什么自动资源管理的替代品存在的卡拉?
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18-09-2019 - |
题
我已经看到许多例子的手臂(自动资源管理)在网络上为拉斯卡拉.这似乎是一个仪式的通道编写一,虽然大多数看起来很像另一个。我 有没有 看到一个很酷的实例中使用的延续,虽然。
在任何速度,很多代有缺陷的一种或另一种,所以我想这将是一个好主意,要有一个参考在这里堆溢出,在这里我们可以投票了最正确和适当的版本。
解决方案
有关现在的Scala 2.13 终于支持:通过使用try with resources
使用 :),例:
val lines: Try[Seq[String]] =
Using(new BufferedReader(new FileReader("file.txt"))) { reader =>
Iterator.unfold(())(_ => Option(reader.readLine()).map(_ -> ())).toList
}
或使用Using.resource
避免Try
val lines: Seq[String] =
Using.resource(new BufferedReader(new FileReader("file.txt"))) { reader =>
Iterator.unfold(())(_ => Option(reader.readLine()).map(_ -> ())).toList
}
您可以找到更多的例子使用 DOC。
一个用于执行自动资源管理实用程序。它可以被用于执行使用资源的操作,在这之后释放资源在其创作的顺序相反。
其他提示
克里斯*汉森的 博客入境'臂块在拉斯卡拉:重新讨论',从3/26/09 谈到有关的滑动21的马丁Odersky的 FOSDEM介绍.这一块是采取直接从滑动21(权限):
def using[T <: { def close() }]
(resource: T)
(block: T => Unit)
{
try {
block(resource)
} finally {
if (resource != null) resource.close()
}
}
--最终报价--
然后我们就可以叫这样的:
using(new BufferedReader(new FileReader("file"))) { r =>
var count = 0
while (r.readLine != null) count += 1
println(count)
}
什么样的缺点的这种方法?这一模式似乎将地址95%的在哪里我将会需要自动资源的管理...
编辑: 加入代码
Edit2: 延伸的设计图案的灵感来自蟒蛇 with
声明和处理:
- 发言之前运行块
- 重新投掷异常,根据管理资源
- 处理两种资源的一个单使用的声明
- 资源的具体处理通过提供一个隐含的转换和一个
Managed
类
这是与卡拉2.8.
trait Managed[T] {
def onEnter(): T
def onExit(t:Throwable = null): Unit
def attempt(block: => Unit): Unit = {
try { block } finally {}
}
}
def using[T <: Any](managed: Managed[T])(block: T => Unit) {
val resource = managed.onEnter()
var exception = false
try { block(resource) } catch {
case t:Throwable => exception = true; managed.onExit(t)
} finally {
if (!exception) managed.onExit()
}
}
def using[T <: Any, U <: Any]
(managed1: Managed[T], managed2: Managed[U])
(block: T => U => Unit) {
using[T](managed1) { r =>
using[U](managed2) { s => block(r)(s) }
}
}
class ManagedOS(out:OutputStream) extends Managed[OutputStream] {
def onEnter(): OutputStream = out
def onExit(t:Throwable = null): Unit = {
attempt(out.close())
if (t != null) throw t
}
}
class ManagedIS(in:InputStream) extends Managed[InputStream] {
def onEnter(): InputStream = in
def onExit(t:Throwable = null): Unit = {
attempt(in.close())
if (t != null) throw t
}
}
implicit def os2managed(out:OutputStream): Managed[OutputStream] = {
return new ManagedOS(out)
}
implicit def is2managed(in:InputStream): Managed[InputStream] = {
return new ManagedIS(in)
}
def main(args:Array[String]): Unit = {
using(new FileInputStream("foo.txt"), new FileOutputStream("bar.txt")) {
in => out =>
Iterator continually { in.read() } takeWhile( _ != -1) foreach {
out.write(_)
}
}
}
丹尼尔,
我最近刚刚部署的自动化资源管理斯卡拉臂库。你可以在这里找到的文档: https://github.com/jsuereth/scala-arm/wiki
此库支持三种样式使用的(目前):
1)势在必行/换表达式:
import resource._
for(input <- managed(new FileInputStream("test.txt")) {
// Code that uses the input as a FileInputStream
}
2)一元式
import resource._
import java.io._
val lines = for { input <- managed(new FileInputStream("test.txt"))
val bufferedReader = new BufferedReader(new InputStreamReader(input))
line <- makeBufferedReaderLineIterator(bufferedReader)
} yield line.trim()
lines foreach println
3)分隔延续式
下面是一个 “回声” TCP服务器:
import java.io._
import util.continuations._
import resource._
def each_line_from(r : BufferedReader) : String @suspendable =
shift { k =>
var line = r.readLine
while(line != null) {
k(line)
line = r.readLine
}
}
reset {
val server = managed(new ServerSocket(8007)) !
while(true) {
// This reset is not needed, however the below denotes a "flow" of execution that can be deferred.
// One can envision an asynchronous execuction model that would support the exact same semantics as below.
reset {
val connection = managed(server.accept) !
val output = managed(connection.getOutputStream) !
val input = managed(connection.getInputStream) !
val writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(output)))
val reader = new BufferedReader(new InputStreamReader(input))
writer.println(each_line_from(reader))
writer.flush()
}
}
}
代码使得一个资源类型性状的用途,所以它能够适应于大多数资源类型。它有一个回退使用针对类结构打字或者紧密或dispose方法。请查看文档,让我知道,如果你想到的任何方便的功能来添加。
下面是使用continuation 詹姆斯IRY 溶液:
// standard using block definition
def using[X <: {def close()}, A](resource : X)(f : X => A) = {
try {
f(resource)
} finally {
resource.close()
}
}
// A DC version of 'using'
def resource[X <: {def close()}, B](res : X) = shift(using[X, B](res))
// some sugar for reset
def withResources[A, C](x : => A @cps[A, C]) = reset{x}
下面是有和没有延续用于比较的解决方案:
def copyFileCPS = using(new BufferedReader(new FileReader("test.txt"))) {
reader => {
using(new BufferedWriter(new FileWriter("test_copy.txt"))) {
writer => {
var line = reader.readLine
var count = 0
while (line != null) {
count += 1
writer.write(line)
writer.newLine
line = reader.readLine
}
count
}
}
}
}
def copyFileDC = withResources {
val reader = resource[BufferedReader,Int](new BufferedReader(new FileReader("test.txt")))
val writer = resource[BufferedWriter,Int](new BufferedWriter(new FileWriter("test_copy.txt")))
var line = reader.readLine
var count = 0
while(line != null) {
count += 1
writer write line
writer.newLine
line = reader.readLine
}
count
}
和这里的改进Tiark Rompf的建议:
trait ContextType[B]
def forceContextType[B]: ContextType[B] = null
// A DC version of 'using'
def resource[X <: {def close()}, B: ContextType](res : X): X @cps[B,B] = shift(using[X, B](res))
// some sugar for reset
def withResources[A](x : => A @cps[A, A]) = reset{x}
// and now use our new lib
def copyFileDC = withResources {
implicit val _ = forceContextType[Int]
val reader = resource(new BufferedReader(new FileReader("test.txt")))
val writer = resource(new BufferedWriter(new FileWriter("test_copy.txt")))
var line = reader.readLine
var count = 0
while(line != null) {
count += 1
writer write line
writer.newLine
line = reader.readLine
}
count
}
我看到一个渐进的4步演变为这样做的手臂,在拉斯卡拉:
- 没有胳膊:污垢
- 只有关闭:好,但多嵌套块
- 继续单:使用对平套,但是非自然分离,在2块
- 直接风格的延续:Nirava,啊哈!这也是大多数类型安全的替代:资源外withResource块将类型的错误。
有重量轻(10行代码)ARM附带更好-文件。请参阅: https://github.com/pathikrit/better-files#lightweight-arm一>
import better.files._
for {
in <- inputStream.autoClosed
out <- outputStream.autoClosed
} in.pipeTo(out)
// The input and output streams are auto-closed once out of scope
下面是它是如何,如果你不希望整个库中实现:
type Closeable = {
def close(): Unit
}
type ManagedResource[A <: Closeable] = Traversable[A]
implicit class CloseableOps[A <: Closeable](resource: A) {
def autoClosed: ManagedResource[A] = new Traversable[A] {
override def foreach[U](f: A => U) = try {
f(resource)
} finally {
resource.close()
}
}
}
如何使用类型的类
trait GenericDisposable[-T] {
def dispose(v:T):Unit
}
...
def using[T,U](r:T)(block:T => U)(implicit disp:GenericDisposable[T]):U = try {
block(r)
} finally {
Option(r).foreach { r => disp.dispose(r) }
}
另一种选择是断断续续的懒惰TryClose单子。这是相当不错的与数据库连接:
val ds = new JdbcDataSource()
val output = for {
conn <- TryClose(ds.getConnection())
ps <- TryClose(conn.prepareStatement("select * from MyTable"))
rs <- TryClose.wrap(ps.executeQuery())
} yield wrap(extractResult(rs))
// Note that Nothing will actually be done until 'resolve' is called
output.resolve match {
case Success(result) => // Do something
case Failure(e) => // Handle Stuff
}
和与流:
val output = for {
outputStream <- TryClose(new ByteArrayOutputStream())
gzipOutputStream <- TryClose(new GZIPOutputStream(outputStream))
_ <- TryClose.wrap(gzipOutputStream.write(content))
} yield wrap({gzipOutputStream.flush(); outputStream.toByteArray})
output.resolve.unwrap match {
case Success(bytes) => // process result
case Failure(e) => // handle exception
}
下面是@ chengpohi的答案,修改,以便它使用Scala 2.8+的作品,而不是仅仅斯卡拉2.13(是的,它的工作原理与斯卡拉2.13还):
def unfold[A, S](start: S)(op: S => Option[(A, S)]): List[A] =
Iterator
.iterate(op(start))(_.flatMap{ case (_, s) => op(s) })
.map(_.map(_._1))
.takeWhile(_.isDefined)
.flatten
.toList
def using[A <: AutoCloseable, B](resource: A)
(block: A => B): B =
try block(resource) finally resource.close()
val lines: Seq[String] =
using(new BufferedReader(new FileReader("file.txt"))) { reader =>
unfold(())(_ => Option(reader.readLine()).map(_ -> ())).toList
}