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A java-native implementation of the zchunk file format
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README.md

zchunk-java

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This is a pure java-only implementation of the ZChunk file format. It does not hard depend on other libraries and can be used without any external or transitive dependencies.

What is zchunk?

zchunk is a compressed file format that splits the file into independent chunks. This allows you to only download changed chunks when downloading a new version of the file, and also makes zchunk files efficient over rsync.

zchunk files are protected with strong checksums to verify that the file you downloaded is, in fact, the file you wanted.

What is the goal of this project?

The goal is to create a pure java implementation of the zchunk file format. This way, java programs or Android apps will be able to use the zchunk file format.

Which dependencies do I need?

zchunk-java can be used without any transitives dependencies/libraries.

However, to have support for compression, you should use the arteifact zchunk-bundle-lib (TBD), which also pulls in support for zstd compression and maybe other compressions later.

The command line application (zchunk-app) is another matter. It uses picocli for parsing command lines, but other than that, it does not have any new dependencies. Since picocli is bundled with the app in an executable one-jar, there is no manual copying of dependencies.

Runtime Requirements and usage

  • Java 8 or newer, any JDK (adopt and adopt-openj9 work well).

To read a file header:

class Main {
  static void main(String[] args) {
    ZChunkFile zck = ZChunk.fromFile(inputFile);
    boolean isValid = ChecksumUtil.isValid(zck.getHeader());   
  }
}

writing a file

=> TBD

verifying checksum data

class Main {
  static void main(String[] args) {
    ZChunkFile zck = ZChunk.fromFile(inputFile);
    // total file validation (header, all single chunks, all chunk data).
    boolean isValid = ZChunk.validateFile(inputFile);
    
    
    // partial checksums:
    boolean isValidHeader = ChecksumUtil.isValidHeader(zck.getHeader(), inputFile);
    boolean isAllChunksValid = ChecksumUtil.allChunksAreValid(zck.getHeader(), inputFile);
    boolean isValidData = ChecksumUtil.isValidData(zck.getHeader(), inputFile);
  }
}

Downloading chunks

  • locally
  • Over HTTPS and HTTP/2.0
  • Over ssh/scp
    • This is not even implemented in the original code.
    • This can be done by executing dd if=file bs=1 count=X or similar.

=> TBD

executable jars

=> TBD

Building from source

Build requirements

  • Maven 3.3.1 or newer, or just use mvnw.

Installation

  • ./mvnw clean install
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