Load config resources from classpath / URL

[nedtwigg]

It comes up from time to time that users want to setup their formatter steps from classpath resources and/or URLs. A few previous requests:

• Gradle plugin does not allow Eclipse configuration file from web? #715
• Load Eclipse configuration from JAR classpath. #744
• License header URL support #1093
• spotless gradle plugin leaks memory (massively) #1194 (comment)

I'll answer it once and for all here. The short answer is use blowdryer.

The longer answer is that File is a good interface. Sometimes the formatting happens within the JVM, but othertimes we are actually passing the file to a different process, and File works for either case.

So instead of overloading every method that takes a File to have another version that takes URL url and another that takes String localResource, what you really want is some function that takes a URL or local classpath resource and returns a File. That's simple to do, but there's some tricky bookkeeping - what should the name be, where should the file live, when do we reuse the same file, etc.

blowdryer answers all those questions with the shortest answer we could think of. It introduces no tasks - it creates the Files at configuration time and uses caching to support fully-offline builds and the best possible performance.