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 withUsing. If you only need to manage a single resource, theapply method is easiest; it wraps the resource opening, operation, and resource releasing in aTry.
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 aTry.
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 touse:
def mkreader(filename: String) = use(new BufferedReader(use(new FileReader(filename))))Custom resources can be registered on construction by requiring an implicitManager. This ensures they will be released even if composition fails:
import scala.util.Usingcase 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 aTry, you can useUsing.resource, which throws any exceptions that occur.
Example:
import java.io.{BufferedReader, FileReader}import scala.util.Usingval 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 isadded 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 aControlThrowable, or if it does not support suppression (seeThrowable's constructor with anenableSuppression parameter), an exception that would have been suppressed is instead discarded.
Exceptions are ranked from highest to lowest severity as follows:
java.lang.VirtualMachineErrorjava.lang.LinkageErrorjava.lang.InterruptedExceptionandjava.lang.ThreadDeathfatal exceptions, excluding
scala.util.control.ControlThrowablescala.util.control.ControlThrowableall 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
Members list
Type members
Classlikes
A resource manager.
Attributes
- Note
It is recommended for API designers to require an implicit
Managerfor the creation of custom resources, and to callacquireduring those resources' construction. Doing so guarantees that the resourcemust 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
Attributes
- Companion
- trait
- Source
- Using.scala
- Supertypes
- Self type
- Releasable.type
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 ofjava.io.OutputStream would be considered a resource, because it is important to close the stream after it is finished being used.
An instance ofReleasable is needed in order to automatically manage a resource withUsing. An implicit instance is provided for all types extendingjava.lang.AutoCloseable.
Type parameters
- R
the type of the resource
Attributes
- Companion
- object
- Source
- Using.scala
- Supertypes
- Known subtypes
Value members
Concrete methods
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 forUsing for full details of suppression behavior.
Attributes
- Returns
aTry containing an exception if one or more were thrown, or the result of the operation if no exceptions were thrown
- Source
- Using.scala
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 forUsing 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
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 forUsing 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
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 forUsing 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
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 forUsing 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