文件在使用FileChannel.map后不能被删除(Windows上)

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同事发现在Windows上使用FileChannel的map方法之后, 不能够删除掉文件. 我在Linux上试了一下, 发现没这个问题。 做个笔记, 记录一下.

import java.io.File;
import java.io.RandomAccessFile;
import java.lang.reflect.Method;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileChannel.MapMode;
 
import sun.nio.ch.FileChannelImpl;
 
public class TestDeleteMappedFile {

  public static void main(String args[]) {
    File f = new File("mapfile");
    RandomAccessFile aFile = null;
    FileChannel inChannel = null;
    try {
      aFile = new RandomAccessFile(f, "rw");
      inChannel = aFile.getChannel();
      ByteBuffer buf = inChannel.map(MapMode.READ_WRITE, 0L, 4);
    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      try {
        inChannel.close();
        aFile.close();
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
    System.out.println("delete " + f + " : " + f.delete());
  }
}

从 Oracle的buglist上看到了这个问题的描述.

引用

We cannot fix this.  Windows does not allow a mapped file to be deleted.  This
problem should be ameliorated somewhat once we fix our garbage collectors to
deallocate direct buffers more promptly (see 4469299), but otherwise there's
nothing we can do about this.

sun.misc.Cleaner为我们干了关闭文件, 释放资源的脏活. 这个类是一个幻引用, 所以会在gc的时候调用到. FileChannel正是调用了这个Cleaner, 在gc的时候做unmap.

// sun.nio.ch.FileChannelImpl类
 Unmapper um = new Unmapper(addr, size + pagePosition);
            if ((!writable) || (imode == MAP_RO))
                return Util.newMappedByteBufferR(isize, addr + pagePosition, um);

// java.nio.DirectByteBuffer类
    // For memory-mapped buffers -- invoked by FileChannelImpl via reflection
    //
    protected DirectByteBuffer(int cap, long addr, Runnable unmapper) {

        super(-1, 0, cap, cap, true);
	address = addr;
	cleaner = Cleaner.create(this, unmapper);
        viewedBuffer = null;



    }

如果finalizer delay的时候不能调unmap, 那么手动调unmap, 应该就可以在windows上释放掉资源了. 我们试试

import java.io.File;
import java.io.RandomAccessFile;
import java.lang.reflect.Method;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileChannel.MapMode;
 
import sun.nio.ch.FileChannelImpl;
 
public class TestDeleteMappedFile {

  public static void main(String args[]) {
    File f = new File("mapfile");
    RandomAccessFile aFile = null;
    FileChannel inChannel = null;
    try {
      aFile = new RandomAccessFile(f, "rw");
      inChannel = aFile.getChannel();
      ByteBuffer buf = inChannel.map(MapMode.READ_WRITE, 0L, 4);
      // 加上这几行代码,手动unmap
      Method m = FileChannelImpl.class.getDeclaredMethod("unmap", MappedByteBuffer.class);
      m.setAccessible(true);
      m.invoke(FileChannelImpl.class, buf);
    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      try {
        inChannel.close();
        aFile.close();
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
    System.out.println("delete " + f + " : " + f.delete());
  }
}

果然, 可以删除

Have fun!