
    private boolean writeHeader = true;

    
Creates a new zlib encoder with the default compression level (6) and the default wrapper (ZlibWrapper.ZLIB). 
Throws:
/**
     * Creates a new zlib encoder with the default compression level ({@code 6})
     * and the default wrapper ({@link ZlibWrapper#ZLIB}).
     *
     * @throws CompressionException if failed to initialize zlib
     */
    public JdkZlibEncoder() {
        this(6);
    }

    
Creates a new zlib encoder with the specified compressionLevel and the default wrapper (ZlibWrapper.ZLIB). 
Params:
  • compressionLevel –  1 yields the fastest compression and 9 yields the best compression. 0 means no compression. The default compression level is 6.
Throws:
/**
     * Creates a new zlib encoder with the specified {@code compressionLevel}
     * and the default wrapper ({@link ZlibWrapper#ZLIB}).
     *
     * @param compressionLevel
     *        {@code 1} yields the fastest compression and {@code 9} yields the
     *        best compression.  {@code 0} means no compression.  The default
     *        compression level is {@code 6}.
     *
     * @throws CompressionException if failed to initialize zlib
     */
    public JdkZlibEncoder(int compressionLevel) {
        this(ZlibWrapper.ZLIB, compressionLevel);
    }

    
Creates a new zlib encoder with the default compression level (6) and the specified wrapper. 
Throws:
/**
     * Creates a new zlib encoder with the default compression level ({@code 6})
     * and the specified wrapper.
     *
     * @throws CompressionException if failed to initialize zlib
     */
    public JdkZlibEncoder(ZlibWrapper wrapper) {
        this(wrapper, 6);
    }

    
Creates a new zlib encoder with the specified compressionLevel and the specified wrapper. 
Params:
  • compressionLevel –  1 yields the fastest compression and 9 yields the best compression. 0 means no compression. The default compression level is 6.
Throws:
/**
     * Creates a new zlib encoder with the specified {@code compressionLevel}
     * and the specified wrapper.
     *
     * @param compressionLevel
     *        {@code 1} yields the fastest compression and {@code 9} yields the
     *        best compression.  {@code 0} means no compression.  The default
     *        compression level is {@code 6}.
     *
     * @throws CompressionException if failed to initialize zlib
     */
    public JdkZlibEncoder(ZlibWrapper wrapper, int compressionLevel) {
        if (compressionLevel < 0 || compressionLevel > 9) {
            throw new IllegalArgumentException(
                    "compressionLevel: " + compressionLevel + " (expected: 0-9)");
        }
        if (wrapper == null) {
            throw new NullPointerException("wrapper");
        }
        if (wrapper == ZlibWrapper.ZLIB_OR_NONE) {
            throw new IllegalArgumentException(
                    "wrapper '" + ZlibWrapper.ZLIB_OR_NONE + "' is not " +
                    "allowed for compression.");
        }

        this.wrapper = wrapper;
        deflater = new Deflater(compressionLevel, wrapper != ZlibWrapper.ZLIB);
    }

    
Creates a new zlib encoder with the default compression level (6) and the specified preset dictionary. The wrapper is always ZlibWrapper.ZLIB because it is the only format that supports the preset dictionary. 
Params:
  • dictionary –  the preset dictionary
Throws:
/**
     * Creates a new zlib encoder with the default compression level ({@code 6})
     * and the specified preset dictionary.  The wrapper is always
     * {@link ZlibWrapper#ZLIB} because it is the only format that supports
     * the preset dictionary.
     *
     * @param dictionary  the preset dictionary
     *
     * @throws CompressionException if failed to initialize zlib
     */
    public JdkZlibEncoder(byte[] dictionary) {
        this(6, dictionary);
    }

    
Creates a new zlib encoder with the specified compressionLevel and the specified preset dictionary. The wrapper is always ZlibWrapper.ZLIB because it is the only format that supports the preset dictionary. 
Params:
  • compressionLevel –  1 yields the fastest compression and 9 yields the best compression. 0 means no compression. The default compression level is 6.
  • dictionary –  the preset dictionary
Throws:
/**
     * Creates a new zlib encoder with the specified {@code compressionLevel}
     * and the specified preset dictionary.  The wrapper is always
     * {@link ZlibWrapper#ZLIB} because it is the only format that supports
     * the preset dictionary.
     *
     * @param compressionLevel
     *        {@code 1} yields the fastest compression and {@code 9} yields the
     *        best compression.  {@code 0} means no compression.  The default
     *        compression level is {@code 6}.
     * @param dictionary  the preset dictionary
     *
     * @throws CompressionException if failed to initialize zlib
     */
    public JdkZlibEncoder(int compressionLevel, byte[] dictionary) {
        if (compressionLevel < 0 || compressionLevel > 9) {
            throw new IllegalArgumentException(
                    "compressionLevel: " + compressionLevel + " (expected: 0-9)");
        }
        if (dictionary == null) {
            throw new NullPointerException("dictionary");
        }

        wrapper = ZlibWrapper.ZLIB;
        deflater = new Deflater(compressionLevel);
        deflater.setDictionary(dictionary);
    }

    @Override
    public ChannelFuture close() {
        return close(ctx().newPromise());
    }

    @Override
    public ChannelFuture close(final ChannelPromise promise) {
        ChannelHandlerContext ctx = ctx();
        EventExecutor executor = ctx.executor();
        if (executor.inEventLoop()) {
            return finishEncode(ctx, promise);
        } else {
            final ChannelPromise p = ctx.newPromise();
            executor.execute(new Runnable() {
                @Override
                public void run() {
                    ChannelFuture f = finishEncode(ctx(), p);
                    f.addListener(new ChannelPromiseNotifier(promise));
                }
            });
            return p;
        }
    }

    private ChannelHandlerContext ctx() {
        ChannelHandlerContext ctx = this.ctx;
        if (ctx == null) {
            throw new IllegalStateException("not added to a pipeline");
        }
        return ctx;
    }

    @Override
    public boolean isClosed() {
        return finished;
    }

    @Override
    protected void encode(ChannelHandlerContext ctx, ByteBuf uncompressed, ByteBuf out) throws Exception {
        if (finished) {
            out.writeBytes(uncompressed);
            return;
        }

        int len = uncompressed.readableBytes();
        if (len == 0) {
            return;
        }

        int offset;
        byte[] inAry;
        if (uncompressed.hasArray()) {
            // if it is backed by an array we not need to to do a copy at all
            inAry = uncompressed.array();
            offset = uncompressed.arrayOffset() + uncompressed.readerIndex();
            // skip all bytes as we will consume all of them
            uncompressed.skipBytes(len);
        } else {
            inAry = new byte[len];
            uncompressed.readBytes(inAry);
            offset = 0;
        }

        if (writeHeader) {
            writeHeader = false;
            if (wrapper == ZlibWrapper.GZIP) {
                out.writeBytes(gzipHeader);
            }
        }

        if (wrapper == ZlibWrapper.GZIP) {
            crc.update(inAry, offset, len);
        }

        deflater.setInput(inAry, offset, len);
        while (!deflater.needsInput()) {
            deflate(out);
        }
    }

    @Override
    protected final ByteBuf allocateBuffer(ChannelHandlerContext ctx, ByteBuf msg,
                                           boolean preferDirect) throws Exception {
        int sizeEstimate = (int) Math.ceil(msg.readableBytes() * 1.001) + 12;
        if (writeHeader) {
            switch (wrapper) {
                case GZIP:
                    sizeEstimate += gzipHeader.length;
                    break;
                case ZLIB:
                    sizeEstimate += 2; // first two magic bytes
                    break;
                default:
                    // no op
            }
        }
        return ctx.alloc().heapBuffer(sizeEstimate);
    }

    @Override
    public void close(final ChannelHandlerContext ctx, final ChannelPromise promise) throws Exception {
        ChannelFuture f = finishEncode(ctx, ctx.newPromise());
        f.addListener(new ChannelFutureListener() {
            @Override
            public void operationComplete(ChannelFuture f) throws Exception {
                ctx.close(promise);
            }
        });

        if (!f.isDone()) {
            // Ensure the channel is closed even if the write operation completes in time.
            ctx.executor().schedule(new Runnable() {
                @Override
                public void run() {
                    ctx.close(promise);
                }
            }, 10, TimeUnit.SECONDS); // FIXME: Magic number
        }
    }

    private ChannelFuture finishEncode(final ChannelHandlerContext ctx, ChannelPromise promise) {
        if (finished) {
            promise.setSuccess();
            return promise;
        }

        finished = true;
        ByteBuf footer = ctx.alloc().heapBuffer();
        if (writeHeader && wrapper == ZlibWrapper.GZIP) {
            // Write the GZIP header first if not written yet. (i.e. user wrote nothing.)
            writeHeader = false;
            footer.writeBytes(gzipHeader);
        }

        deflater.finish();

        while (!deflater.finished()) {
            deflate(footer);
            if (!footer.isWritable()) {
                // no more space so write it to the channel and continue
                ctx.write(footer);
                footer = ctx.alloc().heapBuffer();
            }
        }
        if (wrapper == ZlibWrapper.GZIP) {
            int crcValue = (int) crc.getValue();
            int uncBytes = deflater.getTotalIn();
            footer.writeByte(crcValue);
            footer.writeByte(crcValue >>> 8);
            footer.writeByte(crcValue >>> 16);
            footer.writeByte(crcValue >>> 24);
            footer.writeByte(uncBytes);
            footer.writeByte(uncBytes >>> 8);
            footer.writeByte(uncBytes >>> 16);
            footer.writeByte(uncBytes >>> 24);
        }
        deflater.end();
        return ctx.writeAndFlush(footer, promise);
    }

    private void deflate(ByteBuf out) {
        int numBytes;
        do {
            int writerIndex = out.writerIndex();
            numBytes = deflater.deflate(
                    out.array(), out.arrayOffset() + writerIndex, out.writableBytes(), Deflater.SYNC_FLUSH);
            out.writerIndex(writerIndex + numBytes);
        } while (numBytes > 0);
    }

    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        this.ctx = ctx;
    }
}