Implementing try-with-resources in Xtend

Java 7 and C# provide a convenient syntax that ensures the correct use of Closeable (IDisposable) objects. In Java 7 it is called try-with-resources. Here is an example of its usage (Java code):

try (Reader reader = new FileReader("file.txt")) {

  // do something with reader

}

Xtend does not provide this feature. However, you can implement it easily:

class Using {

  def static <T extends Closeable, R> R using(T resource, (T) => R proc) {

    // This is kept for a case when a Throwable from close()

    // overwrites a throwable from try

    var Throwable mainThrowable = null

    try {

      return proc.apply(resource)

    } catch (Throwable t) {

      mainThrowable = t

      throw t

    } finally {

      if (mainThrowable == null) {

        resource.close

      } else {

        try {

          resource.close

        } catch (Throwable unused) {

          // ignore because mainThrowable is present

        }

      }

    }

  }

}

This is sometimes called a loan pattern which is widely used in Scala. I use using as a method name, because try is a reserved keyword in Java and Xtend. The following example shows how to use the using statement:

import static org.example.xtend.Using.*

val text = using(new FileReader('file.txt')) [

  val buf = CharBuffer::allocate(1024) // this is just an example

  it.read(buf)

  buf.rewind

  buf.toString

]

println(text)

One can use it keyword to refer to a passed instance of Closeable.

The nice feature is that using can also return a value. So, "Everything is an expression" principle works here as well.