As topic, what is the difference between class and delegate expression? Which is better to use?
The class attribute for the Service Task is the FQN of the class that implements JavaDelegate. When this is used Flowable will instantiate the class.
A delegate expression is an expression that resolves a bean that implements JavaDelegate. This means that you can define any bean of yours in there and you have control for its instantiation.
In my opinion it is better to use delegate expression. The reason is that with a delegate expression you can change the implementation between different versions and you won't need to change your BPMN Models.
More detailed explanation can be found in the Java Service Task section of the Flowable documentation.
Related
I am trying to implement a NotificationService in a correct way from the point of view of OOP. For this I have the next interface:
abstract class NotificationsService {
void initNotificationsHandlers();
int sendGeneralNotification({String? title, String? body});
//...
}
And his subclass:
class FirebaseNotificationService extends NotificationsService {
//All implementations...
}
The problem is when I implement it. I have to instance it on the main:
NotificationsService notificationsService = new FirebaseNotificationService();
But I have to use this service in more classes,and I don't want to instance the FirebaseNotificationService in every class because I would be violating the Dependency Inversion Principle. I want other classes just know the abstraction NotificationsService.
I have thought using something like this:
abstract class NotificationsService {
///Current notification service subclass used.
static final NotificationsService instance;
//...
}
And then implementing the class this way:
Main
NotificationsService.instance = new FirebaseNotificationService();
Other class
NotificationsService.instance.initNotificationsHandlers(); // For example, it could be any method
But it doesn't look very clean because I am using the NotificationService interface to "save" the current subclass. I think it shouldn't be his responsibility.
Maybe should I make another class which "saves" the current implementation? Or apply a singleton pattern? What is the OOP most correct way to do this?
Clarification: I am not asking for a personal opinion (otherwise this question should be close). I'm asking about the correct OOP solution.
In which language are you programming? Java probably, by reading your Code.
What you actually want is Dependency Injection and a Singleton (even though I think that Singleton is overkill for a NotificationService)
If we remain at the Java Standard, it works in this way:
The classes that need your NotificationService would have a constructor annotated with #Inject and an agument of type NotificationService (not your Implementation Class) - so your consumer classes rely on something abstract rather than something concrete, which makes it easier to change the implementation.
The Dependency Injection Container or Framework would take care that when your classes are being injected by them self somewhere, that their Dependencies are being satisfied in order to be able to construct this class.
How does it actually know which Implementation belongs to an Interface?
Well it depends on the Framework or Platform you are using but you either define your bindings of the interface to the concrete class or is is looking it up with reflection (if we are using Java)
If a class gets injected with a new Instance every time or always the same instance this depends on your annotations on the class itself. For example you could annotate it with #Singleton.
I hope it helps a bit.
Baeldung has this section:
this limits matching to join points where the bean reference is an
instance of the given type, while target limits matching to join
points where the target object is an instance of the given type. The
former works when Spring AOP creates a CGLIB-based proxy, and the
latter is used when a JDK-based proxy is created. Suppose that the
target class implements an interface:
public class FooDao implements BarDao {
...
}
In this case, Spring AOP will use the JDK-based proxy, and we should
use the target PCD because the proxied object will be an instance of
the Proxy class and implement the BarDao interface:
#Pointcut("target(com.baeldung.pointcutadvice.dao.BarDao)")
On the other hand, if FooDao doesn't implement any interface, or the
proxyTargetClass property is set to true, then the proxied object will
be a subclass of FooDao and we can use the this PCD:
#Pointcut("this(com.baeldung.pointcutadvice.dao.FooDao)")
I'm still confuse why this just works with CGLIB proxy and target just works with JDK proxy. Could you help to tell me the different between them?
Actually, the explanation in the tutorial does not make much sense:
Both this(MyInterface) and target(MyInterface) work for JDK proxies, if the bean is declared as a MyInterface type.
Both this(MyClass) and target(MyClass) work for CGLIB proxies, if the bean is declared as a MyClass type.
Just try, and you will see that I am right. In Spring AOP, there is not real difference between this() and target(), because due to its proxy-based nature, it implicitly only supports execution() pointcuts from AspectJ.
In native AspectJ however, you also have other pointcut types such as call(), and there you would see a difference: this() would match the caller's type, while target() would match the callee's type. E.g., if you intercept a method A.a() calling B.b() via pointcut call(B.b()), this() would return an A instance, while target() would return a B instance. Do not worry, if this is difficult to understand for you, because for Spring AOP and execution pointcuts it really does not matter.
The only subtle difference I noticed in Spring AOP is, that for MyInterfaceImpl implements MyInterface, pointcut target(MyInterfaceImpl) would actually match, while this(MyInterfaceImpl) would not. This is because for JDK proxies, the proxy actually extends java.lang.reflect.Proxy, not MyInterfaceImpl. The proxy only delegates to a MyInterfaceImpl instance.
Edit: If you keep in mind that, in contrast to native AspectJ which involves no dynamic proxies, the semantics in Spring AOP are such that
this() relates to the proxy object, while
target() relates to the proxied object (proxy target),
it becomes clear why in this special case for JDK proxies target() matches, but this() does not.
Reference: Spring manual, section "Declaring a pointcut - examples":
this(com.xyz.service.AccountService): Any join point (method execution only in Spring AOP) where the proxy implements the AccountService interface.
target(com.xyz.service.AccountService): Any join point (method execution only in Spring AOP) where the target object implements the AccountService interface.
Only in this case, we are not asking for the interface class (which both the proxy and the target object implement), but for the implementation class itself.
Bottom line: For all intents and purposes, in normal use cases you can use either this() or target() for both JDK and CGLIB proxies. I recommend to stick with target(), because it best matches the implicit execution() semantics of Spring AOP, and usually you are interested in information about the target rather than about the proxy..
I included a library I'd like to use, but in accessing to one of its classes I get the error message,
"Cannot access '<init>': it is private in [class name]
Is there something I can do to rectify this on my side, or am I just stuck to not use the package?
The error means the constructor is private. Given your comment, I'm assuming you're using a library. If this is the case, you'll have to find a different way to initialize it. Some libraries have factories or builders for classes, so look up any applicable documentation (if it is a library or framework). Others also use the singleton pattern, or other forms of initialization where you, the developer, don't use the constructor directly.
If, however, it is your code, remove private from the constructor(s). If it's internal and you're trying to access it outside the module, remove internal. Remember, the default accessibility is public. Alternatively, you can use the builder pattern, factory pattern, or anything similar yourself if you want to keep the constructor private or internal.
I came across this issue when trying to extend a sealed class in another file. Without seeing the library code it is hard to know if that is also what you are attempting to do.
The sealed classes have the following unique features:
A sealed class can have subclasses, but all of them must be declared in the same file as the sealed class itself.
A sealed class is abstract by itself, it cannot be instantiated directly and can have abstract members.
Sealed classes are not allowed to have non-private constructors (their constructors are private by default).
Classes that extend subclasses of a sealed class (indirect inheritors) can be placed anywhere, not necessarily in the same file.
For more info, have a read at https://www.ericdecanini.com/2019/10/14/kotlins-sealed-class-enums-on-steroids/
Hopefully, this will help others new to Kotlin who are also encountering this issue.
Class constructors are package-private by default. Just add the public keyword before declaring the constructor.
By default constructor is public so need to remove internal keyword.
1) Singleton means the class have one instance. Having private constructer. No way to create object except reflection. No subclassing.
If i want to clone my singleton classthen class must and should implement Cloneable and override clone() right.
am not going to implement Cloneable interface in my Singleton class.
Then how cloning breaks my singleton. is this correct. Please clarify some one. if am wrong.
what is the need of throwing clonenotsupported exception.
No reason to implement Cloneable and override clone() to throw CloneNotSupportedException. Object. Clone, throws expectation when the Cloneable interface is absent.
The correct way to create singleton class using enums can be referred in my favorite java book "Effective Java" . Please read that.
I am learning to use dependency injection with ninject. Most of the properties and methods are fairly intuitive, one that has me though is Kernel.Inject(instance);
What does the Inject method actually do as it doesn't return anything. I've looked around but search for a method called inject on a dependency injection container is a nightmare, I can't find any references to the method specifically.
Kernel.Inject(instance) will inject dependencies into an already existing object.
That's why it returns void because it takes the parameter object instance and starts to investigate it's methods and property setters looking for the [Inject] attribute. Then it will call them with the resolved instances of their parameter types. (this is called Method or Property injection)
So when contructor injection is not enoughpossible you can Kernel.Inject to fill in your dependencies for a given instance.
You can read more about this here: Ninject Injection Patterns