java_croe 学习笔记之新IO---java.nio 之内存映射文件

解释：将文件的一段区域映射到内存中，比传统的文件处理速度要快很多

参考：

无格式输入流 110秒

缓冲输入流     9.9秒

随机存取文件  162秒

内存映射文件   7.2秒

例子

package twelve;

import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.util.zip.CRC32;

/**
@Title NIOTTest.java
@description  TODO
@author qinpeng
@date Aug 25, 2009 10:23:26 PM
*/
public class NIOTTest {

public static void main(String[] args) {

String fileName = "d:\\IOTest.pdf";

System.out.println("inputStream");
long start = System.currentTimeMillis();
long crcValue = checksumInputStreanm(fileName);
long end = System.currentTimeMillis();
System.out.println(Long.toHexString(crcValue));
System.out.println((end - start)+"耗时");

System.out.println("BufferedinputStream");
start = System.currentTimeMillis();
crcValue = checksumInputStreanm(fileName);
end = System.currentTimeMillis();
System.out.println(Long.toHexString(crcValue));
System.out.println((end - start)+"耗时");

System.out.println("RandomAccessFileinputStream");
start = System.currentTimeMillis();
crcValue = checksumInputStreanm(fileName);
end = System.currentTimeMillis();
System.out.println(Long.toHexString(crcValue));
System.out.println((end - start)+"耗时");

System.out.println(" MappedFile inputStream");
start = System.currentTimeMillis();
crcValue = checksumInputStreanm(fileName);
end = System.currentTimeMillis();
System.out.println(Long.toHexString(crcValue));
System.out.println((end - start)+"耗时");
}

public static long checksumInputStreanm(String fileName){
CRC32 crc = new CRC32();
try {
InputStream in = new FileInputStream(fileName);
int c;
while((c=in.read())!=-1){
crc.update(c);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
System.err.print("NIOTTest--checksumInputStreanm--new FileInputStream is not found");
} catch(IOException ioe){
ioe.printStackTrace();
System.err.print("NIOTTest--checksumInputStreanm--new FileInputStream'read append IOException");
}
return crc.getValue();
}

public static long checksumBufferedInputStream(String fileName){
CRC32 crc = new CRC32();
try {
InputStream in = new BufferedInputStream(new FileInputStream(fileName));
int c;
while((c=in.read())!=-1){
crc.update(c);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
System.err.print("NIOTTest--checksumBufferedInputStream--new FileInputStream is not found");
} catch(IOException ioe){
ioe.printStackTrace();
System.err.print("NIOTTest--checksumBufferedInputStream--new FileInputStream'read append IOException");
}
return crc.getValue();
}

public static long checksumRondomAccessFileInputStream(String fileName){
CRC32 crc = new CRC32();
try {
RandomAccessFile file = new RandomAccessFile(fileName,"r");
int c;
while((c=file.read())!=-1){
crc.update(c);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
System.err.print("NIOTTest--checksumRondomAccessFileInputStream--new FileInputStream is not found");
} catch(IOException ioe){
ioe.printStackTrace();
System.err.print("NIOTTest--checksumRondomAccessFileInputStream--new FileInputStream'read append IOException");
}
return crc.getValue();
}

public static long checksumMappedFile(String fileName){
CRC32 crc = new CRC32();
try {
FileInputStream in = new FileInputStream(fileName);
FileChannel channel = in.getChannel();
int length = (int) channel.size();
MappedByteBuffer buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, length);

for(int p = 0;p<length;p++){
int c = buffer.getInt(p);
crc.update(c);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
System.err.print("NIOTTest--checksumRondomAccessFileInputStream--new FileInputStream is not found");
} catch(IOException ioe){
ioe.printStackTrace();
System.err.print("NIOTTest--checksumRondomAccessFileInputStream--new FileInputStream'read append IOException");
}
return crc.getValue();
}

}