Let me preface this post with a single caution. I am a total beginner when it comes to Java. I have been programming PHP on and off for a while, but I was ready to make a desktop application, so I decided to go with Java for various reasons.
The application I am working on is in the beginning stages (less than 5 classes) and I need to read bytes from a local file. Typically, the files are currently less than 512kB (but may get larger in the future). Currently, I am using a FileInputStream to read the file into three byte arrays, which perfectly satisfies my requirements. However, I have seen a BufferedInputStream mentioned, and was wondering if the way I am currently doing this is best, or if I should use a BufferedInputStream as well.
I have done some research and have read a few questions here on Stack Overflow, but I am still having troubles understanding the best situation for when to use and not use the BufferedInputStream. In my situation, the first array I read bytes into is only a few bytes (less than 20). If the data I receive is good in these bytes, then I read the rest of the file into two more byte arrays of varying size.
I have also heard many people mention profiling to see which is more efficient in each specific case, however, I have no profiling experience and I'm not really sure where to start. I would love some suggestions on this as well.
I'm sorry for such a long post, but I really want to learn and understand the best way to do these things. I always have a bad habit of second guessing my decisions, so I would love some feedback. Thanks!
If you are consistently doing small reads then a BufferedInputStream will give you significantly better performance. Each read request on an unbuffered stream typically results in a system call to the operating system to read the requested number of bytes. The overhead of doing a system call is may be thousands of machine instructions per syscall. A buffered stream reduces this by doing one large read for (say) up to 8k bytes into an internal buffer, and then handing out bytes from that buffer. This can drastically reduce the number of system calls.
However, if you are consistently doing large reads (e.g. 8k or more) then a BufferedInputStream slows things. You typically don't reduce the number of syscalls, and the buffering introduces an extra data copying step.
In your use-case (where you read a 20 byte chunk first then lots of large chunks) I'd say that using a BufferedInputStream is more likely to reduce performance than increase it. But ultimately, it depends on the actual read patterns.
If you are using a relatively large arrays to read the data a chunk at a time, then BufferedInputStream will just introduce a wasteful copy. (Remember, read does not necessarily read all of the array - you might want DataInputStream.readFully). Where BufferedInputStream wins is when making lots of small reads.
BufferedInputStream reads more of the file that you need in advance. As I understand it, it's doing more work in advance, like, 1 big continous disk read vs doing many in a tight loop.
As far as profiling - I like the profiler that's built into netbeans. It's really easy to get started with. :-)
I can't speak to the profiling, but from my experience developing Java applications I find that using any of the buffer classes - BufferedInputStream, StringBuffer - my applications are exceptionally faster. Because of which, I use them even for the smallest files or string operation.
import java.io.*;
class BufferedInputStream
{
public static void main(String arg[])throws IOException
{
FileInputStream fin=new FileInputStream("abc.txt");
BufferedInputStream bis=new BufferedInputStream(fin);
int size=bis.available();
while(true)
{
int x=bis.read(fin);
if(x==-1)
{
bis.mark(size);
System.out.println((char)x);
}
}
bis.reset();
while(true)
{
int x=bis.read();
if(x==-1)
{
break;
System.out.println((char)x);
}
}
}
}
来源：https://stackoverflow.com/questions/3122422/usage-of-bufferedinputstream