Simplified example to give you an idea, hope it's be clear.
I've already added inheritance for Service class to avoid switches I'm having now
class Config {}
class ConfigA extends Config {}
class ConfigB extends Config {}
// service class - different implementation for configA and ConfigB
// normally it would look like
class ServiceA {
public String run(ConfigA configA) {}
}
thus next then I need sth like
class ServiceRunner {
public String run(Config config) {
// if or switch doesn't matter now
if (config instanceof ConfigA) {
return serviceA.run((ConfigA)config);
}
}
}
// main
Config config = configFactory.create(...) // returns ConfigA or ConfigB
String result = serviceRunner.run(config);
Is there a better way to code it I mean without casting?
The only solution I can see is:
interface Service { String run(); }
#RequestScope
class ServiceA implements Service {
private ConfigA config;
public ServiceA(ConfigA configA) {this.configA = configA}
public String run() {
...
}
}
but I'm not convinced it's a good idea to implement service beans as state beans and I'm using CDI (quarkus actually) for DI which it seems doesn't support assisted injection via constructor
Why don't you hide the detail about which Config a given Service handles inside the Service itself? By doing so you could have something like the following:
interface Service {
boolean handlesConfig(Config config)
String run(Config config);
}
class ServiceRunner {
private List<Service> services;
public String run(Config config) {
for (service : services) {
if (service.handles(config)) {
return service.run(config);
}
}
}
}
Seems like it's a case for Bridge pattern. You have parallel hierarchies of service and config. So if tomorrow there is a ServiceC, there would supposedly be a ConfigC. There might be an abstraction which is common in both service and config. Try to find it and abstract it out. Then service would be using that abstraction. And ConfigA, ConfigB would be impls of that abstraction.
Or perhaps, as the replier above mentioned, service should be programmed to the abstract config instead of impls.
Does the Configs have different types of interfaces that can't be extracted into a common interface? In that case, it is violating the Liskov's substitution principle, which requires that all subclasses should be interchangeable. e.g. the hierarchy of Shape <- Rectangle <- Square is wrong, because square is not a type of rectangle (programmatically, of course) - because Rectangle has 2 dimensions, namely length and breadth, while square has one dimension of length only. Making this hierarchy would break the abstraction. Similarly, it might be your case that the hierarchy of Config may not be a hierarchy at all.
Related
In a Spring application that uses HTTP remoting, I have a service façade module configured as follows (I made the code generic to improve clarity):
#Configuration
public class MyFacadeConfig {
private HttpInvokerServiceExporter facade(Class<?> cls) {
HttpInvokerServiceExporter bean = new HttpInvokerServiceExporter();
// The service referred to by this exporter is already instantiated as another Spring bean with all its dependencies.
bean.setService(appContext.getBean(cls));
bean.setServiceInterface(cls);
return bean;
}
#Bean("/first.service")
public HttpInvokerServiceExporter firstServiceFacade() {
return facade(FirstService.class);
}
#Bean("/second.service")
public HttpInvokerServiceExporter secondServiceFacade() {
return facade(SecondService.class);
}
// ... and so on for the 37 other services
}
where FirstService and SecondService are interfaces with existing implementations whose detail is not needed here.
I have another module that defines 39 proxies (instances of HttpInvokerProxyFactoryBean) corresponding to each of my services exposed through my façade.
So far, everything works properly.
But I would like to make the code more generic, elegant, and robust while mitigating the risk of error (e.g., a bad mapping between a service and its proxy in the future). The way I would like to do this is as follows:
First, I move the façade/proxy metadata into an enumeration:
public enum ConfigBeansFacade {
FIRST("/first", FirstService.class),
SECOND("/second", SecondService.class)
// ... and so on for the 37 other services
;
private String beanName;
private Class<?> serviceInterface;
// Constructor and getters
public String getCompleteBeanName() {
return beanName + ".service";
}
}
Then the configuration of the façade would be simplified in a style similar to the following:
#Configuration
public class MyFacadeConfig {
#Autowired
private ConfigurableBeanFactory beanFactory;
#Autowired
public void configExporters() {
for (ConfigBeansFacade bean : ConfigBeansFacade.values()) {
HttpInvokerServiceExporter exp = new HttpInvokerServiceExporter();
exp.setService(beanFactory.getBean(bean.getServiceInterface()));
exp.setServiceInterface(bean.getServiceInterface());
beanFactory.registerSingleton(bean.getCompleteBeanName(), exp);
}
}
}
I tried every single recipe I found in online forums, including StackOverflow, but there are two constraints not met elsewhere:
When defining the exporters, the underlying services are other Spring beans that are instantiated, initialized, and registered with their own configuration and dependencies through the standard Spring mechanics. There is no direct class instantiation other than the exporters themselves.
I thought about grouping the exporters into a single collection as suggested by some people. The only problem is that Spring MVC uses the HttpInvokerServiceExporter Spring bean names as endpoint URIs when registering the exporters into its own configuration. I must therefore register each exporter as a “first-class citizen” bean with its own bean name into the application context.
Given these constraints, the problem I have arises in (1) when I try to retrieve the underlying services to be encapsulated into exporters: they are not necessarily ready yet, which results into UnsatisfiedDependencyExceptions.
I tried solutions with a #PostContruct-annotated method, with a BeanPostProcessor, with an #Autowired method (as shown above), nothing is working as required.
Does anyone know about a way or a technique to initialize and register multiple beans inside a single method under my constraints described above? Such a method doesn't need to be annotated with #Bean, #Autowired, or any other specific annotation, it's just an example of what I tried.
In the client module, mercifully, the HttpInvokerProxyFactoryBean instances need only the interfaces and the bean names, so constraint (1) above should not apply.
Thanks in advance for any help you can provide...
I'm not 100% I've understood what you're trying to do but I wonder if you could try autowiring a List of beans that implement an interface?
e.g.
public interface MyService {
String getKey();
void doStuff();
}
Then implement as many of these as you require
e.g.
#Component
public class FirstService implements MyService {
public String getKey() {
return "/first";
}
public void doStuff() {
...
}
}
then have a factory bean with the autowired list
#Component
public class MyServiceFactory {
private final List<MyService> services;
#Autowired
public MyServiceFactory(List<MyService> services) {
this.services = services;
}
}
To add more implementations of MyService, simply add them as #Component and Spring magically picks them up and adds them to the list.
Sometimes I find it useful to access my implementations via a Map
#Component
public class MyServiceFactory {
private final Map<String, MyService> services;
#Autowired
public MyServiceFactory(List<MyService> services) {
this.services = services
.stream()
.collect(toMap(MyService::getKey, Function.identity()));
}
public MyService getServiceByKey(String key) {
return services.get(key);
}
}
I find this keeps each implementation nice and self contained (and easy to test). Spring automatically picks up all the components that implement my interface without the factory having a huge number of imports. And I can test the factory easily by mocking the list of implementations.
I am working on a REST API where I have an interface that defines a list of methods which are implemented by 4 different classes, with the possibility of adding many more in the future.
When I receive an HTTP request from the client there is some information included in the URL which will determine which implementation needs to be used.
Within my controller, I would like to have the end-point method contain a switch statement that checks the URL path variable and then uses the appropriate implementation.
I know that I can define and inject the concrete implementations into the controller and then insert which one I would like to use in each particular case in the switch statement, but this doesn't seem very elegant or scalable for 2 reasons:
I now have to instantiate all of the services, even though I only need to use one.
The code seems like it could be much leaner since I am literally calling the same method that is defined in the interface with the same parameters and while in the example it is not really an issue, but in the case that the list of implementations grows ... so does the number of cases and redundant code.
Is there a better solution to solve this type of situation? I am using SpringBoot 2 and JDK 10, ideally, I'd like to implement the most modern solution.
My Current Approach
#RequestMapping(Requests.MY_BASE_API_URL)
public class MyController {
//== FIELDS ==
private final ConcreteServiceImpl1 concreteService1;
private final ConcreteServiceImpl2 concreteService2;
private final ConcreteServiceImpl3 concreteService3;
//== CONSTRUCTORS ==
#Autowired
public MyController(ConcreteServiceImpl1 concreteService1, ConcreteServiceImpl2 concreteService2,
ConcreteServiceImpl3 concreteService3){
this.concreteService1 = concreteService1;
this.concreteService2 = concreteService2;
this.concreteService3 = concreteService3;
}
//== REQUEST MAPPINGS ==
#GetMapping(Requests.SPECIFIC_REQUEST)
public ResponseEntity<?> handleSpecificRequest(#PathVariable String source,
#RequestParam String start,
#RequestParam String end){
source = source.toLowerCase();
if(MyConstants.SOURCES.contains(source)){
switch(source){
case("value1"):
concreteService1.doSomething(start, end);
break;
case("value2"):
concreteService2.doSomething(start, end);
break;
case("value3"):
concreteService3.doSomething(start, end);
break;
}
}else{
//An invalid source path variable was recieved
}
//Return something after additional processing
return null;
}
}
In Spring you can get all implementations of an interface (say T) by injecting a List<T> or a Map<String, T> field. In the second case the names of the beans will become the keys of the map. You could consider this if there are a lot of possible implementations or if they change often. Thanks to it you could add or remove an implementation without changing the controller.
Both injecting a List or a Map have some benefits and drawbacks in this case. If you inject a List you would probably need to add some method to map the name and the implementation. Something like :
interface MyInterface() {
(...)
String name()
}
This way you could transform it to a Map<String, MyInterface>, for example using Streams API. While this would be more explicit, it would polute your interface a bit (why should it be aware that there are multiple implementations?).
When using the Map you should probably name the beans explicitly or even introduce an annotation to follow the principle of least astonishment. If you are naming the beans by using the class name or the method name of the configuration class you could break the app by renaming those (and in effect changing the url), which is usually a safe operation to do.
A simplistic implementation in Spring Boot could look like this:
#SpringBootApplication
public class DynamicDependencyInjectionForMultipleImplementationsApplication {
public static void main(String[] args) {
SpringApplication.run(DynamicDependencyInjectionForMultipleImplementationsApplication.class, args);
}
interface MyInterface {
Object getStuff();
}
class Implementation1 implements MyInterface {
#Override public Object getStuff() {
return "foo";
}
}
class Implementation2 implements MyInterface {
#Override public Object getStuff() {
return "bar";
}
}
#Configuration
class Config {
#Bean("getFoo")
Implementation1 implementation1() {
return new Implementation1();
}
#Bean("getBar")
Implementation2 implementation2() {
return new Implementation2();
}
}
#RestController
class Controller {
private final Map<String, MyInterface> implementations;
Controller(Map<String, MyInterface> implementations) {
this.implementations = implementations;
}
#GetMapping("/run/{beanName}")
Object runSelectedImplementation(#PathVariable String beanName) {
return Optional.ofNullable(implementations.get(beanName))
.orElseThrow(UnknownImplementation::new)
.getStuff();
}
#ResponseStatus(BAD_REQUEST)
class UnknownImplementation extends RuntimeException {
}
}
}
It passes the following tests:
#RunWith(SpringRunner.class)
#SpringBootTest
#AutoConfigureMockMvc
public class DynamicDependencyInjectionForMultipleImplementationsApplicationTests {
#Autowired
private MockMvc mockMvc;
#Test
public void shouldCallImplementation1() throws Exception {
mockMvc.perform(get("/run/getFoo"))
.andExpect(status().isOk())
.andExpect(content().string(containsString("foo")));
}
#Test
public void shouldCallImplementation2() throws Exception {
mockMvc.perform(get("/run/getBar"))
.andExpect(status().isOk())
.andExpect(content().string(containsString("bar")));
}
#Test
public void shouldRejectUnknownImplementations() throws Exception {
mockMvc.perform(get("/run/getSomethingElse"))
.andExpect(status().isBadRequest());
}
}
Regarding two of your doubts :
1. Instantiating the service object should not be an issue as this is one time job and controller gonna need them to serve all type of request.
2. You can use the exact Path mapping to get rid of switch case. For e.g. :
#GetMapping("/specificRequest/value1")
#GetMapping("/specificRequest/value2")
#GetMapping("/specificRequest/value3")
All of the above mapping will be on separate method which would deal with specific source value and invoke respective service method.
Hope this will help to make code more cleaner and elegant.
There is one more option of separating this on service layer and having only one endpoint to serve all types of source but as you said there is different implementation for each source value then it says that source is nothing but a resource for your application and having separate URI/separate method makes the perfect sense here. Few advantages that I see here with this are :
Makes it easy to write the test cases.
Scaling the same without impacting any other source/service.
Your code dealing the each source as separate entity from other sources.
The above approach should be fine when you have limited source values. If you have no control over source value then we need further redesign here by making source value differentiate by one more value like sourceType etc. and then having separate controller for each group type of source.
How can I get Jersey to inject classes without creating and registering factories on a one-for-one basis?
I have the following config:
public class MyConfig extends ResourceConfig {
public MyConfig() {
register(new AbstractBinder() {
#Override
protected void configure() {
bindFactory(FooFactory.class).to(Foo.class);
bindFactory(BazFactory.class).to(Baz.class);
}
});
}
}
hk2 will now successfully inject Foo and Baz:
// this works; Foo is created by the registered FooFactory and injected
#GET
#Path("test")
#Produces("application/json")
public Response getTest(#Context Foo foo) {
// code
}
But that's not my goal. My goal is to inject objects that wrap these classes. There are many and they each consume different combinations of Foo and Baz. Some examples:
public class FooExtender implements WrapperInterface {
public FooExtender(Foo foo) {
// code
}
}
public class FooBazExtender implements WrapperInterface {
public FooBazExtender(Foo foo, Baz baz) {
// code
}
}
public class TestExtender implements WrapperInterface {
public TestExtender(Foo foo) {
// code
}
// code
}
And so on.
The following does not work:
// this does not work
#GET
#Path("test")
#Produces("application/json")
public Response getTest(#Context TestExtender test) {
// code
}
I could create a factory for each and register it in my application config class, using the bindFactory syntax like I did with Foo and Baz. But that is not a good approach due to the number of objects in question.
I have read much of the hk2 documentation, and tried a variety of approaches. I just don't know enough of how hk2 actually works to come up with the answer, and it seems like a common enough problem that there should be a straightforward solution.
Factories are really only needed for more complex initializations. If you don't need this, all you need to do is bind the service
#Override
protected void configure() {
// bind service and advertise it as itself in a per lookup scope
bindAsContract(TestExtender.class);
// or bind service as a singleton
bindAsContract(TestExtender.class).in(Singleton.class);
// or bind the service and advertise as an interface
bind(TestExtender.class).to(ITestExtender.class);
// or bind the service and advertise as interface in a scope
bind(TestExtender.class).to(ITestExtender.class).in(RequestScoped.class);
}
You also need to add #Inject on the constructors so HK2 knows to inject the Foo and Baz
#Inject
public TestExtender(Foo foo, Baz baz) {}
I wound up using FastClasspathScanner to grab classes from the package(s) I was interested in. Then I called the appropriate bind methods (bindAsContract or bind) in batches, as mentioned in Paul Samsotha's answer (after also adding the appropriate #Inject annotations).
That seemed to be the most expedient method available to emulate autoscanning and avoid having to manually register each class.
It feels like a hack and I'd be surprised if hk2 doesn't have a better method baked in.
I need a suggestion for how to code for multiple implementations for a service using Google-guice. Below is the example
TestService testService =new TestServiceImplOne();
TestService testService =new TestServiceImplTwo();
As Guice doesn't allow binding a type to more than one implementations as the below code results in error
binderObject.bind(SomeType.class).to(ImplemenationOne.class);
binderObject.bind(SomeType.class).to(ImplemenationTwo.class);
we can solve this with named annotations as below
binder.bind(Player.class).annotatedWith(Names.named("Good")).to(GoodPlayer.class);
binder.bind(Player.class).annotatedWith(Names.named("Bad")).to(BadPlayer.class);
#Named("Good") Player goodPlayer = (Player)injector.getInstance(Player.class);
#Named("Bad") Player badPlayer = (Player)injector.getInstance(Player.class);
But the application which iam working is something like this. We are binding all the modules in the init() method and creating the injector modules:
//separate method to bind
protected void configure() {
bind(new TypeLiteral<List<Service>>() {}).toInstance(serviceSets);
}
//separate method to inject
Injector i = Guice.createInjector(modules);
But with the above process I can just bind one implementation class to the interface (service class)
Could you please provide me a way to do this with providers. I would like to do something like this below
class TestServiceProvider extends Provider{
// some code where it returns the instance of impl class needed. In my case TestServiceImplOne and TestServiceImplTwo and provider returns the corresponding instance of service class
}
and bind service class with provider class. Something like this
bind(TestService.class).toProvider(TestServiceProvider.class);
I would appreciate if someone suggests a good example using providers or some other way that I can inject whatever implementation I want in the client.
Note: I am using webservices and I am not sure how I can inject different implementations when a webservice is called to a service class.
First of all thanks very much for responding . Coming straight to the point
Iam working on webservices . Heres's the Flow
// GET URI
GET http://www.google.com:8182/indi/provide/organizations/{ou}
OrganizationsResource -------->OrganizationService------>OrganizationServiceImpl
Iam binding OrganizationService with OrganizationServiceImpl and injecting the OrganizationService in OrganizationsResource
#Inject
public void setOrganizationService(OrganizationService orgService) {
this.orgService= orgService;
}
Its fine till here but i have two implementations for OrganizationService ------>OrgDeatilsServiceImpl which does some other job
Now i want to bind both OrganizationServiceImpl and OrgDeatilsServiceImpl to OrganizationService
Confusions:
1) What procedure i have to use in Guice to bind two implementaions?
2) How exactly i can code in OrganizationsResource to dynamically decide which implementation to call.
I would appreciate if you give a sample example for the above requirement.
As Vladimir noted, you can use binding annotations with Providers...
// in YourModule.configure():
bind(TestService.class)
.annotatedWith(Names.named("foo")
.toProvider(TestServiceProvider.class);
...and generic types using TypeLiterals...
bind(new TypeLiteral<List<Service>>() {})
.annotatedWith(Names.named("bar")
.toInstance(serviceSets);
...as long as you ask for an annotated instance using getInstance(Key<T>)...
List<Service> servicesOne = injector.getInstance(
new Key<List<Service>>(Names.named("bar")) {});
// or
List<Service> servicesTwo = injector.getInstance(
Key.get(new TypeLiteral<List<Service>>() {}, Names.named("bar"));
...or, preferably, keep them as fields and let Guice do the injecting, because Guice can't inject local variables. Remember that Guice can only inject classes that it creates, or that you request specifically.
class MyInjectorCreator {
#Inject #Named("foo") Provider<TestService> fooServiceProvider;
#Inject #Named("bar") List<Service> barServices;
// Guice will also wrap/unwrap Providers automatically.
#Inject #Named("foo") TestService fooService;
#Inject #Named("bar") Provider<List<Service>> barServicesProvider;
public void createInjector() {
Injector injector = Guice.createInjector(getListOfModules());
injector.injectMembers(this);
}
}
Now, that answers the question as you phrased it in the title. That said, it sounds like you actually want to choose between implementations at runtime, which is a slightly different but easy-to-solve problem:
class TestServiceProvider extends Provider<TestService> {
// Injection is allowed here!
#Inject ApplicationSettings settings;
#Inject Provider<TestServiceImplOne> oneProvider;
#Inject Provider<TestServiceImplTwo> twoProvider;
#Override public TestService get() {
if (settings.isInTestMode()) {
return new TestTestServiceImplImpl(); // without injection!
} else if (settings.useNewService()) {
return twoProvider.get(); // with injection!
} else {
return oneProvider.get(); // also with injection!
}
}
}
But I should warn you that if you know at injector creation time which service to use, you should probably just bind it correctly then for the sake of code cleanliness and ease of readability:
// in YourModule.configure():
if (settings.isInTestMode()) {
bind(TestService.class).toInstance(new TestTestServiceImplImpl());
} else if (settings.useNewService()) {
bind(TestService.class).to(TestServiceImplTwo.class);
} else {
bind(TestService.class).to(TestServiceImplOne.class);
}
Say I have these 3 layers of my code:
1. Database layer (ORM)
2. BusinessLogic
3. Application
Now, I write my code as follows:
Database layer:
This mainly has CURD operations over database.
class MyDatabaseLayer {
public int findValue(int k) {
// find v
}
public void insertValue(int k, int v) {
// Insert v
}
}
BusinessLogic:
This holds the actual logic to call database layer and do stuff.
class MyBusinessLogic {
private MyDatabaseLayer dbLayer;
public MyBusinessLogic(MyDatabaseLayer dbLayer) {
this.dbLayer = dbLayer;
}
public int manipulateValue(int k) {
dbLayer.findValue(k);
//do stuff with value
}
}
Application layer:
This calls the business logic and displays the data
MyBusinessLogic logic = new MyBusinessLogic(new MyDatabaseLayer ()); //The problem
logic.manipulateValue(5);
Now if you see in the application layer, it knows about the database layer, which is wrong. It knows too much.
Misko Hevery says: Constructor injection is good. But if I follow that, how will I achieve abstraction? And how can Google Guice help me here?
Note that for the testability Misko refers to, you ideally want to make interfaces for MyDatabaseLayer, MyBusinessLogic, etc. and have the constructors take those interfaces rather than concrete classes so that in testing you can easily pass in fake implementations that don't actually use the database, etc.
With Guice, you would bind the interfaces to concrete classes in a Module or Modules. You would then create an Injector using those Modules and get some root object (for example, your application object) from the Injector.
Injector injector = Guice.createInjector(new AbstractModule() {
#Override protected void configure() {
bind(MyDatabaseLayer.class).to(MyDatabaseLayerImplementation.class);
// etc.
});
MyApplicationLayer applicationLayer = injector.getInstance(MyApplicationLayer.class);
In MyApplicationLayer, you'd inject the business logic:
#Inject
public MyApplicationLayer(MyBusinessLogic logic) {
this.logic = logic;
}
This is of course a very simple example and there are much more complex things you can do. In a web application, for example, you can use constructor injection on servlets using Guice Servlet rather than getting an object directly out of the Injector after creating it.
The part you are missing with inversion-of-control is that the application layer does not call the constructor directly.. It uses a factory (the IoC container) to populate the constructor parameter.
Whatever tool you use, guice / spring / picocontainer / singleton-factories, your application code should look something like:
#Controller
class MyController {
#Resource // Some container knows about this annotation and wires you in
MyBusinessLogic myBusinessLogic;
#RequestMethod("/foo/bar.*")
public MyWebResponse doService(Response resp, long id, String val) {
boolean worked = myBusinessLogic.manipulatevalue(id, val);
return new MyWebResponse(worked);
}
}
Note, the myBusinessLogic could be registered in several ways - java's #Resource, MyBusinessLogicFactory.getMyBusinessLogic(), guice.get(MyBusinessLogic.class), etc.
A poor-mans solution would be:
package foo;
class MyBusinessLogicFactory {
static volatile MyBusinessLogic instance; // package-scoped so unit tests can override
public static MyBusinessLogic getInstance() {
if (instance == null) {
synchronized(MyBusinessLogicFactory.class) {
instance = new MyBusinessLogic(MyDatabaseLayerFactory.getInstance());
}
}
return instance;
}
}
// repeat with MyDatabaseLayerFactory
Note the above singleton model is highly discouraged, since it has no scope. You COULD wrap the above inside a context - soething like
class Context {
Map<Class,Object> class2Instance = new ConcurrentHashMap<>();
public <T> T getInstance(Class<T> clazz) {
Object o = class2Instance.get(clazz);
if (o == null) {
synchronized(this) {
o = class2Instance.get(clazz);
if (o != null) return (T)o;
o = transitivelyLoadInstance(clazz); // details not shown
for (Class c : loadClassTree(clazz)) { // details not shown
class2Instance.put(c, o);
}
}
}
return (T)o;
}
...
}
But at that point, picocontainer, guice and spring can solve the complexities of the above SOOO much better.
Further, things like spring that honor the java 6 annotations mean you can do things other than constructor injection, which is VERY useful if you have multiple configuration items of the same base data-type (e.g. strings).