JAVA NIO FileChannel 内存映射文件
文件通道总是阻塞式的。
文件通道不能创建，只能通过（RandomAccessFile、FileInputStream、FileOutputStream）getChannel()获得，具有与File形同的访问权限。
线程安全。
文件锁：锁的对象是文件。
package org.windwant.nio; import java.io.File; import java.io.FileNotFoundException; import java.io.IOException; import java.io.RandomAccessFile; import java.nio.ByteBuffer; import java.nio.MappedByteBuffer; import java.nio.channels.Channel; import java.nio.channels.FileChannel; import java.nio.channels.FileLock; import java.nio.channels.Selector; import java.util.concurrent.locks.ReadWriteLock; import java.util.concurrent.locks.ReentrantReadWriteLock; /** * Created by windwant on 2016/5/13. */ public class NIOOpt { public static void main(String[] args) { try { MappedPrivateChannel(); } catch (Exception e) { e.printStackTrace(); } } /** * MapMode.PRIVATE 写时拷贝（copy-on-write）映射：通过put()修改的任何修改，会导致产生一个私有的数据 * 拷贝，宝贝中的数据只有MappedByteBuffer实例可以看到。不会对底层文件做任何修改。若缓冲区被回收，修改丢 * 失，read/write方式建立通道。 * 做修改，拷贝副本前，其它方式的映射区的修改，会反映到当前区域。映射相互的修改不可见 * 允许父子进程共享内存页 * 处理一个文件多个映射场景。 * 关闭通道，映射会保持。除非丢弃缓冲区本身。 * MappedByteBuffer 对象是直接的，占用的内存位于jvm堆栈之外。 */ public static void MappedPrivateChannel() throws Exception { // Create a temp file and get a channel connected to it File tempFile = File.createTempFile ("mmaptest", null); RandomAccessFile file = new RandomAccessFile (tempFile, "rw"); FileChannel channel = file.getChannel( ); ByteBuffer temp = ByteBuffer.allocate (100); // Put something in the file, starting at location 0 temp.put ("This is the file content".getBytes( )); temp.flip( ); channel.write (temp, 0); // Put something else in the file, starting at location 8192. // 8192 is 8 KB, almost certainly a different memory/FS page. // This may cause a file hole, depending on the // filesystem page size. temp.clear( ); temp.put ("This is more file content".getBytes( )); temp.flip( ); channel.write (temp, 8192); // Create three types of mappings to the same file MappedByteBuffer ro = channel.map ( FileChannel.MapMode.READ_ONLY, 0, channel.size( )); MappedByteBuffer rw = channel.map ( FileChannel.MapMode.READ_WRITE, 0, channel.size( )); MappedByteBuffer cow = channel.map ( FileChannel.MapMode.PRIVATE, 0, channel.size( )); // the buffer states before any modifications System.out.println ("Begin"); showBuffers (ro, rw, cow); // Modify the copy-on-write buffer cow.position (8); cow.put ("COW".getBytes( )); System.out.println ("Change to COW buffer"); showBuffers (ro, rw, cow); // Modify the read/write buffer92 rw.position (9); rw.put (" R/W ".getBytes( )); rw.position (8194); rw.put (" R/W ".getBytes( )); rw.force( ); System.out.println ("Change to R/W buffer"); showBuffers (ro, rw, cow); // Write to the file through the channel; hit both pages temp.clear( ); temp.put ("Channel write ".getBytes( )); temp.flip( ); channel.write (temp, 0); temp.rewind( ); channel.write (temp, 8202); System.out.println ("Write on channel"); showBuffers (ro, rw, cow); // Modify the copy-on-write buffer again cow.position (8207); cow.put (" COW2 ".getBytes( )); System.out.println ("Second change to COW buffer"); showBuffers (ro, rw, cow); // Modify the read/write buffer rw.position (0); rw.put (" R/W2 ".getBytes( )); rw.position (8210); rw.put (" R/W2 ".getBytes( )); rw.force( ); System.out.println ("Second change to R/W buffer"); showBuffers (ro, rw, cow); // cleanup channel.close( ); file.close( ); tempFile.delete( ); } // Show the current content of the three buffers public static void showBuffers (ByteBuffer ro, ByteBuffer rw, ByteBuffer cow) throws Exception{ dumpBuffer ("R/O", ro); dumpBuffer ("R/W", rw); dumpBuffer ("COW", cow); System.out.println (""); } // Dump buffer content, counting and skipping nulls public static void dumpBuffer (String prefix, ByteBuffer buffer) throws Exception { System.out.print (prefix + ": '"); 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) { System.out.print ("|[" + nulls + " nulls]|"); nulls = 0; } System.out.print (c); } System.out.println ("'"); } /** * channel Gather/Scatter 矢量IO */ public static void channelGatherScatter(){ ByteBuffer head = ByteBuffer.allocate(4); ByteBuffer body = ByteBuffer.allocate(100); RandomAccessFile afile = null; RandomAccessFile bfile = null; ReadWriteLock readWriteLock = new ReentrantReadWriteLock(); try { afile = new RandomAccessFile("hello.txt", "r"); bfile = new RandomAccessFile("hehe.txt", "rw"); readWriteLock.readLock().lock(); FileChannel fileChannel = afile.getChannel(); ByteBuffer[] buffers = {head, body}; while (fileChannel.read(buffers) != -1){ } head.flip(); body.flip(); System.out.println(new String(head.array())); System.out.println(new String(body.array())); readWriteLock.readLock().unlock(); fileChannel.close(); afile.close(); readWriteLock.writeLock().lock(); FileChannel bfileChannel = bfile.getChannel(); while (bfileChannel.write(buffers) > 0){ } bfileChannel.position(bfileChannel.position() + 10); bfileChannel.write(ByteBuffer.wrap("THIS IS THE TEST TEXT!".getBytes())); bfileChannel.truncate(3); //改变文件大小 bfileChannel.force(true); //写入磁盘文件 参数 是否更新文件元数据（所有者、访问权限等） readWriteLock.writeLock().unlock(); bfileChannel.close(); bfile.close(); }catch (Exception e){ e.printStackTrace(); } } /** * 基于MappedFileChannle的文件复制 * 文件锁 */ public static void mappedFileChannelLock(){ RandomAccessFile afile = null; RandomAccessFile bfile = null; FileChannel fc = null; FileChannel fcb = null; try { afile = new RandomAccessFile("hello.txt", "rw"); fc = afile.getChannel(); long length = fc.size(); FileLock fileLock = fc.tryLock(0, length, true);//true共享锁 false 独占锁 从开始 锁定全部内容 如果获取不到锁会返回null if(null != fileLock) { MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, length); byte[] fbo = new byte[(int) length]; mbb.get(fbo); System.out.println(new String(fbo, "UTF-8")); fileLock.release(); bfile = new RandomAccessFile("hehe.txt", "rw"); fcb = bfile.getChannel(); fileLock = fcb.tryLock(0, length, false); MappedByteBuffer mbbb = fcb.map(FileChannel.MapMode.READ_WRITE, 0, length); for (int i = 0; i < length; i++) { mbbb.put(i, fbo[i]); } mbbb.flip(); mbbb.force(); fileLock.release(); } } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); }finally { try { fc.close(); fcb.close(); afile.close(); bfile.close(); } catch (IOException e) { e.printStackTrace(); } } } /** * MappedByteBuffer map(MapMode mode, long position, long size) * size大于文件大小，文件会做扩充 * MappedByteBuffer 内存映射缓冲池 * 基于MappedFileChannle的文件复制 * 读写锁 * 直接读取，修改磁盘上的文件。 * 自动缓存内存页，比较高效。 */ public static void mappedFileChannel(){ RandomAccessFile afile = null; RandomAccessFile bfile = null; FileChannel fc = null; FileChannel fcb = null; ReadWriteLock readWriteLock = new ReentrantReadWriteLock(); try { afile = new RandomAccessFile("hello.txt", "rw"); readWriteLock.readLock().lock(); fc = afile.getChannel(); long length = fc.size(); MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, length); byte[] fbo = new byte[(int) length]; mbb.get(fbo); System.out.println(new String(fbo)); readWriteLock.readLock().unlock(); bfile = new RandomAccessFile("hehe.txt", "rw"); readWriteLock.writeLock().lock(); fcb = bfile.getChannel(); MappedByteBuffer mbbb = fcb.map(FileChannel.MapMode.READ_WRITE, 0, length); for (int i = 0; i < length; i++) { mbbb.put(i, fbo[i]); } mbbb.flip(); mbbb.force(); readWriteLock.writeLock().unlock(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); }finally { try { fc.close(); fcb.close(); afile.close(); bfile.close(); } catch (IOException e) { e.printStackTrace(); } } } /** * FileChannel文件读取 */ public static void fileChannel(){ try { RandomAccessFile afile = new RandomAccessFile("hello.txt", "rw"); FileChannel fc = afile.getChannel(); ByteBuffer bb = ByteBuffer.allocate(48); int byteRead; while ((byteRead = fc.read(bb)) != -1){//确保读完 System.out.println("read:" + byteRead); bb.flip();//翻转为读状态 while (bb.hasRemaining()){//直到没有可读的字节 System.out.println(String.valueOf(bb.get())); } bb.clear(); } fc.close(); afile.close(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } /** * 基于FileChannel transferTo transferFrom 方法文件复制 */ public static void fileTransfer(){ try { RandomAccessFile afile = new RandomAccessFile("hello.txt", "rw"); RandomAccessFile bfile = new RandomAccessFile("hehe.txt", "rw"); FileChannel ac = afile.getChannel(); FileChannel bc = bfile.getChannel(); long position = 0; long count = ac.size(); // bc.transferFrom(ac, position, count); ac.transferTo(position, count, bc); ac.close(); afile.close(); bc.close(); bfile.close(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } public static void fileSelector(){ try { RandomAccessFile afile = new RandomAccessFile("hello.txt", "rw"); Channel c = afile.getChannel(); Selector s = Selector.open(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } /** * 基于基本channel buffer的文件复制操作 */ public static void fileTransferByNormal() { try { RandomAccessFile afile = new RandomAccessFile("hello.txt", "rw"); RandomAccessFile bfile = new RandomAccessFile("hehe.txt", "rw"); FileChannel ac = afile.getChannel(); FileChannel bc = bfile.getChannel(); ByteBuffer bf = ByteBuffer.allocateDirect(16 * 1024); while (ac.read(bf) != -1) { bf.flip(); while (bf.hasRemaining()) {//确保写完 bc.write(bf); } bf.clear(); } ac.close(); afile.close(); bc.close(); bfile.close(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } }
