try-with-resources / use / multiple resources

Question

I'm using a Java-API which heavily uses the Autoclosable-Interface and thus in Java try-with-resources. However in Java you can specify

try (res1, res2, res3...) {
  ...
}

Do we have a way to use more than one resource? It looks like the well known callback-hell:

val database = Databases.openDatabase(dbFile)

database.use {
  database.createResource(ResourceConfiguration.Builder(resPathName, config).build())

  val resMgr = database.getResourceManager(ResourceManagerConfiguration.Builder(resPathName).build())

  resMgr.use {
    val wtx = resMgr.beginNodeWriteTrx()

    wtx.use {
      wtx.insertSubtreeAsFirstChild(XMLShredder.createStringReader(resFileToStore))
    }
  }
}

Answer 1

There is no standard solution for this. If you had all of the Closable instances ready at the start, you could use your own self-defined methods to handle them, like this blog post or this repository shows (and here is the discussion on the official forums that led to the latter).

In your case however, where subsequent objects rely on the previous ones, none of these apply like a regular try-with-resources would.

The only thing I can suggest is trying to define helper functions for yourself that hide the nested use calls, and immediately place you in the second/third/nth layer of these resourcs acquisitions, if that's at all possible.

Answer 2

For simplicity I will use A,B and C for the chained autocloseables.

import java.io.Closeable

open class MockCloseable: Closeable {
    override fun close() = TODO("Just for compilation")
}
class A: MockCloseable(){
    fun makeB(): B = TODO()
}
class B: MockCloseable(){
    fun makeC(): C = TODO()

}
class C: MockCloseable()

Using uses

This would look like this:

A().use {a ->
    a.makeB().use {b -> 
        b.makeC().use {c -> 
            println(c)
        }
    }
}

Making a chain use function with a wrapper

Definition

class ChainedCloseable<T: Closeable>(val payload: T, val parents: List<Closeable>) {
    fun <U> use(block: (T)->U): U {
        try {
            return block(payload)
        } finally {
            payload.close()
            parents.asReversed().forEach { it.close() }
        }
    }

    fun <U: Closeable> convert(block: (T)->U): ChainedCloseable<U> {
        val newPayload = block(payload)
        return ChainedCloseable(newPayload, parents + payload)
    }
}

fun <T: Closeable, U: Closeable> T.convert(block:(T)->U): ChainedCloseable<U> {
    val new = block(this)

}

Usage

A()
    .convert(A::makeB)
    .convert(B::makeC)
    .use { c ->
         println(c)
    }

This allows you to avoid having to nest deeply, at the cost of creating wrapper objects.