I'm new to OSGI framework and I'm trying to access the 'Derived' Class variable 'publicVariable' from another class 'Derived2' like "Derived.publicVariable" but publicVariable is always shows null. I really appreciate if someone can help me out with this.
Thanks
Manifest file - Derived2
Require-Bundle:com.xxxxxx.Derived1
Java code
abstract class Base {
protected Vector <String> supportedCommands = new Vector <String> ();
protected abstract void initialiseCommands();
}
class Derived extends Base {
private static Derived derivedPlugin = null;
public Derived()
{
derivedPlugin = this;
}
public static Derived getPlugin()
{
return derivedPlugin;
}
public String publicVariable = null;
protected void initialiseCommands()
{
publicVariable = "someData";
System.out.println("Derived" + publicVariable);
}
}
class Derived2 extends Base {
protected void initialiseCommands()
{
supportedCommands.add(Derived.getPlugin().publicVariable);
System.out.println("IMRSAUtilitiesPlugin" +supportedCommands);
}
Also referred below link, which is a similar issue but i'm not using any static variable, it is just a public variable.
how use Singleton object in different class loader....?
The code in the question will not compile. You are trying to access an instance field (publicVariable in class Derived) in a static way, i.e. Derived.publicVariable.
OSGi does not change the semantics of the Java language, and if you cannot even compile your code then OSGi will certainly not be able to run it.
Related
I'm a newbie to AspectJ, and trying to understand joinpoint model
Now i have class like this
public class Account {
private static Map<String, PaymentMethod> supportedPayments = new HashMap<>();
static {
supportedPayments.add("COD", new CodPaymentMethod());
supportedPayments.add("ATM", new ATMPaymentMethod());
}
}
as i read from AspectJ In Action, there is a way to define pointcut when class is intialization, but i could not find syntax.
Anyone help me?
This one does not work:
#Pointcut("initialization(com.jas.aop.bean.Payment"))
it say
ERROR] Syntax error on token "initialization(com.jas.aop.bean.Payment)", "name pattern" expected
/Users/admin/eclipse-workspace/aop/src/main/java/com/jas/aop/aspect/ClassInitializationAspect.java:9
#Pointcut("initialization(com.jas.aop.bean.Payment)")
Your pointcut has several problems:
There is a missing closing parenthesis ).
initialization intercepts constructors (i.e. object initialisation) rather than static class initialisation, which is not what you want and also would require a constructor pattern, not a class name pattern.
If your aspect does not happen to be in the exact same package as the target class, you must use a fully qualified class name such as my.package.Account in order to make the pointcut match.
By the way, your code snippets are just pseudo code because a hash map does not have an add method, rather a put method. The sample class does not even compile. Don't just invent code when posting questions here. Make life easier for the people trying to help you.
Now here is an MCVE, something I always suggest you to specify in your question in order to help people reproduce your situation. I did it for you this time, this was your free shot. Next time, please do it yourself.
Dependency classes used by the main class:
package de.scrum_master.app;
public interface PaymentMethod {}
package de.scrum_master.app;
public class ATMPaymentMethod implements PaymentMethod {}
package de.scrum_master.app;
public class CodPaymentMethod implements PaymentMethod {}
Target class with driver application:
package de.scrum_master.app;
import java.util.HashMap;
import java.util.Map;
public class Account {
private static Map<String, PaymentMethod> supportedPayments = new HashMap<>();
static {
System.out.println("Static initialiser block");
supportedPayments.put("COD", new CodPaymentMethod());
supportedPayments.put("ATM", new ATMPaymentMethod());
}
public Account() {
System.out.println("Creating account");
}
public void doSomething() {
System.out.println("Doing something");
}
public static void main(String[] args) {
new Account().doSomething();
}
}
Aspect:
The aspect shows both initialization and staticinitialization in order to show the difference in both syntax and functionality. You can find all of this explained with examples in the AspectJ manual, which I warmly recommend you to read.
package de.scrum_master.aspect;
import org.aspectj.lang.JoinPoint;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
#Aspect
public class AccountAspect {
#Before("initialization(de.scrum_master.app.Account.new(..))")
public void interceptObjectInitialisation(JoinPoint joinPoint) {
System.out.println(joinPoint);
}
#Before("staticinitialization(de.scrum_master.app.Account)")
public void interceptClassInitialisation(JoinPoint joinPoint) {
System.out.println(joinPoint);
}
}
Console log:
staticinitialization(de.scrum_master.app.Account.<clinit>)
Static initialiser block
initialization(de.scrum_master.app.Account())
Creating account
Doing something
I'm developing an application which is using a library and I would like to wrap this library so that it does not goes to deep into my application code. Thanks to that I could change the library I'm using just by re-implementing my wrapper classes.
Suppose that I have a library LibA. It gives me 2 objects to work with, LibAObj1 and LibAObj2. LibAObj2 has a method using LibAObj1.
Here can be a simple definition of their declaration
class LibAObj1 {};
class LibAObj2
{
void action(LibAObj1 &obj);
};
Now I would like to define an interface that my application can use to wrap those objects in my application code
For instance:
class ItfLibAObj1 {};
class ItfLibAObj2
{
public:
void action(ItfLibAObj1 &obj) = 0;
};
The problem comes whenever I want to implement my interface ItfLibAObj2.
class ImplLibAObj2 : public ItfLibAObj2
{
public:
void action(ItfLibAObj1 &itfObj)
{
<how to get my LibAObj1 from itfObj>?
obj.action(LibAObj1);
}
private:
LibObj2 obj;
}
The question is actually in the pseudo code. How to get my LibAObj1 contained in my ItfLibAObj1 reference? I could add a getter function in LibAObj1 interface to return a void pointer that I would cast but I don't find that elegant in C++.
Is there any kind of design pattern I could use to solve my problem? Or do I just have a design issue?
Note that I'm not wishing to select which library to use at run time.
Thanks a lot for your help.
Kind regards
You problem perfectly explains why in Proxy Pattern, both the Real and Proxy must implement the same interface:
And your code should look like this:
// interfaces
class ItfLibAObj1 {};
class ItfLibAObj2
{
public:
void action(ItfLibAObj1 &obj) = 0;
};
// real
class RealLibAObj1 : public ItfLibAObj1 {};
class RealLibAObj2 : public ItfLibAObj2
{
void action(ItfLibAObj1 &obj)
{
...
}
};
// proxy
class ProxyLibAObj1 : public ItfLibAObj1
{
private:
RealLibAObj1 real;
};
class ProxyLibAObj2 : public ItfLibAObj2
{
private:
RealLibAObj2 real;
void action(ItfLibAObj1 &obj)
{
// do something
real.action(obj); // delegate to the real
// do something
}
};
However, if the whole purpose of your "wrapping" is adding a new layer between your core/real and the outside (client), please consider the Facade Pattern which provides a simpler interface to the client, instead of merely mimic the classes/methods of the core.
C++/CLI :
public interface class ITest{
public:
virtual void doSomething (){
}
}
public ref Base {
...........
...........
}
generic <typename T> where T : ITest
public ref Derived : Base{
public:
virtual void doNothing (){
}
}
public ref AnotherClass {
public:
generic<class T> where T : Base
static int justDoThis(){
//Problem!!
}
}
C# :
In C# there are two classes A and B. A inherits from the ITest and B inherits from Derived where A is used as the typename. Also, B has a private variable of type A. So, from main function AnotherClass.justDoThis<B>() is called where B is passed as the generic type.
"//Problem!!" Part :
Now I have to create a new instance of B in this section and also access the A which is private variable in B.
So if I take your paragraph of description of the C# code:
class A : ITest {}
class B : Derived<A>
{
private A someVariableOfTypeA;
}
class Program
{
void Main(string[] args)
{
AnotherClass.justDoThis<B>();
}
}
And the problem is that you want to do this:
public ref AnotherClass {
public:
generic<class T> where T : Base
static int justDoThis()
{
// Problem!!
Something^ actuallyB = gcnew Something();
A^ a = actuallyB->someVariableOfTypeA;
}
}
Issue #1: You can allow creation of new objects of the generic type by specifying gcnew as another generic constraint. (In C#, this would be new.) This will require that the generic type have a default (i.e., parameterless) constructor, which you can access with the normal gcnew.
generic<class T> where T : Base, gcnew
static int justDoThis()
{
T^ t = gcnew T();
}
Issue #2: You cannot access private variables within an object. That's what private means. If you want to give justDoThis access to the A object, then add an appropriate public method or property to Base. The method or property would return type ITest. You could also put that method/property on a new interface (perhaps named IHaveAnITestAccessorMethod), and add that as another generic constraint, and B satisfies all the constraints.
Note that it won't do any good to make the variable public on type B: justDoThis doesn't know about B, it only knows about T, which is a Base with a no parameter constructor.
Disclaimers:
I didn't check my syntax with a compiler.
Yes, you can do anything with reflection, but that's a bad design. Don't do that, fix your code the right way.
I created a project to test the dependency injection offered by Google Guice in my Jax-rs resources, using Resteasy.
My intentions are:
Use multiple #ApplicationPath for the versions of my API. In each class annotated with #ApplicationPath I load a set of classes for the specific version.
Each resource have a #Inject (from Google Guice) in his constructor to inject some services.
I created two classes annotated with #ApplicationPath: ApplicationV1RS and ApplicationV2RS. In both I added the same resources classes (UserResource and HelloResource), only for my test.
My Module is configured like this:
public class HelloModule implements Module
{
public void configure(final Binder binder)
{
binder.bind(IGreeterService.class).to(GreeterService.class);
binder.bind(IUserService.class).to(UserService.class);
}
}
When I call http://localhost:9095/v1/hello/world or http://localhost:9095/v2/hello/world, I receive the same error:
java.lang.RuntimeException: RESTEASY003190: Could not find constructor
for class: org.jboss.resteasy.examples.guice.hello.HelloResource
Well, as I expected, this not works. The Google Guice is not "smart" to instantiate the resource classes using the construtor for me.
But I can't find a way to work. To be really honest, I'm really confuse about how the Google Guice, Jetty and Resteasy play with each other in this scenario.
If I abandon the idea of use #ApplicationPath, my resources work with Google Guice configuring my HelloModule like this:
public class HelloModule implements Module
{
public void configure(final Binder binder)
{
binder.bind(HelloResource.class);
binder.bind(IGreeterService.class).to(GreeterService.class);
binder.bind(UserResource.class);
binder.bind(IUserService.class).to(UserService.class);
}
}
But in this case, I'm passing the control to register my resources (HelloResource and UserResource) to Guice. It's not flexible for me, I can't setup my multiple #ApplicationPath.
So, what I'm missing or not understanding?
I created a project with the problemetic code. Is very easy to setup and test: https://github.com/dherik/resteasy-guice-hello/tree/so-question/README.md
Thanks!
When you have getClasses method in your Application then it tries to create instance for all the registered resources using the default constructor which is missing in our Resources class. One way is to create a default constructor and Inject the dependencies through setter Injection.
And then instead of overriding getClasses in ApplicationV1RS and ApplicationV2RS you override getSingletons. Since Resources can be Singleton.
Below are the changes that I made to make it work the way you want.
ApplicationV1RS.java
#ApplicationPath("v1")
public class ApplicationV1RS extends Application {
private Set<Object> singletons = new HashSet<Object>();
public ApplicationV1RS(#Context ServletContext servletContext) {
}
#Override
public Set<Object> getSingletons() {
Injector injector = Guice.createInjector(new HelloModule());
HelloResource helloResource = injector.getInstance(HelloResource.class);
UserResource userResource = injector.getInstance(UserResource.class);
singletons.add(helloResource);
singletons.add(userResource);
return singletons;
}
}
ApplicationV2RS.java
#ApplicationPath("v2")
public class ApplicationV2RS extends Application {
private Set<Object> singletons = new HashSet<Object>();
public ApplicationV2RS(#Context ServletContext servletContext) {
}
#Override
public Set<Object> getSingletons() {
Injector injector = Guice.createInjector(new HelloModule());
HelloResource helloResource = injector.getInstance(HelloResource.class);
UserResource userResource = injector.getInstance(UserResource.class);
singletons.add(helloResource);
singletons.add(userResource);
return singletons;
}
}
HelloResource.java
#Path("hello")
public class HelloResource {
#Inject
private IGreeterService greeter;
public HelloResource() {
}
#GET
#Path("{name}")
public String hello(#PathParam("name") final String name) {
return greeter.greet(name);
}
}
UserResource.java
#Path("user")
public class UserResource {
#Inject
private IUserService userService;
public UserResource() {
}
#GET
#Path("{name}")
public String hello(#PathParam("name") final String name) {
return userService.getUser(name);
}
}
Add #Singleton to your Service Classes.
Hope it helps.
I have also pushed the code to forked repo. check it out
i have a question regarding design patterns.
suppose i want to design pig killing factory
so the ways will be
1) catch pig
2)clean pig
3) kill pig
now since these pigs are supplied to me by a truck driver
now if want to design an application how should i proceed
what i have done is
public class killer{
private Pig pig ;
public void catchPig(){ //do something };
public void cleanPig(){ };
public void killPig(){};
}
now iam thing since i know that the steps will be called in catchPig--->cleanPig---->KillPig manner so i should have an abstract class containing these methods and an execute method calling all these 3 methods.
but i can not have instance of abstract class so i am confused how to implement this.
remenber i have to execute this process for all the pigs that comes in truck.
so my question is what design should i select and which design pattern is best to solve such problems .
I would suggest a different approach than what was suggested here before.
I would do something like this:
public abstract class Killer {
protected Pig pig;
protected abstract void catchPig();
protected abstract void cleanPig();
protected abstract void killPig();
public void executeKillPig {
catchPig();
cleanPig();
killPig();
}
}
Each kill will extend Killer class and will have to implement the abstract methods. The executeKillPig() is the same for every sub-class and will always be performed in the order you wanted catch->clean->kill. The abstract methods are protected because they're the inner implementation of the public executeKillPig.
This extends Avi's answer and addresses the comments.
The points of the code:
abstract base class to emphasize IS A relationships
Template pattern to ensure the steps are in the right order
Strategy Pattern - an abstract class is as much a interface (little "i") as much as a Interface (capital "I") is.
Extend the base and not use an interface.
No coupling of concrete classes. Coupling is not an issue of abstract vs interface but rather good design.
public abstract Animal {
public abstract bool Escape(){}
public abstract string SaySomething(){}
}
public Wabbit : Animal {
public override bool Escape() {//wabbit hopping frantically }
public override string SaySomething() { return #"What's Up Doc?"; }
}
public abstract class Killer {
protected Animal food;
protected abstract void Catch(){}
protected abstract void Kill(){}
protected abstract void Clean(){}
protected abstract string Lure(){}
// this method defines the process: the methods and the order of
// those calls. Exactly how to do each individual step is left up to sub classes.
// Even if you define a "PigKiller" interface we need this method
// ** in the base class ** to make sure all Killer's do it right.
// This method is the template (pattern) for subclasses.
protected void FeedTheFamily(Animal somethingTasty) {
food = somethingTasty;
Catch();
Kill();
Clean();
}
}
public class WabbitHunter : Killer {
protected override Catch() { //wabbit catching technique }
protected override Kill() { //wabbit killing technique }
protected override Clean() { //wabbit cleaning technique }
protected override Lure() { return "Come here you wascuhwy wabbit!"; }
}
// client code ********************
public class AHuntingWeWillGo {
Killer hunter;
Animal prey;
public AHuntingWeWillGo (Killer aHunter, Animal aAnimal) {
hunter = aHunter;
prey = aAnimal;
}
public void Hunt() {
if ( !prey.Escape() ) hunter.FeedTheFamily(prey)
}
}
public static void main () {
// look, ma! no coupling. Because we pass in our objects vice
// new them up inside the using classes
Killer ElmerFudd = new WabbitHunter();
Animal BugsBunny = new Wabbit();
AHuntingWeWillGo safari = new AHuntingWeWillGo( ElmerFudd, BugsBunny );
safari.Hunt();
}
The problem you are facing refer to part of OOP called polymorphism
Instead of abstract class i will be using a interface, the difference between interface an abstract class is that interface have only method descriptors, a abstract class can have also method with implementation.
public interface InterfaceOfPigKiller {
void catchPig();
void cleanPig();
void killPig();
}
In the abstract class we implement two of three available methods, because we assume that those operation are common for every future type that will inherit form our class.
public abstract class AbstractPigKiller implements InterfaceOfPigKiller{
private Ping pig;
public void catchPig() {
//the logic of catching pigs.
}
public void cleanPig() {
// the logic of pig cleaning.
}
}
Now we will create two new classes:
AnimalKiller - The person responsible for pig death.
AnimalSaver - The person responsible for pig release.
public class AnimalKiller extends AbstractPigKiller {
public void killPig() {
// The killing operation
}
}
public class AnimalSaver extends AbstractPigKiller {
public void killPing() {
// The operation that will make pig free
}
}
As we have our structure lets see how it will work.
First the method that will execute the sequence:
public void doTheRequiredOperation(InterfaceOfPigKiller killer) {
killer.catchPig();
killer.cleanPig();
killer.killPig();
}
As we see in the parameter we do not use class AnimalKiller or AnimalSever. Instead of that we have the interface. Thank to this operation we can operate on any class that implement used interface.
Example 1:
public void test() {
AnimalKiller aKiller = new AnimalKiller();// We create new instance of class AnimalKiller and assign to variable aKiller with is type of `AnimalKilleraKiller `
AnimalSaver aSaver = new AnimalSaver(); //
doTheRequiredOperation(aKiller);
doTheRequiredOperation(aSaver);
}
Example 2:
public void test() {
InterfaceOfPigKiller aKiller = new AnimalKiller();// We create new instance of class AnimalKiller and assign to variable aKiller with is type of `InterfaceOfPigKiller `
InterfaceOfPigKiller aSaver = new AnimalSaver(); //
doTheRequiredOperation(aKiller);
doTheRequiredOperation(aSaver);
}
The code example 1 and 2 are equally in scope of method doTheRequiredOperation. The difference is that in we assign once type to type and in the second we assign type to interface.
Conclusion
We can not create new object of abstract class or interface but we can assign object to interface or class type.