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I've been thinking about checking jar file's hash value to determine if it has changed or not, but as it turns out the same jar file has different hashes every time I build it (export as jar file from eclipse, or build it using maven). I've removed manifest file's date values and stuff but it still is different. Is there something in bytecode generation which includes a timestamp or something?
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So, a Word file and the PDF file published from the Word file may contain the same content, but the HASH value will be different. Even copying the content from one file to another in the same software program can result in different HASH values, or even different file sizes.
Unless the file has changed its own content upon execution, or you have written to the file in some way (even metadata within the file itself), then the hash will not change. One flipped bit in a file will cause an entirely new hash in a secure hashing algorithm.
Application metadata resides within the file and moves with the file, not changing unless the contents of the file are altered. System metadata resides outside the file and can be altered without impacting the contents of the file. Hashing the file hashes its contents, not information about the file.
The contents of a file are processed through a cryptographic algorithm, and a unique numerical value – the hash value - is produced that identifies the contents of the file. If the contents are modified in any way, the value of the hash will also change significantly.
A JAR file is a ZIP file and it contains a last modified date in its local file headers and central directory file header. This will lead to different hashes of your builds.
If you run the JAR command on the exact same set of files (with same file dates) and skip manifest file creation it should give you the exact same JAR file (if the order of files inside the ZIP does not change).
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