java nio file 内存映射文件

亦凉 2022-06-09 13:54 151阅读 0赞

原理：[内存映射文件原理探索][Link 1]

java对象解析：[java内存映射文件][java]

java对象中，介绍是用普通的bytebuffer来做的，而我们在实际生产环境中使用MappedByteBuffer。

介绍中是操作FileChannel的write、force、close等方法。使用MappedByteBuffer = FileChannel.map(..)产生，只要操作MappedByteBuffer的put、force方法就好了

MappedByteBuffer代码：

package file.ram.map;
    
    import sun.misc.Cleaner;
    import sun.nio.ch.DirectBuffer;
    import sun.nio.ch.FileChannelImpl;
    
    import java.io.File;
    import java.io.IOException;
    import java.lang.reflect.InvocationTargetException;
    import java.lang.reflect.Method;
    import java.nio.MappedByteBuffer;
    import java.nio.channels.FileChannel;
    import java.nio.charset.StandardCharsets;
    import java.nio.file.Files;
    import java.nio.file.StandardCopyOption;
    import java.nio.file.StandardOpenOption;
    import java.util.concurrent.atomic.AtomicInteger;
    
    public class FileMMap {
        private static AtomicInteger pos = new AtomicInteger();
        private static FileChannel channel;
        private static MappedByteBuffer mmap;
        private static long size = 1024L;
    
        public static void main(String[] args) throws IOException, InterruptedException, NoSuchMethodException, InvocationTargetException, IllegalAccessException {
            File file = new File("./test.dump");
            channel = FileChannel.open(file.toPath(), StandardOpenOption.CREATE,
                    StandardOpenOption.READ, StandardOpenOption.WRITE);
            mmap = channel.map(FileChannel.MapMode.READ_WRITE, 0, size);
            byte[] len = new byte[8];
            mmap.get(len);
            System.out.println(mmap.capacity());
            System.out.println(mmap.limit());
            System.out.println(mmap.remaining());
            mmap.put("abc".getBytes(StandardCharsets.UTF_8));
            // channel.close();
            // System.out.println(mmap.slice());
            for (int i = 0; i < 10000; i++) {
    //            channel.transferTo(0, channel.size(), FileChannel.open(new File("./test1.dump").toPath(),
    //                    StandardOpenOption.CREATE, StandardOpenOption.WRITE));
                // 移动
                Files.move(file.toPath(), new File("./test1.dump").toPath(),
                        StandardCopyOption.REPLACE_EXISTING, StandardCopyOption.ATOMIC_MOVE);
                channel.close();
                // 新建
                channel = FileChannel.open(file.toPath(), StandardOpenOption.CREATE,
                        StandardOpenOption.READ, StandardOpenOption.WRITE);
                // 手动unmap
    //            Method m = FileChannelImpl.class.getDeclaredMethod("unmap",
    //                    MappedByteBuffer.class);
    //            m.setAccessible(true);
    //            m.invoke(FileChannelImpl.class, mmap);
                // 可以直接释放MappedByteBuffer
                Cleaner var1 = ((DirectBuffer)mmap).cleaner();
                if(var1 != null) {
                    var1.clean();
                }
    
                mmap = channel.map(FileChannel.MapMode.READ_WRITE, 0, size);
                mmap.put(String.valueOf(i + 1).getBytes(StandardCharsets.UTF_8));
            }
        }
    }

[Link 1]: http://blog.csdn.net/mg0832058/article/details/5890688
[java]: http://www.cnblogs.com/huane/p/6072668.html