FileChannel和MappedByteBuffer


使用内存映射文件可以高效访问文件。下面代码做了一个示例,对比内存映射文件、filechannel的操作和FileOutputStream写文件的效率差异。


import java.io.File;

import java.io.FileNotFoundException;

import java.io.FileOutputStream;

import java.io.IOException;

import java.io.RandomAccessFile;

import java.nio.MappedByteBuffer;

import java.nio.channels.FileChannel;

import java.nio.channels.FileChannel.MapMode;


import org.slf4j.Logger;

import org.slf4j.LoggerFactory;


public class FileClient {

private FileChannel fileChannel;

private MappedByteBuffer mappedByteBuffer;

private File file;


private static int mapedFileSizeCommitLog = 1024 * 1024 * 1024;


private static final String fileName = "D:\\temp\\testfilecache\\a.dat";

private static final String fileName2 = "D:\\temp\\testfilecache\\b.dat";


private static final Logger logger = LoggerFactory.getLogger(FileClient.class.getName());


public static void main(String[] args) throws IOException {

FileClient fclient = new FileClient();

fclient.start();

fclient.start2();

}


@SuppressWarnings("resource")

public void start() throws IOException {

file = new File(fileName);

String testString = "hello";

boolean ok = false;

try {

this.fileChannel = new RandomAccessFile(this.file, "rw").getChannel();

this.mappedByteBuffer = this.fileChannel.map(MapMode.READ_WRITE, 0, mapedFileSizeCommitLog);

int curPos = 0;

long startTime = System.currentTimeMillis();

for (int i = 0; i < 1000000; i++) {

this.mappedByteBuffer.position(curPos);

this.mappedByteBuffer.put(testString.getBytes());

curPos += 6;

}

this.mappedByteBuffer.force();

this.fileChannel.close();

long stopTime = System.currentTimeMillis();

logger.debug("time :  " + (stopTime - startTime));

ok = true;

} catch (FileNotFoundException e) {

logger.error("create file channel " + fileName + " Failed. ", e);

throw e;

} catch (IOException e) {

logger.error("map file " + fileName + " Failed. ", e);

throw e;

} finally {

if (!ok && this.fileChannel != null) {

this.fileChannel.close();

}

}

}


public void start2() throws IOException {

FileOutputStream fos = new FileOutputStream(fileName2);

String testString = "hello";


long startTime = System.currentTimeMillis();

for (int i = 0; i < 1000000; i++) {

fos.write(testString.getBytes());

}

fos.close();

long stopTime = System.currentTimeMillis();

logger.debug("time2 :  " + (stopTime - startTime));


}


public void start3() throws IOException {

String input = "hello";


byte[] inputBytes = input.getBytes();

ByteBuffer buffer = ByteBuffer.wrap(inputBytes);


FileOutputStream fos = new FileOutputStream(fileName3);

FileChannel fileChannel = fos.getChannel();


long startTime = System.currentTimeMillis();

for (int i = 0; i < 1000000; i++) {

fileChannel.write(buffer);

}

fileChannel.close();

fos.close();

long stopTime = System.currentTimeMillis();

logger.debug("time2 :  " + (stopTime - startTime));


}



}


服务器sata盘,写入性能:

[root@ospservice74 ~]# dd if=/dev/zero of=kwxgd bs=64k count=4k oflag=dsync

4096+0 records in

4096+0 records out

268435456 bytes (268 MB) copied, 8.93065 seconds, 30.1 MB/s

运行后,可以看到对比结果:

[root@74 liujingyu]# java -jar filechannel-test.jar 

08:25:36.029 [main] DEBUG o.l.filechannel_test.FileClient - time :  177

08:25:37.628 [main] DEBUG o.l.filechannel_test.FileClient - time2 :  1596

08:25:37.932 [main] DEBUG o.l.filechannel_test.FileClient - time2 :  304

性能相差8倍


代码可以从https://github.com/mfcliu/netty-learning下载。




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