2019年9月1日更新：
- 从Scala 2.13开始，最好使用scala.util.Using
- 修复了如果代码抛出错误
finally会吞下原始文件的错误ExceptiontryfinallyException
在回顾了有关如何在Scala中轻松编写文件的所有这些答案之后，其中一些非常不错，我遇到了三个问题：
- 在Jus12的答案中，对于Scala / FP初学者来说，使用辅助方法使用curring并不明显
- 需要封装较低级别的错误
scala.util.Try
- 需要向刚接触Scala / FP的Java开发人员展示如何正确嵌套依赖资源，以便
close以相反的顺序对每个依赖资源执行该方法- 注意：尤其是在失败的情况下，以相反的顺序关闭依赖资源是很少了解的要求该java.lang.AutoCloseable规范往往会导致非常有害且难以发现的错误和运行时故障
在开始之前，我的目标不是简洁。这是为了使Scala / FP初学者（通常是Java初学者）更容易理解。最后，我将所有内容都放在一起，然后增加简洁性。
首先，using需要更新方法以使用Try（同样，简洁不是这里的目标）。它将重命名为tryUsingAutoCloseable：
def tryUsingAutoCloseable[A <: AutoCloseable, R]
(instantiateAutoCloseable: () => A) //parameter list 1
(transfer: A => scala.util.Try[R]) //parameter list 2
: scala.util.Try[R] =
Try(instantiateAutoCloseable())
.flatMap(
autoCloseable => {
var optionExceptionTry: Option[Exception] = None
try
transfer(autoCloseable)
catch {
case exceptionTry: Exception =>
optionExceptionTry = Some(exceptionTry)
throw exceptionTry
}
finally
try
autoCloseable.close()
catch {
case exceptionFinally: Exception =>
optionExceptionTry match {
case Some(exceptionTry) =>
exceptionTry.addSuppressed(exceptionFinally)
case None =>
throw exceptionFinally
}
}
}
)
上面tryUsingAutoCloseable方法的开头可能会令人困惑，因为它似乎有两个参数列表，而不是通常的单个参数列表。这称为“咖喱”。而且，我不会详细介绍curry的工作原理或在什么地方有用。事实证明，对于这个特定的问题空间，这是完成任务的正确工具。
接下来，我们需要创建方法，tryPrintToFile该方法将创建一个（或覆盖现有的）File并编写一个List[String]。它使用FileWriter由BufferedWriter封装的，由封装的PrintWriter。和提升性能，默认缓冲区大小比默认为大得多的BufferedWriter定义，defaultBufferSize以及分配的值65536。
这是代码（同样，简洁不是这里的目标）：
val defaultBufferSize: Int = 65536
def tryPrintToFile(
lines: List[String],
location: java.io.File,
bufferSize: Int = defaultBufferSize
): scala.util.Try[Unit] = {
tryUsingAutoCloseable(() => new java.io.FileWriter(location)) { //this open brace is the start of the second curried parameter to the tryUsingAutoCloseable method
fileWriter =>
tryUsingAutoCloseable(() => new java.io.BufferedWriter(fileWriter, bufferSize)) { //this open brace is the start of the second curried parameter to the tryUsingAutoCloseable method
bufferedWriter =>
tryUsingAutoCloseable(() => new java.io.PrintWriter(bufferedWriter)) { //this open brace is the start of the second curried parameter to the tryUsingAutoCloseable method
printWriter =>
scala.util.Try(
lines.foreach(line => printWriter.println(line))
)
}
}
}
}
上面的tryPrintToFile方法很有用，因为它将a List[String]作为输入并将其发送到File。现在让我们创建一个tryWriteToFile方法，该方法采用a String并将其写入a File。
这是代码（我在这里让您猜测简洁的优先级）：
def tryWriteToFile(
content: String,
location: java.io.File,
bufferSize: Int = defaultBufferSize
): scala.util.Try[Unit] = {
tryUsingAutoCloseable(() => new java.io.FileWriter(location)) { //this open brace is the start of the second curried parameter to the tryUsingAutoCloseable method
fileWriter =>
tryUsingAutoCloseable(() => new java.io.BufferedWriter(fileWriter, bufferSize)) { //this open brace is the start of the second curried parameter to the tryUsingAutoCloseable method
bufferedWriter =>
Try(bufferedWriter.write(content))
}
}
}
最后，能够以a的File形式获取a的内容很有用String。虽然scala.io.Source提供了一种方便的方法来轻松获取a的内容File，但close必须在上使用该方法Source来释放基础JVM和文件系统句柄。如果不这样做，则直到JVM GC（垃圾收集器）释放Source实例本身之前，资源才被释放。即使这样，JVM仍然只能保证finalize将GC调用该方法到close资源。这意味着，显式调用该close方法是客户的责任，就像客户close对某个实例的责任是相同的。java.lang.AutoCloseable。为此，我们需要对handles的using方法的第二个定义scala.io.Source。
这是此代码（仍然不够简洁）：
def tryUsingSource[S <: scala.io.Source, R]
(instantiateSource: () => S)
(transfer: S => scala.util.Try[R])
: scala.util.Try[R] =
Try(instantiateSource())
.flatMap(
source => {
var optionExceptionTry: Option[Exception] = None
try
transfer(source)
catch {
case exceptionTry: Exception =>
optionExceptionTry = Some(exceptionTry)
throw exceptionTry
}
finally
try
source.close()
catch {
case exceptionFinally: Exception =>
optionExceptionTry match {
case Some(exceptionTry) =>
exceptionTry.addSuppressed(exceptionFinally)
case None =>
throw exceptionFinally
}
}
}
)
这是在超级简单的行流文件读取器中的用法示例（当前用于从数据库输出中读取制表符分隔的文件）：
def tryProcessSource(
file: java.io.File
, parseLine: (String, Int) => List[String] = (line, index) => List(line)
, filterLine: (List[String], Int) => Boolean = (values, index) => true
, retainValues: (List[String], Int) => List[String] = (values, index) => values
, isFirstLineNotHeader: Boolean = false
): scala.util.Try[List[List[String]]] =
tryUsingSource(scala.io.Source.fromFile(file)) {
source =>
scala.util.Try(
( for {
(line, index) <-
source.getLines().buffered.zipWithIndex
values =
parseLine(line, index)
if (index == 0 && !isFirstLineNotHeader) || filterLine(values, index)
retainedValues =
retainValues(values, index)
} yield retainedValues
).toList //must explicitly use toList due to the source.close which will
//occur immediately following execution of this anonymous function
)
)
提供了上述功能的更新版本，作为对其他但相关的StackOverflow问题的解答。
现在，将所有内容与提取的导入内容结合在一起（使粘贴到Eclipse ScalaIDE和IntelliJ Scala插件中的Scala工作表中变得更加容易，从而可以很容易地将输出转储到桌面上，以便使用文本编辑器进行检查），这是代码的样子（更加简洁）：
import scala.io.Source
import scala.util.Try
import java.io.{BufferedWriter, FileWriter, File, PrintWriter}
val defaultBufferSize: Int = 65536
def tryUsingAutoCloseable[A <: AutoCloseable, R]
(instantiateAutoCloseable: () => A) //parameter list 1
(transfer: A => scala.util.Try[R]) //parameter list 2
: scala.util.Try[R] =
Try(instantiateAutoCloseable())
.flatMap(
autoCloseable => {
var optionExceptionTry: Option[Exception] = None
try
transfer(autoCloseable)
catch {
case exceptionTry: Exception =>
optionExceptionTry = Some(exceptionTry)
throw exceptionTry
}
finally
try
autoCloseable.close()
catch {
case exceptionFinally: Exception =>
optionExceptionTry match {
case Some(exceptionTry) =>
exceptionTry.addSuppressed(exceptionFinally)
case None =>
throw exceptionFinally
}
}
}
)
def tryUsingSource[S <: scala.io.Source, R]
(instantiateSource: () => S)
(transfer: S => scala.util.Try[R])
: scala.util.Try[R] =
Try(instantiateSource())
.flatMap(
source => {
var optionExceptionTry: Option[Exception] = None
try
transfer(source)
catch {
case exceptionTry: Exception =>
optionExceptionTry = Some(exceptionTry)
throw exceptionTry
}
finally
try
source.close()
catch {
case exceptionFinally: Exception =>
optionExceptionTry match {
case Some(exceptionTry) =>
exceptionTry.addSuppressed(exceptionFinally)
case None =>
throw exceptionFinally
}
}
}
)
def tryPrintToFile(
lines: List[String],
location: File,
bufferSize: Int = defaultBufferSize
): Try[Unit] =
tryUsingAutoCloseable(() => new FileWriter(location)) { fileWriter =>
tryUsingAutoCloseable(() => new BufferedWriter(fileWriter, bufferSize)) { bufferedWriter =>
tryUsingAutoCloseable(() => new PrintWriter(bufferedWriter)) { printWriter =>
Try(lines.foreach(line => printWriter.println(line)))
}
}
}
def tryWriteToFile(
content: String,
location: File,
bufferSize: Int = defaultBufferSize
): Try[Unit] =
tryUsingAutoCloseable(() => new FileWriter(location)) { fileWriter =>
tryUsingAutoCloseable(() => new BufferedWriter(fileWriter, bufferSize)) { bufferedWriter =>
Try(bufferedWriter.write(content))
}
}
def tryProcessSource(
file: File,
parseLine: (String, Int) => List[String] = (line, index) => List(line),
filterLine: (List[String], Int) => Boolean = (values, index) => true,
retainValues: (List[String], Int) => List[String] = (values, index) => values,
isFirstLineNotHeader: Boolean = false
): Try[List[List[String]]] =
tryUsingSource(() => Source.fromFile(file)) { source =>
Try(
( for {
(line, index) <- source.getLines().buffered.zipWithIndex
values = parseLine(line, index)
if (index == 0 && !isFirstLineNotHeader) || filterLine(values, index)
retainedValues = retainValues(values, index)
} yield retainedValues
).toList
)
}
作为Scala / FP的新手，我已经花了很多时间（大部分时间都在抓挠自己的挫败感），获得了上述知识和解决方案。我希望这可以帮助其他Scala / FP新手更快克服这种特殊的学习障碍。