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。