[PATCH] Performance bug in String(byte[],int,int,Charset)

[Markus Gaisbauer]

A bug in java.lang.StringCoding causes a full and unnecessary copy of
the byte array given as the first argument.
This results in severe slow down of the Constructor if the byte array is
big.

The attached patch, should fix the problem.


Unfortunately I do not (yet) have an official bug id for this, as this
seems to take a while (reported 2 weeks ago).

To reproduce the problem run the following test program:

import java.nio.charset.Charset;

public class StringTest {

       public static void main(String[] args) throws Exception {
               long before;
               long after;
               byte[] data;

               data = new byte[1024*1024*16]; // 16 megabyte
               data[0] = 'X';

               // warmup
               new String(data, 0, 1);
               new String(data, 0, 1, Charset.forName("UTF8"));
               new String(data, 0, 1, "UTF8");

               before = System.nanoTime();
               new String(data, 0, 1);
               after = System.nanoTime();
               System.out.println((after - before) / 1000000 + "ms");

               before = System.nanoTime();
               new String(data, 0, 1, Charset.forName("UTF8"));
               after = System.nanoTime();
               System.out.println((after - before) / 1000000 + "ms");

               before = System.nanoTime();
               new String(data, 0, 1, "UTF8");
               after = System.nanoTime();
               System.out.println((after - before) / 1000000 + "ms");
       }

}

Index: StringCoding.java
===================================================================
--- StringCoding.java   (revision 258)
+++ StringCoding.java   (working copy)
@@ -193,7 +193,6 @@
 
     static char[] decode(Charset cs, byte[] ba, int off, int len) {
         StringDecoder sd = new StringDecoder(cs, cs.name());
-        byte[] b = Arrays.copyOf(ba, ba.length);
         return sd.decode(b, off, len);
     }