note 29- a stupid mistake of IO;

byte[] buffer=new byte[4*1024];
            
            int len;
            while((len=input.read(buffer))!=-1){
                //the 0 is the starting of buffer,not the starting of ouput;
                output.write(buffer,0,len);
            }
            output.flush();

the 0 and len means that the section of the buffer, not the section of the output stream. The outputStream pointer will keep go on as long as something write to it .

Every one while loop write a 4*1024 buffer to buffer .  When it touch the final part of the file . it will return the real length of the final part of the file .

If not point out the len in the output stream write(). It will write 1024 bytes instead writing the real length.