Packages

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

  1. 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)
    }
  2. 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

  1. 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
  2. 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
  3. 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
  4. 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

  5. final def asInstanceOf[T0]: T0

    Cast the receiver object to be of type T0.

    Cast the receiver object to be of type T0.

    Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

    returns

    the receiver object.

    Definition Classes
    Any
    Exceptions thrown

    ClassCastException if the receiver object is not an instance of the erasure of type T0.

  6. 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

  7. 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
  8. 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
  9. 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

  10. 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()
  11. 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()
  12. 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.

  13. 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
  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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
  21. 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
  22. final def wait(): Unit

    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

  23. final def wait(arg0: Long, arg1: Int): Unit

    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

  24. final def wait(arg0: Long): Unit

    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

  25. object Manager
  26. object Releasable

Inherited from AnyRef

Inherited from Any

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