Is there a way to Autowire a bean that requires constructor arguments

Question

I m using Spring 3 0 5 and am using  Autowire annotation for my class members as much as possible   One of the beans that I need to autowire requires arguments to its constructor   I ve looked through the Spring docs  but cannot seem to find any reference to how to annotate constructor arguments   In XML  I can use  as part of the bean definition   Is there a similar mechanism for  Autowire annotation   Ex    Component public class MyConstructorClass     String var    public MyConstructorClass  String constrArg        this var   var               Service public class MyBeanService     Autowired   MyConstructorClass myConstructorClass              In this example  how do I specify the value of  constrArg  in MyBeanService with the  Autowire annotation   Is there any way to do this   Thanks   Eric

User · Answer

In this example  how do I specify the value of  constrArg  in MyBeanService with the  Autowire annotation  Is there any way to do this    No  not in the way that you mean  The bean representing MyConstructorClass must be configurable without requiring any of its client beans  so MyBeanService doesn t get a say in how MyConstructorClass is configured   This isn t an autowiring problem  the problem here is how does Spring instantiate MyConstructorClass  given that MyConstructorClass is a  Component  and you re using component-scanning  and therefore not specifying a MyConstructorClass explicitly in your config    As  Sean said  one answer here is to use  Value on the constructor parameter  so that Spring will fetch the constructor value from a system property or properties file  The alternative is for MyBeanService to directly instantiate MyConstructorClass  but if you do that  then MyConstructorClass is no longer a Spring bean

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Another alternative  if you already have an instance of the object created and you want to add it as an  autowired dependency to initialize all the internal  autowired variables  could be the following    Autowired  private AutowireCapableBeanFactory autowireCapableBeanFactory   public void doStuff        YourObject obj   new YourObject  Value X    etc       autowireCapableBeanFactory autowireBean obj

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I like Zakaria s answer  but if you re in a project where your team doesn t want to use that approach  and you re stuck trying to construct something with a String  integer  float  or primative type from a property file into the constructor  then you can use Spring s  Value annotation on the parameter in the constructor  For example  I had an issue where I was trying to pull a string property into my constructor for a class annotated with  Service  My approach works for  Service  but I  think this approach should work with any spring java class  if it has an annotation  such as  Service   Component  etc   which indicate that Spring will be the one constructing instances of the class  Let s say in some yaml file  or whatever configuration you re using   you have something like this  some      custom          envProperty   quot property-for-dev-environment quot   and you ve got a constructor   Service    I think this should work for  Component  or any annotation saying Spring is the one calling the constructor  class MyClass           MyClass String property                        This won t run as Spring won t be able to find the string envProperty  So  this is one way you can get that value  class MyDynamoTable import org springframework beans factory annotation Value          MyDynamoTable  Value  quot   some custom envProperty   String property                      In the above constructor  Spring will call the class and know to use the String  quot property-for-dev-environment quot  pulled from my yaml configuration when calling it  NOTE  this I believe  Value annotation is for strings  intergers  and I believe primative types  If you re trying to pass custom classes  beans   then approaches in answers defined above work

User · Answer

You need to use  Autowired and  Value  Refer this post for more information on this topic

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An alternative would be instead of passing the parameters to the constructor you might have them as getter and setters and then in a  PostConstruct initialize the values as you want  In this case Spring will create the bean using the default constructor  An example is below   Component public class MyConstructorClass     String var     public void setVar String var        this var   var         public void getVar        return var          PostConstruct   public void init         setVar  var                 Service public class MyBeanService      field autowiring    Autowired   MyConstructorClass myConstructorClass

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Well  from time to time I run into the same question  As far as I know  one cannot do that when one wants to add dynamic parameters to the constructor  However  the factory pattern may help   public interface MyBean          here be my fancy stuff    public interface MyBeanFactory       public MyBean getMyBean    bean parameters          Component public class MyBeanFactoryImpl implements MyBeanFactory        Autowired     WhateverIWantToInject somethingInjected       public MyBean getMyBean    params               return new MyBeanImpl    params                 private class MyBeanImpl implements MyBean           public MyBeanImpl    params                      let s do whatever one has to                     Component public class MyConsumerClass        Autowired     private MyBeanFactory beanFactory       public void myMethod                here one has to prepare the parameters         MyBean bean   beanFactory getMyBean    params                Now  MyBean is not a spring bean per se  but it is close enough  Dependency Injection works  although I inject the factory and not the bean itself - one has to inject a new factory on top of his own new MyBean implementation if one wants to replace it   Further  MyBean has access to other beans - because it may have access to the factory s autowired stuff   And one might apparently want to add some logic to the getMyBean function  which is extra effort I allow  but unfortunately I have no better solution  Since the problem usually is that the dynamic parameters come from an external source  like a database  or user interaction  therefore I must instantiate that bean only in mid-run  only when that info is readily available  so the Factory should be quite adequate

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Most answers are fairly old  so it might have not been possible back then  but there actually is a solution that satisfies all the possible use-cases   So right know the answers are    Not providing a real Spring component  the factory design  or does not fit every situation  using  Value you have to have the value in a configuration file somewhere    The solution to solve those issues is to create the object manually using the ApplicationContext    Component public class MyConstructorClass       String var       public MyConstructorClass          public MyConstructorClass String constrArg            this var   var            Service public class MyBeanService implements ApplicationContextAware       private static ApplicationContext applicationContext       MyConstructorClass myConstructorClass       public MyBeanService                    Creating the object manually         MyConstructorClass myObject   new MyConstructorClass  hello world               Initializing the object as a Spring component         AutowireCapableBeanFactory factory   applicationContext getAutowireCapableBeanFactory            factory autowireBean myObject           factory initializeBean myObject  myObject getClass   getSimpleName                 Override     public void setApplicationContext ApplicationContext context  throws BeansException           applicationContext   context            This is a cool solution because    It gives you access to all the Spring functionalities on your object   Autowired obviously  but also  Async for example   You can use any source for your constructor arguments  configuration file  computed value  hard-coded value        It only requires you to add a few lines of code without having to change anything  It can also be used to dynamically create a unknown number of instances of a Spring-managed class  I m using it to create multiple asynchronous executors on the fly for example    The only thing to keep in mind is that you have to have a constructor that takes no arguments  and that can be empty  in the class you want to instantiate  or an  Autowired constructor if you need it

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You need the  Value annotation   A common use case is to assign default field values using  quot   systemProperties myProp  quot  style expressions   public class SimpleMovieLister      private MovieFinder movieFinder    private String defaultLocale      Autowired   public void configure MovieFinder movieFinder                            Value  quot    systemProperties  user region     quot   String defaultLocale          this movieFinder   movieFinder        this defaultLocale   defaultLocale                   See  Expression Language  gt  Annotation Configuration  To be more clear  in your scenario  you d wire two classes  MybeanService and MyConstructorClass  something like this   Component public class MyBeanService implements BeanService       Autowired     public MybeanService MyConstructorClass foo              do something with foo           Component public class MyConstructorClass      public MyConstructorClass  Value  quot   some expression here  quot   String value               do something with value          Update  if you need several different instances of MyConstructorClass with different values  you should use Qualifier annotations

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You can also configure your component like this    package mypackage  import org springframework context annotation Configuration      Configuration    public class MyConstructorClassConfig         Bean    public MyConstructorClass myConstructorClass          return new myConstructorClass  foobar                 With the Bean annotation  you are telling Spring to register the returned bean in the BeanFactory   So you can autowire it as you wish

[spring] Is there a way to @Autowire a bean that requires constructor arguments?

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