Java.util.ConcurrentModification Exception

Question

Hi
I am trying to write a caching object for my web application. I have a CacheMAnger thread which is started when Tomcat starts. I put objects onto the cache and give them a TTL. My thread occasionaly goes through and checks all the cached objects to see if they are expired. I thought this was a syncronization problem but I can't find where.
It bombs when an object expires and gets removed form the cache. The error output is below as well as a copy of the CacheManager source.
Thanks
Michael
<pre>
2003-04-15 12:55:15,161 DEBUG cache.CacheManager$1 82 - CacheManager: CustomerList in Cache... expi
ry?
EXPIRY:Tue Apr 15 17:55:04 GMT 2003CURRENT:Tue Apr 15 17:55:15 GMT 2003
2003-04-15 12:55:15,161 INFO cache.CacheManager$1 90 - CacheManager Removed CustomerList
java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:750)
at java.util.HashMap$KeyIterator.next(HashMap.java:786)
at net.talisen.tsr.cache.CacheManager$1.run(CacheManager.java:77)
at java.lang.Thread.run(Thread.java:536)
</pre>
=====================================================================
<pre>
package net.talisen.tsr.cache;
import java.util.HashMap;
import java.util.Set;
import java.util.Iterator;
import org.apache.log4j.Logger;
* Title: Caching
* Description:
* Copyright: Copyright (c) 2001
* @author Michael Holly based on code from Jonathan Lurie in July 2001 Article in JavaWorld
* @version 1.0
public class CacheManager
// This is the HashMap that contains all objects in the cache.
private static java.util.HashMap cacheHashMap = new java.util.HashMap();
// This object acts as a semaphore, which protects the HashMap
// RESERVED FOR FUTURE USE private static Object lock = new Object();
static
try
//Create background thread, which will be responsible for purging expired items.
Thread threadCleanerUpper = new Thread( new Runnable()
// The default time the thread should sleep between scans. The sleep
// method takes in a millisecond value so 5000 = 5 Seconds.
final static int milliSecondSleepTime = 20000;
public void run()
Logger log = Logger.getLogger(CacheManager.class);
try
// Sets up an infinite loop.
while (true)
log.debug( "CacheManager Scanning For Expired Objects...");
// Get the set of all keys that are in cache. These are the unique
// identifiers
Set keySet = null;
Iterator keys = null;
Object key = null;
Cacheable value = null;
synchronized ( cacheHashMap )
keySet = cacheHashMap.keySet();
// An iterator is used to move through the Keyset //
synchronized( keySet )
keys = keySet.iterator();
// Sets up a loop that will iterate through each key in the KeySet
while(keys.hasNext())
// Get the individual key. We need to hold on to this key
// in case it needs to be removed
synchronized ( keys )
key = keys.next();
// Get the cacheable object associated with the key inside the cache
value = (Cacheable)cacheHashMap.get(key);
log.debug( "CacheManager: " + value.getIdentifier() + " in Cache... expiry?");
// Is the cacheable object expired?
if (value.isExpired())
//Yes it's expired! Remove it from the cache
cacheHashMap.remove(key);
cacheHashMap.notifyAll();
log.info( "CacheManager Removed " + key.toString() );
***** A LRU (least-recently used) or
LFU (least-frequently used) *****
******* would best be inserted here *****
// Puts the thread to sleep. Don't need to check it continuously
Thread.sleep( this.milliSecondSleepTime);
catch (Exception e)
e.printStackTrace();
return;
} // End run method
}); // End class definition and close new thread definition
// Sets the thread's priority to the minimum value.
threadCleanerUpper.setPriority( Thread.MIN_PRIORITY );
// Starts the thread.
threadCleanerUpper.start();
catch(Exception e)
System.out.println( "CacheManager.Static Block: " + e);
} // End static block
// default constructor
public CacheManager()
* Used to add an object to the Cache
* @param object the object to be added to the cache
public static void putCache(Cacheable object)
Logger log = Logger.getLogger(CacheManager.class);
// Remember if the HashMap previously contains a mapping for the key, the old value
// will be replaced. This is valid functioning.
if ( object == null ) {
log.debug("object to be cached is null. Caching was not attempted");
return;
if ( object.getIdentifier() == null ) {
log.debug("object to be cached has a null key. Caching was not attempted");
return;
else
synchronized ( cacheHashMap ) {
cacheHashMap.put(object.getIdentifier(), object);
log.debug( object.getIdentifier() + " cached");
* Used to get an object from the cache
* @param object the object to be retrieved from the cache
* @return the cached object
public static Cacheable getCache(Object identifier)
Logger log = Logger.getLogger(CacheManager.class);
if (identifier == null ) {
log.debug( "Cache key was null" );
return null;
log.debug("Looking for " + identifier.toString() );
//synchronized (lock) // UNCOMMENT LINE XXX
//{                    // UNCOMMENT LINE XXX
Cacheable object = null;
synchronized ( cacheHashMap ) {
object = ( Cacheable )cacheHashMap.get(identifier);
// The code to create the object would be placed here.
//} // UNCOMMENT LINE XXX
if (object == null) {
log.debug( "object not found in cache");
return null;
if (object.isExpired())
log.debug( "object in cache was expired");
synchronized ( cacheHashMap ) {
cacheHashMap.remove(identifier);
return null;
else
log.debug( object.getIdentifier() + " found");
object.incrementCacheHits(); // track the performance
return object;
* Used to clear all objects from the cache
public static void emptyCache()
Logger log = Logger.getLogger(CacheManager.class);
synchronized ( cacheHashMap ) {
cacheHashMap.clear();
log.debug( "Cache cleared");
* Used to remove a particular object from the cache
* @param identifier the object to be removed from the cache
public static void removeEntry(Object identifier)
Logger log = Logger.getLogger(CacheManager.class);
if (identifier == null ) {
log.debug( "Cache key was null" );
return;
synchronized ( cacheHashMap ) {
cacheHashMap.remove(identifier);
log.debug( identifier + " removed");
* Used to get a count of cached objects
* @return number of objects in the cache
public static int getCount()
synchronized ( cacheHashMap ) {
return cacheHashMap.size();
* Used to get a set of the entries in the cache
* @param Set a set of the objects in the cache
public static Set getEntrySet()
synchronized ( cacheHashMap ) {
return cacheHashMap.entrySet();
* Used to get a html page on the status of the cache
public static String getStatus()
Set keySet = null;
Iterator keys = null;
synchronized (cacheHashMap) {
keySet = cacheHashMap.keySet();
keys = keySet.iterator();
String status = new String();
status += ( "<html><head>");
status += ( "<meta http-equiv=\"Cache-Control\" content=\"no-cache\" />");
status += ( "<meta http-equiv=\"Pragma\" content=\"no-cache\" />");
status += ( "<meta http-equiv=\"Expires\" content=\"0\" /> ");
status += ( "<meta http-equiv=\"refresh\" content=\"10\" /> ");
status += ( "</head><body>" );
status += ( "<p><h4>Cache Status " + (new java.util.Date()).toString() + "</h4>" );
status += ( "<br>Page automagically refreshed every 10 seconds<br>" );
status += ( getCount() + " object(s) cached" );
status += ( "<p>Cache contains: " );
while(keys.hasNext())
Object key = keys.next();
// Get the cacheable object associated with the key inside the cache
Cacheable object = null;
synchronized ( cacheHashMap ) {
object = (Cacheable)cacheHashMap.get(key);
status += ( "<br><b>ID:</b><i>" + object.getIdentifier() + "</i>" );
status += ( "<br>   <b>Class of:</b><i>" + object.toString() + "</i>");
status += ( "<br>   <b> Expires at:</b><i>" + object.getExpireDate() + "</i>");
status += ( "<br>   <b> Cache Hits:</b><i>" + object.getCacheHits().toString() + "</i>");
status += ( "<br>   <b> Object Size:</b><i>" + object.getByteCount(object).toString() + "</i>");
int bytesServed = object.getCacheHits().intValue() * object.getByteCount(object).intValue();
double eff = 0.0;
// handle divide by 0
if ( bytesServed > 0 ) {
eff = ( bytesServed - object.getByteCount(object).intValue() ) / bytesServed;
status += ( "<br>   <b> Cache Efficiency:</b><i>" + eff + "</i><br><br>");
status += ( "</body></html>" );
return status;
* Used to get a html page on the status of the cache
public static String flushCache()
Set keySet = null;
Iterator keys = null;
synchronized ( cacheHashMap ) {
keySet = cacheHashMap.keySet();
keys = keySet.iterator();
String status = new String();
status += ( "<html><head>");
status += ( "<meta http-equiv=\"Cache-Control\" content=\"no-cache\" />");
status += ( "<meta http-equiv=\"Pragma\" content=\"no-cache\" />");
status += ( "<meta http-equiv=\"Expires\" content=\"0\" /> ");
status += ( "</head><body>" );
status += ( "<p><h4>Cache status before cache reset " + (new java.util.Date()).toString() + "</h4>" );
status += ( "<p>Cache size: " + getCount() + " object(s) cached" );
status += ( "<p>Cache contains: " );
while(keys.hasNext())
Object key = keys.next();
// Get the cacheable object associated with the key inside the cache
Cacheable object = null;
synchronized ( cacheHashMap ) {
object = (Cacheable)cacheHashMap.get(key);
status += ( "<br><b>ID:</b><i>" + object.getIdentifier() + "</i> <b>Class of:</b><i>" + object.toString() );
status += ( "</i><b> Expires at:</b>" + object.getExpireDate() + "</i>");
CacheManager.emptyCache();
status += ( "<p><h4>Cache status after cache reset " + (new java.util.Date()).toString() + "</h4>" );
status += ( "<p>Cache size: " + CacheManager.getCount() + " object(s) cached" );
status += ( "<p>Cache contains: " );
Set newKeySet = null;
Iterator newKeys = null;
synchronized ( cacheHashMap ) {
newKeySet = cacheHashMap.keySet();
newKeys = newKeySet.iterator();
// this really never gets executed
while(newKeys.hasNext())
Object key = newKeys.next();
// Get the cacheable object associated with the key inside the cache
Cacheable object = null;
synchronized ( cacheHashMap ) {
object = (Cacheable)cacheHashMap.get(key);
status += ( "<br><b>ID:</b><i>" + object.getIdentifier() + "</i> <b>Class of:</b><i>" + object.toString() );
status += ( "</i><b> Expires at:</b>" + object.getExpireDate() + "</i>");
status += ( "</body></html>" );
return status;
</pre>

Answer 1

I think my problem is that I am iterating through all the values of the hashMap and when I find one and delete it then the iterator becomes invalid? Checkout the while loop in the thread.
I'm not sure that this scenario is consistant with the error I'm getting.
I had tried something similar to what you described... maybe I missed something. Let me try it again...
I have reposted my code below with the proper tags to render the code if you need to reference for any more suggestions.
Michael
package net.talisen.tsr.cache;
import java.util.HashMap;
import java.util.Set;
import java.util.Iterator;
import org.apache.log4j.Logger;
* Title:        Caching
* Description:
* Copyright:    Copyright (c) 2001
* @author Michael Holly based on code from Jonathan Lurie in July 2001 Article in JavaWorld
* @version 1.0
public class CacheManager
   // This is the HashMap that contains all objects in the cache.
   private static java.util.HashMap cacheHashMap = new java.util.HashMap();
   // This object acts as a semaphore, which protects the HashMap
   // RESERVED FOR FUTURE USE  private static Object lock = new Object();
   static
      try
         //Create background thread, which will be responsible for purging expired items.
         Thread threadCleanerUpper = new Thread( new Runnable()
            //  The default time the thread should sleep between scans.  The sleep
            //  method takes in a millisecond value so 5000 = 5 Seconds.
            final static int milliSecondSleepTime = 20000;
            public void run()
               Logger log = Logger.getLogger(CacheManager.class);
               try
                  // Sets up an infinite loop.
                  while (true)
                     log.debug( "CacheManager Scanning For Expired Objects...");
                     // Get the set of all keys that are in cache.  These are the unique
                     // identifiers
                     Set keySet      = null;
                     Iterator keys   = null;
                     Object key      = null;
                     Cacheable value = null;
                     synchronized ( cacheHashMap )
                        keySet = cacheHashMap.keySet();
                        // An iterator is used to move through the Keyset //
                        synchronized( keySet )
                           keys = keySet.iterator();
                        // Sets up a loop that will iterate through each key in the KeySet
                        while(keys.hasNext())
                           // Get the individual key. We need to hold on to this key
                           // in case it needs to be removed
                           synchronized ( keys )
                              key = keys.next();
                           // Get the cacheable object associated with the key inside the cache
                           value = (Cacheable)cacheHashMap.get(key);
                           log.debug( "CacheManager: " + value.getIdentifier() + " in Cache... expiry?");
                           // Is the cacheable object expired?
                           if (value.isExpired())
                              //Yes it's expired! Remove it from the cache
                              cacheHashMap.remove(key);
                              cacheHashMap.notifyAll();
                              log.info( "CacheManager Removed " + key.toString() );
                         ***** A LRU (least-recently used) or
                                              LFU (least-frequently used) *****
                         ******* would best be inserted here              *****
                     // Puts the thread to sleep. Don't need to check it continuously
                     Thread.sleep( this.milliSecondSleepTime);
               catch (Exception e)
                  e.printStackTrace();
               return;
            } // End run method
         }); // End class definition and close new thread definition
         // Sets the thread's priority to the minimum value.
         threadCleanerUpper.setPriority( Thread.MIN_PRIORITY );
         // Starts the thread.
         threadCleanerUpper.start();
      catch(Exception e)
         System.out.println( "CacheManager.Static Block: " + e);
   } // End static block
   // default constructor
   public CacheManager()
   * Used to add an object to the Cache
   * @param object the object to be added to the cache
   public static void putCache(Cacheable object)
      Logger log = Logger.getLogger(CacheManager.class);
      // Remember if the HashMap previously contains a mapping for the key, the old value
      // will be replaced.  This is valid functioning.
      if ( object == null ) {
         log.debug("object to be cached is null. Caching was not attempted");
         return;
      if ( object.getIdentifier() == null ) {
         log.debug("object to be cached has a null key. Caching was not attempted");
         return;
      else
         synchronized ( cacheHashMap ) {
            cacheHashMap.put(object.getIdentifier(), object);
            log.debug( object.getIdentifier() + " cached");
   * Used to get an object from the cache
   * @param object the object to be retrieved from the cache
   * @return the cached object
   public static Cacheable getCache(Object identifier)
      Logger log = Logger.getLogger(CacheManager.class);
      if (identifier == null ) {
         log.debug( "Cache key was null" );
         return null;
      log.debug("Looking for " + identifier.toString() );
      Cacheable object = null;
      synchronized ( cacheHashMap )  {
         object = ( Cacheable )cacheHashMap.get(identifier);
      if (object == null) {
         log.debug( "object not found in cache");
         return null;
      if (object.isExpired())
         log.debug( "object in cache was expired");
         synchronized ( cacheHashMap )  {
            cacheHashMap.remove(identifier);
         return null;
      else
         log.debug( object.getIdentifier() + " found");
         object.incrementCacheHits(); // track the performance
         return object;
   * Used to clear all objects from the cache
   public static void emptyCache()
      Logger log = Logger.getLogger(CacheManager.class);
      synchronized ( cacheHashMap )  {
         cacheHashMap.clear();
      log.debug( "Cache cleared");
   * Used to remove a particular object from the cache
   * @param identifier the object to be removed from the cache
   public static void removeEntry(Object identifier)
      Logger log = Logger.getLogger(CacheManager.class);
      if (identifier == null ) {
         log.debug( "Cache key was null" );
         return;
      synchronized ( cacheHashMap )  {
         cacheHashMap.remove(identifier);
      log.debug( identifier + " removed");
   * Used to get a count of cached objects
   * @return number of objects in the cache
   public static int getCount()
      synchronized ( cacheHashMap )  {
         return cacheHashMap.size();
   * Used to get a set of the entries in the cache
   * @param Set a set of the objects in the cache
   public static Set getEntrySet()
      synchronized ( cacheHashMap )  {
         return cacheHashMap.entrySet();
   * Used to get a html page on the status of the cache
   public static String getStatus()
      Set keySet = null;
      Iterator keys = null;
      synchronized (cacheHashMap) {
         keySet = cacheHashMap.keySet();
         keys = keySet.iterator();
      String status = new String();
      status += ( "<html><head>");
      status += ( "<meta http-equiv=\"Cache-Control\" content=\"no-cache\" />");
      status += ( "<meta http-equiv=\"Pragma\" content=\"no-cache\" />");
      status += ( "<meta http-equiv=\"Expires\" content=\"0\" /> ");
      status += ( "<meta http-equiv=\"refresh\" content=\"10\" /> ");
      status += ( "</head><body>" );
      status += ( "<p><h4>Cache Status " + (new java.util.Date()).toString() + "</h4>" );
      status += ( "<br>Page automagically refreshed every 10 seconds<br>" );
      status += ( getCount() + " object(s) cached" );
      status += ( "<p>Cache contains: " );
      while(keys.hasNext())
         Object key = keys.next();
         // Get the cacheable object associated with the key inside the cache
         Cacheable object = null;
         synchronized ( cacheHashMap )  {
            object = (Cacheable)cacheHashMap.get(key);
         status += ( "<br><b>ID:</b><i>" + object.getIdentifier() + "</i>"  );
         status += ( "<br>���<b>Class�of:</b><i>" + object.toString() + "</i>");
         status += ( "<br>���<b> Expires at:</b><i>" + object.getExpireDate() + "</i>");
         status += ( "<br>���<b> Cache Hits:</b><i>" + object.getCacheHits().toString() + "</i>");
         status += ( "<br>���<b> Object Size:</b><i>" + object.getByteCount(object).toString() + "</i>");
         int bytesServed = object.getCacheHits().intValue() * object.getByteCount(object).intValue();
         double eff = 0.0;
         // handle divide by 0
         if ( bytesServed > 0 ) {
            eff = ( bytesServed - object.getByteCount(object).intValue() ) / bytesServed;
         status += ( "<br>���<b> Cache Efficiency:</b><i>" + eff + "</i><br><br>");
      status += ( "</body></html>" );
      return status;
   * Used to get a html page on the status of the cache
   public static String flushCache()
      Set keySet = null;
      Iterator keys = null;
      synchronized ( cacheHashMap )  {
         keySet = cacheHashMap.keySet();
         keys   = keySet.iterator();
      String status = new String();
      status += ( "<html><head>");
      status += ( "<meta http-equiv=\"Cache-Control\" content=\"no-cache\" />");
      status += ( "<meta http-equiv=\"Pragma\" content=\"no-cache\" />");
      status += ( "<meta http-equiv=\"Expires\" content=\"0\" /> ");
      status += ( "</head><body>" );
      status += ( "<p><h4>Cache status before cache reset " + (new java.util.Date()).toString() + "</h4>" );
      status += ( "<p>Cache size: " + getCount() + " object(s) cached" );
      status += ( "<p>Cache contains: " );
      while(keys.hasNext())
         Object key = keys.next();
         // Get the cacheable object associated with the key inside the cache
         Cacheable object = null;
         synchronized ( cacheHashMap )  {
            object = (Cacheable)cacheHashMap.get(key);
         status += ( "<br><b>ID:</b><i>" + object.getIdentifier() + "</i>  <b>Class of:</b><i>" + object.toString() );
         status += ( "</i><b> Expires at:</b>" + object.getExpireDate() + "</i>");
      CacheManager.emptyCache();
      status += ( "<p><h4>Cache status after cache reset " + (new java.util.Date()).toString() + "</h4>" );
      status += ( "<p>Cache size: " + CacheManager.getCount() + " object(s) cached" );
      status += ( "<p>Cache contains: " );
      Set newKeySet = null;
      Iterator newKeys = null;
      synchronized ( cacheHashMap )  {
         newKeySet = cacheHashMap.keySet();
         newKeys   = newKeySet.iterator();
      // this really never gets executed
      while(newKeys.hasNext())
         Object key = newKeys.next();
         // Get the cacheable object associated with the key inside the cache
         Cacheable object = null;
         synchronized ( cacheHashMap )  {
            object = (Cacheable)cacheHashMap.get(key);
         status += ( "<br><b>ID:</b><i>" + object.getIdentifier() + "</i>  <b>Class of:</b><i>" + object.toString() );
         status += ( "</i><b> Expires at:</b>" + object.getExpireDate() + "</i>");
      status += ( "</body></html>" );
      return status;