Beanshell self-reference causes memory leak

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Beanshell self-reference causes memory leak

Stewart Cambridge
Dear Beanshell Users,

I have come across a phenomena in Beanshell, which I am pretty sure is
a bug. I have put together a very simple test. I wanted to know if
anyone has already come across this, before I submit a bug report.

I have included below the source for a very simple test which can be
run from the command line. Although the test prints out memory usage,
the phenomena is best observed using a memory tool such a JProfiler. I
have also attached a screen-shot from JProfiler to graphically
illustrate the problem.

The core issue is this:
If I setup a self-referencing circle with the Interpreter like so (in
standard Java code):
{
    this.interpreter = new bsh.Interpreter();
    this.interpreter.set( "ref", this );
}
It sets up objects in the JVM memory which can never ever be garbage
collected, even though there is no reference them, and "this" obejct
is out of scope.

This is in contrast to, say a HashSet:
{
    this.set = new HashSet();
    onethis.set.add( this );
}
This setup does appear to garbage collect when the "this" object goes
out of scope.

The code for Test.java is below. Its output follows. Note that after
the self-referencing setup, roughly 90Kb has not been garbage
collected, compared with 20Kb for the HashSet and 200 bytes for two
java objects.

Before tests
------------
10528
40
------------
Waiting  ...


After object ref test
---------------------
26944
208
---------------------
Waiting  ...


After HashSet test
------------------
158280
22760
------------------
Waiting  ...


After Interpreter test
----------------------
362648
91352
----------------------
Waiting  ...

If this test is run with JProfiler, it can be seen that both the
self-referencing Test objects, and the self-referencing HashSet setup
are successfully garbage collected.

The screen shot from JProfiler shows the objects in memory after the
object of class Test has gone out of scope - at the very final part of
the program.

The Interpreter is not garbage collected.
Obviously, if you do this enough in an application, it will cripple it
for speed and eventually bomb out with an "Out of heap space" error.

Any thoughts?

Regards,

Stewart

import java.io.*;
import java.util.*;
import bsh.*;

public class Test
{
  private static Runtime r = Runtime.getRuntime();
  private static long baseline = 0L;

  public static void main(String[] args)
    throws Exception
  {
    System.gc();
    baseline = (r.maxMemory() - r.freeMemory());

    report("Before tests");

    twoObjectTest();

    report("After object ref test");

    hashSetTest();

    report("After HashSet test");

    interpreterTest();

    report("After Interpreter test");
  }

  private Test ref = null;
  private HashSet<Test> s = null;
  private Interpreter i = null;

  private static void twoObjectTest()
  {
    Test one = new Test();
    Test two = new Test();

    one.ref = two;
    two.ref = one;
  }

  private static void hashSetTest()
  {
    Test one = new Test();

    one.s = new HashSet<Test>();
    one.s.add( one );
  }

  private static void interpreterTest()
    throws EvalError
  {
    Test one = new Test();

    one.i = new Interpreter();
    one.i.set( "ref", one );
  }

  private static void report(String string)
    throws IOException
  {
    char[] lines = new char[string.length()];
    Arrays.fill(lines, '-');
    System.out.println( );
    System.out.println( string );
    System.out.println( lines );
    System.out.println( (r.maxMemory() - r.freeMemory() - baseline) );
    System.gc();
    System.out.println( (r.maxMemory() - r.freeMemory() - baseline) );
    System.out.println( lines );
    System.out.println("Waiting  ...");
    new BufferedReader( new InputStreamReader( System.in )).readLine();
  }

  /**
   * I thought this might solve the problem, but of course, it never
gets called.
   */
  protected void finalize()
    throws Throwable
  {
    i.set( "ref", null );
    i.unset( "ref" );
    i = null;
  }
}

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