nio~FileChannel

*Character

---scatter/gather

---always blocking

---attempts to use native I/O services when possible

---ThreadSafe,but Operations that affect the channel's position or the file size are single-threaded

 

*API

---truncate(long size)

if size<CurrentSize then discarding any bytes beyond the new end of the file

if size>=CurrentSize then the file is not modified.

if position>size ,then position=size

---force(boolean metaData)

将缓存内容同步到磁盘

metaData的含义,true表示将file的meta更新信息也保存到文件,false表示只更新文件的内容.

这个方法将引起可能的IO操作.

---FileChannel.position(long newPosition)

设置读取或修改的位置,注意FileChannel的position和文件流的position是同步的.

/**
 * Mar 11, 2011 by dzh
 */
package nio.channel;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.channels.FileChannel;

/**
 * @author dzh
 * 
 */
public class FileChannelTest {

	/**
	 * @param args
	 * @throws Exception 
	 */
	public static void main(String[] args) throws Exception {
		// InputStream is =FileChannelTest.class.getResourceAsStream("file.txt");
		if(!new File("file.txt").exists()){ //内容:123abc兵者,诡道也.故能而示之不能,用而示之不用//编码utf-8
			System.out.println("Not found!");
			return;
		}
		RandomAccessFile raf = new RandomAccessFile(new File("file.txt"), "rw");
		FileChannel fc =raf.getChannel();
		System.out.println(raf.length());	//63
		//first position
		raf.seek(1);
		System.out.println(fc.position());	//1
		printCurrentContent(raf);	//23abc兵
		//second 
		fc.position(8);
		System.out.println(raf.getFilePointer());	//8
		printCurrentContent(raf);	//�者,诡
		//truncate
		fc.truncate(6);
		System.out.println(fc.size());	//6
		System.out.println(fc.position());	//16 why?
		System.out.println(raf.getFilePointer());	//16
		
		fc.force(true);
		printFileContent(raf);	//123abc
		
		fc.close();
		raf.close();
	}
	
	private static void printCurrentContent(RandomAccessFile raf){
		byte[] b =new byte[8];
		try {
			raf.read(b);
			System.out.println(new String(b,"utf-8"));
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	
	private static void printFileContent(RandomAccessFile raf){
		try {
			raf.seek(0);
			byte[] buf =new byte[32];
			int len =-1;
			ByteArrayOutputStream bos =new ByteArrayOutputStream();
			while((len=raf.read(buf))!=-1){
				bos.write(buf, 0, len);
			}
			System.out.println(bos.toString("utf-8"));
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	
}

*文件锁FileLock

---用于进程间并发

Locks are associated with files, not channels. Use locks to coordinate
with external processes, not between threads in the same JVM.

---概念

共享锁:共享读操作，但只能一个写

排它锁:读写一时一个．

---FileLock FileChannel.lock(long position, long size, boolean shared) ,文件锁

shared的含义:是否使用共享锁,一些不支持共享锁的操作系统,将自动将共享锁改成排它锁.

---lock()和tryLock()的区别

lock()阻塞的方法

tryLock()非阻塞,当未获得锁时,返回null.

---FileLock的生命周期

在调用FileLock.release(),或者Channel.close(),或者JVM关闭

---FileLock是线程安全的

---boolean java.nio.channels.FileLock.overlaps(long position, long size)

true表示当前锁在区域内,false表示当前锁的区域与参数区域不重叠