如何正确关闭MappedByteBuffer？

Question

这是我正在运行的代码：

import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;

public class Main {
    public static void main(String[] args) throws Exception {
        String filePath = "D:/temp/file";
        RandomAccessFile file = new RandomAccessFile(filePath, "rw");

        try {
            MappedByteBuffer buffer = file.getChannel().map(FileChannel.MapMode.READ_WRITE, 0, 128);

            // Do something
            buffer.putInt(4);
        } finally {
            file.close();
            System.out.println("File closed");
        }

        System.out.println("Press any key...");
        System.in.read();

        System.out.println("Finished");
    }
}

在按下键之前，我正尝试在FAR Manager中手动删除该文件。但是远远地说文件是被锁定的：

 The process cannot access the file because it is being used by another process.
                     Cannot delete the file
                         D:\temp\file
                    Object is being opened in:
 Java(TM) Platform SE binary (PID: 5768, C:\Program Files\Java\jdk1.8.0_05\bin\javaw.exe)

只有在按下键后，应用程序才会终止，而我可以删除该文件。

我的密码怎么了？

Answer 1

试试这个。

public class Test
{
    public static void main(String[] args) throws Exception {
        String filePath = "D:/temp/file";
        RandomAccessFile file = new RandomAccessFile(filePath, "rw");
        FileChannel chan = file.getChannel();
        try {
            MappedByteBuffer buffer = chan.map(FileChannel.MapMode.READ_WRITE, 0, 128);

            // Do something
            buffer.putInt(4);
            buffer.force();
            Cleaner cleaner = ((sun.nio.ch.DirectBuffer) buffer).cleaner();
            if (cleaner != null) {
                cleaner.clean();
            }
        } finally {
            chan.close();
            file.close();
            System.out.println("File closed");
        }

        System.out.println("Press any key...");
        System.in.read();

        System.out.println("Finished");
    }
}

Answer 2

@SANN3 3的答案不再适用于Java 9。在Java9中，有一个可以使用的新方法sun.misc.Unsafe.invokeCleaner。下面是一个工作代码：

MappedByteBuffer buffer = ...

// Java 9+ only:
Class<?> unsafeClass = Class.forName("sun.misc.Unsafe");
Field unsafeField = unsafeClass.getDeclaredField("theUnsafe");
unsafeField.setAccessible(true);
Object unsafe = unsafeField.get(null);
Method invokeCleaner = unsafeClass.getMethod("invokeCleaner", ByteBuffer.class);
invokeCleaner.invoke(unsafe, buffer);

Answer 3

如果您使用的是java1.8，并且不能直接使用sun.nio.ch.DirectBuffer和Cleaner，您可以尝试：

public void clean(final ByteBuffer buffer) {
    AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
        try {
            Field field = buffer.getClass().getDeclaredField("cleaner");
            field.setAccessible(true);
            Object cleaner = field.get(buffer);

            Method cleanMethod = cleaner.getClass().getMethod("clean");
            cleanMethod.invoke(cleaner);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    });
}