If you want to use it for your own projects, you can download it on Curse (http://www.curse.com/mc-mods/minecraft/226991-java-version-checker) or bundled with one of my other mods, Hardcore Ender Expansion (http://hee.chylex.com/)

The mod is in Public Domain, feel free to do whatever you want with it! Hugs and mentions appreciated but not required.

Shading Java 8 Checker into your mod

Warning: this is experimental, if you find any issues, please report them immediately.

You can insert Java Checker into your mod, so that it will alert the user if they are using outdated Java. You will need to get a modified version of the Java Checker mod, and place it in the libs folder of your mod setup.

You can download Java 8+ or Java 7+ checkers. Currently they are both for 1.7.10 only, there will be a more proper update later (the currently released versions have a bug that prevents shading).

Then you need to configure your build.gradle file to shade the checker into your own mod files. Use this as a guide, make sure to update everything in <> brackets.

configurations{
  shade
  compile.extendsFrom shade
}

<your buildscript info is somewhere here>

dependencies{
  shade files('libs/Java<version>Checker MC-1.7.10 v1.1-shade.jar')
}

<your mod info is somewhere here>

minecraft{
  srgExtra "PK: chylex/javacheck <yourmodname>/shade/javacheck"
}

jar{
  configurations.shade.each{ dep ->
    from(project.zipTree(dep)){
      exclude 'META-INF', 'META-INF/**', 'mcmod.info'
    }
  }
  
  manifest{
    attributes 'FMLCorePlugin': '<yourmodname>.shade.javacheck.Java<version>CheckerCoremod'
    attributes 'FMLCorePluginContainsFMLMod': 'true'
  }
}

You can also use Hardcore Ender Expansion's build.gradle as a reference.

Now build the mod and run it to make sure nothing messed up. If you need help with something, ask AbrarSyed because I have absolutely no idea what I'm doing :P.