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.
Related
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.
In our application we embed groovy 1.8.9 jar and invoke a groovy script from Java code at run time. The groovy scripts are not changing in each call and can be considered fixed logic.
When we run the application for a long time we see too many classes in org/codehaus/groovy/runtime/callsite/CallSiteClassLoader
in fact for each call, we see a class being generated. This leads to GC pressure and unpleasant side effects.
since our groovy scripts are unchanged from call to call, is there a way to reuse the callsite classes generated once.
I have an app which uses Core Data. My app does some processing in background threads and saves the output to Core Data, and simultaneously, my main/UI thread performs reading operations on Core Data.
Now, I am trying to synthesize one attribute in my managed object in order to create custom getters and setters (I wanna automatically save long strings in a file, and then simply save the path instead). But then, when reading these objects later in the main thread, they became nil. I tried removing my custom getters and setters and simply leaving #dynamic attr replaced with #synthesize attr = _attr, but even then it stopped working. (I cleaned my project and removed it from both the simulator and the devices I have been trying it out on).
The moment I restored the original syntax, everything started working once again. Any idea what could be causing such behavior?
Refer to Managed Object Accessor Methods section of Core Data Programming Guide for the information on how to correctly implement custom accessor methods.
By the way, in general it is recommended not to override managed object’s accessor methods because the existing implementation is highly optimized.
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?
A lot of Java ME tutorials use the startApp() method for creating and initializing objects and the constructor is left blank. However, the startApp() method is also invoked when the MIDlet resumes from a paused state. This causes all the objects to be re-initialized and any changes made are lost.
I have also noticed that the netbeans IDE, in its auto-generated code, uses many if(object==null) statements in startApp() to check if the object was created earlier.
Would it not make sense to simply do all the object creation and initialization in the constructor itself? Is there any reason for not doing this?
This is in part about understanding the MIDP threading model.
What thread the MIDlet constructor is called in depends on who developed the Java Virtual Machine on the phone.
Developers have a tendency to rely only on what the MIDP specification says in that area, which is how startApp, pauseApp and lcdui event handling should behave.
However there are only a few cases where this is important.
The second aspect to this issue is software developers trying to free as much resources (memory, file handles, sockets...) as possible when the MIDlet is paused.
Again, there are cases when MIDlets should hang on to some resources even when paused but you really need to think about what you're doing (and understand it better than casually) when coding that kind of behavior.
It is also worth remembering that some phones always keep the JVM process running. When they also have a JVM that doesn't support class unloading (as is usual in a J2ME world), this means that static variables can remain in memory even after the MIDlet has been completely destroyed.