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Working on performance in one of java projects I found that the slowest part is copy method from file utility class. Method was implemented without buffering. Implementation was next: just using ‘while’ operator reading input stream and writing to output stream.

There are good article about i/o performance on sun website.

Listing 4-4 from article looks good, but provided method not perfect too. I have next reason: my application is a multithreaded application. And anytime I synchronize on a static field, I make a bottleneck, because all threads must block while waiting to enter the synchronized block of code.

There is solution for this. If you want to use a cache that is not initialized every time method invoked, you should use ThreadLocal, and a weak reference to the buffer, which allows the memory to be collected when memory is tight.

So result code next:

import java.io.*;

public class CopyExample {
	// Thread buffer size
	private static final int BUFF_SIZE = 1024 * 4;

	// Thread buffer variable
	private static final ThreadLocal<byte[]> buffCache = new ThreadLocal<byte[]>() {
		@Override
		protected byte[] initialValue() {
			return new byte[BUFF_SIZE];
		}
	};

	private CopyExample() {
	}

	public static void copy(File from, File to) throws IOException {
		InputStream in = null;
		OutputStream out = null;
		try {
			in = new FileInputStream(from);
			out = new FileOutputStream(to);
			byte[] buffer = buffCache.get();
			int amountRead;
			while ((amountRead = in.read(buffer)) != -1) {
				out.write(buffer, 0, amountRead);
			}
		} finally {
			// close in and out streams
			// ...
		}
	}
}

If you have some notes about this code, please make comment to this post. It's intresting to know other opinions.