My Requirement:
I have a service which takes care of persistence in my project. Let me call this service as PersistenceProvider service and lets assume it's residing in "my.persistenceservice" bundle.
Now, I have another bundle named "my.persitenceconsumer" bundle which referencing PersistenceProvider service using bind() unbind() method of one of the class named MyPersistenseConsumer. So when "my.persistenceconsumer" bundle starts, it will get reference of PersistenceProvider service using bind() method and MyPersistenceConsumer can use PersistenceProvider service
But, I also need to use this PersistenceProvider service from different classes in "my.persitenceconsumer" bundle.
My QUESTION is:
What is the best way to use such shared service within different classes (in same bundle)
One of the solution:
I can add Activator class in "my.persitenceconsumer" bundle .. having static getInstance() method. Which can be called by MyPersistenceConsumer.bind() and stores PersistenceProvider with Activator. Latter all classes in "my.persitenceconsumer" bundle can use PersistenceProvider using Activator class.
Here is the code:
public class MyPersistenceConsumer {
public void bindPersistenceProvider(PersistenceProvider ppRef) {
MyPersistenceConsumerActivator.getInstance().bindPersistenceProvider(ppRef);
}
}
public class MyPersistenceConsumerActivator {
static MyPersistenceConsumerActivator instance;
PersistenceProvider ppRef;
public static getInstance() {
return instance;
}
public void bindPersistenceProvider(PersistenceProvider ppRef) {
this.ppRef = ppRef;
}
public PersistenceProvider getPersistenceProvider() {
return ppRef;
}
public void start(BundleContext context) throws Exception {
instance = this;
}
}
public class MyClass1 {
public void usePersistenceProvider(){
PersistenceProvider pp Ref =
MyPersistenceConsumerActivator.getInstance().getPersistenceProvider();
}
}
public class MyClass2 {
public void usePersistenceProvider(){
PersistenceProvider pp Ref =
MyPersistenceConsumerActivator.getInstance().getPersistenceProvider();
}
}
At Last:
Does above is good way .. or is there a better way?
Singletons are an evilness that DS tries to eliminate. Singletons create brittle systems, they have the same problems as global vars.
The best solution is no-coupling. With DS I would use:
#Component
public class MyClass1 {
#Reference
void setPP( PersistenceProvider pp ) { ... }
}
And the same for MyClass2. If there is an instance relation between classes, pass the object around since you're already coupled then. As always, minimise coupling and maximise cohesion.
Factories and singletons are the exact evils OSGi tries to prevent for very good reasons.
There are two possibilities I would recommend:
Make the objects that use the persistence-service declarative services. This the SCR will handle all the lifecycle-management and you will get very modular and testable classes.
Alternatively, you can pass a reference to the persistence-service in the constructor of these objects. The only problem here is that you have to make sure that these objects to not try to access the persistence-service after it has gone away.
Either of these approaches is to be preferred over the singleton-based solution you proposed.
Related
If there are more than one implementation of an interface, then I would have to use #Qualifier to indicate which class I want to autowire with. Wouldn't it make more sense to autowire the class directly instead of interface?
This is what object oriented programming (especially abstraction and polymorphism) is about.
You build the classes independendent of concrete implementations and use an interface.
This allows you to change what implementation you use at any point.
Assume you have this:
public interface I{
void doSomething();
}
#Component("a")
public class A implements I{
public void doSomething(){
//...
}
public void doSomethingElse(){
//...
}
}
#Component("b")
public class B implements I{
public void doSomething(){
//...
}
}
If you use
#Autowired
private A yourVariable;
you might be tempted to use the doSomethingElse() method that is an implementation detail (by what reason whatsoever).
However, if you use
#Qualifier("a")
private I yourVariable;
this cannot happen as the method is not present in the interface.
This gives you the ability to swap the implementation at any time.
Wouldn't it make more sense to autowire the class directly instead of interface?
This pretty much depends on how exactly do you work with qualifiers but in general, the answer is "No", autowiring the class is a bad thing to do if you have an interface - you should always work by interface.
Here are the examples:
interface I { void foo(); }
class IImpl1 implements I {
void foo() {System.out.println("Impl1");
pubic void bar() {// another method - why not?}
}
class IImpl2 implements I { void foo() {System.out.println("Impl2"); }
Note, that an implementation IImpl1 has an additional method bar that doesn't belong to the interface I
Case 1:
#Component
public class Driver {
#Autowired
private IImpl1 i;
}
Here class Driver is tightly could to the concrete implementation IImpl1 in general I can call method bar from the Driver class because I have this implementation, but in this case if I'll have to switch the implementation of Driver in future you'll have to also change the code: both change the reference, and also get rid of calls to IImpl1.bar() that might be not that easy to do - its a change in logic. As a result, the whole polymorphic behavior is lost.
By far its the worst way to program.
Now, consider Case 2:
#Component
public class Driver {
#Autowired
#Qualifier("someLogicalName") // under the hood it spring will map it to IImpl1 but you don't know that here
I i;
}
In this case - the things are much better - you never couple the driver to the concrete implementation at the level of code. This means that in general its enough to change the qualifier to get another implementation. You can't call bar anymore - it won't compile.
You also do not know what the implementation will be injected. If in future the implementation of the component that actually implements the logic of "someLogicalName" will change (by that I mean that you'll create a new class IImpl3), you won't feel it here - you'll place that qualifier on that new class and all the places that use it will get that implementation automatically.
Case 3:
#Configuration
public class MyConfiguration {
#Bean
public Driver driver(#Qualifier("someLogicalName") I i) {
return new Driver(i);
}
}
public class Driver {
private final I i;
public Driver(I i) { // in real life this constructor can be generated by
// lombok or something, put it here for the sake of completeness
this.i = i;
}
}
This by far the most "clean" approach - conceptually its really similar to case 2, however in this case you don't place the qualifier at the level of Driver class at all, in fact, the class Driver is not even aware at the level of code / metadata that its related to Spring anyhow. Of course all the "advantages" of case 2 are also applicable here as well.
Context
An application that utilizes Spring 4.1.7. All configurations are in XML files (not using annotations) and I rather keep it that way (but I can change the ways things are done if I must).
Problem
I have created a new class that comes with a builder class.
Now I'd like to inject other beans into this new class. I can probably use lookup-methods and similar solutions to do that and then use the new class's builder in the caller beans to create an instance. However, I rather an instance of this new class to be injected to its caller beans then they creating one through the builder. This is where I'm not sure how I can do that. For example, this looks like an Abstract Factory to me, but I don't know how I can pass those parameters (which are passed to the builder) at runtime to the Abstract Factory and subsequently the factories it builds.
Some code snippets to make the question clearer:
public final class Processor {
private final StatusEnum newStatus;
private final Long timeOut;
// I'd like this to be be injected by Spring through its setter (below)
private DaoBean daoInstance;
private Processor() {
this.newStatus = null;
this.timeOut = null;
}
private Processor(Builder builder) {
this.newStatus = builder.getNewStatus();
this.timeOut = builder.getTimeOut();
}
// To be called by Spring
public void setDaoInstance(DaoBean instance) {
this.daoInstance = instance;
}
public void updateDatabase() {
daoInstance.update(newStatus, timeOut);
}
// Builder class
public static final class Builder {
private StatusEnum newStatus;
private Long timeOut;
// lots of other fields
public Long getTimeOut() {
return this.timeOut;
}
public StatusEnum getNewStatus() {
return this.newStatus;
}
public Builder withTimeOut(Long timeOut) {
this.timeOut = timeOut;
return this;
}
public Builder withNewStatus(StatusEnum newStatus) {
this.newStatus = newStatus;
return this;
}
public Processor build() {
return new Processor(this);
}
}
}
I'd like an instance of "DaoBean" to be injected to the "Processor" class. But to do that, Processor will have to be a bean or otherwise I have to utilize something like lookup-methods. On the other hand, wherever I want to use processor, I have to do something like this:
new Processor.Builder()
.withTimeOut(1000L)
.withNewStatus(StatusEnum.UPDATED)
.build()
.updateDatabase();
Instead of this, I wonder if I can make the Processor a bean that Spring can inject to its callers whilst maintaining its immutability. An instance of DaoBean can then be injected to the Processor by Spring. That way I'd be able to segregate the wiring code and the business logic.
It's worth mentioning that the Builder has a lot more than 2 fields and not all of them have to be set. This is why I thought an abstract factory is the way to go (building instances of the Processor in different ways).
One solution, while keeping the builder, would probably be to simply making the Builder itself a Spring bean...
This allows something like this..
#Autowired
private Builder builder;
public void someMethod() {
Result = builder.withX(...).doSomething();
}
This way, your Result object is immutable, can be created via a nice builder and the builder can inject the Spring bean (dao, in your case) into it without anyone even noticing that it's there.
And the only thing that changes is, that you don't create the builder yourself, but let Spring create it for you...
#Component
#Scope("prototype") // normally a good idea
public static class Builder {
#Autowired
private DaoBean dao;
// your logic here
}
(Same works with JavaConfig or XML config, if you don't want to scan.)
Especially with many combinations, I prefer a builder pattern, since a factory would need complex method signatures. Of course, the builder has the disadvantage that you cannot check at compile time if a given combination of attribute types is at least theoretically acceptable. Ok, you could simulate that with various builders, but that would probably be overkill.
I am trying to implement DI with Castle Windsor. Currently I have a controller with overloaded constructors like this (this is an antipattern as described here: https://www.cuttingedge.it/blogs/steven/pivot/entry.php?id=97):
public class MyController : ApiController
{
protected IStorageService StorageService;
protected MyController()
{
StorageService = StorageServiceFactory.CreateStorageService(User.Identity as ClaimsIdentity);
}
protected MyController(IStorageService storageService)
{
StorageService = storageService;
}
}
I am trying to get rid of the first constructor and have Castle Windsor handle the resolution of the storage service dependency.
I created a Castle Windsor installer class like this:
public class StorageServiceInstaller : IWindsorInstaller
{
public void Install(IWindsorContainer container, IConfigurationStore store)
{
container.Register(
Component.For<IStorageService>()
.UsingFactoryMethod(
() => StorageServiceFactory.CreateStorageService(User.Identity as ClaimsIdentity)));
}
}
The problem is that User (which has type IPrincipal) is a property on ApiController, so it's not accessible from the installer. How can I make this work?
Update:
#PatrickQuirk seems to be implying that there is a better way to do this using Castle Windsor without needing a factory at all.
My StorageServiceFactory looks like this:
public static class StorageServiceFactory
{
public static IStorageService CreateStorageService(ClaimsIdentity identity)
{
if (identity == null)
{
return null;
}
Claim providerKeyClaim = identity.FindFirst(ClaimTypes.NameIdentifier);
if (providerKeyClaim == null || string.IsNullOrEmpty(providerKeyClaim.Value))
{
return null;
}
StorageProviderType storageProviderType;
string storageProviderString = identity.FindFirstValue("storage_provider");
if (string.IsNullOrWhiteSpace(storageProviderString) || !Enum.TryParse(storageProviderString, out storageProviderType))
{
return null;
}
string accessToken = identity.FindFirstValue("access_token");
if (string.IsNullOrWhiteSpace(accessToken))
{
return null;
}
switch (storageProviderType)
{
// Return IStorageService implementation based on the type...
}
}
}
Is there a way to incorporate selecting the correct IStorageService into Windsor's dependency resolution and avoid the factory altogether? Or do I still need it?
I like #PatrickQuirk's solution, except that it seems odd to have to create a wrapper and corresponding wrapper interface for the factory just for the sake of dependency injection. Ideally I'd have the api controller's constructor take in an IStorageService as a parameter, which seems more intuitive/consistent with the field that actually needs to be set.
I don't think the multiple constructors is as much of a sin as the hidden dependency on StorageServiceFactory is, but I agree with your approach for the most part.
Instead of a factory method, pass a factory object into the class and have it create the storage service:
public class MyController : ApiController
{
protected IStorageService StorageService;
protected MyController(IStorageServiceFactory storageServiceFactory)
{
StorageService = storageServiceFactory.CreateStorageService(User.Identity as ClaimsIdentity);
}
}
And then define your factory interface and implementation:
public interface IStorageServiceFactory
{
IStorageService Create(ClaimsIdentity claimsIdentity);
}
public class StorageServiceFactoryImpl : IStorageServiceFactory
{
public IStorageService Create(ClaimsIdentity claimsIdentity)
{
return StorageServiceFactory.CreateStorageService(claimsIdentity);
}
}
This way, you have a single constructor and the dependency on the storage service factory is explicit.
Regarding your update:
...it seems odd to have to create a wrapper and corresponding wrapper interface for the factory just for the sake of dependency injection.
Well, that's kind of the point of dependency injection.
The wrapper I propose is solving two problems: it removes the need to call a static method from inside your class (hiding a dependency), and allows for delayed resolution (because your dependency relies on member data to be created).
If you have a way to change the dependencies of creating an IStorageService to not rely on a member of the class you're giving it to, then you could pass one in directly (provided you can tell Windsor how to create one).
Update: Is there a way to achieve what I'm trying to do in an IoC framework other than Windsor? Windsor will handle the controllers fine but won't resolve anything else. I'm sure it's my fault but I'm following the tutorial verbatim and objects are not resolving with ctor injection, they are still null despite doing the registers and resolves. I've since scrapped my DI code and have manual injection for now because the project is time sensitive. Hoping to get DI worked out before deadline.
I have a solution that has multiple classes that all implement the same interface
As a simple example, the Interface
public interface IMyInterface {
string GetString();
int GetInt();
...
}
The concrete classes
public class MyClassOne : IMyInterface {
public string GetString() {
....
}
public int GetInt() {
....
}
}
public class MyClassTwo : IMyInterface {
public string GetString() {
....
}
public int GetInt() {
....
}
}
Now these classes will be injected where needed into layers above them like:
public class HomeController {
private readonly IMyInterface myInterface;
public HomeController() {}
public HomeController(IMyInterface _myInterface) {
myInterface = _myInterface
}
...
}
public class OtherController {
private readonly IMyInterface myInterface;
public OtherController() {}
public OtherController(IMyInterface _myInterface) {
myInterface = _myInterface
}
...
}
Both controllers are getting injected with the same interface.
When it comes to resolving these interfaces with the proper concrete class in my IoC, how do I differentiate that HomeController needs an instance of MyClassOne and OtherController needs an instance of MyClassTwo?
How do I bind two different concrete classes to the same interface in the IoC? I don't want to create 2 different interfaces as that breaks the DRY rule and doesn't make sense anyway.
In Castle Windsor I would have 2 lines like this:
container.Register(Component.For<IMyInterface>().ImplementedBy<MyClassOne>());
container.Register(Component.For<IMyInterface>().ImplementedBy<MyClassTwo>());
This won't work because I will only ever get a copy of MyClassTwo because it's the last one registered for the interface.
Like I said, I don't get how I can do it without creating specific interfaces for each concrete, doing that breaks not only DRY rules but basic OOP as well. How do I achieve this?
Update based on Mark Polsen's answer
Here is my current IoC, where would the .Resolve statements go? I don' see anything in the Windsor docs
public class Dependency : IDependency {
private readonly WindsorContainer container = new WindsorContainer();
private IDependency() {
}
public IDependency AddWeb() {
...
container.Register(Component.For<IListItemRepository>().ImplementedBy<ProgramTypeRepository>().Named("ProgramTypeList"));
container.Register(Component.For<IListItemRepository>().ImplementedBy<IndexTypeRepository>().Named("IndexTypeList"));
return this;
}
public static IDependency Start() {
return new IDependency();
}
}
I hope you can use service overrides.
Ex.
container.Register(
Component.For<IMyService>()
.ImplementedBy<MyServiceImpl>()
.Named("myservice.default"),
Component.For<IMyService>()
.ImplementedBy<OtherServiceImpl>()
.Named("myservice.alternative"),
Component.For<ProductController>()
.ServiceOverrides(ServiceOverride.ForKey("myService").Eq("myservice.alternative"))
);
public class ProductController
{
// Will get a OtherServiceImpl for myService.
// MyServiceImpl would be given without the service override.
public ProductController(IMyService myService)
{
}
}
You should be able to accomplish it with named component registration.
container.Register(Component.For<IMyInterface>().ImplementedBy<MyClassOne>().Named("One"));
container.Register(Component.For<IMyInterface>().ImplementedBy<MyClassTwo>().Named("Two"));
and then resolve them with
kernel.Resolve<IMyInterface>("One");
or
kernel.Resolve<IMyInterface>("Two");
See: To specify a name for the component
Typically DI containers follow Register, Resolve and Release patterns. During the register phase there are two steps. The first is to specify the mapping as you are doing. The second step is to specify the rules which govern which to inject where.
This problem is very common when we try to address Cross cutting concerns using decorators. In these situations, you have multiple classes(decorators) implementing a single interface.
Briefly, we need to implement IModelInterceptorsSelector which allows you to write imperative code that decides which Interceptor to apply to which types or members.
This is elaborately described in the book Dependency Injection in .Net book by Mark Seemann. Look for chapter 9 interception or search for the above interface.
I am not an expert at this, but was searching for the exact same problem and found the ans in the above book.
Hope this helps.
Regards
Dev1
In Spring 3 it is not possible to set #Autowired in either static fields or methods, so since I want to declare an utility class such as:
public class SchoolYearServiceUtil {
private static SchoolYearService schoolYearService;
public static SchoolYear getSchoolYear(Long id) {
return schoolYearService.get(id);
}
}
to avoid having to inject the schoolYearService everywhere (jsp, command class...) in which I need it. In this case, I don't need an interface to be implemented by SchoolYearServiceUtil.
I don't want to have to initialize the object through code but getting the same instance as the Spring's one.
Which would be the best option to implement the getSchoolYear as a static method?
Thanks.
Would this be conceptually wrong?:
#Component
public class SchoolYearServiceUtil {
private static SchoolYearService schoolYearService;
#Autowired(required = true)
private SchoolYearServiceUtil(#Qualifier("schoolYearServiceImpl") SchoolYearService schoolYearService) {
SchoolYearServiceUtil.schoolYearService = schoolYearService;
}
public static SchoolYearService getSchoolYearService() {
return schoolYearService;
}
public static SchoolYear getSchoolYear(Long id) {
return getSchoolYearService().get(id);
}
}
I would have to make sure that only Spring calls once the constructor and the constructor is called nowhere else, that's why I declared the constructor as private.
I fully support skaffman's comment. You don't need static fields with DI. You just define a bean of scope singleton (default).
There is a way to obtain a bean statically, but you should be aware that it is not to be used in regular situations. (there are some valid applications). It is to use the WebApplicationContextUtils.getRequiredWebApplicationContext(servletContext)
You notice that you need to pass a ServletContext argument.