Implement a simple factory pattern with Spring 3 annotations

Question

I was wondering how I could implement the simple factory pattern with Spring 3 annotations   I saw in the documentation that you can create beans that call the factory class and run a factory method   I was wondering if this was possible using annotations only   I have a controller that currently calls   MyService myService   myServiceFactory getMyService test   result   myService checkStatus      MyService is an interface with one method called checkStatus     My factory class looks like this    Component public class MyServiceFactory        public static MyService getMyService String service            MyService myService           service   service toLowerCase             if  service equals  one                  myService   new MyServiceOne              else if  service equals  two                  myService   new MyServiceTwo              else if  service equals  three                  myService   new MyServiceThree              else               myService   new MyServiceDefault                       return myService            MyServiceOne class looks like this     Autowired private LocationService locationService   public boolean checkStatus             do stuff     When I run this code the locationService variable is alwasy null   I beleive this is because I am creating the objects myself inside the factory and autowiring is not taking place   Is there a way to add annotations to make this work correctly   Thanks

User · Answer

Try this   public interface MyService      Code     Component  One   public class MyServiceOne implements MyService      Code     Component  Two   public class MyServiceTwo implements MyService      Code

User · Answer

You could also declaratively define a bean of type ServiceLocatorFactoryBean that will act as a Factory class  it supported by Spring 3   A FactoryBean implementation that takes an interface which must have one or more methods with the signatures  typically  MyService getService   or MyService getService String id   and creates a dynamic proxy which implements that interface  Here s an example of implementing the Factory pattern using Spring  One more clearly example

User · Answer

Why not add the interface FactoryBean to MyServiceFactory  to tell Spring that it s a factory   add a register String service  MyService instance  then  have each of the services call    Autowired MyServiceFactory serviceFactory    PostConstruct public void postConstruct         serviceFactory register myName  this       This way  you can separate each service provider into modules if necessary  and Spring will automagically pick up any deployed and available service providers

User · Answer

The following worked for me   The interface consist of you logic methods plus additional identity method   public interface MyService       String getType        void checkStatus        Some implementations    Component public class MyServiceOne implements MyService        Override     public String getType             return  one               Override     public void checkStatus              Your code           Component public class MyServiceTwo implements MyService        Override     public String getType             return  two               Override     public void checkStatus              Your code           Component public class MyServiceThree implements MyService        Override     public String getType             return  three               Override     public void checkStatus              Your code           And the factory itself as following    Service public class MyServiceFactory         Autowired     private List lt MyService gt  services       private static final Map lt String  MyService gt  myServiceCache   new HashMap lt  gt           PostConstruct     public void initMyServiceCache             for MyService service   services                myServiceCache put service getType    service                        public static MyService getService String type            MyService service   myServiceCache get type           if service    null  throw new RuntimeException  Unknown service type      type           return service            I ve found such implementation easier  cleaner and much more extensible  Adding new MyService is as easy as creating another spring bean implementing same interface without making any changes in other places

User · Answer

You are right  by creating object manually you are not letting Spring to perform autowiring  Consider managing your services by Spring as well    Component public class MyServiceFactory         Autowired     private MyServiceOne myServiceOne        Autowired     private MyServiceTwo myServiceTwo        Autowired     private MyServiceThree myServiceThree        Autowired     private MyServiceDefault myServiceDefault       public static MyService getMyService String service            service   service toLowerCase             if  service equals  one                  return myServiceOne            else if  service equals  two                  return myServiceTwo            else if  service equals  three                  return myServiceThree            else               return myServiceDefault                      But I would consider the overall design to be rather poor  Wouldn t it better to have one general MyService implementation and pass one two three string as extra parameter to checkStatus    What do you want to achieve    Component public class MyServiceAdapter implements MyService         Autowired     private MyServiceOne myServiceOne        Autowired     private MyServiceTwo myServiceTwo        Autowired     private MyServiceThree myServiceThree        Autowired     private MyServiceDefault myServiceDefault       public boolean checkStatus String service            service   service toLowerCase             if  service equals  one                  return myServiceOne checkStatus              else if  service equals  two                  return myServiceTwo checkStatus              else if  service equals  three                  return myServiceThree checkStatus              else               return myServiceDefault checkStatus                        This is still poorly designed because adding new MyService implementation requires MyServiceAdapter modification as well  SRP violation   But this is actually a good starting point  hint  map and Strategy pattern

User · Answer

Based on solution by Pavel Cern   here we can make an universal typed implementation of this pattern  To to it  we need to introduce NamedService interface       public interface NamedService          String name            and add abstract class   public abstract class AbstractFactory lt T extends NamedService gt         private final Map lt String  T gt  map       protected AbstractFactory List lt T gt  list            this map   list                  stream                    collect Collectors toMap NamedService  name  Function identity                            Factory method for getting an appropriate implementation of a service         param name name of service impl          return concrete service impl               public T getInstance  NonNull final String name            T t   map get name           if t    null              throw new RuntimeException  Unknown service name      name           return t            Then we create a concrete factory of specific objects like MyService    public interface MyService extends NamedService              String name               void doJob         Component public class MyServiceFactory extends AbstractFactory lt MyService gt          Autowired     protected MyServiceFactory List lt MyService gt  list            super list             where List the list of implementations of MyService interface at compile time   This approach works fine if you have multiple similar factories across app that produce objects by name  if producing objects by a name suffice you business logic of course   Here map works good with String as a key  and holds all the existing implementations of your services   if you have different logic for producing objects  this additional logic can be moved to some another place and work in combination with these factories  that get objects by name

User · Answer

You can manually ask Spring to Autowire it   Have your factory implement ApplicationContextAware  Then provide the following implementation in your factory    Override public void setApplicationContext final ApplicationContext applicationContext        this applicationContext   applicationContext      and then do the following after creating your bean   YourBean bean   new YourBean    applicationContext getAutowireCapableBeanFactory   autowireBean bean   bean init      If it has an init   method    This will autowire your LocationService perfectly fine

User · Answer

You could instantiate  AnnotationConfigApplicationContext  by passing all your service classes as parameters    Component public class MyServiceFactory        private ApplicationContext applicationContext       public MyServiceFactory             applicationContext   new AnnotationConfigApplicationContext                  MyServiceOne class                  MyServiceTwo class                  MyServiceThree class                  MyServiceDefault class                  LocationService class                        I have added LocationService class because this component is also autowired               public MyService getMyService String service             if   one  equalsIgnoreCase service                 return applicationContext getBean MyServiceOne class                       if   two  equalsIgnoreCase service                 return applicationContext getBean MyServiceTwo class                       if   three  equalsIgnoreCase service                 return applicationContext getBean MyServiceThree class                       return applicationContext getBean MyServiceDefault class

User · Answer

Following answer of DruidKuma Litte refactor of his factory with autowired constructor   Service public class MyServiceFactory        private static final Map lt String  MyService gt  myServiceCache   new HashMap lt  gt           Autowired     private MyServiceFactory List lt MyService gt  services            for MyService service   services                myServiceCache put service getType    service                        public static MyService getService String type            MyService service   myServiceCache get type           if service    null  throw new RuntimeException  quot Unknown service type   quot    type           return service

User · Answer

Following the answer from DruidKuma and jumping monkey You can also include optional and make your code a bit nicer and cleaner   public static MyService getService String type            return Optional ofNullable myServiceCache get type                    orElseThrow    - gt  new RuntimeException  quot Unknown service type   quot    type

User · Answer

I suppose you to use org springframework beans factory config ServiceLocatorFactoryBean  It will much simplify your code  Except MyServiceAdapter u can only create interface MyServiceAdapter  with method  MyService getMyService and with alies to register your classes  Code  bean id  printStrategyFactory  class  org springframework beans factory config ServiceLocatorFactoryBean  gt           lt property name  YourInterface  value  factory MyServiceAdapter    gt       lt  bean gt        lt alias name  myServiceOne  alias  one    gt       lt alias name  myServiceTwo  alias  two    gt

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