How to list the JDK compatibility (like “sourceCompatibility”) of all of the dependencies with a gradle project?

Question

I need to know the Java runtime compatibility of all of the dependencies I'm using in my gradle project (as if it were checking each project's "sourceCompatibility" setting so to speak), preferrably without perusing each dependency's documentation to find it, if it's there at all. Is there a way to do this with gradle? Or even some other automating tool?

(Specifically I'm trying to see which dependencies might be using Java 8 features like Streams, since I'm trying to compile for Android with retrolambda, and iOS with RoboVM.)

Answer 1

Such information probably does not exist, and is very tedious to generate.

"sourceCompatibility" does not exist after the source is compiled-- it's just to tell the compiler how to interpret the source syntax. What you're more likely to be interested in is the "targetCompatibility", or the class file format major version: Java 8 is 52, Java 7 is 51, etc.

This tells Java that Java 8 is required to understand the class format and bytecodes contained in the class, so you could download and unpack every dependency in your project, and all of their dependencies, and then look at the version number of every class file, except...

Simply looking at the class file format version doesn't tell you whether the class makes reference to methods and fields that exist only in specific versions of the JDK. I've not tested with Java 8, but in Java 7 it's possible to set the source/target compatibility to 1.6 and still reference new methods that were added in Java 7. A Java 6 JVM will load and run the file, but fail with NoSuchMethodException despite otherwise looking perfectly fine.

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The only way to realistically check if a dependency is completely compatible with a different version of Java than the one it was compiled for is to go through the constant pool of every class, find every class and method reference, and then verify that they are valid for the desired JRE.

You will want an automated tool for this (The JVM could do it if you have the desired version of Java installed and 100% coverage in your unit tests...), but I don't know of a standalone tool that does, and neither gradle nor project documentation usually includes this sort of info.