Java Q&A
by Ethan Henry and Ed Lycklama


Example 1:
Public void useless() {
MyObject m1 = new MyObject();
MyObject m2 = new MyObject();
m1.ref = m2;
global.ref = m2;
return;
}

Example 2:
public class PrintService {
static PrintService singleton;
Printable target;
public PrintService getPrintService() {
  return singleton;
}
public void setTarget(Printable p) {
  target = p;
}
public void doPrint() {
  // set stuff up
  // print target
}
}

Example 3:
Void method() {
  // this creates a large object 
  Biggie big = readIt();
  // this condenses it
  Item item = findIt(big);
  // we'd really like to reuse big's memory
  // big = null;
  // this method is going to run a long time
  parseIt(item);
}

Listing One
/*****************************************************************************
 * Copyright (c) 1999, KL GROUP INC.  All Rights Reserved.
 * http://www.klgroup.com
 * The Software is provided "AS IS," without a warranty of any kind.
 * ALL EXPRESS OR IMPLIED CONDITIONS, REPRESENTATIONS AND WARRANTIES,
 * INCLUDING ANY IMPLIED WARRANTY OF MERCHANTABILITY, FITNESS FOR A
 * PARTICULAR PURPOSE OR NON-INFRINGEMENT, ARE HEREBY EXCLUDED.
 * KL GROUP AND ITS LICENSORS SHALL NOT BE LIABLE FOR ANY DAMAGES
 * SUFFERED BY LICENSEE AS A RESULT OF USING, MODIFYING OR DISTRIBUTING
 * THE SOFTWARE OR ITS DERIVATIVES. IN NO EVENT WILL KL GROUP OR ITS
 * LICENSORS BE LIABLE FOR ANY LOST REVENUE, PROFIT OR DATA, OR FOR DIRECT,
 * INDIRECT, SPECIAL, CONSEQUENTIAL, INCIDENTAL OR PUNITIVE DAMAGES,
 * HOWEVER CAUSED AND REGARDLESS OF THE THEORY OF LIABILITY, ARISING OUT
 * OF THE USE OF OR INABILITY TO USE SOFTWARE, EVEN IF KL GROUP HAS
 * BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
 *****************************************************************************/

import java.lang.reflect.*;
import java.util.*;

/*************************************************************
 * Utility class for identifying loitering objects. Objects are tracked by 
 * calling ObjectTracker.add() when instantiated, and calling 
 * ObjectTracker.remove() when finalized. Only classes that implement 
 * ObjectTracker.Tracked can be tracked. As instances are created and 
 * destroyed, they are reported to the stdout. Summaries by class can also be 
 * reported on demand. To enable this functionality, add -DObjectTracker 
 * when running your program. This will track all classes that implement
 * ObjectTracker.Tracked and call add/remove as indicated in the
 * previous paragraph.
 * For a finer degree of control, specify a list of filters
 * when setting the <code>ObjectTracker</code> property. For instance,
 * -DObjectTracker=+MySpecialClass,-ClassFoo will only report o
 * on instances of classes whose name contains MySpecialClass
 * but not ClassFoo. Hence MySpecialClassBar will be tracked, while 
 * MySpecialClassFoo will not be. See <A HREF="ObjectTracker.html#start()"> 
 * start()</A> for more details.
 * Limitations
 * Since you must add instrumentation to all the classes you want to track,
 * this is not nearly as useful as a Memory Profiler/Debugger like
 * JProbe Profiler. Also, since it cannot tell you which references
 * are causing the object to loiter, it doesn't help you remove the loiterers.
 * If you want to solve the problem, you really need to use a Memory 
 * Profiler/Debugger like JProbe Profiler. The only thing ObjectTracker can 
 * help with is testing whether an instance of a known class goes away.
 * Implementation Notes
 * The current implementation assumes that every object has a unique
 * hashcode. A false assumption in general, but does work in JavaSoft's Win32 
 * VM for JDK1.1. This implementation will definitely not work in JavaSoft's
 * implementation of the Java 2 VM, including the HotSpot VM.
 ************************************************************* */
public class ObjectTracker {

// Property ObjectTracker turns this on when set
private final static boolean ENABLED = 
    System.getProperty("ObjectTracker") != null;
// Classes are hashed by name into this table.
private static Hashtable classReg;
private static Vector    patterns;

/** Record info about an object.  Class and ordinal number are stored. */
private static class ObjectEntry {
    int     ordinal;        // distinguishes between mult. instances
    String  clazz;          // classname
    String  name;           // name (may be null)

    public ObjectEntry(int ordinal, String clazz, String name) {
        this.ordinal = ordinal;
        this.clazz   = clazz;
        this.name    = name;
    }
    public String toString() {
        return clazz + ":#" + ordinal + " ("+name+")";
    }
} // ObjectEntry
/** Records info about a class. Within each class, a table of objects is 
 * maintained, along with next ordinal to use to stamp next object 
 * of this class. */
private static class ClassEntry {
    String      clazz;          // class name
    Hashtable   objects;        // list of ObjectEntry
    int         ordinal;        // last instance of this class created
    public ClassEntry(String clazz) {
        this.clazz = clazz;
        objects    = new Hashtable();
        ordinal    = 1;
    }
    public String toString() {
        return clazz;
    }
    /** Get the name of the object by invoking getName().
     * Uses reflection to find the method. */
    private String getName(Object o) {
        String name = null;
        try {
            Class cl = o.getClass();
            Method m = cl.getMethod("getName", null);
            name = (m.invoke(o, null)).toString();
        }
        catch (Exception e) { }
        return name;
    }
    public void addObject(Object obj) {
        // Store this object in the object table
        Integer id = new Integer(System.identityHashCode(obj));
        ObjectEntry entry = new ObjectEntry(ordinal, clazz, getName(obj));
        objects.put(id, entry);
        ordinal++;
        System.out.println("    added: " +entry);
    }
    public void removeObject(Object obj) {
        // Removes this object from the object table
        Integer id = new Integer(System.identityHashCode(obj));
        ObjectEntry entry = (ObjectEntry) objects.get(id);
        objects.remove(id);
        System.out.println("    removed: " +entry);
    }
    /** Dump out a list of all object in this table */
    public void listObjects() {
        if (objects.size() == 0)  {
            // skip empty tables
            return;
        }
        System.out.println("For class: " + clazz);
        Enumeration objs = objects.elements();
        while (objs.hasMoreElements()) {
            ObjectEntry entry = (ObjectEntry) objs.nextElement();
            System.out.println("    " +entry);
        }
    }
} // ClassEntry
/** No constructor */
private ObjectTracker() {}
/**  Determine is this class name should be tracked. 
 * @return true if this class should be tracked. @see start */
private static boolean isIncluded(String clazz) {
    int i=0, size = patterns.size();
    if (size == 0) {
        // always match if list is empty
        return true;
    }
    boolean flag = false;
    for (; i<size; i++) {
        String pat = (String) patterns.elementAt(i);
        String op = pat.substring(0, 1);        // + or -
        String name = pat.substring(1);
        if (name.equals("all")) {
            if (op.equals("+"))
                flag = true;        // match all, unless told otherwise
            else if (op.equals("-"))
                flag = false;       // match nothing, unless told otherwise
        }
        else if (clazz.indexOf(name) != -1) {
            // match if any of the filter names is a substring of
            // the class name
            if (op.equals("+"))
                return true;
            else if (op.equals("-"))
                return false;
        }
    }
    return flag;
}
/** Must be called before any objects can be tracked. Turns on object tracking
 * if property <code>ObjectTracker</code> is set.  In addition, the list of
 * patterns assigned to this property is stored for future pattern matching
 * by <code>isIncluded()</code>. This list of patterns must be supplied as a
 * comma-separated list, each preceded by <code>+</code> or <code>-</code>,
 * which indicates whether or not the pattern should cause matching classes to
 * be tracked or not. If property <code>ObjectTracker</code> has no values, 
 * it is equivalent to <code>+all</code>. */
public static void start() {
    if (ENABLED) {
        classReg = new Hashtable();
        patterns = new Vector();

        String targets = System.getProperty("ObjectTracker");
        StringTokenizer parser = new StringTokenizer(targets, ",");
        while (parser.hasMoreTokens()) {
            String token = parser.nextToken();
            patterns.addElement(token);
        }
    }
}
/** Add object to the tracked list. Will only be added if object's class has
 * not been filtered out. @param obj object to be added to tracking list */
public static void add(Tracked obj) {
    if (ENABLED) {
        String clazz = obj.getClass().getName();
        if (isIncluded(clazz)) {
            ClassEntry entry = (ClassEntry) classReg.get(clazz);
            if (entry == null) {
                // first one for this class
                entry = new ClassEntry(clazz);
                classReg.put(clazz, entry);
            }
            entry.addObject(obj);
        }
    }
}
/** Removes object from tracked list.  This method should be called 
 * from the finalizer. @param obj object to be removed from tracking list  */
public static void remove(Tracked obj) {
    if (ENABLED) {
        String clazz = obj.getClass().getName();
        if (isIncluded(clazz)) {
            ClassEntry entry = (ClassEntry) classReg.get(clazz);
            entry.removeObject(obj);
        }
    }
}
/** Print tracked objects, summarized by class.  Also prints a
 * summary of free/total memory.  */
public static void dump() {
    if (ENABLED) {
        Enumeration e = classReg.elements();
        while (e.hasMoreElements()) {
            ClassEntry entry = (ClassEntry) e.nextElement();
            entry.listObjects();
        }
        System.out.println("==================================");
        System.out.println("Total Memory: " +
            Runtime.getRuntime().totalMemory());
        System.out.println("Free  Memory: " +
            Runtime.getRuntime().freeMemory());
        System.out.println("==================================");
        System.out.println("");
    }
}
/**  All classes that want to use this service must implement this
 * interface.  This forces this class to implement Object's finalize
 * method, which should call <code>ObjectTracker.remove()</code>. */
public interface Tracked {
    /**  All classes that use ObjectTracker must implement a finalizer. */
    void finalize();
}
}


Listing Two
/***************************************************************************
 * Copyright (c) 1999, KL GROUP INC.  All Rights Reserved.
 * http://www.klgroup.com
 * The Software is provided "AS IS," without a warranty of any kind.
 * ALL EXPRESS OR IMPLIED CONDITIONS, REPRESENTATIONS AND WARRANTIES,
 * INCLUDING ANY IMPLIED WARRANTY OF MERCHANTABILITY, FITNESS FOR A
 * PARTICULAR PURPOSE OR NON-INFRINGEMENT, ARE HEREBY EXCLUDED.
 * KL GROUP AND ITS LICENSORS SHALL NOT BE LIABLE FOR ANY DAMAGES
 * SUFFERED BY LICENSEE AS A RESULT OF USING, MODIFYING OR DISTRIBUTING
 * THE SOFTWARE OR ITS DERIVATIVES. IN NO EVENT WILL KL GROUP OR ITS
 * LICENSORS BE LIABLE FOR ANY LOST REVENUE, PROFIT OR DATA, OR FOR DIRECT,
 * INDIRECT, SPECIAL, CONSEQUENTIAL, INCIDENTAL OR PUNITIVE DAMAGES,
 * HOWEVER CAUSED AND REGARDLESS OF THE THEORY OF LIABILITY, ARISING OUT
 * OF THE USE OF OR INABILITY TO USE SOFTWARE, EVEN IF KL GROUP HAS
 * BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
 ***************************************************************************/

public class tester implements ObjectTracker.Tracked {
    private int[] junk = new int[5000];
    public static void main(String args[]) {
        ObjectTracker.start();
        for (int i=0; i<1000; i++) {
            tester t = new tester();
            t.doNothing();
            if (i%100 == 0) {
                System.gc();
            }
        }
        ObjectTracker.dump();
    }
    public tester() {
        ObjectTracker.add(this);
    }
    public void finalize() {
        ObjectTracker.remove(this);
    }
    public void doNothing() {
    }
}








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