This post was inspired by a hypothetical closed source piece of software from a hardware vendor, written in Java, which has unusable font rendering that makes it inaccessible for me, but I need to use it for class, so what am I to do? I want to write evil LD_PRELOAD hacks but it's probably easier to patch the program itself, so that's what we're going to do.

I use IntelliJ IDEA for my Java work. It includes quite a nice Java decompiler, which is (probably) intentionally not exposed to the user in its full functionality, but includes a main class that lets us access it anyway.

First, make an IntelliJ project for your sources. Include all the libraries that they depend on. Now, time for some mild reversing!

Decompile the bad JAR file (hat tip to StackOverflow):

PS> $p = 'C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.2.1\plugins\java-decompiler\lib\java-decompiler.jar'
PS> mkdir decomp
PS> java -cp $p .\ProblemProgram.jar decomp

Then, you will get a source JAR with all the sources in it. You can just unzip this with whatever tool you prefer:

PS> Expand-Archive decomp/ProblemProgram.jar -dest src

You should have all the files in your source directory and can work on them!

There are probably a pile of compile errors, because decompilers aren't perfect. They are, however, likely fairly easy to fix to convince the project to build. In the tool I patched, it was primarily mysteriously inserted redefinitions and javac getting confused about generics.

Time to patch!

Classes that you are looking for are subclassing JPanel or similar AWT classes. They should have a setFont call you can patch, and an implementation of paint(Graphics). First, patch the setFont in the sources to use a better font (because their choice is probably not good):

this.setFont(new Font("Iosevka", 0, 14));

Then, for the magic incantations to patch the actual rendering (thanks again, StackOverflow):

// at the top of the file
import java.awt.Graphics2D;
import java.awt.RenderingHints;

// in paint(Graphics g)
((Graphics2D) g).setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_LCD_HRGB);

This enables subpixel antialiasing (which is superior to the default antialiasing type that ends up rather blurry).

Recompile, and you can do the final stage of patching:

Reincorporating the patches

You can use the jar tool included with your Java Development Kit to update the file. Note that the path to the class file must have the package name at its base:

# Make a backup!!
PS> cp ProblemProgram.jar ProblemProgram-orig.jar
# Patch it!
PS> jar uf ../ProblemProgram.jar com/problemcompany/problemprogram/UI.class

We replace only the class that is causing us problems to reduce exposure to anything bad that happened in the round trip through the decompiler.

Now, for the result:

Bonus fun

Font rendering may not be the only thing wrong with this closed source program, and you have to figure out some weird behaviours or find a configuration file. A debugger can be fantastically useful for this purpose. IntelliJ provides quite a smooth experience at debugging closed source code.

If you don't want to commit to fixing any decompilation errors, you can add the program's JAR as a library in File>Project Structure in IDEA, and it will let you set breakpoints in arbitrary class files without having to decompile and recompile them.

Run the program with a similar Java command to this:

PS> java '-agentlib:jdwp=transport=dt_socket,address=,server=y,suspend=y' -jar C:\ThatVendor\ProblematicProgram.jar

Once you have the configuration set up in IDEA, you can click the "Debug" button and it will connect to your JVM and start running the remote program.

In case we're thinking of the same program from a blue FPGA vendor

Monitor_Program/amp.config has a setting debug yes to enable a debug console, though it doesn't have much of interest in it. has a setting YOURUSERNAME-enable-source-level-debugging that, if disabled, as it seems to have done to itself initially, it disables all the file related functionality in the program, which is quite confusing indeed (and was the reason I first got out the decompiler).