Scala有哪些自动资源管理替代方案？

我在Web上看到了很多Scala的ARM（自动资源管理）示例。写一个似乎很容易，尽管大多数看上去很像。我确实看到了一个使用延续的非常酷的示例。

无论如何，很多代码都存在一种或另一种类型的缺陷，因此我认为在Stack Overflow上引用一个参考是一个好主意，我们可以在其中引用最正确和最合适的版本。

目前，Scala 2.13最终支持：try with resources通过使用Using :)，例如：

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
  }

您可以从使用文档中找到更多示例。

用于执行自动资源管理的实用程序。它可以用来执行使用资源的操作，然后以创建资源的相反顺序释放资源。

克里斯·汉森（Chris Hansen）在2009年3月26日发表的博客文章“斯卡拉的ARM块：再访”中谈到了马丁· 奥德斯基（Martin Odersky）的FOSDEM演示文稿的幻灯片21 。下一个步骤直接摘自幻灯片21（经许可）：

def using[T <: { def close() }]
    (resource: T)
    (block: T => Unit) 
{
  try {
    block(resource)
  } finally {
    if (resource != null) resource.close()
  }
}

-报价-

然后我们可以这样调用：

using(new BufferedReader(new FileReader("file"))) { r =>
  var count = 0
  while (r.readLine != null) count += 1
  println(count)
}

这种方法的缺点是什么？这种模式似乎可以解决我需要自动资源管理的95％...

编辑：添加了代码段

Edit2：扩展设计模式-从python with语句中获取灵感并解决：

• 语句在块之前运行
• 重新抛出异常取决于托管资源
• 使用一个using语句处理两个资源
• 通过提供隐式转换和Managed类来进行资源特定的处理

这是在Scala 2.8中。

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

我最近刚刚部署了scala-arm库来进行自动资源管理。您可以在这里找到文档：https : //github.com/jsuereth/scala-arm/wiki

该库支持三种使用方式（当前）：

1）命令式/表达式：

import resource._
for(input <- managed(new FileInputStream("test.txt")) {
// Code that uses the input as a FileInputStream
}

2）单子风格

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）分隔延续样式

这是一个“ echo” 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()
    }
  }
}

该代码利用了资源类型特征，因此能够适应大多数资源类型。使用带有close或dispose方法的类对类使用结构化类型具有后备功能。请查阅文档，如果您想添加任何方便的功能，请告诉我。

这是使用延续的James Iry解决方案：

// 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}

以下是有或没有继续进行比较的解决方案：

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
}

这是Tiark Rompf的改进建议：

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
}

我看到在Scala中进行ARM的4步逐步演进：

1. 没有ARM：土
2. 仅闭包：更好，但有多个嵌套块
3. Continuation Monad：用于使拼合变平，但不自然地分成2个块
4. 直接样式延续：Nirava，啊哈！这也是最安全的类型选择：withResource块之外的资源将是类型错误。

更好的文件包含轻量级（10行代码）ARM。参见：https : //github.com/pathikrit/better-files#lightweight-arm

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

如果您不希望使用整个库，可以通过以下方式实现：

  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Type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Choppy的Lazy TryClose monad。与数据库连接非常好：

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
}

更多信息在这里：https : //github.com/choppythelumberjack/tryclose

这是@chengpohi的答案，已修改，因此它可以与Scala 2.8+一起使用，而不仅仅是Scala 2.13（是的，它也可以与Scala 2.13一起使用）：

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
  }