MappedByteBuffer 继承自ByteBuffer ，与 BufferReader等等类似，但是性能上比BufferReader这些高。
使用方法：
private void writeToCaches(ResponseBody body,DownInfo info){
RandomAccessFile randomAccessFile=null;
FileChannel fileCahnnel=null;
InputStream inputStream=null;
File file=new File(info.getSavePath());
if(!file.getParentFile().exists()){
file.getParentFile().mkdirs();
}
try {
long allLength = 0 == info.getCountLength() ? body.contentLength() : info.getReadLength() + body
.contentLength();
randomAccessFile=new RandomAccessFile(file,"rwd"); //rwd 表示文件可读写
fileCahnnel=randomAccessFile.getChannel();
inputStream=body.byteStream();
MappedByteBuffer byteBuffer=fileCahnnel.map(FileChannel.MapMode.READ_WRITE,info.getReadLength(), allLength - info.getReadLength());
byte[] buffer=new byte[1024*4];
int len;
while((len=inputStream.read(buffer))!=-1){
byteBuffer.put(buffer,0,len);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}finally {
try {
if(inputStream!=null) {
inputStream.close();
}
if(fileCahnnel!=null){
fileCahnnel.close();
}
if(randomAccessFile!=null){
randomAccessFile.close();
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
