Java IO读写大文件的几种方式及测试

读取文件大小：1.45G
第一种，OldIO:

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:

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:

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：

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

前三种读法可以通过jconsole得到资源占用图。
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：

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：

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;
}

});
}

以上两种方法感觉都别扭，还有就是可以自己分割成物理文件再循环调用，这个也不太美观。
速度也会减慢好多。