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.

Source: Using.scala

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Type Members

final class Manager extends AnyRef
A resource 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.
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) }
trait Releasable[-R] extends AnyRef
A type class describing how to release a particular type of resource.
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.
R
the type of the resource

Value Members

final def !=(arg0: Any): Boolean
Test two objects for inequality.
Test two objects for inequality.
returns
true if !(this == that), false otherwise.
Definition Classes
AnyRef → Any
final def ##: Int
Equivalent to x.hashCode except for boxed numeric types and null.
Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.
returns
a hash value consistent with ==
Definition Classes
AnyRef → Any
final def ==(arg0: Any): Boolean
The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).
The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).
returns
true if the receiver object is equivalent to the argument; false otherwise.
Definition Classes
AnyRef → Any
def apply[R, A](resource: => R)(f: (R) => A)(implicit arg0: Releasable[R]): Try[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.
See the main doc for Using for full details of suppression behavior.
returns
a Try containing an exception if one or more were thrown, or the result of the operation if no exceptions were thrown
final def asInstanceOf[T0]: T0
Forces the compiler to treat the receiver object as having type T0, even though doing so may violate type safety.
Forces the compiler to treat the receiver object as having type T0, even though doing so may violate type safety.
This method is useful when you believe you have type information the compiler doesn't, and it also isn't possible to check the type at runtime. In such situations, skipping type safety is the only option.
It is platform dependent whether asInstanceOf has any effect at runtime. It might do a runtime type test on the erasure of T0, insert a conversion (such as boxing/unboxing), fill in a default value, or do nothing at all.
In particular, asInstanceOf is not a type test. It does **not** mean:
```
this match {
 case x: T0 => x
 case _     => throw ClassCastException("...")
```
Use pattern matching or isInstanceOf for type testing instead.
Situations where asInstanceOf is useful:
- when flow analysis fails to deduce T0 automatically
- when down-casting a type parameter or an abstract type member (which cannot be checked at runtime due to type erasure) If there is any doubt and you are able to type test instead, you should do so.
Be careful of using asInstanceOf when T0 is a primitive type. When T0 is primitive, asInstanceOf may insert a conversion instead of a type test. If your intent is to convert, use a toT method (x.toChar, x.toByte, etc.).
returns
the receiver object.
Definition Classes
Any
Exceptions thrown
ClassCastException if the receiver is not an instance of the erasure of T0, if that can be checked on this platform
def clone(): AnyRef
Create a copy of the receiver object.
Create a copy of the receiver object.
The default implementation of the clone method is platform dependent.
returns
a copy of the receiver object.
Attributes
protected[lang]
Definition Classes
AnyRef
Annotations
@throws(classOf[java.lang.CloneNotSupportedException]) @native()
Note
not specified by SLS as a member of AnyRef
final def eq(arg0: AnyRef): Boolean
Tests whether the argument (that) is a reference to the receiver object (this).
Tests whether the argument (that) is a reference to the receiver object (this).
The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:
- It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
- For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
- null.eq(null) returns true.
When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).
returns
true if the argument is a reference to the receiver object; false otherwise.
Definition Classes
AnyRef
def equals(arg0: AnyRef): Boolean
The equality method for reference types.
The equality method for reference types. Default implementation delegates to eq.
See also equals in scala.Any.
returns
true if the receiver object is equivalent to the argument; false otherwise.
Definition Classes
AnyRef → Any
def finalize(): Unit
Called by the garbage collector on the receiver object when there are no more references to the object.
Called by the garbage collector on the receiver object when there are no more references to the object.
The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.
Attributes
protected[lang]
Definition Classes
AnyRef
Annotations
@throws(classOf[java.lang.Throwable])
Note
not specified by SLS as a member of AnyRef
final def getClass(): Class[_ <: AnyRef]
Returns the runtime class representation of the object.
Returns the runtime class representation of the object.
returns
a class object corresponding to the runtime type of the receiver.
Definition Classes
AnyRef → Any
Annotations
@native()
def hashCode(): Int
The hashCode method for reference types.
The hashCode method for reference types. See hashCode in scala.Any.
returns
the hash code value for this object.
Definition Classes
AnyRef → Any
Annotations
@native()
final def isInstanceOf[T0]: Boolean
Test whether the dynamic type of the receiver object has the same erasure as T0.
Test whether the dynamic type of the receiver object has the same erasure as T0.
Depending on what T0 is, the test is done in one of the below ways:
- T0 is a non-parameterized class type, e.g. BigDecimal: this method returns true if the value of the receiver object is a BigDecimal or a subtype of BigDecimal.
- T0 is a parameterized class type, e.g. List[Int]: this method returns true if the value of the receiver object is some List[X] for any X. For example, List(1, 2, 3).isInstanceOf[List[String]] will return true.
- T0 is some singleton type x.type or literal x: this method returns this.eq(x). For example, x.isInstanceOf[1] is equivalent to x.eq(1)
- T0 is an intersection X with Y or X & Y: this method is equivalent to x.isInstanceOf[X] && x.isInstanceOf[Y]
- T0 is a union X | Y: this method is equivalent to x.isInstanceOf[X] || x.isInstanceOf[Y]
- T0 is a type parameter or an abstract type member: this method is equivalent to isInstanceOf[U] where U is T0's upper bound, Any if T0 is unbounded. For example, x.isInstanceOf[A] where A is an unbounded type parameter will return true for any value of x.
This is exactly equivalent to the type pattern _: T0
returns
true if the receiver object is an instance of erasure of type T0; false otherwise.
Definition Classes
Any
Note
due to the unexpectedness of List(1, 2, 3).isInstanceOf[List[String]] returning true and x.isInstanceOf[A] where A is a type parameter or abstract member returning true, these forms issue a warning.
final def ne(arg0: AnyRef): Boolean
Equivalent to !(this eq that).
Equivalent to !(this eq that).
returns
true if the argument is not a reference to the receiver object; false otherwise.
Definition Classes
AnyRef
final def notify(): Unit
Wakes up a single thread that is waiting on the receiver object's monitor.
Wakes up a single thread that is waiting on the receiver object's monitor.
Definition Classes
AnyRef
Annotations
@native()
Note
not specified by SLS as a member of AnyRef
final def notifyAll(): Unit
Wakes up all threads that are waiting on the receiver object's monitor.
Wakes up all threads that are waiting on the receiver object's monitor.
Definition Classes
AnyRef
Annotations
@native()
Note
not specified by SLS as a member of AnyRef
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.
R
the type of the resource
A
the return type of the operation
resource
the resource
body
the operation to perform with the resource
returns
the result of the operation, if neither the operation nor releasing the resource throws
def resources[R1, R2, R3, R4, A](resource1: R1, resource2: => R2, resource3: => R3, resource4: => R4)(body: (R1, R2, R3, R4) => A)(implicit arg0: Releasable[R1], arg1: Releasable[R2], arg2: Releasable[R3], arg3: Releasable[R4]): 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.
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
A
the return type of the operation
resource1
the first resource
resource2
the second resource
resource3
the third resource
resource4
the fourth resource
body
the operation to perform using the resources
returns
the result of the operation, if neither the operation nor releasing the resources throws
def resources[R1, R2, R3, A](resource1: R1, resource2: => R2, resource3: => R3)(body: (R1, R2, R3) => A)(implicit arg0: Releasable[R1], arg1: Releasable[R2], arg2: Releasable[R3]): 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.
R1
the type of the first resource
R2
the type of the second resource
R3
the type of the third resource
A
the return type of the operation
resource1
the first resource
resource2
the second resource
resource3
the third resource
body
the operation to perform using the resources
returns
the result of the operation, if neither the operation nor releasing the resources throws
def resources[R1, R2, A](resource1: R1, resource2: => R2)(body: (R1, R2) => A)(implicit arg0: Releasable[R1], arg1: Releasable[R2]): 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.
R1
the type of the first resource
R2
the type of the second resource
A
the return type of the operation
resource1
the first resource
resource2
the second resource
body
the operation to perform using the resources
returns
the result of the operation, if neither the operation nor releasing the resources throws
final def synchronized[T0](arg0: => T0): T0
Executes the code in body with an exclusive lock on this.
Executes the code in body with an exclusive lock on this.
returns
the result of body
Definition Classes
AnyRef
def toString(): String
Creates a String representation of this object.
Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.
returns
a String representation of the object.
Definition Classes
AnyRef → Any
final def wait(): Unit
See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait--.
See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait--.
Definition Classes
AnyRef
Annotations
@throws(classOf[java.lang.InterruptedException])
Note
not specified by SLS as a member of AnyRef
final def wait(arg0: Long, arg1: Int): Unit
See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-int-
See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-int-
Definition Classes
AnyRef
Annotations
@throws(classOf[java.lang.InterruptedException])
Note
not specified by SLS as a member of AnyRef
final def wait(arg0: Long): Unit
See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-.
See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-.
Definition Classes
AnyRef
Annotations
@throws(classOf[java.lang.InterruptedException]) @native()
Note
not specified by SLS as a member of AnyRef
object Manager
object Releasable

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