Using
A utility for performing automatic resource management. It can be used to perform an operation using resources, after which it releases the resources in reverse order of their creation.
Usage
There are multiple ways to automatically manage resources with Using. If you only need to manage a single resource, the apply method is easiest; it wraps the resource opening, operation, and resource releasing in a Try.
Example:
import java.io.{BufferedReader, FileReader}
import scala.util.{Try, Using}
val lines: Try[Seq[String]] =
Using(new BufferedReader(new FileReader("file.txt"))) { reader =>
Iterator.continually(reader.readLine()).takeWhile(_ != null).toSeq
}
If you need to manage multiple resources, Using.Manager should be used. It allows the managing of arbitrarily many resources, whose creation, use, and release are all wrapped in a Try.
Example:
import java.io.{BufferedReader, FileReader}
import scala.util.{Try, Using}
val files = List("file1.txt", "file2.txt", "file3.txt", "file4.txt")
val lines: Try[Seq[String]] = Using.Manager { use =>
// acquire resources
def mkreader(filename: String) = use(new BufferedReader(new FileReader(filename)))
// use your resources here
def lines(reader: BufferedReader): Iterator[String] =
Iterator.continually(reader.readLine()).takeWhile(_ != null)
files.map(mkreader).flatMap(lines)
}
Composed or "wrapped" resources may be acquired in order of construction, if "underlying" resources are not closed. Although redundant in this case, here is the previous example with a wrapped call to use:
def mkreader(filename: String) = use(new BufferedReader(use(new FileReader(filename))))
Custom resources can be registered on construction by requiring an implicit Manager. This ensures they will be released even if composition fails:
import scala.util.Using
case class X(x: String)(implicit mgr: Using.Manager) extends AutoCloseable {
override def close() = println(s"CLOSE $x")
mgr.acquire(this)
}
case class Y(y: String)(x: String)(implicit mgr: Using.Manager) extends AutoCloseable {
val xres = X(x)
override def close() = println(s"CLOSE $y")
// an error during construction releases previously acquired resources
require(y != null, "y is null")
mgr.acquire(this)
}
Using.Manager { implicit mgr =>
val y = Y("Y")("X")
println(s"USE $y")
}
println {
Using.Manager { implicit mgr =>
Y(null)("X")
}
} // Failure(java.lang.IllegalArgumentException: requirement failed: y is null)
If you wish to avoid wrapping management and operations in a Try, you can use Using.resource, which throws any exceptions that occur.
Example:
import java.io.{BufferedReader, FileReader}
import scala.util.Using
val lines: Seq[String] =
Using.resource(new BufferedReader(new FileReader("file.txt"))) { reader =>
Iterator.continually(reader.readLine()).takeWhile(_ != null).toSeq
}
Suppression Behavior
If two exceptions are thrown (e.g., by an operation and closing a resource), one of them is re-thrown, and the other is added to it as a suppressed exception. If the two exceptions are of different 'severities' (see below), the one of a higher severity is re-thrown, and the one of a lower severity is added to it as a suppressed exception. If the two exceptions are of the same severity, the one thrown first is re-thrown, and the one thrown second is added to it as a suppressed exception. If an exception is a ControlThrowable, or if it does not support suppression (see Throwable's constructor with an enableSuppression parameter), an exception that would have been suppressed is instead discarded.
Exceptions are ranked from highest to lowest severity as follows:
-
java.lang.VirtualMachineError -
java.lang.LinkageError -
java.lang.InterruptedExceptionandjava.lang.ThreadDeath -
fatal exceptions, excluding
scala.util.control.ControlThrowable -
scala.util.control.ControlThrowable -
all other exceptions
When more than two exceptions are thrown, the first two are combined and re-thrown as described above, and each successive exception thrown is combined as it is thrown.
Attributes
- Source
- Using.scala
- Graph
-
- Supertypes
- Self type
-
Using.type