Java NIO——与传统IO文件复制的效率比较（channel.transferTo、Files.copy 、channel.write）
IO与NIO操作效率对比（网上的说法千篇一律，99%都是错的，没经过自己的验证，正确的解释如下）
复制文件的情况：
注意：Files.copy和传统IO代码基本一样，只是它开辟的缓冲区大小较大，为8192（基本是缓冲区开的越大越快，但是也吃内存）- 当缓冲区（<1024）和数据量比较小（<10KB）时，传统IO 最为高效
效率上：传统IO > channel.transferTo > Files.copy > NIO::channel.write - 其他情况：
效率上：channel.transferTo > Files.copy > 传统IO > NIO::channel.write
- 当缓冲区（<1024）和数据量比较小（<10KB）时，传统IO 最为高效
多线程共用同一个流追加数据的情况：“NIO”比“IO”效率高
- 多线程使用不同的流追加数据的情况：“NIO+锁”比“IO+锁”效率高
注意：这种情况必须枷锁，否则会引起数据混乱
package com.demo.test;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileWriter;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
import java.nio.channels.OverlappingFileLockException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Date;
import java.util.concurrent.CountDownLatch;
public class FileChannelDemo {
private static void createFile(String path, long numKB) {
FileWriter fw = null;
BufferedWriter bw = null;
try {
fw = new FileWriter(path);
bw = new BufferedWriter(fw);
for (long i = 0; i < numKB * 16 * 4; i++) {
for (int j = 0; j < 16; j++) {
String hex = Integer.toHexString(j);
bw.write(hex);
}
}
} catch (IOException e) {
e.printStackTrace();
} finally {
if (bw != null) {
try {
bw.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (fw != null) {
try {
fw.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
private static void copyByIO(String srcPath, String dstPath) {
byte[] buffer = new byte[bufferSize];
FileInputStream fis = null;
FileOutputStream fos = null;
try {
fis = new FileInputStream(srcPath);
fos = new FileOutputStream(dstPath);
int len;
while ((len = fis.read(buffer)) != -1) {
fos.write(buffer, 0, len);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if (fis != null) {
try {
fis.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (fos != null) {
try {
fos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
private static void copyByNIO(String srcPath, String dstPath) {
FileInputStream fis = null;
FileOutputStream fos = null;
FileChannel fisChannel = null;
FileChannel fosChannel = null;
ByteBuffer buffer = ByteBuffer.allocateDirect(bufferSize);
try {
fis = new FileInputStream(srcPath);
fos = new FileOutputStream(dstPath);
fisChannel = fis.getChannel();
fosChannel = fos.getChannel();
while (fisChannel.read(buffer) != -1) {
buffer.flip();
fosChannel.write(buffer);
buffer.clear();
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if (fisChannel != null) {
try {
fisChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (fis != null) {
try {
fis.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (fosChannel != null) {
try {
fosChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (fos != null) {
try {
fos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
private static void copyByNIOTransfer(String srcPath, String dstPath) {
FileInputStream fis = null;
FileOutputStream fos = null;
FileChannel fisChannel = null;
FileChannel fosChannel = null;
try {
fis = new FileInputStream(srcPath);
fos = new FileOutputStream(dstPath);
fisChannel = fis.getChannel();
fosChannel = fos.getChannel();
long len = fisChannel.transferTo(0, fisChannel.size(), fosChannel);
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if (fisChannel != null) {
try {
fisChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (fis != null) {
try {
fis.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (fosChannel != null) {
try {
fosChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (fos != null) {
try {
fos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
private static void copyByFiles(String srcPath, String dstPath) {
Path path = Paths.get(srcPath);
FileOutputStream fos = null;
try {
fos = new FileOutputStream(dstPath);
/*
* private static final int BUFFER_SIZE = 8192;
* private static long copy(InputStream source, OutputStream sink) throws IOException
* {
* long nread = 0L;
* byte[] buf = new byte[BUFFER_SIZE];
* int n;
* while ((n = source.read(buf)) > 0) {
* sink.write(buf, 0, n);
* nread += n;
* }
* return nread;
* }
*/
long len = Files.copy(path, fos);
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if (fos != null) {
try {
fos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
private static void multiThreadWriteIO(String path) {
FileOutputStream fos = null;
int num = 1000;
final CountDownLatch countDownLatch = new CountDownLatch(num);
try {
fos = new FileOutputStream(path);
final FileOutputStream thisFos = fos;
Thread[] threads = new Thread[num];
for (int i = 0; i < num; i++) {
final int index = i;
threads[i] = new Thread(new Runnable() {
@Override
public void run() {
byte[] buffer = (index + ". " + (new Date()).toString() + " hello world!xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx welcome to Newland\r\n").getBytes();
try {
thisFos.write(buffer);
} catch (IOException e) {
e.printStackTrace();
}
countDownLatch.countDown();
}
});
}
for (int i = 0; i < num; i++) {
threads[i].start();
}
countDownLatch.await();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
if (fos != null) {
try {
fos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
private static void multiThreadWriteNIO(String path) {
FileOutputStream fos = null;
FileChannel fosChannel = null;
int num = 1000;
final CountDownLatch countDownLatch = new CountDownLatch(num);
try {
fos = new FileOutputStream(path);
fosChannel = fos.getChannel();
final FileChannel thisFosChannel = fosChannel;
for (int i = 0; i < num; i++) {
final int index = i;
new Thread(new Runnable() {
@Override
public void run() {
ByteBuffer buffer = ByteBuffer.wrap((index + ". " + (new Date()).toString() + " hello world!xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx welcome to Newland\r\n").getBytes());
try {
thisFosChannel.write(buffer);
} catch (IOException e) {
e.printStackTrace();
}
countDownLatch.countDown();
}
}).start();
}
countDownLatch.await();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
if (fosChannel != null) {
try {
fosChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (fos != null) {
try {
fos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
private static void multiThreadOpenWriteIO(final String path) {
int num = 1000;
final CountDownLatch countDownLatch = new CountDownLatch(num);
final Object obj = new Object();
for (int i = 0; i < num; i++) {
final int index = i;
new Thread(new Runnable() {
@Override
public void run() {
RandomAccessFile raf = null;
synchronized (obj) {
try {
raf = new RandomAccessFile(path, "rw");
raf.seek(raf.length());
byte[] buffer = (index + ". " + (new Date()).toString() + " hello world!xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx welcome to Newland\r\n").getBytes();
raf.write(buffer);
} catch (FileNotFoundException e1) {
e1.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if (raf != null) {
try {
raf.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
countDownLatch.countDown();
}
}).start();
}
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private static void multiThreadOpenWriteNIO(final String path, long sleepTimeMillis) {
int num = 1000;
final CountDownLatch countDownLatch = new CountDownLatch(num);
for (int i = 0; i < num; i++) {
final int index = i;
new Thread(new Runnable() {
@Override
public void run() {
RandomAccessFile raf = null;
FileChannel rafChannel = null;
FileLock fl = null;
try {
raf = new RandomAccessFile(path, "rw");
rafChannel = raf.getChannel();
while (true) {
try {
fl = rafChannel.tryLock();
if (fl != null) {
break;
}
} catch (OverlappingFileLockException e) {
try {
Thread.sleep(sleepTimeMillis);
} catch (InterruptedException e1) {
e1.printStackTrace();
}
}
}
rafChannel.position(rafChannel.size());
ByteBuffer buffer = ByteBuffer.wrap((index + ". " + (new Date()).toString() + " hello world!xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx welcome to Newland\r\n").getBytes());
rafChannel.write(buffer);
} catch (FileNotFoundException e1) {
e1.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if (fl != null) {
try {
fl.release();
} catch (IOException e) {
e.printStackTrace();
}
}
if (rafChannel != null) {
try {
rafChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (raf != null) {
try {
raf.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
countDownLatch.countDown();
}
}).start();
}
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private static final int[] NUM_KB_ARR = { 1, 10, 100, 1024, 10 * 1024, 100 * 1024 };
private static final int[] BUFF_SIZE_ARR = { 256, 1024, 8192 };
private static int bufferSize = 8192;
private static long beginTime;
private static void calcCostTime(String msg) {
if (msg == null) {
beginTime = System.nanoTime();
} else {
System.out.println(msg + "\tcost\t" + String.format("%011d", System.nanoTime() - beginTime) + "us");
}
}
public static void main(String[] args) {
String testPath = "test.txt";
String ioOutputPath = "io.txt";
String nioOutputPath = "nio.txt";
String nioTransferOutputPath = "nioTransfer.txt";
String nioFilesPath = "nioFiles.txt";
String ioMultiThreadWritePath = "multiThreadIoWritePath.txt";
String nioMultiThreadNioWritePath = "multiThreadNioWritePath.txt";
String iomultiThreadIoOpenWritePath = "multiThreadIoOpenWritePath.txt";
String nioMultiThreadNioOpenWritePath = "multiThreadNioOpenWritePath.txt";
Runtime runtime = Runtime.getRuntime();
int availableProcessors = runtime.availableProcessors();
System.out.println("availableProcessors = " + availableProcessors);
System.out.println("vm's freeMemory = " + runtime.freeMemory() + ", vm's totalMemory = " + runtime.totalMemory());
System.out.println();
for (int i = 0; i < BUFF_SIZE_ARR.length; i++) {
bufferSize = BUFF_SIZE_ARR[i];
System.out.println("BUFF_SIZE = " + bufferSize);
// 复制文件的情况
for (int j = 0; j < NUM_KB_ARR.length; j++) {
System.out.println(j + ". FILE_SIZE = " + NUM_KB_ARR[j] + " KB");
String numKBStr = String.format("%09d", NUM_KB_ARR[j]);
calcCostTime(null);
createFile(testPath, NUM_KB_ARR[j]);
calcCostTime("createFile\t\t" + numKBStr);
calcCostTime(null);
copyByIO(testPath, bufferSize+"_"+ioOutputPath);
calcCostTime("copyByIO\t\t" + numKBStr);
calcCostTime(null);
copyByNIO(testPath, bufferSize+"_"+nioOutputPath);
calcCostTime("copyByNIO\t\t" + numKBStr);
calcCostTime(null);
copyByNIOTransfer(testPath, bufferSize+"_"+nioTransferOutputPath);
calcCostTime("copyByNIOTransfer\t" + numKBStr);
calcCostTime(null);
copyByFiles(testPath, bufferSize+"_"+nioFilesPath);
calcCostTime("copyByFiles\t\t" + numKBStr);
}
System.out.println();
}
// 多线程共用同一个流追加数据的情况
calcCostTime(null);
multiThreadWriteIO(ioMultiThreadWritePath);
calcCostTime("multiThreadWriteIO\t\t");
calcCostTime(null);
multiThreadWriteNIO(nioMultiThreadNioWritePath);
calcCostTime("multiThreadWriteNIO\t\t");
// 多线程使用不同的流追加数据的情况
calcCostTime(null);
multiThreadOpenWriteIO(iomultiThreadIoOpenWritePath);
calcCostTime("multiThreadOpenWriteIO\t\t");
calcCostTime(null);
multiThreadOpenWriteNIO(nioMultiThreadNioOpenWritePath, 15);
calcCostTime("multiThreadOpenWriteNIO\t\t");
System.out.println();
File file = new File(".");
for (String path : file.list()) {
if (path.endsWith(".txt")) {
try {
System.out.println("prepare delete "+path);
Files.delete(Paths.get(path));
System.out.println("delete succ");
} catch (IOException e) {
e.printStackTrace();
System.out.println("delete fail");
}
}
}
}
}
注意到：在“多线程使用不同的流追加数据”的情况下，“NIO+锁”每次循环都需要休眠一定时间再判断是否可以成功获得锁。因此继续修改代码来测试这个最优时间。
结论：可以看出，在休眠时间为15ms的时候所消耗的时间最少，效率最高。
（这里需要根据具体问题具体分析，和很多环境因素都有关，没有定论）
//（变量、函数定义，略……）
public satic void main(String[] args) {
for(int i=0;inull);
multiThreadOpenWriteNIO(SLEEP_TIME_MILLIS_ARR[i] + "_" + nioMultiThreadNioOpenWritePath, SLEEP_TIME_MILLIS_ARR[i]);
calcCostTime("multiThreadOpenWriteNIO\t" + SLEEP_TIME_MILLIS_ARR[i] + "\t");
System.out.println();
}
//(删除文件部分，略……)
} 
通过轮询，然后不断尝试无形中浪费了很多时间。后面我想：是否可以走“事件-通知”的模式，即“wait-notify”，来提高执行效率呢？
很不幸，结论是否定的。因为多线程只能通过“wait-notify”来暂停和唤醒，而“wait-notify”又必须引入同步锁，同步锁增加了程序的开销。
记得《Java并发编程》中大意是这么说的：“同步代码块会强制跨越内存栅栏（强制将寄存器中的数据更新到主存中）……引入锁会带来跟多的开销，并且容易引起不必要的错误，因此能不用锁尽量不用锁”。
//（变量、函数定义，略……）
private static void multiThreadOpenWriteNIOEx(final String path) {
int num = 1000;
final CountDownLatch countDownLatch = new CountDownLatch(num);
final Object object = new Object();
for (int i = 0; i < num; i++) {
final int index = i;
new Thread(new Runnable() {
@Override
public void run() {
RandomAccessFile raf = null;
FileChannel rafChannel = null;
FileLock fl = null;
try {
raf = new RandomAccessFile(path, "rw");
rafChannel = raf.getChannel();
if( index != 0) {
synchronized (object) {
try {
object.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
while(true) {
try {
fl = rafChannel.tryLock();
if(fl != null) {
break;
}
} catch (OverlappingFileLockException e) {
e.printStackTrace();
return;
}
}
rafChannel.position(rafChannel.size());
ByteBuffer buffer = ByteBuffer.wrap((index + ". " + (new Date()).toString() + " hello world!xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx welcome to Newland\r\n").getBytes());
rafChannel.write(buffer);
} catch (FileNotFoundException e1) {
e1.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if (fl != null) {
try {
fl.release();
} catch (IOException e) {
e.printStackTrace();
}
}
if (rafChannel != null) {
try {
rafChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
synchronized (object) {
object.notify();
}
if (raf != null) {
try {
raf.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
countDownLatch.countDown();
}
}).start();
}
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
//(执行其它函数代码，略……)
calcCostTime(null);
multiThreadOpenWriteNIOEx(nioMultiThreadNioOpenWriteExPath);
calcCostTime("multiThreadOpenWriteNIOEx\t");
System.out.println();
//(删除文件部分，略……)
}