无法从jar获取资源

Question

In my project i need to take jsons from folder located in resources. For it I implemented this method:

protected def getListOfJsonFromResources(path: String): List[String] = {
  val source = Source.fromInputStream(Thread.currentThread().getContextClassLoader.getResourceAsStream(path))
  println(source)
  val list = source.getLines().map{file =>
    Source.fromInputStream(Thread.currentThread().getContextClassLoader.getResourceAsStream(path + file))
      .mkString
  }.toList
  source.close()
  list
}

It's work fine in IDE but return nothing when I run my jar. I know that problem is in val source as in jar it return empty iterator. I tried already use this.getClass().getResourceAsStream() , this.getClass().getClassLoader().getResourceAsStream() and rewrite this method as java method.

I generate JAR in Idea (Build>Build Artifacts>Build). This files is inside JAR with correct path. I use Apache Maven 3.6.3 My res folder: resources/folder/json/jsonFolder path:folder/json/jsonFolder/

How should I change this method to get list of jsons from given folder?

Thank, in advance.

Answer 1

The proper usage is actually MyClass.class.getResourceAsStream . After all, getClass().getRAS() would fail if your code is subclassed. However, I assume it isn't - so that change will improve the style of this code and future proof it, but it probably won't directly fix the issue.

What MyClass.class.getRAS does, is look in the exact same place that the MyClass.class file is at. For example, if you have a jar with:

META-INF/MANIFEST.MF
com/foo/app/MyClass.class
app-icon.png
com/foo/app/save.png

And you run MyClass, then MyClass.class.gRAS("save.png") will work out. As weill MyClass.class.gRAS("/app-icon.png") - note the leading slash.

Thus, if gRAS isn't working for you here, the conclusion is that you're either specifying the wrong path, or the file you need to be included with your app is not being included by your build tooling.

You haven't shown anything relevant from your build scripts, so there's not much to say about how to fix this. But I can give you the tools you need to debug the problem so you can fix it yourself, with this useful trick:

MyClass.class.getResource("MyClass.class")

works for any class. Print it ( System.out.println the URL that falls out of the above call), and you will see the URL, which tells you exactly which of the no doubt many variants of MyClass is actually being used (by your IDE's build-on-save, by sbt's build tooling, in the jar file produced by sbt during the dist phase, and who knows how many more exist somewhere on disk.).

Now use standard computer tools (Mac: Finder, windows: Explorer, or just use the command line) to navigate to the actual location and check for the existence of this resource file. Note that jar files are just zip files; unzip -l somejar.jar is useful here.

If you see the file, well, now you know where that file is relative to MyClass.class , and now you know what to feed to the gRAS call. Note that .. is not a good idea; if it's not in the same dir as your class and not in any subdir, then use the leading slash to go off of the 'root' of the jar file or classpath root directory.

If you don't see the file, well, now you know that your build is broken. Search the web (or ask in a separate question here on SO) on how to fix that, and now you don't need to repeatedly build and run the entire app to figure it out; you can simply keep scanning the jar file produced by the build tool until the resource file shows up in the right spot.

NB: ClassLoader.getSystemCLassLoader.getResource, getThreadContext().getResource, getClass().getResource - these are all wrong. Don't do that. It'll probably also work, but it's more code and will break in various circumstances. MyClass.class.getResource does exactly what you want, is idiomatic, and is one of the shortest forms.