java – Deflater.deflate和小输出缓冲区

我看到一个奇怪的情况,使用 Java 8u45的小输出缓冲区和 java.util.Deflater.deflate(byte[] b,int off,int len,int flush)方法与小输出缓冲区一起使用时.

(我正在开发一些与WebSocket即将推出的扩展 – 扩展相关的低级网络代码,所以小缓冲区对我来说是现实的)

示例代码：

package deflate;

import java.nio.charset.StandardCharsets;
import java.util.zip.Deflater;

public class DeflaterSmallBufferBug
{
    public static void main(String[] args)
    {
        boolean nowrap = true;
        Deflater deflater = new Deflater(Deflater.DEFAULT_COMPRESSION,nowrap);

        byte[] input = "Hello".getBytes(StandardCharsets.UTF_8);

        System.out.printf("input is %,d bytes - %s%n",input.length,getHex(input,input.length));

        deflater.setInput(input);

        byte[] output = new byte[input.length];

        // break out of infinite loop seen with bug
        int maxloops = 10;

        // Compress the data
        while (maxloops-- > 0)
        {
            int compressed = deflater.deflate(output,output.length,Deflater.SYNC_FLUSH);
            System.out.printf("compressed %,compressed,getHex(output,compressed));

            if (compressed < output.length)
            {
                System.out.printf("Compress success");
                return;
            }
        }

        System.out.printf("Exited compress (maxloops left %d)%n",maxloops);
    }

    private static String getHex(byte[] buf,int offset,int len)
    {
        StringBuilder hex = new StringBuilder();
        hex.append('[');
        for (int i = offset; i < (offset + len); i++)
        {
            if (i > offset)
            {
                hex.append(' ');
            }
            hex.append(String.format("%02X",buf[i]));
        }
        hex.append(']');
        return hex.toString();
    }
}

在上面的例子中,我试图使用长度为5个字节的输出缓冲区为输入“Hello”生成压缩字节.

我假设以下结果字节：

buffer 1 [ F2 48 CD C9 C9 ]
buffer 2 [ 07 00 00 00 FF ]
buffer 3 [ FF ]

翻译为

[ F2 48 CD C9 C9 07 00 ] <-- the compressed data
[ 00 00 FF FF ]          <-- the deflate tail bytes

但是,当Deflater.deflate()与一个小缓冲区一起使用时,这个正常循环会无限延续5个字节的压缩数据(似乎只显示在5个字节或更低的缓冲区).

产生上述演示的结果输出…

input is 5 bytes - [48 65 6C 6C 6F]
compressed 5 bytes - [F2 48 CD C9 C9]
compressed 5 bytes - [07 00 00 00 FF]
compressed 5 bytes - [FF 00 00 00 FF]
compressed 5 bytes - [FF 00 00 00 FF]
compressed 5 bytes - [FF 00 00 00 FF]
compressed 5 bytes - [FF 00 00 00 FF]
compressed 5 bytes - [FF 00 00 00 FF]
compressed 5 bytes - [FF 00 00 00 FF]
compressed 5 bytes - [FF 00 00 00 FF]
compressed 5 bytes - [FF 00 00 00 FF]
Exited compress (maxloops left -1)

如果输入/输出大于5个字节,则问题似乎消失了. (只需输入字符串“Hellox”即可自行测试)

使缓冲区为6个字节的结果(输入为“Hellox”)

input is 6 bytes - [48 65 6C 6C 6F 78]
compressed 6 bytes - [F2 48 CD C9 C9 AF]
compressed 6 bytes - [00 00 00 00 FF FF]
compressed 5 bytes - [00 00 00 FF FF]
Compress success

即使这些结果对我来说也有点古怪,因为它似乎存在2个缩减的尾字节序列.

所以,我想我的最终问题是,我是否遗漏了一些关于Deflater用法的东西,这对我来说很奇怪,或者这是否指向JVM Deflater实现本身可能存在的错误？

更新：2015年8月7日

该发现已被接受为bugs.java.com/JDK-8133170

解决方法

In the case of a Z_FULL_FLUSH or Z_SYNC_FLUSH,make sure that
avail_out is greater than six to avoid repeated flush markers due to
avail_out == 0 on return.

发生的事情是每次使用Z_SYNC_FLUSH调用deflate()都会插入一个五字节的flush标记.由于您没有提供足够的输出空间来获取标记,因此您再次调用以获得更多输出,但要求它同时插入另一个刷新标记.

您应该做的是使用Z_SYNC_FLUSH调用deflate()一次,然后使用Z_NO_FLUSH(或Java中的NO_FLUSH),在必要时使用额外的deflate()调用获取所有可用输出.

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