java - JVM 在哪里持有我的文件锁？

Question

我正在处理 java.nio.file.AccessDeniedException 问题。

我有一个 Scala 程序，如果我这样做:

java.nio.file.Files.delete(FileSystems.getDefault().getPath("""D:\Users\Eric\Google Drive (New)\Music\Downloaded\Foreigner [Discography HQ]\1977 - Foreigner\03 - Starrider.mp3""")) 

一切正常。我这里有一些代码

def delete(path : Path) {
  try {
    println("deleting " + path)
    java.nio.file.Files.delete(path)
  } catch {
    case exception: Exception => System.err.println(exception)
  }
}

val google1 = FileSystems.getDefault().getPath("""D:\Users\Eric\Google Drive\Music\Downloaded\Foreigner [Discography HQ]""")
val google2 = FileSystems.getDefault().getPath("""D:\Users\Eric\Google Drive (New)\Music\Downloaded\Foreigner [Discography HQ]""")

val duplicates = TraversablePaths(List(google1, google2)).duplicateFilesList

println("deleting duplicate files")
duplicates.foreach(_.filter(!_.startsWith(google1)).foreach(delete))

但是当我尝试删除同一个文件时，我得到了

java.nio.file.AccessDeniedException: D:\Users\Eric\Google Drive (New)\Music\Downloaded\Foreigner [Discography HQ]\1977 - Foreigner\03 - Starrider.mp3

我能说的最好的是，JVM 要么锁定了文件，要么锁定了文件所在的目录，但我无法确定位置。检查文件是否相同的代码看起来像

def identical(file1 : Path, file2 : Path) : Boolean = {

  require(isRegularFile(file1), file1 + " is not a file")
  require(isRegularFile(file2), file2 + " is not a file")

  val size1 = size(file1)
  val size2 = size(file2)

  if (size1 != size2) return false

  var position : Long = 0
  var length = min(Integer.MAX_VALUE, size1 - position)

  val channel1 = FileChannel.open(file1)
  val channel2 = FileChannel.open(file2)

  try {
    while (length > 0) {
      val buffer1 = channel1.map(MapMode.READ_ONLY, position, length)
      val buffer2 = channel2.map(MapMode.READ_ONLY, position, length)
      if (!buffer1.equals(buffer2)) return false
        position += length
    length = min(Integer.MAX_VALUE, size1 - position)
    }
    true
    } finally {
    channel1.close()
    channel2.close()
  }
}

我原以为关闭 channel 会释放 JVM 需要的任何文件锁。这是代码中我实际打开文件进行读取的唯一部分，尽管代码的其他部分会检查文件长度，但我不希望 JVM 为此需要文件锁。

JVM 持有文件锁还有哪些其他原因？我怎样才能找到并释放它们？

干杯，埃里克

Answer 1

我只知道 JavaDoc 是怎么说的:

A mapping, once established, is not dependent upon the file channel that was used to create it. Closing the channel, in particular, has no effect upon the validity of the mapping.

和

A mapped byte buffer and the file mapping that it represents remain valid until the buffer itself is garbage-collected.

你可能没有持有缓冲区，但也可能它也没有被 GC。

更新:稍后我会重新启动到 Windows 来尝试一下，但这在 Linux 上不是问题。

更新:...但是在 Windows 上，是的，这就是问题所在。

package niolock

import java.nio.channels._
import java.nio.file._
import FileChannel.MapMode.{ READ_ONLY => RO }

import scala.util._

object Test extends App {
  val p = FileSystems.getDefault getPath "D:/tmp/mapped"
  val c = FileChannel open p
  var b = c map (RO, 0L, 100L)
  c.close

  Console println Try(Files delete p)
  b = null
  System.gc()
  Console println Try(Files delete p)
}

尝试一下:

$ scalac niolock.scala ; scala niolock.Test
Failure(java.nio.file.AccessDeniedException: D:\tmp\mapped)
Success(())

或者:

Release Java file lock in Windows

How to unmap a file from memory mapped using FileChannel in java?