Beanshell self-reference causes memory leak

"Stewart" <>
27 Jul 2006 09:43:27 -0700
Dear All,

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.
Unfortunately, I can't attach here 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 the "this"
object is out of scope in the calling method.

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 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
Waiting ...

After object ref test
Waiting ...

After HashSet test
Waiting ...

After Interpreter test
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.

But JProfiler shows the Interpreter still in memory after the object of
class Test has gone out of scope - at the very final part of the

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?



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
   baseline = (r.maxMemory() - r.freeMemory());

   report("Before tests");


   report("After object ref test");


   report("After HashSet test");


   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.out.println( (r.maxMemory() - r.freeMemory() - baseline) );
   System.out.println( lines );
   System.out.println("Waiting ...");
   new BufferedReader( new InputStreamReader( )).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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