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.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileChannel.MapMode;

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.?

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