Is it possible to write Thread creation listener in java? For example using aop?!
I mean something like this that if my application creates a thread I would like to register this object in my own table, container or something.
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3 Answers
I would create a thread that continously lists all running threads on the JVM.
Then each time it noticies that a new thread has appeared, it would notify in either way a class in your code.
Here are some links about how to list all threads currently running on the JVM :
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A starting code :
ThreadCreationListener.java
public interface ThreadCreationListener { public void onThreadCreation(Thread newThread); } ThreadCreationMonitor.java
public class ThreadCreationMonitor extends Thread { private List<ThreadCreationListener> listeners; private boolean canGo; public ThreadCreationMonitor() { listeners = new Vector<ThreadCreationListener>();//Vector class is used because many threads may use a ThreadCreationMonitor instance. canGo = true; // Initialize the rest of the class here... } // Most important methods public void addListener(ThreadCreationListener tcl) { listeners.add(tcl); } public void removeListener(ThreadCreationListener tcl) { listeners.remove(tcl); } public void run() { List<Thread> runningThreads; List<Thread> lastRunningThreads = new ArrayList<>(); while(canGo) { // Step 1 - List all running threads (see previous links) // runningThreads = ... // Step 2 - Check for new threads and notify all listeners if necessary if (runningThreads.removeAll(lastRunningThreads)==true) { for(Thread t : runningThreads) { for(ThreadCreationListener tcl : listeners) { lastRunningThreads.add(t); tcl.onThreadCreation(t); //Notify listener } } } } } public void shutdown() { canGo = false; } } MyThreadInfoConsumer.java
public class MyThreadInfoConsumer implements ThreadCreationListener { public void onThreadCreation(Thread newThread) { // Process here the notification... } } Main.java
public class Main { public static void main(String[] args) { ThreadCreationMonitor tcm = new ThreadCreationMonitor(); tcm.start(); MyThreadInfoConsumer myTIC = new MyThreadInfoConsumer(); tcm.addListener(myTIC); // rest of your code... // Don't forget to call tcm.shutdown() when exiting your application ! } } 
2 Comments
Łukasz Rzeszotarski
These examples are good and useful, but there is no listener behaviour. I would like to have the information that new thread is gonna created by jvm let's say 'instatntly' without overriding anything.
user454322
@ŁukaszRzeszotarski you can implement a listener behavior from the examples above though, they are about running threads not created threads (as you asked).
I think this would be possible with AOP (aspectj for instance). But it is still required to create your own Thread and ThreadGroup/Executor types, unless you can recompile the JDK classes with the aspect compiler. Define the pointcut on your thread's start method if you want to register on thread launching or on the createThread of your pool if you want to register on the creation of the thread objects.
The following works only if you recompiled the JDK with the aspect compiler: All threads are started with Thread.start, so write a pointcut for that method then you can use advices to do what you'd like to. Of course this is not perfect since e.g. a cachedThreadPool executor might not start a new thread for each task, but maybe if you register a pointcut on Runnable.run and Callable.call rather than on Thread.start, that might be sufficient enough.
6 Comments
user454322
This might work for running threads but, the question is about listening on threads created not threads running.
zeller
@user454322 No. A before advice will get called when the threads are started. (ie any of these 3 methods are called)
user454322
I understand that it will be called before threads are started, but would be called when threads are created? e.g., what would happen if a thread is created but never started?
zeller
@user454322. I see, I interpreted the question as if he wanted to register on the ' thread start' event. I edited my answer.
Łukasz Rzeszotarski
@zeller sounds good. And what about registering pointcut on a Thread class constructor? Would it work - something like this? What do you think?
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Perhaps a ThreadGroup is what you need. All Threads are members of a ThreadGroup and when you start a new Thread it is added to the same group as its parent by default.
In theory its possible (but not recommended) to sub-class to be notified when a Thread is added or removed from the group.
It is likely that polling the threads of this groups, or polling all threads is a better solution.
2 Comments
Łukasz Rzeszotarski
A thread is added to the thread group on creation or when it is started?
Peter Lawrey
From the code, it suggests when it is started. If you want to know when a Thread is created, you need to sub-class Thread (and change all the thread creating code)
ClassLoaderin the main thread, hopefully that will be inherited by all new threads, and again hopefully, any thread will need to load some class, which means yourClassLoaderwill be consulted and you have an opportunity to register the thread....Threadwith the required methods for listeners. Of course, will work only for your code.