Memory leak in 16 lines

Question

Here is a trivial program (16 lines) that repeatedly writes the same string into an ObjectOutputStream. It has a memory leak so severe that it not able to iterate more than 1.5 million times with 256M of memory on Windows/2000. Calling System.gc() periodically doesn't make any difference. I'm using 1.3.1_01. Does anyone have any idea how to avoid this?
Here is the code:
import java.io.*;
public class LeakTest {
public static void main(String[] args) throws IOException
int i, j;     
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("mine.tmp"));     
for(i = 0, j = 0; i < 10000000; i++) {
String s = new String("hello");
out.writeObject(s);
if(++j == 10000) {
j = 0;
//System.gc();
System.out.println(i);

Answer 1

$java -Xrunhprof:heap=sites,thread=y LeakTest
SITES BEGIN (ordered by live bytes) Thu Jan 10 11:13:45 2002
          percent         live       alloc'ed  stack class
rank   self  accum    bytes objs   bytes objs trace name
    1 41.51% 41.51%  6291468 1048578 6291468 1048578   591 java.lang.String
    2 41.51% 83.02%  6291462 1048577 6291462 1048577   592 [C
    3  6.92% 89.94%  1048580    1 2097216   18   640 [I
    4  6.92% 96.86%  1048580    1 2097216   18   641 java.lang.ObjectThese four allocation sites took up 96.86% of the live memory when the OutOfMemoryException was thrown. Consider that the first two allocation sites are found in the new String("hello") constructor... the other two allocation sites are found in ObjectOutputStream$HandleTable.growEntries().
Recall that an ObjectOutputStream writes a connected map of all of the objects written to the stream. To correctly back-reference already written objects, the stream needs to keep a reference to every object already written... which means that none of the Strings created in your program ever become eligible for garbage collection, until the ObjectOutputStream no longer needs its handle table - when it is closed.
Conclusion? ObjectOutputStream is a memory leak if not closed. If you do not require a connected map of your serialised objects, consider wrapping an ObjectOutputStream around a ByteArrayOutputStream to serialise each request, and then send the resulting set of bytes to the FileOutputStream. This way you aren't preventing large numbers of objects from being collected.
Hope this helps,
-Troy
PS- unless you were passing the -Xmx runtime option to java, you were probably only using 32Mb of your 256Mb of memory :-)