How to allocate the memory from OS instead of increasing the JVM’s heap size?

I need to detect whether the开发者_JAVA技巧 file I am attaching to an email is exceeding the server limit. I am not allowed to increase the JVM heap size to do this since it is going to affect the application performance.

If I don’t increase the JVM heap size, I will run into OutOfMemoryError directly.

I would like to know how do allocate the memory from OS instead of increasing the JVM’s heap size?

Thanks a lot!

Are you really trying to read the entire file to determine its size to check if it is less than some configured value (your question is not too easy to understand)? If so, why are you not using File#length() instead?

If you need to stream the file to the server in order to find out whether it's too big, you still don't need to read the whole file into memory.

Instead, read maybe 10-100k into memory. Fill the buffer, send it to the server. Repeat until the file is done or the server complains. Then you don't need enough memory for the whole file.

If you write your own stream handling code, you could create your own counter to track the number of bytes transmitted. I'd be surprised if there wasn't already some sort of Filter class that does this for you. Sun has a page about this. Search for 'CountReader'.

You could allocate the memory natively via native code and JNI. However that sounds a painful way to do this.

Instead can't you give the JVM suitable memory configurations (via -Xmx) ? If the document you're mailing is so big that you can't easily handle it, then I'm not sure email is the correct medium to transfer it, and you should instead host it and send a link to it, or perhaps FTP it.

If all the other solutions turn out to be unusable (and I would encourage you to find a better way than requiring the entire file to fit in memory!) you could consider using a direct ByteBuffer. It has the option of using mmap() or other system calls to map a file into your memory without actually reading / allocating space in the heap. You can do this by calling map() on a FileChannel -- API documentation. Note that this is potentially expensive and/or not supported on some platforms, so it should be considered suboptimal compared to any solution which does not require the entire file to be in memory.

Socket s = /* go get your socket to the server */
InputStream is = new FileInputStream("foo.txt");
OutputStream os = s.getOutputStream();
byte[] buf = new byte[4096];
for(int len=-1;(len=is.read(buf))!=-1;) os.write(buf,0,len);
os.close();
is.close();

Of course handle your Exceptions.

If you're not allowed to increase the heap size because of memory constaints, doing an "under the table" memory allocation would cause the same problems. It sounds like you're looking for a loophole in the rules. Like, "My doctor says to cut down on how much I eat at each meal, so I'm eating more between meals to make up for it."

The only way I know of to allocate memory without using the Java heap would be to write JNDI calls to malloc the memory with C. But then how would you use this memory? You'd have to write more JNDI calls to interact with it. I think you'd end up basically re-inventing Java.

If the goal here is to send a large file, just use buffered streams and read/write it one byte at a time. A buffered stream, as the name implies, will take care of buffering for you so you're not really hitting the hard drive one byte at a time. It will really read, I think the default is 8k at a time, and then pass these bytes to you as you ask for them. Likewise, on the write side it will save up a few kb and and send them all in chunks.

So all you should have to do is open a BufferedInputStream and a BufferedOutputStream. Then write a loop that reads one byte from the input stream and writes it to the output stream until you hit end-of-file.

Something like:

OutputStream os=... however you're getting your socket ...
BufferedInputStream bis=new BufferedInputStream(new FileInputStream(fileObject));
BufferedOutputStream bos=new BufferedOutputStream(os);
int b;
while ((b=bis.read())!=-1)
  bos.write(b);
bis.close();
bos.close();

No need to make life complicated for yourself by re-inventing buffering. while (