干法清理文件资源

Question

我编写了一个函数，生成报告，然后将其作为电子邮件的附件发送。如果在此过程中出现错误，我不想用大量的关闭文件系统的调用来干扰这个函数。你们都怎么想？

// attempt to generate the report, any error will be wrapped in Failure(ex)
val op1 = Try {
  DB.withConnection{ implicit c =>
    generateReport(command.programId, token.claims)(output, c)
  }
}

// use (abuse?) pattern matching to send the report if it generated succesfully
val op2 = op1 match {
  case Success(_) => Try(sendReport(Address(user.email, user.first), file, command.created))

  // if the op1 value is of type Failure(ex), carry it forward until after the temp files have been removed
  case Failure(ex) => Failure(ex)
}

// the operations have either succeeded or failed, so clean up the filesystem
cleanUp(output, file)

// throw any exception that occurred along the way
op2.get

假设抛出异常是此函数的合理设计选择，则调用过程期望抛出异常并优雅地处理它们。

Answer 1

您可以考虑使用ARM (自动资源管理)来清理资源。在Scala中，有一个名为scala臂的库。

然后，您的代码可以被构造成如下所示：

import resource._
import scala.util.Try

for {
  connection ← managed(DB.getConnection())
  output ← managed(...)
  file ← managed(...)
} {
  val result = for {
    report ← Try(generateReport(...)(output, connection))
    sendResult ← Try(sendReport(..., file))
  } yield sendResult
  // throws exception from generateReport or sendReport
  result.get 
}

清理资源的细节将由scala中的默认行为处理，或者如果需要定义自定义清理行为，则可以创建自己的资源特性实现。可以在资源的伴生对象中创建隐式对象。下面的代码片段显示了哪些方法可以被重写；只有close()实现是必需的：

class MyOutput {}

object MyOutput {
  implicit object OutputResource extends Resource[MyOutput] {
    override def open(r: MyOutput): Unit = super.open(r)
    override def closeAfterException(r: MyOutput, t: Throwable): Unit = super.closeAfterException(r, t)
    override def isFatalException(t: Throwable): Boolean = super.isFatalException(t)
    override def isRethrownException(t: Throwable): Boolean = super.isRethrownException(t)
    override def close(r: MyOutput): Unit = ???
  }
}