jdk中的MappedByteBuffer非常类似linux中的mmap将文件映射到虚拟内存，这样可以利用操作系统自带的虚拟内存实现来
提高io效率, 很多文档中提到这一点，具体大家可以自行测试。
MappedByteBuffer有3种映射模式：
MapMode.READ_ONLY:只读映射
MapMode.READ_WRITE?：读写映射
MapMode.PRIVATE?：copy on write映射,一旦需要改变原有buffer，拷贝一份原来的
本文通过一种测试代码来理解MappedByteBuffer使用MapMode.READ_WRITE以及MapMode.PRIVATE，特别是MAP_MODE.PRIVATE模式?
public class MMapTest {private static FileChannel channel;@BeforeClasspublic static void runBeforeClass() throws FileNotFoundException {String path = MMapTest.class.getResource("/test.txt").getPath();File file = new File(path);channel = new RandomAccessFile(file, "rw").getChannel();}@AfterClasspublic static void runAfterClass() throws IOException {channel.close();}@Beforepublic void runBeforeEveryTest() throws IOException {ByteBuffer temp = ByteBuffer.allocate(100);temp.put("12345".getBytes());temp.flip();channel.truncate(5);channel.write(temp);}/** * * @author weip.pengw * @time 2012-3-19 下午02:53:21 * @param buffer * @throws Exception */public static String dumpBuffer(ByteBuffer buffer) throws Exception {StringBuffer sb = new StringBuffer();int nulls = 0;int limit = buffer.limit();for (int i = 0; i < limit; i++) {char c = (char) buffer.get(i);if (c == '\u0000') {nulls++;continue;}if (nulls != 0) {sb.append("|[" + nulls + " nulls]|");nulls = 0;}sb.append(c);}return sb.toString();}/** * * @author weip.pengw * @throws Exception * @time 2012-3-19 下午02:45:28 */@Testpublic void testCopyOnWrite() throws Exception {// 产生一个文件，文件跨内存页ByteBuffer temp = ByteBuffer.allocate(100);temp.put("This is the file content".getBytes());temp.flip();channel.write(temp, 0);temp.clear();temp.put("This is more file content".getBytes());temp.flip();channel.write(temp, 8192);// 产生两个MAPFILEMappedByteBuffer rw = channel.map(FileChannel.MapMode.READ_WRITE, 0,channel.size());MappedByteBuffer cow = channel.map(FileChannel.MapMode.PRIVATE, 0,channel.size());// 测试之前assertThat(dumpBuffer(rw),is("This is the file content|[8168 nulls]|This is more file content"));assertThat(dumpBuffer(cow),is("This is the file content|[8168 nulls]|This is more file content"));// 测试step1，修改rw前几个字节rw.position(0);rw.put("RW".getBytes());rw.force();assertThat(dumpBuffer(rw),is("RWis is the file content|[8168 nulls]|This is more file content"));assertThat(dumpBuffer(cow),is("RWis is the file content|[8168 nulls]|This is more file content"));// 测试step2，修改cow前几个字节,触发copy on writecow.position(0);cow.put("COW".getBytes());assertThat(dumpBuffer(rw),is("RWis is the file content|[8168 nulls]|This is more file content"));assertThat(dumpBuffer(cow),is("COWs is the file content|[8168 nulls]|This is more file content"));// 测试step3，修改rw的最后几个字节,cow后面的字节反应了改变rw.position(8192);rw.put("RW".getBytes());rw.force();assertThat(dumpBuffer(rw),is("RWis is the file content|[8168 nulls]|RWis is more file content"));assertThat(dumpBuffer(cow),is("COWs is the file content|[8168 nulls]|RWis is more file content"));// 测试step4，修改cow的最后几个字节,再次触发copy on writecow.position(8192);cow.put("COW".getBytes());assertThat(dumpBuffer(rw),is("RWis is the file content|[8168 nulls]|RWis is more file content"));assertThat(dumpBuffer(cow),is("COWs is the file content|[8168 nulls]|COWs is more file content"));// 测试step5，再次修改rw的前后几个字节,对cow没有了影响rw.position(0);rw.put("RW2".getBytes());rw.position(8192);rw.put("RW2".getBytes());rw.force();assertThat(dumpBuffer(rw),is("RW2s is the file content|[8168 nulls]|RW2s is more file content"));assertThat(dumpBuffer(cow),is("COWs is the file content|[8168 nulls]|COWs is more file content"));// cleanup// channel.close();}
重点看看代码中的测试方法testCopyOnWrite
1）产生一个较大空洞文件(本文操过8k)，使得操作系统（代码在ubuntu10.04测试通过）无法在一页能映射该文件（跨内存页），这样才会有测试效果
2）将文件映射到两个MappedByteBuffer实例，一个是MapMode.READ_WRITE模式简称rw,一个是FileChannel.MapMode.PRIVATE模式简称cow
3) 测试之前，两个buff实例内容一致
4）第1步测试修改rw实例，由于没有触发copy on write,所以两个buff实例内容一致
5) 第2步测试修改cow实例的前几个字节，cow的修改只影响它本身，因为它修改的时候实际上重新拷贝了一份
6）第3步测试修改rw实例的后几个字节，居然在cow实例中能看到，之前不是说cow拷贝了一份吗？原因后面说
7）第4步测试修改cow实例的后几个字节，?cow的修改只影响它本身
8）第5步测试修改rw实例的前后字节，发现都没反映在cow实例中，的确和6）有所不同。
这说明：
一开始没有修改cow,会共用rw的内存，当cow修改了前几个字节，cow会拷贝内存，但明显cow实例是按内存页拷贝的，由于文件较长，?所以只拷贝了前一个部分，前面的是自己的，后面是共享的?当cow修改了后几个字节?，cow才拷贝后面的内存页，这样的话，bw对文件后面的修改，cow也就看不到了
躲在某一时间想念一段时光的掌纹，
