Using

scala.util.Using

object 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.InterruptedException and java.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: class Object

trait Matchable

class Any
Self type: Using.type

Members list

Type members

Classlikes

final class Manager

A resource manager.

Resources can be registered with the manager by calling acquire; such resources will be released in reverse order of their acquisition when the manager is closed, regardless of any exceptions thrown during use.

See the main doc for Using for full details of suppression behavior.

Attributes

Note

It is recommended for API designers to require an implicit Manager for the creation of custom resources, and to call acquire during those resources' construction. Doing so guarantees that the resource must be automatically managed, and makes it impossible to forget to do so. Example:

class SafeFileReader(file: File)(implicit manager: Using.Manager)
 extends BufferedReader(new FileReader(file)) {
 def this(fileName: String)(implicit manager: Using.Manager) = this(new File(fileName))
 manager.acquire(this)
}

Companion

object

Source

Using.scala

Supertypes

class Object

trait Matchable

class Any

object Manager

Attributes

Companion: class
Source: Using.scala
Supertypes: class Object

trait Matchable

class Any
Self type: Manager.type

object Releasable

Attributes

Companion: trait
Source: Using.scala
Supertypes: class Object

trait Matchable

class Any
Self type: Releasable.type

trait Releasable[-R]

A type class describing how to release a particular type of resource.

A resource is anything which needs to be released, closed, or otherwise cleaned up in some way after it is finished being used, and for which waiting for the object's garbage collection to be cleaned up would be unacceptable. For example, an instance of java.io.OutputStream would be considered a resource, because it is important to close the stream after it is finished being used.

An instance of Releasable is needed in order to automatically manage a resource with Using. An implicit instance is provided for all types extending java.lang.AutoCloseable.

Type parameters

R: the type of the resource

Attributes

Companion: object
Source: Using.scala
Supertypes: class Object

trait Matchable

class Any
Known subtypes: object AutoCloseableIsReleasable

Value members

Concrete methods

def apply[R : Releasable, A](resource: => R)(f: R => A): Try[A]

Performs an operation using a resource, and then releases the resource, even if the operation throws an exception.

See the main doc for Using for full details of suppression behavior.

Attributes

Returns: a Try containing an exception if one or more were thrown, or the result of the operation if no exceptions were thrown
Source: Using.scala

def resource[R, A](resource: R)(body: R => A)(implicit releasable: Releasable[R]): A

Performs an operation using a resource, and then releases the resource, even if the operation throws an exception.

Performs an operation using a resource, and then releases the resource, even if the operation throws an exception. This method behaves similarly to Java's try-with-resources.

See the main doc for Using for full details of suppression behavior.

Type parameters

A: the return type of the operation
R: the type of the resource

Value parameters

body: the operation to perform with the resource
resource: the resource

Attributes

Returns: the result of the operation, if neither the operation nor releasing the resource throws
Source: Using.scala

def resources[R1 : Releasable, R2 : Releasable, A](resource1: R1, resource2: => R2)(body: (R1, R2) => A): A

Performs an operation using two resources, and then releases the resources in reverse order, even if the operation throws an exception.

Performs an operation using two resources, and then releases the resources in reverse order, even if the operation throws an exception. This method behaves similarly to Java's try-with-resources.

See the main doc for Using for full details of suppression behavior.

Type parameters

A: the return type of the operation
R1: the type of the first resource
R2: the type of the second resource

Value parameters

body: the operation to perform using the resources
resource1: the first resource
resource2: the second resource

Attributes

Returns: the result of the operation, if neither the operation nor releasing the resources throws
Source: Using.scala

def resources[R1 : Releasable, R2 : Releasable, R3 : Releasable, A](resource1: R1, resource2: => R2, resource3: => R3)(body: (R1, R2, R3) => A): A

Performs an operation using three resources, and then releases the resources in reverse order, even if the operation throws an exception.

Performs an operation using three resources, and then releases the resources in reverse order, even if the operation throws an exception. This method behaves similarly to Java's try-with-resources.

See the main doc for Using for full details of suppression behavior.

Type parameters

A: the return type of the operation
R1: the type of the first resource
R2: the type of the second resource
R3: the type of the third resource

Value parameters

body: the operation to perform using the resources
resource1: the first resource
resource2: the second resource
resource3: the third resource

Attributes

Returns: the result of the operation, if neither the operation nor releasing the resources throws
Source: Using.scala

def resources[R1 : Releasable, R2 : Releasable, R3 : Releasable, R4 : Releasable, A](resource1: R1, resource2: => R2, resource3: => R3, resource4: => R4)(body: (R1, R2, R3, R4) => A): A

Performs an operation using four resources, and then releases the resources in reverse order, even if the operation throws an exception.

Performs an operation using four resources, and then releases the resources in reverse order, even if the operation throws an exception. This method behaves similarly to Java's try-with-resources.

See the main doc for Using for full details of suppression behavior.

Type parameters

A: the return type of the operation
R1: the type of the first resource
R2: the type of the second resource
R3: the type of the third resource
R4: the type of the fourth resource

Value parameters

body: the operation to perform using the resources
resource1: the first resource
resource2: the second resource
resource3: the third resource
resource4: the fourth resource

Attributes

Returns: the result of the operation, if neither the operation nor releasing the resources throws
Source: Using.scala

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