MappedByteBuffer定义

Java NIO ByteBuffer详解：http://donald-draper.iteye.com/blog/2357084
引言：
在上一篇文章中我们看了HeapByteBuffer，今天来看另外一个DirectByteBuffer。在看DirectByteBuffer之前，我们先来看一下DirectByteBuffer的父类MappedByteBuffer。

//DirectByteBuffer
package java.nio;
import java.io.FileDescriptor;
import sun.misc.Cleaner;
import sun.misc.Unsafe;
import sun.misc.VM;
import sun.nio.ch.DirectBuffer;
class DirectByteBuffer
    extends MappedByteBuffer
    implements DirectBuffer
{

下面来看MappedByteBuffer

package java.nio;
import java.io.FileDescriptor;
import sun.misc.Unsafe;

/**
 * A direct byte buffer whose content is a memory-mapped region of a file.
 *MappedByteBuffer的内容为文件的内存映射region。
 *  Mapped byte buffers are created via the {@link
 * java.nio.channels.FileChannel#map FileChannel.map} method.  This class
 * extends the {@link ByteBuffer} class with operations that are specific to
 * memory-mapped file regions.
 *MappedByteBuffer通过java.nio.channels.FileChannel#map方法创建。MappedByteBuffer
 拓展的ByteBuffer，添加了内存映射文件regions的相关操作。
 * <p> A mapped byte buffer and the file mapping that it represents remain
 * valid until the buffer itself is garbage-collected.
 *在缓存被被垃圾回收器，回收之前，MappedByteBuffer和文件的映射都是有效的。
 * <p> The content of a mapped byte buffer can change at any time, for example
 * if the content of the corresponding region of the mapped file is changed by
 * this program or another.  Whether or not such changes occur, and when they
 * occur, is operating-system dependent and therefore unspecified.
 *MappedByteBuffer的内容可以在任何时候修改，比如映射文件相关的region内容可以被
 应用或其他应用修改。修改是否起作用，依赖于具体的操作系统，因此是不确定的。
 * <a name="inaccess"><p> All or part of a mapped byte buffer may become
 * inaccessible at any time, for example if the mapped file is truncated.  An
 * attempt to access an inaccessible region of a mapped byte buffer will not
 * change the buffer's content and will cause an unspecified exception to be
 * thrown either at the time of the access or at some later time.  It is
 * therefore strongly recommended that appropriate precautions be taken to
 * avoid the manipulation of a mapped file by this program, or by a
 * concurrently running program, except to read or write the file's content.
 *如果映射文件被删除，MappedByteBuffer的所有parts都是不可访问的。尝试访问
 不会改变buffer的内容，无论在访问的时间，还是访问后，将会引起一个不确定的异常抛出。
所以强烈建议不要通过应用或并发应用程序直接操作一个映射文件，除了读写文件内容之外。
 * <p> Mapped byte buffers otherwise behave no differently than ordinary direct
 * byte buffers. 

 *除了上述的可读文件内容，应用不可直接操作文件映射这个不同之外，MappedByteBuffer
 与一般的DirectByteBuffer没有什么不同。
 *
 * @author Mark Reinhold
 * @author JSR-51 Expert Group
 * @since 1.4
 */

public abstract class MappedByteBuffer
    extends ByteBuffer
{

    // This is a little bit backwards: By rights MappedByteBuffer should be a
    // subclass of DirectByteBuffer, but to keep the spec clear and simple, and
    // for optimization purposes, it's easier to do it the other way around.
    // This works because DirectByteBuffer is a package-private class.
    //如果想要使用MappedByteBuffer，应该是DirectByteBuffer的子类，为了保证干净简单和
    //最优化的目的，我们应该可以很容易地实现一个继承DirectByteBuffer的子类。
    为什么是DirectByteBuffer的子类呢，这是由于DirectByteBuffer是包私有的类。
     
    // For mapped buffers, a FileDescriptor that may be used for mapping
    // operations if valid; null if the buffer is not mapped.
    //在映射缓存中，如果文件描述符有效，文件描述可以用于映射操作。为null，则
    //缓存不能映射
    private final FileDescriptor fd;

    // This should only be invoked by the DirectByteBuffer constructors
    //此方法通过DirectByteBuffer的构造方法调用
    MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
                     FileDescriptor fd)
    {
        super(mark, pos, lim, cap);
        this.fd = fd;
    }

    MappedByteBuffer(int mark, int pos, int lim, int cap) { // package-private
        super(mark, pos, lim, cap);
        this.fd = null;
    }
   //检查文件描述符是否为null
    private void checkMapped() {
        if (fd == null)
            // Can only happen if a luser explicitly casts a direct byte buffer
            throw new UnsupportedOperationException();
    }

    // Returns the distance (in bytes) of the buffer from the page aligned address
    // of the mapping. Computed each time to avoid storing in every direct buffer.
    //获取起始地址
    private long mappingOffset() {
        int ps = Bits.pageSize();
        long offset = address % ps;
        return (offset >= 0) ? offset : (ps + offset);
    }
    //获取实际的起始地址
    private long mappingAddress(long mappingOffset) {
        return address - mappingOffset;
    }
   //返回映射地址长度
    private long mappingLength(long mappingOffset) {
        return (long)capacity() + mappingOffset;
    }

    // not used, but a potential target for a store, see load() for details.
    private static byte unused;//记录，not used

    /**
     * Loads this buffer's content into physical memory.
     *
     *  This method makes a best effort to ensure that, when it returns,
     * this buffer's content is resident in physical memory.  Invoking this
     * method may cause some number of page faults and I/O operations to
     * occur. 

     *
     * @return  This buffer
     */
    public final MappedByteBuffer load() {
        checkMapped(); //检查文件描述是否为null
        if ((address == 0) || (capacity() == 0))//如果地址或容量为0，返回true
            return this;
        long offset = mappingOffset();//起始地址
        long length = mappingLength(offset);//计算需要的地址长度，用于分配内存
        load0(mappingAddress(offset), length);

        // Read a byte from each page to bring it into memory. A checksum
        // is computed as we go along to prevent the compiler from otherwise
        // considering the loop as dead code.
        Unsafe unsafe = Unsafe.getUnsafe();
        int ps = Bits.pageSize();//获取分页大小
        int count = Bits.pageCount(length);//获取分页数量
        long a = mappingAddress(offset);
        byte x = 0;
	//将物理内存地址与MappedByteBuffer建立映射
        for (int i=0; i<count; i++) {
            x ^= unsafe.getByte(a);
            a += ps;
        }
        if (unused != 0)
            unused = x;

        return this;
    }
     private native void load0(long address, long length);
     /**
     * Tells whether or not this buffer's content is resident in physical
     * memory.
     *判断缓存的内容是否存在与实际的物理内存中
     *  A return value of <tt>true</tt> implies that it is highly likely
     * that all of the data in this buffer is resident in physical memory and
     * may therefore be accessed without incurring any virtual-memory page
     * faults or I/O operations.  A return value of <tt>false</tt> does not
     * necessarily imply that the buffer's content is not resident in physical
     * memory.
     *当返回值为true时，缓存中数据存在物理内存中，因此访问数据不会引起虚拟机分页
     或IO操作错误。false，即不在物理内存中
     * <p> The returned value is a hint, rather than a guarantee, because the
     * underlying operating system may have paged out some of the buffer's data
     * by the time that an invocation of this method returns.  

     *返回的结果是不能保证却对的正确，因为在方法调用的时候，底层的操作系统可能会
     分页取出缓存中的数据。
     * @return  <tt>true</tt> if it is likely that this buffer's content
     *          is resident in physical memory
     */
    public final boolean isLoaded() {
        //检查文件描述是否为null
        checkMapped();
	//如果地址或容量为0，返回true
        if ((address == 0) || (capacity() == 0))
            return true;
	//起始地址
        long offset = mappingOffset();
	//长度
        long length = mappingLength(offset);
        return isLoaded0(mappingAddress(offset), length, Bits.pageCount(length));
    }
    private native boolean isLoaded0(long address, long length, int pageCount);

    /**
     * Forces any changes made to this buffer's content to be written to the
     * storage device containing the mapped file.
     *强制将缓冲区的数据改变和映射文件，写到存储设备上。
     *  If the file mapped into this buffer resides on a local storage
     * device then when this method returns it is guaranteed that all changes
     * made to the buffer since it was created, or since this method was last
     * invoked, will have been written to that device.
     *如果缓存的文件映射已经存储在本地设备上，调用此方法可以保证从MappedByteBuffer创建，
     到当前时间，缓存的所有数据变化，写到设备上。
     * <p> If the file does not reside on a local device then no such guarantee
     * is made.
     *如果文件 不存在本地设备上，则方法不能保证
     * <p> If this buffer was not mapped in read/write mode ({@link
     * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
     * method has no effect. 

     *如果缓存没有映射为java.nio.channels.FileChannel.MapMode#READ_WRITE模式，则调用方法无效
     * @return  This buffer
     */
    public final MappedByteBuffer force() {
        checkMapped();
        if ((address != 0) && (capacity() != 0)) {
            long offset = mappingOffset();
            force0(fd, mappingAddress(offset), mappingLength(offset));
        }
        return this;
    }
   
    private native void force0(FileDescriptor fd, long address, long length);
}

来看这个方法中的地址address从何而来
//获取实际的起始地址

 private long mappingAddress(long mappingOffset) {
     return address - mappingOffset;
 }

//Buffer

 public abstract class Buffer {

    // Invariants: mark <= position <= limit <= capacity
    private int mark = -1;
    private int position = 0;
    private int limit;
    private int capacity;

    // Used only by direct buffers
    // NOTE: hoisted here for speed in JNI GetDirectBufferAddress
    //Direct buffer的物理地址
    long address;
}

再来建立MappedByteBuffer与物理内存文件映射方法load中的Bits.pageCount，Bits.pageSize()

public final MappedByteBuffer load() {
        checkMapped(); //检查文件描述是否为null
        if ((address == 0) || (capacity() == 0))//如果地址或容量为0，返回true
            return this;
        long offset = mappingOffset();//起始地址
        long length = mappingLength(offset);//计算需要的地址长度，用于分配内存
        load0(mappingAddress(offset), length);

        // Read a byte from each page to bring it into memory. A checksum
        // is computed as we go along to prevent the compiler from otherwise
        // considering the loop as dead code.
        Unsafe unsafe = Unsafe.getUnsafe();
        int ps = Bits.pageSize();//获取分页大小
        int count = Bits.pageCount(length);//获取分页数量
        long a = mappingAddress(offset);
        byte x = 0;
	//将物理内存地址与MappedByteBuffer建立映射
        for (int i=0; i<count; i++) {
            x ^= unsafe.getByte(a);
            a += ps;
        }
        if (unused != 0)
            unused = x;

        return this;
    }

//Bits

  private static final Unsafe unsafe = Unsafe.getUnsafe();
    static int pageCount(long size) {
        return (int)(size + (long)pageSize() - 1L) / pageSize();
    }
    private static int pageSize = -1;
    static int pageSize() {
        if (pageSize == -1)
            pageSize = unsafe().pageSize();
        return pageSize;
    }
  static Unsafe unsafe() {
        return unsafe;
    }

//Unsafe

public native int pageSize();

总结：
MappedByteBuffer将缓存区数据分页存放到实际的物理内存中，并建立映射。我们一般不直接使用MappedByteBuffer，而是使用MappedByteBuffer的子类DirectByteBuffer。在后面的java.nio.channels.FileChannel相关文章中，我们回再次提到MappedByteBuffer。