# How to do binary diffs with Java jars

This is a simple example how to generate and apply a binary patch to a jar file.
In the hope that some tooling vendor or gradle / maven / repository backend guru
might pick this up to unban us from too long dependency download times :)

# Download test jars:
```
$ wget http://central.maven.org/maven2/com/google/guava/guava/18.0/guava-18.0.jar
$ wget http://central.maven.org/maven2/com/google/guava/guava/16.0.1/guava-16.0.1.jar
```

# Abtain bsdiff / bspatch:
http://www.daemonology.net/bsdiff/

#Create the binary patch file
```
$ bsdiff guava-16.0.1.jar guava-18.0.jar guava-16.0.1-18.0.patch
```

#List directory:
```
$ ll
total 10520
-rw-r--r--  1 tom  staff   878K Mar 12 10:45 guava-16.0.1-18.0.patch
-rw-r--r--  1 tom  staff   2.1M Feb  3  2014 guava-16.0.1.jar
-rw-r--r--  1 tom  staff   2.2M Aug 25  2014 guava-18.0.jar
```

As one can see the binary patch file is aroung 900kb

#Rename the original jar file
```
$ mv guava-18.0.jar guava-18.0.jar.orig
```

#compute the MD5 hash of the original file
```
$ md5 guava-18.0.jar.orig

MD5 (guava-18.0.jar.orig) = 947641f6bb535b1d942d1bc387c45290
```

# Apply the binary patch file
```
$ bspatch guava-16.0.1.jar guava-18.0.jar guava-16.0.1-18.0.patch
```

# List the directory again
```
$ ll
total 14928
-rw-r--r--  1 tom  staff   878K Mar 12 10:45 guava-16.0.1-18.0.patch
-rw-r--r--  1 tom  staff   2.1M Feb  3  2014 guava-16.0.1.jar
-rw-r--r--  1 tom  staff   2.2M Mar 12 10:51 guava-18.0.jar
-rw-r--r--  1 tom  staff   2.2M Aug 25  2014 guava-18.0.jar.orig
```

#Compute the MD5 hash of the newly created file
```
$ md5 guava-18.0.jar
MD5 (guava-18.0.jar) = 947641f6bb535b1d942d1bc387c45290
```

The checksums match as one can see :)

I think it would be a nice feature and a huge bandwith saver if maven / gradle and the repository backends would
support binary diff for jars.