import java.io.File;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;

public class t {
	public static void main(String[] args) throws Exception {
		final int BUFFER_SIZE = 0x300000;// 缓冲区大小为3M
		File f = new File("C:\\Documents and Settings\\XHY\\桌面\\a.txt");

		/**
		 * map(FileChannel.MapMode mode,long position, long size)
		 * mode - 根据是按只读、读取/写入或专用（写入时拷贝）来映射文件，分别为 FileChannel.MapMode 类中所定义的
		 * READ_ONLY、READ_WRITE 或 PRIVATE 之一
		 * position - 文件中的位置，映射区域从此位置开始；必须为非负数
		 * size - 要映射的区域大小；必须为非负数且不大于 Integer.MAX_VALUE
		 * 所以若想读取文件后半部分内容，如例子所写；若想读取文本后1/8内容，需要这样写map(FileChannel.MapMode.
		 * READ_ONLY, f.length()*7/8,f.length()/8)
		 * 想读取文件所有内容，需要这样写map(FileChannel.MapMode.READ_ONLY, 0,f.length())
		 */

		MappedByteBuffer inputBuffer = new RandomAccessFile(f, "r")
				.getChannel().map(FileChannel.MapMode.READ_ONLY,
						f.length() / 2, f.length() / 2);

		byte[] dst = new byte[BUFFER_SIZE];// 每次读出3M的内容
		long start = System.currentTimeMillis();
		for (int offset = 0; offset < inputBuffer.capacity(); offset += BUFFER_SIZE) {
			if (inputBuffer.capacity() - offset >= BUFFER_SIZE) {
				for (int i = 0; i < BUFFER_SIZE; i++)
					dst[i] = inputBuffer.get(offset + i);
			} else {
				for (int i = 0; i < inputBuffer.capacity() - offset; i++)
					dst[i] = inputBuffer.get(offset + i);
			}

			int length = (inputBuffer.capacity() % BUFFER_SIZE == 0) ? BUFFER_SIZE
					: inputBuffer.capacity() % BUFFER_SIZE;

			// System.out.println(new String(dst,0,length));//new
			// String(dst,0,length)这样可以取出缓存保存的字符串，可以对其进行操作
		}
		long end = System.currentTimeMillis();
		System.out.println("读取文件文件一半内容花费：" + (end - start) + "毫秒");
	}
}

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