Java IO读写大文件的几种方式及测试 (2012-04-21 22:58:06)

原文：http://blog.sina.com.cn/s/blog_6699fade01012bvo.html

读取文件大小：1.45G

第一种，OldIO:

Java代码

public static void oldIOReadFile() throws IOException{


BufferedReader br = new BufferedReader(new FileReader("G://lily_947.txt"));


PrintWriter pw = new PrintWriter("G://oldIO.tmp");


char[] c = new char[100*1024*1024];


for(;;){

if(br.read(c)!=-1){


pw.print(c);

}else{

break;

}

}

pw.close();

br.close();

}

耗时70.79s

第二种，newIO:

Java代码

public static void newIOReadFile() throws IOException{


FileChannel read = new RandomAccessFile("G://lily_947.txt","r").getChannel();


FileChannel writer = new RandomAccessFile("G://newIO.tmp","rw").getChannel();


ByteBuffer bb = ByteBuffer.allocate(200*1024*1024);


while(read.read(bb)!=-1){


bb.flip();

writer.write(bb);

bb.clear();

}

read.close();

writer.close();



}

耗时47.24s

第三种，RandomAccessFile:

Java代码

public static void randomReadFile() throws IOException{


RandomAccessFile read = new RandomAccessFile("G://lily_947.txt","r");


RandomAccessFile writer = new RandomAccessFile("G://random.tmp","rw");


byte[] b = new byte[200*1024*1024];


while(read.read(b)!=-1){


writer.write(b);

}

writer.close();

read.close();

}

耗时46.65

第四种，MappedByteBuffer：

Java代码

public static void mappedBuffer() throws IOException{


FileChannel read = new FileInputStream("G://lily_947.txt").getChannel();


FileChannel writer = new RandomAccessFile("G://buffer.tmp","rw").getChannel();


long i = 0;


long size = read.size()/30;


ByteBuffer bb,cc = null;

while(i<read.size()&&(read.size()-i)>size){

bb = read.map(FileChannel.MapMode.READ_ONLY, i, size);

cc = writer.map(FileChannel.MapMode.READ_WRITE, i, size);

cc.put(bb);

i+=size;

bb.clear();

cc.clear();

}

bb = read.map(FileChannel.MapMode.READ_ONLY, i, read.size()-i);

cc.put(bb);

bb.clear();

cc.clear();

read.close();

writer.close();



}

耗时：36

前三种读法对应的资源占用图如下：

相对于最后一种内存直接映射方式前面的测试其实无意义，基本秒杀。。。。。

对于很大的文件直接分块映射时内存会不够，这是因为MappedByteBuffer未被释放造成的，sun未提供直接回收MappedByteBuffer区域的方法，这个时候有两种方法解决，第一种比较愚笨的：

Java代码

System.gc();

System.runFinalization();

try {

Thread.sleep(3000);


} catch (InterruptedException e) {



e.printStackTrace();

}

第二种网上找来的，利用反射调用clean方法：

Java代码

public static void unmap(final MappedByteBuffer buffer) {


if (buffer == null) {


return;

}

AccessController.doPrivileged(new PrivilegedAction<Object>() {


public Object run() {


try {


Method getCleanerMethod = buffer.getClass().getMethod("cleaner", new Class[0]);


if (getCleanerMethod != null) {


getCleanerMethod.setAccessible(true);


Object cleaner = getCleanerMethod.invoke(buffer, new Object[0]);


Method cleanMethod = cleaner.getClass().getMethod("clean", new Class[0]);


if (cleanMethod != null) {


cleanMethod.invoke(cleaner, new Object[0]);


}

}

} catch (Exception e) {


e.printStackTrace();

}

return null;


}



});

}

转自http://aronlulu.iteye.com/blog/1018370