I have two plugins pluginA, and pluginB, which are using SDK from a platform C. Obviously pluginA and pluginB would not be able to access methods from each other, and any communication between pluginA and pluginB must be via C.
pluginA has a utility class utilA , which has a static method getMethod() which will be used in pluginB. and returns an object of a class which is also in pluginB itself. I can create any interface/factory class , in platform C, so that this communication can be done.
Can someone suggest how to tackle this problem.
This is a Factory design pattern. The factory should be a resource accessible from the SDK to all "plugins" and the SDK should decide (perhaps with a settings file or annotations) which plugin becomes the supplier for the factory.
One other design thought. I prefer to use interfaces as the output of a Factory object. That way one class can implement the interface any way it likes, and every other class (no matter when created, or how loaded by the ClassLoader) can use that factory. This may save you a bunch of headaches at testing or runtime.
Related
Asume the following code:
public class Main {
public static final List<Object> configuration = new ArrayList<>();
public static void main(String[] args) {
System.out.println(configuration);
}
}
I now want to be able, to provide "self-configuring" classes. This means, they should be able to simply provide something like a static block, that will get called automatically like this:
public class Custom {
static {
Main.configuration.add(Custom.class);
}
}
If you execute this code, the configuration list is empty (because of the way static blocks are executed). The class is "reachable", but not "loaded". You could add the following to the Main class before the System.out
Class.forName("Custom");
and the list would now contain the Custom class object (since the class is not initialized yet, this call initializes it). But because the control should be inverse (Custom should know Main and not the other way around), this is not a usable approach. Custom should never be called directly from Main or any class, that is associated with Main.
What would be possible though is the following: You could add an Annotation to the class and collect all classes with said annotation, using something like the ClassGraph framework and call Class.forName on each of them.
TL;DR
Is there a way, to automatically call the static block without the need to analyze all classes and the need of knowing the concrete, "self configuring" class? Perfect would be an approach, that, upon starting the application, automatically initializes a classes (if they are annotated with a certain annotation). I thought about custom ClassLoaders, but from what i understand, they are lazy and therefor not usable for this approach.
The background of this is, that i want to incorporate it into an annotation processor, which creates "self configuring code".
Example (warning: design-talk and in depth)
To make this a little less abstract, imagine the following:
You develop a Framework. Let's call it Foo. Foo has the classes GlobalRepository and Repository. GlobalRepository follows the Singleton design pattern (only static methods). The Repository as well as the GlobalRepository have a method "void add(Object)" and " T get(Class)". If you call get on the Repository and the Class cannot be found, it calls GlobalRepository.get(Class).
For convenience, you want to provide an Annotation called #Add. This Annotation can be placed on Type-Declarations (aka Classes). An annotation-processor creates some configurations, which automatically add all annotated classes to the GlobalRepository and therefor reduce boilerplate code. It should only (in all cases) happen once. Therefor the generated code has a static initializer, in which the GlobalRepository is filled, just like you would do with the local repository. Because your Configurations have names that are designed to be as unique as possible and for some reason even contain the date of creation (this is a bit arbitrary, but stay with me), they are nearly impossible to guess.
So, you also add an annotation to those Configurations, which is called #AutoLoad. You require the using developer to call GlobalRepository.load(), after which all classes are analyzed and all classes with this annotation are initialized, and therefor their respective static-blocks are called.
This is a not very scalable approach. The bigger the application, the bigger the realm to search, the longer the time and so on. A better approach would be, that upon starting the application, all classes are automatically initialized. Like through a ClassLoader. Something like this is what i am looking for.
First, don’t hold Class objects in your registry. These Class objects would require you to use Reflection to get the actual operation, like instantiating them or invoking certain methods, whose signature you need to know before-hand anyway.
The standard approach is to use an interface to describe the operations which the dynamic components ought to support. Then, have a registry of implementation instances. These still allow to defer expensive operations, if you separate them into the operational interface and a factory interface.
E.g. a CharsetProvider is not the actual Charset implementation, but provides access to them on demand. So the existing registry of providers does not consume much memory as long as only common charsets are used.
Once you have defined such a service interface, you may use the standard service discovery mechanism. In case of jar files or directories containing class files, you create a subdirectory META-INF/services/ containing a file name as the qualified name of the interface containing qualified names of implementation classes. Each class path entry may have such a resource.
In case of Java modules, you can declare such an implementation even more robust, using
provides service.interface.name with actual.implementation.class;
statements in your module declaration.
Then, the main class may lookup the implementations, only knowing the interface, as
List<MyService> registered = new ArrayList<>();
for(Iterator<MyService> i = ServiceLoader.load(MyService.class); i.hasNext();) {
registered.add(i.next());
}
or, starting with Java 9
List<MyService> registered = ServiceLoader.load(MyService.class)
.stream().collect(Collectors.toList());
The class documentation of ServiceLoader contains a lot more details about this architecture. When you go through the package list of the standard API looking for packages have a name ending with .spi, you get an idea, how often this mechanism is already used within the JDK itself. The interfaces are not required to be in packages with such names though, e.g. implementations of java.sql.Driver are also searched through this mechanism.
Starting with Java 9, you could even use this to do something like “finding the Class objects for all classes having a certain annotation”, e.g.
List<Class<?>> configuration = ServiceLoader.load(MyService.class)
.stream()
.map(ServiceLoader.Provider::type)
.filter(c -> c.isAnnotationPresent(MyAnnotation.class))
.collect(Collectors.toList());
but since this still requires the classes to implement a service interface and being declared as implementations of the interface, it’s preferable to use the methods declared by the interface for interacting with the modules.
So I'm using a third party library in a Play application (namely the Echo Nest Java API). There are some oversights in how some methods are exposed by the library, and I need to modify one method in particular, which involves using a constructor with default access.
I had initially thought that creating a class in the same package that extends the class that I need to modify would do the trick, but it seems Java's runtime package handling is thwarting that attempt (that is, since different classloaders are being used for the two different classes, their being in the same package is not enough). I have some notion of potentially modifying the classloader for either my subclass or the Echo Nest library, but have just about no idea how to go about doing that or whether there's a better solution available.
Any pointers in the right direction would be appreciated!
So it turns out the solution was to load the library class using Play's default classloader, set the constructor to be accessible, and then use newInstance() to instantiate the library class. Some code, in case any one else runs into a similar problem:
// Use the default application classloader to load the library class
Class artistClazz = Play.application().classloader().loadClass("com.echonest.api.v4.Artist");
// Get the package private constructor that we need
Constructor<Artist> constructor = artistClazz.getDeclaredConstructor(EchoNestAPI.class, Map.class);
// Make sure it's accessible to this class
constructor.setAccessible(true);
return constructor.newInstance(this, (Map) mq.getObject("artist"));
I'm not by any means convinced that this was the best or cleanest solution to this problem, but it minimally affected the code outside of this modified subclass so I'll probably stick to it for now.
I have a library with several packages-
lets say
package a;
package b;
inside package a I have public a_class
inside package b I have public b_class
a_class uses b_class.
I need to generate a library from this , but I do not want the Client to see b_class.
The only solution I know of is to flatten my beautifully understandable packages to single package and to use default package access for b_class.
Is there another way to do so ? maybe using interfaces or some form of design pattern ??
If you reject to move the code to an individual, controlled server, all you can do is to hinder the client programmer when trying to use your APIs. Let's begin applying good practices to your design:
Let your packages organized as they are now.
For every class you want to "hide":
Make it non-public.
Extract its public API to a new, public interface:
public interface MyInterface {...}
Create a public factory class to get an object of that interface type.
public class MyFactory
{
public MyInterface createObject();
}
So far, you have now your packages loosely coupled, and the implementation classes are now private (as good practices preach, and you already said). Still, they are yet available through the interfaces and factories.
So, how can you avoid that "stranger" clients execute your private APIs? What comes next is a creative, a little complicated, yet valid solution, based on hindering the client programmers:
Modify your factory classes: Add to every factory method a new parameter:
public class MyFactory
{
public MyInterface createObject(Macguffin parameter);
}
So, what is Macguffin? It is a new interface you must define in your application, with at least one method:
public interface Macguffin
{
public String dummyMethod();
}
But do not provide any usable implementation of this interface. In every place of your code you need to provide a Macguffin object, create it through an anonymous class:
MyFactory.getObject(new Macguffin(){
public String dummyMethod(){
return "x";
}
});
Or, even more advanced, through a dynamic proxy object, so no ".class" file of this implementation would be found even if the client programmer dares to decompile the code.
What do you get from this? Basically is to dissuade the programmer from using a factory which requires an unknown, undocumented, ununderstandable object. The factory classes should just care not to receive a null object, and to invoke the dummy method and check the return value it is not null either (or, if you want a higher security level, add an undocumented secret-key-rule).
So this solution relies upon a subtle obfuscation of your API, to discourage the client programmer to use it directly. The more obscure the names of the Macguffin interface and its methods, the better.
I need to generate a library from this , but I do not want the Client to see b_class. The only solution I know of is to flatten my beautifully understandable packages to single package and to use default package access for b_class. Is there another way to do so ?
Yes, make b_class package-private (default access) and instantiate it via reflection for use in a_class.
Since you know the full class name, reflectively load the class:
Class<?> clz = Class.forName("b.b_class")
Find the constructor you want to invoke:
Constructor<?> con = clz.getDeclaredConstructor();
Allow yourself to invoke the constructor by making it accessible:
con.setAccessible(true);
Invoke the constructor to obtain your b_class instance:
Object o = con.newInstance();
Hurrah, now you have an instance of b_class. However, you can't call b_class's methods on an instance of Object, so you have two options:
Use reflection to invoke b_class's methods (not much fun, but easy enough and may be ok if you only have a few methods with few parameters).
Have b_class implement an interface that you don't mind the client seeing and cast your instance of b_class to that interface (reading between the lines I suspect you may already have such an interface?).
You'll definitely want to go with option 2 to minimise your pain unless it gets you back to square one again (polluting the namespace with types you don't want to expose the client to).
For full disclosure, two notes:
1) There is a (small) overhead to using reflection vs direct instantiation and invocation. If you cast to an interface you'll only pay the cost of reflection on the instantiation. In any case it likely isn't a problem unless you make hundreds of thousands of invocations in a tight loop.
2) There is nothing to stop a determined client from finding out the class name and doing the same thing, but if I understand your motivation correctly you just want expose a clean API, so this isn't really a worry.
When using Kotlin, you can use the internal modifier for your library classes.
If I understand correctly you are asking about publishing your library for 3rd party usage without disclosing part of your source? If that's the case you can use proguard, which can obfuscate your library. By default everything will be excluded/obfuscated, unless you specify things you want to exclude from being obfuscated/excluded.
If you want to distribute [part of] your code without the client being able to access it at all, that means that the client won't be able to execute it either. :-O
Thus, you just have one option: Put the sensible part of your code into a public server and distribute a proxy to access it, so that your code would be kept and executed into your server and the client would still be able to execute it through the proxy but without accessing it directly.
You might use a servlet, a webservice, a RMI object, or a simple TCP server, depending on the complexity level of your code.
This is the safest approach I can think of, but it also deserves a price to pay: In addition to complexing your system, it would introduce a network delay for each remote operation, which might be big deal depending on the performance requirements. Also, you should securize the server itself, to avoid hacker intrussions. This could be a good solution if you already have a server that you could take advantage of.
I am making some modifications to already existing code and what I need to do is call a method from class A of project X into a method of class B of project Y, without explicitly importing the class A into class B. Creating an import of class A in class B will create a cycle in the build path, which I have to avoid at all costs. Can anyone help?
Have a look at Dependency Inversion which is basically a principle/technique to manage the directionality of dependencies by using abstract classes or interfaces along with your concrete implementations:
Here's a beginner's tutorial (the examples are in C# but Java doesn't differ much)
You can solve that by an Interface (to call the method), a factory pattern to create the object which method you want to call, and or Reflection which allows to create objects and call methods for which you only know the name (e,g given in an config file).
However try it first without reflection.
Is there a feasible way to get my own code run whenever any class is loaded in Java, without forcing the user explicitly and manually loading all classes with a custom classloader?
Without going too much into the details, whenever a class implementing a certain interface read its annotation that links it with another class, and give the pair to a third class.
Edit: Heck, I'll go to details: I'm doing an event handling library. What I'm doing is having the client code do their own Listener / Event pairs, which need to be registered with my library as a pair. (hm, that wasn't that long after all).
Further Edit: Currently the client code needs to register the pair of classes/interfaces manually, which works pretty well. My intent is to automate this away, and I thought that linking the two classes with annotations would help. Next, I want to get rid of the client code needing to keeping the list of registrations up to date always.
PS: The static block won't do, since my interface is bundled into a library, and the client code will create further interfaces. Thus, abstract classes won't do either, since it must be an interface.
If you want to base the behavior on an interface, you could use a static initializer in that interface.
public interface Foo{
static{
// do initializing here
}
}
I'm not saying it's good practice, but it will definitely initialize the first time one of the implementing classes is loaded.
Update: static blocks in interfaces are illegal. Use abstract classes instead!
Reference:
Initializers (Sun Java Tutorial)
But if I understand you right, you want the initialization to happen once per implementing class. That will be tricky. You definitely can't do that with an interface based solution. You could do it with an abstract base class that has a dynamic initializer (or constructor), that checks whether the requested mapping already exists and adds it if it doesn't, but doing such things in constructors is quite a hack.
I'd say you cleanest options are either to generate Code at build time (through annotation processing with apt or through bytecode analysis with a tool like asm) or to use an agent at class load time to dynamically create the mapping.
Ah, more input. Very good. So clients use your library and provide mappings based on annotations. Then I'd say your library should provide an initializer method, where client code can register classes. Something like this:
YourLibrary.getInstance().registerMappedClasses(
CustomClass1.class,
CustomClass2.class,
CustomClass3.class,
CustomClass4.class
)
Or, even better, a package scanning mechanism (example code to implement this can be found at this question):
YourLibrary.getInstance().registerMappedClassesFromPackages(
"com.mycompany.myclientcode.abc",
"com.mycompany.myclientcode.def"
)
Anyway, there is basically no way to avoid having your clients do that kind of work, because you can't control their build process nor their classloader for them (but you could of course provide guides for classloader or build configuration).
If you want some piece of code to be run on any class loading, you should:
overwrite the ClassLoader, adding your own custom code at the loadClass methods (don't forget forwarding to the parent ClassLoader after or before your custom code).
Define this custom ClassLoader as the default for your system (here you got how to do it: How to set my custom class loader to be the default?).
Run and check it.
Depending on what kind of environment you are, there are chances that not all the classes be loaded trouugh your custom ClassLoader (some utility packages use their own CL, some Java EE containers handle some spacific areas with specific classLoaders, etc.), but it's a kind of aproximation to what you are asking.