JAVA I/O（三）内存映射文件

《Java编程思想》中对内存映射文件有详细的介绍，此处仅做简单记录和总结。内存映射文件允许创建和修改因为太大而不能放入内存的文件。

1. 内存映射文件简单实例

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

public class LargeMappedFiles {
    
    private static int LENGTH = 0x0000FFF;

    public static void main(String[] args) throws IOException{
        MappedByteBuffer out = new RandomAccessFile("test.dat", "rw")
　　　　　　　　　 .getChannel()
                .map(FileChannel.MapMode.READ_WRITE, 0, LENGTH);

        for(int i = 0; i < LENGTH; i++) {
            out.put((byte)'x');
        }
        
        for(int i = LENGTH/2; i < LENGTH/2 + 6; i++) {
            System.out.print((char)out.get(i));
        }
    }
}

输出：

xxxxxx

通过RandomAccessFile类获取FileChannel，使其具备读写功能。
通过FileChannel的map方法，获取MappedByteBuffer，该方法包含三个参数，MapMode映射类型、开始位置、映射总数量，意味着可以映射大文件的较小部分。
MappedByteBuffer是一个特殊的直接缓冲器，对该缓冲器的修改会反映到对应文件中；另外，其继承ByteBuffer，具有ByteBuffer的所有方法。

本例中首先创建MappedByteBuffer，并设置为读写模式；然后往缓冲器中写入字符x；最后在文件中间开始读取6个字符。

2. 内存映射文件源码

以下是FileChannel.map()方法的解释：

/**
     * Maps a region of this channel's file directly into memory.
     *
     * <p> A region of a file may be mapped into memory in one of three modes:
     * </p>
     *
     * <ul>
     *
     *   <li><p> <i>Read-only:</i> Any attempt to modify the resulting buffer
     *   will cause a {@link java.nio.ReadOnlyBufferException} to be thrown.
     *   ({@link MapMode#READ_ONLY MapMode.READ_ONLY}) </p></li>
     *
     *   <li><p> <i>Read/write:</i> Changes made to the resulting buffer will
     *   eventually be propagated to the file; they may or may not be made
     *   visible to other programs that have mapped the same file.  ({@link
     *   MapMode#READ_WRITE MapMode.READ_WRITE}) </p></li>
     *
     *   <li><p> <i>Private:</i> Changes made to the resulting buffer will not
     *   be propagated to the file and will not be visible to other programs
     *   that have mapped the same file; instead, they will cause private
     *   copies of the modified portions of the buffer to be created.  ({@link
     *   MapMode#PRIVATE MapMode.PRIVATE}) </p></li>
     *
     * </ul>
     *
     * <p> For a read-only mapping, this channel must have been opened for
     * reading; for a read/write or private mapping, this channel must have
     * been opened for both reading and writing.
     *
     * <p> The {@link MappedByteBuffer <i>mapped byte buffer</i>}
     * returned by this method will have a position of zero and a limit and
     * capacity of <tt>size</tt>; its mark will be undefined.  The buffer and
     * the mapping that it represents will remain valid until the buffer itself
     * is garbage-collected.
     *
     * <p> 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.
     *
     * <p> Many of the details of memory-mapped files are inherently dependent
     * upon the underlying operating system and are therefore unspecified.  The
     * behavior of this method when the requested region is not completely
     * contained within this channel's file is unspecified.  Whether changes
     * made to the content or size of the underlying file, by this program or
     * another, are propagated to the buffer is unspecified.  The rate at which
     * changes to the buffer are propagated to the file is unspecified.
     *
     * <p> For most operating systems, mapping a file into memory is more
     * expensive than reading or writing a few tens of kilobytes of data via
     * the usual {@link #read read} and {@link #write write} methods.  From the
     * standpoint of performance it is generally only worth mapping relatively
     * large files into memory.  </p>
     *
     * @param  mode
     *         One of the constants {@link MapMode#READ_ONLY READ_ONLY}, {@link
     *         MapMode#READ_WRITE READ_WRITE}, or {@link MapMode#PRIVATE
     *         PRIVATE} defined in the {@link MapMode} class, according to
     *         whether the file is to be mapped read-only, read/write, or
     *         privately (copy-on-write), respectively
     *
     * @param  position
     *         The position within the file at which the mapped region
     *         is to start; must be non-negative
     *
     * @param  size
     *         The size of the region to be mapped; must be non-negative and
     *         no greater than {@link java.lang.Integer#MAX_VALUE}
     *
     * @return  The mapped byte buffer
     *
     * @throws NonReadableChannelException
     *         If the <tt>mode</tt> is {@link MapMode#READ_ONLY READ_ONLY} but
     *         this channel was not opened for reading
     *
     * @throws NonWritableChannelException
     *         If the <tt>mode</tt> is {@link MapMode#READ_WRITE READ_WRITE} or
     *         {@link MapMode#PRIVATE PRIVATE} but this channel was not opened
     *         for both reading and writing
     *
     * @throws IllegalArgumentException
     *         If the preconditions on the parameters do not hold
     *
     * @throws IOException
     *         If some other I/O error occurs
     *
     * @see java.nio.channels.FileChannel.MapMode
     * @see java.nio.MappedByteBuffer
     */
    public abstract MappedByteBuffer map(MapMode mode,
                                         long position, long size)
        throws IOException;

该方法直接将通道对应文件的一部分映射到内存，并返回MappedByteBuffer
有3种模式：READ_ONLY（只读）、READ_WRITE（读写）、PRIVATE（私有，用于copy-on-write）
MappedByteBuffer一旦建立，就与创建它的通道无关，即通道关闭时，不影响该缓冲器
内存映射需要依赖于底层操作系统；另外，对大部分操作系统，内存映射要比直接读写昂贵，故一般都映射较大的文件。
该方法的参数包括读写模式（由FileChannel内部类MapMode定义，如下）、开始位置position、映射大小size

/**
     * A typesafe enumeration for file-mapping modes.
     *
     * @since 1.4
     *
     * @see java.nio.channels.FileChannel#map
     */
    public static class MapMode {

        /**
         * Mode for a read-only mapping.
         */
        public static final MapMode READ_ONLY
            = new MapMode("READ_ONLY");

        /**
         * Mode for a read/write mapping.
         */
        public static final MapMode READ_WRITE
            = new MapMode("READ_WRITE");

        /**
         * Mode for a private (copy-on-write) mapping.
         */
        public static final MapMode PRIVATE
            = new MapMode("PRIVATE");

        private final String name;

        private MapMode(String name) {
            this.name = name;
        }

        /**
         * Returns a string describing this file-mapping mode.
         *
         * @return  A descriptive string
         */
        public String toString() {
            return name;
        }

    }

3. 文件加锁

JDK1.4引入文件加锁机制，允许同步访问共享资源文件。文件锁对其他操作系统进程是可见的，因为Java的文件加锁直接映射到本地操作系统的加锁工具。

可以通过FileChannel的tryLock()和lock()方法获取整个文件的FileLock。接口如下，tryLock()是非阻塞的，如果不能获取锁，则返回null；lock()是阻塞的，一直等待文件锁。另外，SocketChannel、DatgramChannel、ServerSocketChannel不需要加锁，因为他们是从单进程实体继承而来；并且通常不会在两个进程间共享socket。

public abstract FileLock lock(long position, long size, boolean shared)
        throws IOException;

public final FileLock lock() throws IOException {
        return lock(0L, Long.MAX_VALUE, false);
    }

public abstract FileLock tryLock(long position, long size, boolean shared)
        throws IOException;

public final FileLock tryLock() throws IOException {
        return tryLock(0L, Long.MAX_VALUE, false);
    }

FileLock是对文件某区域进行标识的（A token representing a lock on a region of a file.），可以通过FileChannel和AsynchronousFileChannel的加锁方法创建，包含四个成员：

public abstract class FileLock implements AutoCloseable {

    private final Channel channel;
    private final long position;
    private final long size;
    private final boolean shared;

加锁区域由size-position决定，不会根据文件大小变化而变化。shared为共享锁和排它锁标识。

对映射文件的部分加锁

文件映射通常应用于极大的文件，对其一部分进行加锁，其他进程可以对其他部分文件进行操作。数据库就是这样，多个用户可以同时访问。下边用2个线程分别对文件不同部分加锁。

import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileChannel.MapMode;
import java.nio.channels.FileLock;

/**
 * 对映射文件加锁
 * 映射文件：MappedByteBuffer out = fc.map(MapMode.READ_WRITE, 0, LENGTH);
 * 加锁：FileLock fl = fc.lock(start, end, false);    fl.release();
 * @author bob
 *
 */
public class LockingMappedFIles {
    
    private static final int LENGTH = 0x0000FFF;//128M
    static FileChannel fc;

    public static void main(String[] args) throws IOException{
        //1.获取读写FileChannel
        fc = new RandomAccessFile("data.dat", "rw").getChannel();
        //2.根据FileChannel获取MappedByteBuffer，读写模式、全文件
        MappedByteBuffer out = fc.map(MapMode.READ_WRITE, 0, LENGTH);
        //3.写入字符  x
        for(int i = 0; i < LENGTH; i++) {
            out.put((byte)'x');
        }
        //4.启动线程1，对文件的前1/3加锁，通过缓冲器操作文件
        Thread thread1 = new LockAndModify(out, 0, LENGTH/3);
        thread1.start();
        //5.启动线程2，对文件的后1/3加锁，通过缓冲器操作文件
        Thread thread2 = new LockAndModify(out, LENGTH*2/3, LENGTH);
        thread2.start();

    }
    
    static class LockAndModify extends Thread{
        private ByteBuffer byteBuffer;
        private int start, end;
        
        public LockAndModify(ByteBuffer byteBuffer, int start, int end) {
            //记录加锁位置的起始位置
            this.start = start;
            this.end = end;
            /**
             * 1. 设置MappedByteBuffer的position和limit
             * 2. 调slice()方法，创建新ByteBuffer，映射原ByteBuffer；其position为0，limit为缓冲器容量
             *       由slice()方法创建的ByteBuffer是直接的、只可读的
             *       修改会映射到原ByteBuffer中
             * 3. 另外，limit 和 position不可颠倒顺序，否则position可能比limit大，报错
             */
            byteBuffer.limit(end);
            byteBuffer.position(start);
            this.byteBuffer = byteBuffer.slice();
        }
        
        public void run() {
            try {
                //加排它锁
                FileLock fl = fc.lock(start, end, false);
                System.out.println("Locked: " + start + " to " + end);
                //修改内容
                while(byteBuffer.position() < byteBuffer.limit()+1) {
                    byteBuffer.put(byteBuffer.position(), (byte)(byteBuffer.get()+1));
                }
                fl.release();
                System.out.println("release: " + start + " to " + end);
            } catch (Exception e) {
                // TODO: handle exception
            }
        }
    }
}