将此Java Deflater / Inflater与字典一起使用有什么问题

Question

I'm trying to use Inflater and Deflater with a dictionary but it's not working. When I run this simple test program:

import java.io.*;
import java.util.*;
import java.util.zip.*;

public class DictTest {

    public static void main(String[] args) throws Exception {

        final int level = 9;
        final boolean nowrap = true;

        // compress
        final Deflater def = new Deflater(level, nowrap);
        final byte[] abcd = new byte[] { 0x41, 0x42, 0x43, 0x44 };
        def.setDictionary(abcd);
        def.setInput(abcd);
        def.finish();
        final byte[] buf = new byte[1024];
        final int nbytes = def.deflate(buf);
        assert def.finished();
        def.end();

        // decompress
        final Inflater inf = new Inflater(nowrap);
        inf.setInput(buf, 0, nbytes + 1);       // include extra "dummy" byte
        while (true) {
            while (inf.inflate(buf) != 0) {
                // discard
            }
            assert !inf.needsInput();
            if (inf.finished())
                break;
            assert inf.needsDictionary();
            inf.setDictionary(abcd);
            continue;
        }
        inf.end();
    }
}

I get this exception:

$ javac DictTest.java && java -ea DictTest
Exception in thread "main" java.util.zip.DataFormatException: invalid distance too far back
    at java.util.zip.Inflater.inflateBytes(Native Method)
    at java.util.zip.Inflater.inflate(Inflater.java:259)
    at java.util.zip.Inflater.inflate(Inflater.java:280)
    at DictTest.main(DictTest.java:27)

What am I doing wrong? Thanks.

Answer 1

Set the inflater dictionary before you set input. Also, your infinite loop will run (forever). You wanted something like

final int level = 9;
final boolean nowrap = true;

// compress
final Deflater def = new Deflater(level, nowrap);
final byte[] abcd = new byte[] { 0x41, 0x42, 0x43, 0x44 };
def.setDictionary(abcd);
def.setInput(abcd);
def.finish();
final byte[] buf = new byte[1024];
final int nbytes = def.deflate(buf);
assert def.finished();
def.end();

// decompress
final Inflater inf = new Inflater(nowrap);
inf.setDictionary(abcd);
inf.setInput(buf); // include extra "dummy" byte
while (inf.inflate(buf) != 0) {
    // discard
}
assert !inf.needsInput();
assert inf.needsDictionary();
inf.end();

And it then runs without error here.