Posts Tagged ‘architecture’
Thread leaks in Mule ESB 2.2.1
Abstract
The application I work on packages Mule ESB 2.2.1 in a WAR and deploys it under a WebLogic 10.3 server. My team mates and I noticed that, on multiple deploy/undeploy cycles, the PermGen size dramatically decreased. The cause of this was the number of threads, which hardly decreased on undeployment phases, unlike the expected behaviour.
Indeed, Mule is seldom deployed as a WebApp. Rather, it is designed to be run as a standalone application, within a Tanuki wrapper. When the JVM is killed, all the threads are killed, too, and therefore no thread survives ; hence, the memory is freed and there is no reason to fear a thread leak.
Moreover, when the application is redeployed, new threads -with the same names as the “old” threads- are created. The risk is that, for any reason, a thread-name-based communication between threads may fail, because the communication pipe may be read by the wrong thread.
In my case: on WebLogic startup, there are 31 threads ; when the application is deployed, there are 150 ; when the application works (receives and handles messages), the number of threads climbs to 800 ; when the application is undeployed, only 12 threads are killed, the other remaining alive.
The question is: how to kill Mule-created threads, in order to avoid a Thread leak?
WebLogic Threads
I performed a thread dump at WebLogic startup. Here are WebLogic threads, created before any deployment occurs:
Attach Listener DoSManager DynamicListenThread[Default[1]] DynamicListenThread[Default] ExecuteThread: '0' for queue: 'weblogic.socket.Muxer' ExecuteThread: '1' for queue: 'weblogic.socket.Muxer' ExecuteThread: '2' for queue: 'weblogic.socket.Muxer' Finalizer JMX server connection timeout 42 RMI Scheduler(0) RMI TCP Accept-0 RMI TCP Connection(1)-127.0.0.1 RMI TCP Connection(2)-127.0.0.1 Reference Handler Signal Dispatcher Thread-10 Thread-11 Timer-0 Timer-1 VDE Transaction Processor Thread [ACTIVE] ExecuteThread: '0' for queue: 'weblogic.kernel.Default (self-tuning)' [ACTIVE] ExecuteThread: '2' for queue: 'weblogic.kernel.Default (self-tuning)' [STANDBY] ExecuteThread: '1' for queue: 'weblogic.kernel.Default (self-tuning)' [STANDBY] ExecuteThread: '3' for queue: 'weblogic.kernel.Default (self-tuning)' [STANDBY] ExecuteThread: '4' for queue: 'weblogic.kernel.Default (self-tuning)' [STANDBY] ExecuteThread: '5' for queue: 'weblogic.kernel.Default (self-tuning)' main weblogic.GCMonitor weblogic.cluster.MessageReceiver weblogic.time.TimeEventGenerator weblogic.timers.TimerThread
Dispose Disposables, Stop Stoppables…
The application being deployed in a WAR, I created a servlet implementing ServletContextListener
. In the method contextDestroyed()
, I destroy Mule objects (Disposable, Stoppable, Model, Service, etc.) one per one.
Eg#1:
final Collection<Model> allModels; try { allModels = MuleServer.getMuleContext().getRegistry().lookupObjects(Model.class); if (LOGGER.isDebugEnabled()) { LOGGER.debug("Disposing models " + allModels.size()); } for (Model model : allModels) { model.dispose(); } allModels.clear(); } catch (Exception e) { LOGGER.error(e); }
Eg#2:
private void stopStoppables() { final Collection<Stoppable> allStoppables; try { allStoppables = MuleServer.getMuleContext().getRegistry().lookupObjects(Stoppable.class); if (LOGGER.isDebugEnabled()) { LOGGER.debug("Stopping stoppables " + allStoppables.size()); } for (Stoppable stoppable : allStoppables) { stoppable.stop(); } allStoppables.clear(); } catch (MuleException e) { LOGGER.error(e); } }
This first step is needed because default mechanism is flawed: Mule re-creates objects that were destroyed.
Kill Threads
The general idea to kill Mule threads is the following: perform a Unix-style “diff” between WebLogic native threads, and the threads still alive once all Mule objects have been stopped and disposed.
On Application Startup
In the ServletContextListener
, I add a field that will be set in a method called in the constructor:
private List<String> threadsAtStartup; (...) /** * This method retrieves the Threads present at startup: mainly speaking, they are Threads related to WebLogic. */ private void retrieveThreadsOnStartup() { final Thread[] threads; final ThreadGroup threadGroup; threadGroup = Thread.currentThread().getThreadGroup(); try { threads = retrieveCurrentActiveThreads(threadGroup); } catch (NoSuchFieldException e) { LOGGER.error("Could not retrieve initial Threads list. The application may be unstable on shutting down ", e); threadsAtStartup = new ArrayList<String>(); return; } catch (IllegalAccessException e) { LOGGER.error("Could not retrieve initial Threads list. The application may be unstable on shutting down ", e); threadsAtStartup = new ArrayList<String>(); return; } threadsAtStartup = new ArrayList<String>(threads.length); for (int i = 0; i < threads.length; i++) { final Thread thread; try { thread = threads[i]; if (null != thread) { threadsAtStartup.add(thread.getName()); if (LOGGER.isDebugEnabled()) { LOGGER.debug("This Thread was available at startup: " + thread.getName()); } } } catch (RuntimeException e) { LOGGER.error("An error occured on initial Thread statement: ", e); } } } /** * Hack to retrieve the field ThreadGroup.threads, which is package-protected and therefore not accessible * * @param threadGroup * @return * @throws NoSuchFieldException * @throws IllegalAccessException */ private Thread[] retrieveCurrentActiveThreads(ThreadGroup threadGroup) throws NoSuchFieldException, IllegalAccessException { final Thread[] threads; final Field privateThreadsField; privateThreadsField = ThreadGroup.class.getDeclaredField("threads"); privateThreadsField.setAccessible(true); threads = (Thread[]) privateThreadsField.get(threadGroup); return threads; }
On application shutdown
In the method ServletContextListener.contextDestroyed()
, let’s call this method:
/** * Cleanses the Threads on shutdown: theorically, when the WebApp is undeployed, should remain only the threads * that were present before the WAR was deployed. Unfornately, Mule leaves alive many threads on shutdown, reducing * PermGen size and recreating new threads with the same names as the old ones, inducing a kind of instability. */ private void cleanseThreadsOnShutdown() { final Thread[] threads; final ThreadGroup threadGroup; final String currentThreadName; currentThreadName = Thread.currentThread().getName(); if (LOGGER.isDebugEnabled()) { LOGGER.debug("On shutdown, currentThreadName is: " + currentThreadName); } threadGroup = Thread.currentThread().getThreadGroup(); try { threads = retrieveCurrentActiveThreads(threadGroup); } catch (NoSuchFieldException e) { LOGGER.error("An error occured on Threads cleaning at shutdown", e); return; } catch (IllegalAccessException e) { LOGGER.error("An error occured on Threads cleaning at shutdown", e); return; } for (Thread thread : threads) { final String threadName = thread.getName(); final Boolean shouldThisThreadBeKilled; shouldThisThreadBeKilled = isThisThreadToBeKilled(currentThreadName, threadName); if (LOGGER.isDebugEnabled()) { LOGGER.info("should the thread named " + threadName + " be killed? " + shouldThisThreadBeKilled); } if (shouldThisThreadBeKilled) { thread.interrupt(); thread = null; } } } /** * Says whether a thread is to be killed<br/> * Rules: * <ul><li>a Thread must NOT be killed if:</li> * <ol> * <li>it was among the threads available at startup</li> * <li>it is a Thread belonging to WebLogic (normally, WebLogic threads are among the list in the previous case</li> * <li>it is the current Thread (simple protection against unlikely situation)</li> * </ol> * <li>a Thread must be killed: in all other cases</li> * </ul> * * @param currentThreadName * @param threadName * @return */ private Boolean isThisThreadToBeKilled(String currentThreadName, String threadName) { final Boolean toBeKilled; toBeKilled = !threadsAtStartup.contains(threadName) && !StringUtils.contains(threadName, "weblogic") && !threadName.equalsIgnoreCase(currentThreadName); return toBeKilled; }
EhCache
My application uses an EhCache. Its threads names usually end with “.data”. They are not killed by the previous actions. To get rid of them, the most elegant way is to add this block in the web.xml
:
<listener> <listener-class>net.sf.ehcache.constructs.web.ShutdownListener</listener-class> </listener>
With all these operations, almost all threads are killed. But Java VisualVM still displays 34, vs. 31 at startup.
Tough Threads
A thread dump confirms that, at this point, 3 rebellious threads still refuse to be kill:
MuleServer.1 SocketTimeoutMonitor-Monitor.1 SocketTimeoutMonitor-Monitor.1
Let’s examine them:
MuleServer.1
: This thread is an instance of the inner classMuleServer.ShutdownThread
. Indeed, this is the first thread created by Mule, and therefore appears among the threads available at startup, before theServletContextListener
is called… I did not succeed in killing it, even why trying to kill it namely, which makes sense: killing the father thread looks like suiciding theServletContextListener
.SocketTimeoutMonitor-Monitor.1
: This thread is created by Mule’sTcpConnector
and its daughter classes:HttpConnector
,SslConnector
, etc. Again, I could not kill them.
Conclusion
We have seen Mule suffers of major thread leaks when deployed as a WAR. Anyway, most of these leaks may be sealed.
I assume MuleSoft was aware of this issue: in the version 3 of Mule, the deployment of webapps was refactored.
Tutorial: an Event Bus Handler for GWT / GXT
Overview
Introduction
Let’s consider a application, JonathanGwtApplication, divided in three main panels
- a panel to select animal name name
- a panel to display, expand and collapse trees of the animal ancestors
- a panel of information to display many indicators (colors, ages, etc.).
An issue we encounter is: how to make the different panels communicate? In more technical terms, how to fire events from a panel to another one?
A first solution would be to declare each panel as listener to the other panels. Indeed, this principle may go further, and declare each component as listener to a list of other components…
Main drawbacks:
- the code becomes hard to read
- adding or removing a component requires to modify many parts of the code
- we don’t follow GWT 2’s “philosophy”, which is to use
Handlers
rather thanListeners
.
Hence, these reasons incited us to provide a global EventBusHandler
.
The EventBusHandler
concept
The EventBusHandler
is a global bus which is aware of all events that should be shared between different panels, and fires them to the right components.
The EventBusHandler
is a field of JonathanGwtApplicationContext
.
Intrastructure
lalou.jonathan.application.web.gwt.animal.events.HandledEvent
: generic interface for a event. Abstract method:EventTypeEnum getEventEnum();
lalou.jonathan.application.web.gwt.animal.handler.EventHandler
: generic interface for a component able to handle an event. Abstract method:void handleEvent(HandledEvent handledEvent);
lalou.jonathan.application.web.gwt.animal.handler.EventHandlerBus
: the actual bus. As a concrete class, it has two methods:/** * Fires an event to all components declared as listening to this event * event type. * * @param baseEvent */ public void fireEvent(HandledEvent baseEvent) { // ... } /** * Adds an listener/handler for the event type given as parameter * * @param eventTypeEnum * @param eventHandler * @return The List of handlers for the key given as parameter. This list * contains the eventHandler that was given as second parameter */ public List<EventHandler> put(EventTypeEnum eventTypeEnum, EventHandler eventHandler) { // ... }
How to use the bus?
- Define an event: in JonathanGwtApplication, an event is decribed by two elements:
- a functionnal entity: eg: “animal”, “food”, “tree node”. The functionnal entity must be isomorph to a technical DTO, eg:
AnimalDTO
for an entity Animal.(in the scope of this turoriel we assume to have DTOs, even though the entities may ne sufficient) - a technical description of the event: “selection changed”, “is expanded”
- a functionnal entity: eg: “animal”, “food”, “tree node”. The functionnal entity must be isomorph to a technical DTO, eg:
- Add an entry in the enum
EventTypeEnum
. Eg: “ANIMAL_SELECTION_CHANGED
“ - in
lalou.jonathan.application.web.gwt.animal.events
, create an event, implementingHandledEvent
and its methodgetEventEnum()
. The match betweenEventTypeEnum
and DTO is achieved here. Eg:public class AnimalSelectionChangedEvent extends SelectionChangedEvent<AnimalDTO> implements HandledEvent { public AnimalSelectionChangedEvent( SelectionProvider<AnimalDTO> provider, List<AnimalDTO> selection) { super(provider, selection); } public EventTypeEnum getEventEnum() { return EventTypeEnum.ANIMAL_SELECTION_CHANGED; } }
animalComboBox.addSelectionChangedListener(new SelectionChangedListener<AnimalDTO>() { @Override public void selectionChanged(SelectionChangedEvent<AnimalDTO> se) { final AnimalDTO selectedAnimalVersion; selectedAnimalVersion= se.getSelectedItem(); JonathanGwtApplicationContext.setSelectedAnimal(selectedAnimal); final AnimalSelectionChangedEvent baseEvent = new AnimalSelectionChangedEvent( se.getSelectionProvider(), se.getSelection()); JonathanGwtApplicationContext.getEventHandlerBus() .fireEvent(baseEvent); } });
- easy case: the component handles only one type of event: this handler must implement the right interface (eg:
AnimalSelectionChangedEventHandler
) and its method, eg:protected void handleAnimalSelectionChangedEvent(HandledEvent handledEvent) { return; }
- frequent case: the component handles two or more event types. No matter, make the component implement all the needed interfaces (eg:
AnimalSelectionChangedEventHandler
,FoodSelectionChangedEventHandler
). Provide a unique entry point for the method to implement, which is common to both interfaces. Retrieve the event type, and handle it with ad hoc methods. Eg:public void handleEvent(HandledEvent handledEvent) { final EventTypeEnum eventTypeEnum; eventTypeEnum = handledEvent.getEventEnum(); switch (eventTypeEnum) { case ANIMAL_SELECTION_CHANGED: handleAnimalSelectionChangedEvent(handledEvent); return; case FOOD_SELECTION_CHANGED: handleFoodSelectionChangedEvent(handledEvent); return; default: break; } } protected void handleAnimalSelectionChangedEvent(HandledEvent handledEvent) { // do something } protected void handleFoodSelectionChangedEvent(HandledEvent handledEvent) { // do something else }