Context
This is a unit test scenario.
The methods of the test target class can be called concurrently from different threads, so instead of guarding the logger implementation instance itself with locks, I've chosen to have a thread bound singleton loggers. The methods under test always creating their thread bound loggers via service locator pattern (please do no hijack the question about is this an antipattern or not).
Ninject is programmed as follows in the Arrange part of the test:
kernel.Bind<ILogger>().To<MyLogger>().InThreadScope();
Question
During the Act part of the test, one or more thread is created by the instance under test (inside).
In the Assert part of the test, I would like to access to the one or more loggers what were created an used by the threads in the class under test, and examine that loggers in the purpose of assertion.
How can I accomplish this task? (access the loggers what was created)
Ninject does not offer a specific API for this, however, you can make use of "OnActivation".
Either add it to your existing binding, or use Rebind in the unit test, as follows:
kernel.Rebind<ILogger>().To<MyLogger>().OnActivation(createdInstance => ...do something...);
Replace the "...do something..." with an Action<ILogger> that adds the instance to a (concurrency-safe?) list or similar.
Also see Intercept creation of instances in Ninject for further information.
Related
I'm running a profiler against a running Java service (Spring Boot framework), containing multiple Groovy files all with the #CompileStatic annotation.
Now one of the most time consuming methods is an internal Groovy method (getSAMMethodImpl()). I've been unsuccessful tracking down what this method is actually doing under the covers.
What exactly does this method do, and is there any way to prevent it from being called?
This method gets executed when a CachedClass for a class with a single abstract method (aka SAM) is created. Cached classes are Groovy mechanism to deal with a reflection in a more effective way - instead of always retrospecting classes from the beginning at the runtime it remembers e.g. modifications applied with metaprogramming (adding new methods to classes for instance), so it gets all class information very quickly. Of course it comes with some overhead.
For instance, when the meta class registry is initialized (once), it registers about 1180 methods. About 190 of them cause CachedSAMClass.getSAMMethodImpl(Class<?> c) to be executed. It happens, because ClassInfo.isSAM(Class<?> c) which checks if given class is a single abstract method class calls this method. And if you take a look at ClassInfo.createCachedClass(Class klazz, ClassInfo classInfo) you will see that this isSAM() method gets called always as a last check.
In most cases creating a registry of cached classes shouldn't be a problem - it happens one time for each class. Most of them get registered when you simply access metaClass property of any class. Or when you create a first closure. When it comes to performance, many different factors matter. For instance, Spring Boot uses hot swapping to reload classes at the runtime. In this case Groovy meta class registry gets recreated and all cached classes have to be recreated as well. The same thing may happen when you run a Spring Boot application with spring-boot-devtools dependency added - it uses additional class loader called RestartClassLoader which requires additional meta class registry to be initialized. Actually the number of class loaders you have, that number of times meta class registry will be initialized (once for each class loader). This RestartClassLoader also causes recreating cached classes when it restarts.
And last but not least - if you want to measure performance correctly, try doing it on a production server instead of a local dev environment. If you can attach a debugger to the running process on a server and put a breakpoint in CachedSAMClass.getSAMMethodImpl(Class<?> c) at line 169, you can see how many times and for what classes this method gets executed. If it happens that it gets executed multiple times for the same class, it may suggest that your application is restarting class loader and Groovy has to rebuild meta class registry. It shouldn't happen - production application once started should not make any changes to class loader without a purpose. It is acceptable on a local dev - devtools and hot swapping will force meta class registry to be recreated any time class loader gets refreshed.
Let's have a worker thread which is accessed from a wide variety of objects. This worker object has some public slots, so anyone who connects its signals to the worker's slots can use emit to trigger the worker thread's useful tasks.
This worker thread needs to be almost global, in the sense that several different classes use it, some of them are deep in the hierarchy (child of a child of a child of the main application).
I guess there are two major ways of doing this:
All the methods of the child classes pass their messages upwards the hierarchy via their return values, and let the main (e.g. the GUI) object handle all the emitting.
All those classes which require the services of the worker thread have a pointer to the Worker object (which is a member of the main class), and they all connect() to it in their constructors. Every such class then does the emitting by itself. Basically, dependency injection.
Option 2. seems much more clean and flexible to me, I'm only worried that it will create a huge number of connections. For example, if I have an array of an object which needs the thread, I will have a separate connection for each element of the array.
Is there an "official" way of doing this, as the creators of Qt intended it?
There is no magic silver bullet for this. You'll need to consider many factors, such as:
Why do those objects emit the data in the first place? Is it because they need to do something, that is, emission is a “command”? Then maybe they could call some sort of service to do the job without even worrying about whether it's going to happen in another thread or not. Or is it because they inform about an event? In such case they probably should just emit signals but not connect them. Its up to the using code to decide what to do with events.
How many objects are we talking about? Some performance tests are needed. Maybe it's not even an issue.
If there is an array of objects, what purpose does it serve? Perhaps instead of using a plain array some sort of “container” class is needed? Then the container could handle the emission and connection and objects could just do something like container()->handle(data). Then you'd only have one connection per container.
Let's say I have some class, TMaster, which asa field includes a TIdTCPServer. Some method of the TMaster class is responsible for the OnExecute event of the TIdTCPServer.
Firstly, is this threadsafe and acceptible? Secondly, let's assume my class has many other private fields (Name, Date, anything...) can the OnExecute event - which is really a method INSIDE the TMaster class, write to these variables safely?
I guess I mean to ask if private fields are threadsafe in this situation?
I am really new to threading and any help will be greatly appreciated!
Thanks,
Adrian!
The way I approach this is not to have the fields used by the events belong to the TidTCPServer
owner, but define a custom TidContext descendant and add the fields to that class.
Then you simply set the ContextClass property on the server class to the type of the of your custom context. This way each Connection/Thread will get its own custom context containing its own private fields, this way there is no issue with concurrent thread access to the same fields.
if you have a list of objects that need to be accessed by the different contexts you have two options.
1) create copies the objects and store them in a private field in for each context instance
This can be done in the OnConnect Event.
2) Protect the objects from concurrent thread access using a synchroniser eg TIdCriticalSection, TMultiReadExclusiveWriteSynchronizer or semaphore,
which method you use depends on each individual situation.
if you need to manipulate any vcl components remember this can't safely be done outside the main vcl thread therefore you should create your own tidnotify decendants for this. performing this sort of operation using tidsynch can lead to deadlocks when stoping the tidtcpserver if it is in the middle of a vclsynch operation.
this is just some of what I have learned over the course of a few years using Indy.
TIdTCPServer is a multi-threaded component. No matter what you wrap it in, the OnExecute event will always be triggered in the context of worker threads, one for each connected client, so any code you put inside the handler must be thread-safe. Members of the TMaster class need adequate protection from concurrent access by multiple threads at the same time.
Why is the struts action class is singleton ?
Actually I am getting point that it is multithreaded. but at time when thousand of request hitting same action, and we put synchronized for preventing threading issue, then it not give good performance bcoz thread going in wait state and it take time to proced.
Is that any way to remove singleton from action class ?
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You are asking about why the Action class is a singleton but I think you also have some issues understanding thread safety so I will try to explain both.
First of all, a Struts Action class is not implemented as a singleton, the framework just uses one instance of it. But because only one instance is used to process all incoming requests, care must be taken not to do something with in the Action class that is not thread safe. But the thing is: by default, Struts Action classes are not thread safe.
Thread safety means that a piece of code or object can be safely used in a multi-threaded environment. An Action class can be safely used in a multi-threaded environment and you can have it used in a thousand threads at the same time with no issues... that is if you implement it correctly.
From the Action class JavaDoc:
Actions must be programmed in a thread-safe manner, because the controller will share the same instance for multiple simultaneous requests. This means you should design with the following items in mind:
Instance and static variables MUST NOT be used to store information related to the state of a particular request. They MAY be used to share global resources across requests for the same action.
Access to other resources (JavaBeans, session variables, etc.) MUST be synchronized if those resources require protection. (Generally, however, resource classes should be designed to provide their own protection where necessary.
You use the Struts Action by deriving it and creating your own. When you do that, you have to take care to respect the rules above. That means something like this is a NO-NO:
public class MyAction extends Action {
private Object someInstanceField;
public ActionForward execute(...) {
// modify someInstanceField here without proper synchronization ->> BAD
}
}
You don't need to synchronize Action classes unless you did something wrong with them like in the code above. The thing is that the entry point of execution into your action is the execute method.
This method receives all it needs as parameters. You can have a thousand threads executed at the same time in the execute method with no issues because each thread has its own execution stack for the method call but not for data that is in the heap (like someInstanceField) which is shared between all threads.
Without proper synchronization when modifying someInstanceField all threads will do as they please with it.
So yes, Struts 1 Action classes are not thread safe but this is in the sense that you can't safely store state in them (i.e.make them statefulf) or if you do it must be properly synchronized.
But if you keep your Action class implementation stateless you are OK, no synchronization needed and threads don't wait for one another.
Why is the struts action class is singleton ?
It's by design. Again the JavaDoc explains it:
An Action is an adapter between the contents of an incoming HTTP request and the corresponding business logic that should be executed to process this request
The request parameters are tied to the web tier and you don't want to send that type of data into your business logic classes because that will create a tight coupling
between the two layers which will then make it impossible to easily reuse your business layer.
Because transforming web objects into model objects (and I don't mean the ActionForm beans here) should be the main purpose of Action classes, they don't need to maintain any state (and shoudn't) and also, there is no reason to have more instances of these guys, all doing the same thing. Just one will do.
If you want you can safely maintain state in your model by persisting info to a database for example, or you can maintain web state by using the http session. It is wrong to maintain state in the Action classes as this introduces the need for syncronisation as explained above.
Is there a way to remove singleton from action class?
I guess you could extend Struts and override the default behavior of RequestProcessor.processActionCreate to create yourself an Action per request
but that means adding another layer on top of Struts to change its "normal" behavior. I've already seen stuff like this go bad in a few applications so I would not recommend it.
My suggestion is to keep your Action classes stateless and go for the single instance that is created for it.
If your app is new and you absolutely need statefull Actions, I guess you could go for Struts 2 (they changed the design there and the Action instances are now one per request).
But Struts 2 is very different from Struts 1 so if you app is old it might be difficult to migrate to Struts 2.
Hope this makes it clear now.
This has changed in Struts2 http://struts.apache.org/release/2.1.x/docs/comparing-struts-1-and-2.html
*Struts 2 Action objects are instantiated for each request, so there are no thread-safety issues. (In practice, servlet containers generate many throw-away objects per request, and one more object does not impose a performance penalty or impact garbage collection.) *
I don't know much about struts, but it appears that this changed in Struts 2, so perhaps you should switch to Struts 2?
It seems that all Guice's out-of-the-box Scope implementations are inherently Thread-based (or ignore Threads entirely):
Scopes.SINGLETON and Scopes.NO_SCOPE ignore Threads and are the edge cases: global scope and no scope.
ServletScopes.REQUEST and ServletScopes.SESSION ultimately depend on retrieving scoped objects from a ThreadLocal<Context>. The retrieved Context holds a reference to the HttpServletRequest that holds a reference to the scoped objects stored as named attributes (where name is derived from com.google.inject.Key).
Class SimpleScope from the custom scope Guice wiki also provides a per-Thread implementation using a ThreadLocal<Map<Key<?>, Object>> member variable.
With that preamble, my question is this: how does one go about creating a non-Thread-based Scope? It seems that something that I can use to look up a Map<Key<?>, Object> is missing, as the only things passed in to Scope.scope() are a Key<T> and a Provider<T>.
Thanks in advance for your time.
It's a bit unclear what you want - you don't want scopes that are based on threads, and you don't want scopes that ignore threads.
But yes, scopes are intended to manage the lifecycle of an object and say when an instance should be reused. So really you're asking "what are the other possibilities for re-using an instance beyond 'always use the same instance', 'never use the same instance', and 'use an instance depending on the execution environment of the current thread'?"
Here's what comes to mind:
Use the same instance for a fixed amount of time. The example here would be of a configuration file that's reloaded and reparsed every ten minutes.
Perform some network call to query whether a given object should be re-used (maybe it's a fast call to determine whether we need to reconstruct the object, but the call for reconstructing the object is slow)
Re-use the same object until some outside call comes in telling us to reload
Re-use the same object per thread, but not with a scope that's explicitly entered and left like the servlet scopes. (So one instance per thread)
A "this thread and child threads" scope that is based on an InheritableThreadLocal, not a plain ThreadLocal.
Related to that, a Scope and a threadpool-based ExecutorService that work togehter so that instances are shared between a thread and jobs it submits for background execution.
Pull instances out of a pool; this is tricky, since we'd need a good way to return objects to the pool when finished. (Maybe you could combine this idea with something like the request scope, so that objects can be returned to the pool when the request ends)
A scope that composes two or more other scopes, so for example we could get a configuration object that is re-read every 10 minutes except that the same instance is used through the lifetime of a given request.