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 lines: Try[Seq[String]] = Using.Manager { use => val r1 = use(new BufferedReader(new FileReader("file1.txt"))) val r2 = use(new BufferedReader(new FileReader("file2.txt"))) val r3 = use(new BufferedReader(new FileReader("file3.txt"))) val r4 = use(new BufferedReader(new FileReader("file4.txt"))) // use your resources here def lines(reader: BufferedReader): Iterator[String] = Iterator.continually(reader.readLine()).takeWhile(_ != null) (lines(r1) ++ lines(r2) ++ lines(r3) ++ lines(r4)).toList }
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
andjava.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
- Alphabetic
- By Inheritance
- Using
- AnyRef
- Any
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- Public
- Protected
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 callacquire
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 withUsing
. 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 andnull
.Equivalent to
x.hashCode
except for boxed numeric types andnull
. 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. Fornull
returns a hashcode wherenull.hashCode
throws aNullPointerException
.- returns
a hash value consistent with ==
- Definition Classes
- AnyRef → Any
- final def ==(arg0: Any): Boolean
The expression
x == that
is equivalent toif (x eq null) that eq null else x.equals(that)
.The expression
x == that
is equivalent toif (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
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 aClassCastException
at runtime, while the expressionList(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 typeT0
.
- 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.
- 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 ofAnyRef
, and has three additional properties:- It is consistent: for any non-null instances
x
andy
of typeAnyRef
, multiple invocations ofx.eq(y)
consistently returnstrue
or consistently returnsfalse
. - For any non-null instance
x
of typeAnyRef
,x.eq(null)
andnull.eq(x)
returnsfalse
. null.eq(null)
returnstrue
.
When overriding the
equals
orhashCode
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
- It is consistent: for any non-null instances
- def equals(arg0: AnyRef): Boolean
The equality method for reference types.
- 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 betweenfinalize
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.
- def hashCode(): Int
The hashCode method for reference types.
- 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 returnstrue
if the value of the receiver object is aBigDecimal
or a subtype ofBigDecimal
.T0
is a parameterized class type, e.g.List[Int]
: this method returnstrue
if the value of the receiver object is someList[X]
for anyX
. For example,List(1, 2, 3).isInstanceOf[List[String]]
will return true.T0
is some singleton typex.type
or literalx
: this method returnsthis.eq(x)
. For example,x.isInstanceOf[1]
is equivalent tox.eq(1)
T0
is an intersectionX with Y
orX & Y: this method is equivalent to
x.isInstanceOf[X] && x.isInstanceOf[Y]T0
is a unionX | Y
: this method is equivalent tox.isInstanceOf[X] || x.isInstanceOf[Y]
T0
is a type parameter or an abstract type member: this method is equivalent toisInstanceOf[U]
whereU
isT0
's upper bound,Any
ifT0
is unbounded. For example,x.isInstanceOf[A]
whereA
is an unbounded type parameter will return true for any value ofx
.
This is exactly equivalent to the type pattern
_: T0
- returns
true
if the receiver object is an instance of erasure of typeT0
;false
otherwise.
- Definition Classes
- Any
- Note
due to the unexpectedness of
List(1, 2, 3).isInstanceOf[List[String]]
returning true andx.isInstanceOf[A]
whereA
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 onthis
.Executes the code in
body
with an exclusive lock onthis
.- 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--.
- 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-
- 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-.
- object Manager
- object Releasable
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int
,Float
,Array
orOption
which are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collection
and its sub-packages contain Scala's collections frameworkscala.collection.immutable
- Immutable, sequential data-structures such asVector
,List
,Range
,HashMap
orHashSet
scala.collection.mutable
- Mutable, sequential data-structures such asArrayBuffer
,StringBuilder
,HashMap
orHashSet
scala.collection.concurrent
- Mutable, concurrent data-structures such asTrieMap
scala.concurrent
- Primitives for concurrent programming such asFutures
andPromises
scala.io
- Input and output operationsscala.math
- Basic math functions and additional numeric types likeBigInt
andBigDecimal
scala.sys
- Interaction with other processes and the operating systemscala.util.matching
- Regular expressionsOther packages exist. See the complete list on the right.
Additional parts of the standard library are shipped as separate libraries. These include:
scala.reflect
- Scala's reflection API (scala-reflect.jar)scala.xml
- XML parsing, manipulation, and serialization (scala-xml.jar)scala.collection.parallel
- Parallel collections (scala-parallel-collections.jar)scala.util.parsing
- Parser combinators (scala-parser-combinators.jar)scala.swing
- A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)Automatic imports
Identifiers in the scala package and the
scala.Predef
object are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
List
is an alias forscala.collection.immutable.List
.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
String
is an alias forjava.lang.String
.