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。