java - Version issue in custom jar

Question

I am posting this question as I have limited resources to test or confirm by my own. Problem statement : I have an old custom Jar working with java 1.6. We don't have source code as well. To fix one of the issue we updated 2 .class file of this jar using 7-zip.The code change was just to update the existing loggers with more meaningful messages. These 2 .class file compiled with java 1.7. Now the questions is whether this jar will work correctly with java 1.6 or will generate java version issue as 2 of the .class files are compiled with java 1.7. Note - I know it's easy to test this rather then asking here but my test environment is only supporting java 1.8 and it's working perfectly but whether it will run with java 1.6 or not I can't test.

Answer 1

If you compile using: javac Foo.java where that javac is from a JDK1.7 installation, and you then attempt to run the class file that results on a JDK1.6, it will not work.

However, all you need to do is this:

javac -source 1.6 -target 1.6 Foo.java, and then it will, unless you used features from 1.7. If you use language features (I can't think of any, so I doubt it), it won't compile, and thus you know. If you use API, it will compile and you won't know. There is no easy solution to this other than compiling with JDK1.6 (or compiling with javac7 against a bootcp of JDK1.6, but you need to download and install a JDK1.6 to get that; might as well just use javac6 then).