Is there any way we can inject new methods and properties into classes during run-time.
http://nurkiewicz.blogspot.com/2009/09/injecting-methods-at-runtime-to-java.html states we may do that by using Groovy.
Is it possible by just doing using Java?
Is it possible by just doing using
Java?
The simple answer is an emphatic "You don't want to do that!".
It is technically possible, but not without resorting to extremely complex, expensive and fragile tricks like bytecode modification1. And even then, you have to rely on dynamic loading to access the modified type and (probably) reflection to make use of its new members. In short, you would be creating lots of pain for yourself, for little if any gain.
Java is a statically typed language, and adding / modifying class type signatures can break the static typing contract of a class.
1 - AspectJ and the like allow you to inject additional behaviour into a class, but it is probably not the "runtime" injection that you are after. Certainly, the injected methods won't be available for statically compiled code to call.
So if you were really crazy, you could do something like what they outline here. What you could do is load the .java file, find the correct insertion point, add whatever methods you need to, call the java compiler and reload the class. Good luck debugging that mess though :)
Edit This actually might be of some use...
You can do some quite funky things with AOP, although genuine modification of classes at runtime is a pretty hairy technique that needs a lot of classloading magic and sleight of hand.
What is easier is using AOP techniques to generate a subclass of your target class and to introduce new methods into this instead, what AOP called a "mixin" or "introduction". See here to read how Spring AOP does it, although this may be quite lame compared to what you're actually trying to achieve.
Is it possible by just doing using Java?
Quite so, the "only" thing you have to do is define an instrumentation agent which supplies an appropriate ClassFileTransformer, and you'll have to use reflection to invoke the added methods. Odds are this isn't what you want to do, though, but it's doable and there's a well-defined interface for it. If you want to modify existing methods you may be interested in something like AspectJ.
While it might be possible, it is not useful.
How would you access these new fields and methods?
You could not use these methods and fields directly (as "ordinary" fields and methods), since they wouldn't be compiled in.
If all you want is the possibility to add "properties" and "methods", you can use a Map<String, Object> for the "dynamic properties", and a Map<String, SuitableInterface> for the "dynamic methods", and look them up by name.
If you need an extension language for Java, an embedded dynamic language (such as Javascript, or Groovy) can be added; most of these can access arbitrary java objects and methods.
Related
In golang, interfaces are extremely important for decoupling and composing code, and thus, an advanced go program might easily define 1000s of interfaces .
How do we evolve these interfaces over time, to ensure that they remain minimal?
Are there commonly used go tools which check for unused functions ?
Are there best practices for annotating go functions with something similar to java's #Override, which ensures that a declared function is properly implementing a expected contract?
Typically in the java language, it is easy to keep code tightly bound to an interface specification because the advanced tooling allows us to find and remove functions which aren't referenced at all (usually this is highlighted automatically for you in any common IDE).
Are there commonly used go tools which check for unused functions ?
Sort of, but it is really hard to be sure for exported interfaces. oracle can be used to find references to types or methods, but only if you have all of the code that references you availible on your gopath.
can you ensure a type implements a contract?
If you attempt to use a type as an interface, the compiler will complain if it does not have all of the methods. I generally do this by exporting interfaces but not implementations, and making a constructor:
type MyInterface interface{
Foo()
}
type impl struct{}
func (i *impl) Foo(){}
func NewImpl() MyInterface{
return &impl{}
}
This will not compile if impl does not implement all of the required functions.
In go, it is not needed to declare that you implement an interface. This allows you to implement an interface without even referencing the package it is defined in. This is pretty much exactly the opposite of "tightly binding to an interface specification", but it does allow for some interesting usage patterns.
What your asking for isn't really a part of Go. There are no best practices for annotating that a function satisfies an interface. I would personally say the only clear best practice is to document which interfaces your types implement so that people can know. If you want to test explicitly (at compile time) if a type implements an interface you can do so using assignment, check out my answer here on the topic; How to check if an object has a particular method?
If you're just looking to take inventory of your code base to do some clean up I would recommend using that assignment method for all your types to generate compile time errors regarding what they don't implement, scale down the declarations until it compiles. In doing so you should become aware of the disparity between what might be implemented and what actually is.
Go is also lacking in IDE options. As a result some of those friendly features like "find all references" aren't there. You can use text searching tricks to get around this, like searching func TheName to get only the declaration and .TheName( to get all invocations. I'm sure you'll get used to it pretty quickly if you continue to use this tooling.
From wikipedia:
reflection is the ability of a computer program to examine and modify the structure and behavior (specifically the values, meta-data, properties and functions) of an object at runtime.
Can anyone give me a concrete example of modifying the structure of an object? I'm aware of the following example.
Object foo = Class.forName("complete.classpath.and.Foo").newInstance();
Method m = foo.getClass().getDeclaredMethod("hello", new Class<?>[0]);
m.invoke(foo);
Other ways to get the class and examine structures. But the questions is how modify is done?
Just an additional hint since the previous answers and comments answer the question concerning reflection.
To really change the structur of a class and therefore its behaviour during runtime look at Byte code instrumentaion and in this case javassist and asm libs. In any case this is not trivial task.
Additionally you might have a look at aspect programming technic, which enables you to enhance methods with some functionallity. Often used to introduce logging without the need to have a dependency of the logging classes within your class and also dont have the invocations of the logging methods between the problem related code.
In English reflection means "mirror image".
So I'd disagree with the Wikipedia definition. For me, reflection is about runtime inspection of code, not manipulation.
In java, you can modify the bytecode at runtime using byte code manipulation. One well known library and in wide spread use is CGLIB.
In java, reflection is not fully supported as defined by the wikipedia.
Only Field.setAccessible(true) or Method.setAccessible(true) really modifies a class, and still it only changes security, not behaviour.
Frameworks like e.g. hibernate use this to add behaviour to a class by e.g. generating a subclass in bytecode that accesses private fields in the parent class.
Java is still a static typed language, unlike javascript where you can change any behaviour at runtime.
The only method in reflection (java.lang.reflect) to modify object's class behaviour is to change the accessibility flag of Constructor, Method and Field - setAccessible, whatever wiki says. Though there are libraries like http://ru.wikipedia.org/wiki/Byte_Code_Engineering_Library for decomposing, modifying, and recomposing binary Java classes
After comming from Ruby world, I'm having little problems doing TDD in Java. The biggest issue is when I have application that is just communicating with external API.
Say I want to just fetch some data from Google Calendar, or 5 tweets from some Twitter user and display it.
In Ruby, I don't have any problems, because I can monkey-patch the API library in tests directly, but I have no such option in Java.
If I think about this in terms of MVC, my model objects are directly accessing the API through some library. The question is, is this bad design? Should I always wrap any API library in some interface, so I can mock/stub it in Java?
Because when I think about this, the only purpose of that interface would be to simulate (please don't kill me for saying this) the monkey-patch. Meaning that any time I use any external resource, I have to wrap each layer in interface that can be stubbed out.
# do I have to abstract everything just to do this in Java?
Twitter.stub!(:search)
Now you might say that I should always abstract away the interface, so I can change the underlying layer to anything else. But if I'm writing twitter app, I'm not going to change it to RSS reader.
Yes, I can add for example Facebook and then it would make sense to have interface. But when there is no other resource that can be substituted for the one I'm using, than I still have to wrap everything in interfaces to make it testable.
Am I missing something, or is this just a way to test in the Java world?
Using interfaces is just generally good practice in Java. Some languages have multiple inheritance, others have duck typing, Java has interfaces. It's a key feature of the language, it lets me use
different aspects of a class in different contexts and
different implementations of the same contract without changing client code.
So interfaces are a concept you should embrace in general, and then you would reap the benefits in situations like this where you could substitute your services by mock objects.
One of the most important books about Java best practices is Effective Java by Joshua Bloch. I would highly suggest you to read it. In this context the most important part is Item 52: Refer to objects by their interfaces. Quote:
More generally, you should favor the use of interfaces rather than
classes to refer to objects. If appropriate interface types exist, then parameters, return values, variables, and fields should all be declared using interface
types. The only time you really need to refer to an object’s class is when you’re
creating it with a constructor.
And if you take things even further (e.g. when using dependency injection), you aren't even calling the constructor.
One of the key problems of switching languages is that you have to switch the way of thinking too. You can't program language x effectively while thinking in language y. You can't program C effectively without using pointers, Ruby not without duck typing and Java not without Interfaces.
Wrapping the external API is the way I would do this.
So, as you already said, you would have an interface and two classes: the real one and the dummy implementation.
Yes, it may seem unreasonable from the perspective of some services indeed being specific, like Twitter. But, this way your build process doesn't depend on external resources. Depending on external libraries isn't all that bad, but having your tests depend on actual data present or not present out there on the web can mess up the build process.
The easiest way is to wrap the API service with your interface/class pair and use that throughout your code.
I understand that what you want are Mock objects.
As you described it, one of the ways one can generate "test versions" of objects is by implementing a common interface and using it.
However, what you are missing is to simply extend the class (provided that it is not declared final) and override the methods that you want to mock. (NB: the possibility of doing that is the reason why it is considered bad form for a library to declare its classes final - it can make testing considerably harder.)
There is a number of Java libraries that aim in facilitating the use of Mock objects - you can look at Mockito or EasyMock.
Mockito is more handy and like your ruby mocks.
You can "monkey-patch" an API in Java. The Java language itself does not provide specific means to do it, but the JVM and the standard libraries do. In Ruby, developers can use the Mocha library for that. In Java, you can use the JMockit library (which I created because of limitations in older mocking tools).
Here is an example JMockit test, equivalent to the test_should_calculate_value_of_unshipped_orders test available in Mocha documentation:
#Test
public void shouldCalculateValueOfUnshippedOrders()
{
final Order anOrder = new Order();
final List<Order> orders = asList(anOrder, new Order(), new Order());
new NonStrictExpectations(Order.class)
{{
Order.findAll(); result = orders;
anOrder.getTotalCost(); result = 10;
}};
assertEquals(30, Order.unshippedValue());
}
Suppose I have an interface with lots of methods that I want to mock for a test, and suppose that I don't need it to do anything, I just need the object under test to have an instance of it. For example, I want to run some performance testing/benchmarking over a certain bit of code and don't want the methods on this interface to contribute.
There are plenty of tools to do that easily, for example
Interface mock = Mockito.mock(Interface.class);
ObjectUnderTest obj = ...
obj.setItem(mock);
or whatever.
However, they all come with some runtime overhead that I would rather avoid:
Mockito records all calls, stashing the arguments for verification later
JMock and others (I believe) require you to define what they going to do (not such a big deal), and then execution goes through a proxy of various sorts to actual invoke the method.
Good old java.lang.reflect.Proxy and friends all go through at least a few more method calls on the stack before getting to the method to be invoked, often reflectively.
(I'm willing to be corrected on any of the details of those examples, but I believe the principle holds.)
What I'm aiming for is a "real" no-op implementation of the interface, such as I could write by hand with everything returning null, false or 0. But that doesn't help if I'm feeling lazy and the interface has loads of methods. So, how can I generate and instantiate such a no-op implementation of an arbitrary interface at runtime?
There are tools available such as Powermock, CGLib that use bytecode generation, but only as part of the larger mocking/proxying context and I haven't yet figured out what to pick out of the internals.
OK, so the example may be a little contrived and I doubt that proxying will have too substantial an impact on the timings, but I'm curious now as to how to generate such a class. Is it easy in CGLib, ASM?
EDIT: Yes, this is premature optimisation and there's no real need to do it. After writing this question I think the last sentence didn't quite make my point that I'm more interested in how to do it in principle, and easy ways into dynamic class-generation than the actual use-case I gave. Perhaps poorly worded from the start.
Not sure if this is what you're looking for, but the "new class" wizard in Eclipse lets you build a new class and specify superclass and/or interface(s). If you let it, it will auto-code up dummy implementations of all interface/abstract methods (returning null unless void). It's pretty painless to do.
I suspect the other "big name" IDEs, such as NetBeans and Idea, have similar facilities.
EDIT:
Looking at your question again, I wonder why you'd be concerned about performance of auto proxies when dealing with test classes. It seems to me that if performance is an issue, you should be testing "real" functionality, and if you're dealing with mostly-unimplemented classes anyway then you shouldn't be in a testing situation where performance matters.
It would take a little work to build the utility, but probably not too hard for basic vanilla Java interface without "edge cases" (annotations, etc), to use Javassist code generation to textually create a class at runtime that implements null versions of every method defined on the interface. This would be different from Javassist ProxyFactory (Or CGLib Enhancer) proxy objects which would still have a few layers of indirection. I think there would be no overhead in the resulting class from the direct bytecode generation mode. If you are brave you could also dive into ASM to do the same thing.
I have read about Java Reflections but till date it has been a vague concept to me. Can someone give a brief details with short example on how to use reflections in Java ?
Thanks.
I have read about Java Reflections but
till date it has been a vague concept
to me.
Here is a quick into to reflection in java:
Structural introspection. Basic reflection deals with the introspection of object at run-time. This means that you can learn the structure of objects and classes at run-time programmatically, e.g. get the class of the object, list the methods of the class, list the fields defined in the class, etc.
Reflective invocation and instantiation. With reflection you can invoke a method at run-time which is not defined at compile-time, e.g. invoke method named M on object O, where M is read in a configuration file. You can also instantiate object dynamically without knowing the class at compile-time.
Annotations. Then you can move one level up in the meta levels, and play with annotations. Annotations describe other elements such as class, method and fields. Many framework rely on this.
Dynamic proxy. Dynamic proxy can be generated at run-time. In this case, it's really like if you create a class dynamically at run-time. To use with care, but definitively handy and powerful in some cases.
I guess you will start with structural introspection. There are links to tutorials in the other answers, but I hope this gives you an overview of what else can be done.
I guess the article 'Using Java Reflection' found on sun.com might be a good starting point.
It's primarily to be used to access classes/methods/fields programmatically (i.e. during runtime instead of compiletime). Good real world API's which uses reflection intensively are ORM's like Hibernate/JPA.
You can find here a Sun tutorial on the subject (click Next link at the bottom to paginate through it).
Something worth mentioning as well is Javassist. Not only does it have reflective abilities, but it also allows run-time bytecode manipulation using ordinary source syntax! Once you've gotten into reflection a bit more (which you probably have by now), you'll truly appreciate it's beauty.