Difference between context annotation-config and context component-scan

Question

I m learning Spring 3 and I don t seem to grasp the functionality behind  lt context annotation-config gt  and  lt context component-scan gt    From what I ve read they seem to handle different annotations   Required   Autowired etc vs  Component   Repository   Service etc   but also from what I ve read they register the same bean post processor classes   To confuse me even more  there is an annotation-config attribute on  lt context component-scan gt    Can someone shed some light on these tags  What s similar  what s different  is one superseded by the other  they complete each other  do I need one of them  both

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A  lt context component-scan  gt  custom tag registers the same set of bean definitions as is done by   apart from its primary responsibility of scanning the java packages and registering bean definitions from the classpath   If for some reason this registration of default bean definitions are to be avoided  the way to do that is to specify an additional  annotation-config  attribute in component-scan  this way    lt context component-scan basePackages    annotation-config  false   gt    Reference  http   www java-allandsundry com 2012 12 contextcomponent-scan-contextannotation html

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The difference between the two is really simple     lt context annotation-config   gt     Enables you to use annotations that are restricted to wiring up properties and constructors only of beans    Where as   lt context component-scan base-package  org package   gt     Enables everything that  lt context annotation-config   gt  can do  with addition of using stereotypes eg    Component   Service    Repository  So you can wire entire beans and not just restricted to constructors or properties

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lt context component-scan   gt  implicitly enables  lt context annotation-config  gt    try with  lt context component-scan base-package       annotation-config  false   gt    in your configuration  Service   Repository   Component works fine  but  Autowired  Resource and  Inject doesn t work   This means AutowiredAnnotationBeanPostProcessor will not be enabled and Spring container will not process the Autowiring annotations

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I found this nice summary of which annotations are picked up by which declarations  By studying it you will find that  lt context component-scan  gt  recognizes a superset of annotations recognized by  lt context annotation-config  gt   namely     Component   Service   Repository   Controller   Endpoint  Configuration   Bean   Lazy   Scope   Order   Primary   Profile   DependsOn   Import   ImportResource   As you can see  lt context component-scan  gt  logically extends  lt context annotation-config  gt  with CLASSPATH component scanning and Java  Configuration features

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lt context annotation-config gt  activates many different annotations in beans  whether they are defined in XML or through component scanning    lt context component-scan gt  is for defining beans without using XML  For further information  read    3 9  Annotation-based container configuration 3 10  Classpath scanning and managed components

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Spring allows you to do two things    Autowiring of beans Autodiscovery of beans   1  Autowiring Usually in applicationContext xml you define beans and other beans are wired using constructor or setter methods  You can wire beans using XML or annotations  In case you use annotations  you need to activate annotations and you have to add  lt context annotation-config   gt  in applicationContext xml  This will simplify the structure of the  tag from applicationContext xml  because you will not have to manually wire beans  constructor or setter   You can use  Autowire annotation and the beans will be wired by type   A step forward for escaping the manual XML configuration is    2  Autodiscovery Autodiscovery is simplifying the XML one step further  in the sense that you don t even need too add the  lt bean gt  tag in applicationContext xml  You just mark the specific beans with one of the following annotation and Spring will automatically wire the marked beans and their dependencies into the Spring container  The annotations are as follow    Controller   Service   Component   Repository  By using  lt context component-scan gt  and pointing the base package  Spring will auto-discover and wire the components into Spring container     As a conclusion     lt context annotation-config   gt  is used in order to be able to use  Autowired annotation  lt context component-scan   gt  is used to determine the search of specific beans and attempt of autowiring

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The  lt context annotation-config gt  tag tells Spring to scan the codebase for automatically resolving dependency requirements of the classes containing  Autowired annotation   Spring 2 5 also adds support for JSR-250 annotations such as  Resource   PostConstruct  and  PreDestroy Use of these annotations also requires that certain BeanPostProcessors be registered within the Spring container  As always  these can be registered as individual bean definitions  but they can also be implicitly registered by including  lt context annotation-config gt  tag in spring configuration   Taken from Spring documentation of Annotation Based Configuration    Spring provides the capability of automatically detecting  stereotyped  classes and registering corresponding BeanDefinitions with the ApplicationContext   According to javadoc of org springframework stereotype   Stereotypes are Annotations denoting the roles of types or methods in the overall architecture  at a conceptual  rather than implementation  level    Example   Controller  Service  Repository etc  These are intended for use by tools and aspects  making an ideal target for pointcuts    To autodetect such  stereotype  classes   lt context component-scan gt  tag is required   The  lt context component-scan gt  tag also tells Spring to scan the code for injectable beans under the package  and all its subpackages  specified

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lt context annotation-config gt   Scanning and activating annotations for already registered beans in spring config xml    lt context component-scan gt   Bean registration    lt context annotation-config gt      Autowired and  Required are targets property level so bean should register in spring IOC before use these annotations  To enable these annotations either have to register respective beans or include  lt context annotation-config   gt   i e   lt context annotation-config   gt  works with registered beans only    Required enables  RequiredAnnotationBeanPostProcessor  processing tool    Autowired enables  AutowiredAnnotationBeanPostProcessor processing tool  Note  Annotation itself nothing to do  we need a Processing Tool  which is a class underneath  responsible for the core process      Repository   Service and  Controller  are  Component  and they targets class level    lt context component-scan gt  it scans the package and find and register the beans  and it includes the work done by  lt context annotation-config   gt    Migrating XML to Annotations

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lt context component-scan base-package  package name    gt    This is used to tell the container that there are bean classes in my package scan those bean classes  In order to scan  bean classes by container on top of the bean we have to write one of the stereo type annotation like following    Component   Service   Repository   Controller   lt context annotation-config   gt    If we don t want to write bean tag explicitly in XML then how the container knows if there is a auto wiring in the bean  This is possible by using  Autowired annotation  we have to inform to the container that there is auto wiring in my bean by context annotation-config

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lt context annotation-config gt  is used to activate annotations in beans already registered in the application context  no matter if they were defined with XML or by package scanning     lt context component-scan gt  can also do what  lt context annotation-config gt  does but  lt context component-scan gt  also scans packages to find and register beans within the application context   I ll use some examples to show the differences similarities    Lets start with a basic setup of three beans of type A  B and C  with B and C being injected into A   package com xxx  public class B     public B         System out println  creating bean B      this          package com xxx  public class C     public C         System out println  creating bean C      this          package com yyy  import com xxx B  import com xxx C  public class A      private B bbb    private C ccc    public A         System out println  creating bean A      this         public void setBbb B bbb        System out println  setting A bbb with     bbb       this bbb   bbb        public void setCcc C ccc        System out println  setting A ccc with     ccc       this ccc   ccc           With the following XML configuration     lt bean id  bBean  class  com xxx B    gt   lt bean id  cBean  class  com xxx C    gt   lt bean id  aBean  class  com yyy A  gt     lt property name  bbb  ref  bBean    gt     lt property name  ccc  ref  cBean    gt   lt  bean gt    Loading the context produces the following output   creating bean B  com xxx B c2ff5 creating bean C  com xxx C 1e8a1f6 creating bean A  com yyy A 1e152c5 setting A bbb with com xxx B c2ff5 setting A ccc with com xxx C 1e8a1f6   OK  this is the expected output  But this is  old style  Spring  Now we have annotations so lets use those to simplify the XML   First  lets autowire the bbb and ccc properties on bean A like so   package com yyy  import org springframework beans factory annotation Autowired  import com xxx B  import com xxx C  public class A      private B bbb    private C ccc    public A         System out println  creating bean A      this          Autowired   public void setBbb B bbb        System out println  setting A bbb with     bbb       this bbb   bbb         Autowired   public void setCcc C ccc        System out println  setting A ccc with     ccc       this ccc   ccc          This allows me to remove the following rows from the XML    lt property name  bbb  ref  bBean    gt   lt property name  ccc  ref  cBean    gt    My XML is now simplified to this    lt bean id  bBean  class  com xxx B    gt   lt bean id  cBean  class  com xxx C    gt   lt bean id  aBean  class  com yyy A    gt    When I load the context I get the following output   creating bean B  com xxx B 5e5a50 creating bean C  com xxx C 54a328 creating bean A  com yyy A a3d4cf   OK  this is wrong  What happened  Why aren t my properties autowired   Well  annotations are a nice feature but by themselves they do nothing whatsoever  They just annotate stuff  You need a processing tool to find the annotations and do something with them     lt context annotation-config gt  to the rescue  This activates the actions for the annotations that it finds on the beans defined in the same application context where itself is defined   If I change my XML to this    lt context annotation-config   gt   lt bean id  bBean  class  com xxx B    gt   lt bean id  cBean  class  com xxx C    gt   lt bean id  aBean  class  com yyy A    gt    when I load the application context I get the proper result   creating bean B  com xxx B 15663a2 creating bean C  com xxx C cd5f8b creating bean A  com yyy A 157aa53 setting A bbb with com xxx B 15663a2 setting A ccc with com xxx C cd5f8b   OK  this is nice  but I ve removed two rows from the XML and added one  That s not a very big difference  The idea with annotations is that it s supposed to remove the XML   So let s remove the XML definitions and replace them all with annotations   package com xxx  import org springframework stereotype Component   Component public class B     public B         System out println  creating bean B      this          package com xxx  import org springframework stereotype Component   Component public class C     public C         System out println  creating bean C      this          package com yyy  import org springframework beans factory annotation Autowired  import org springframework stereotype Component  import com xxx B  import com xxx C   Component public class A      private B bbb    private C ccc    public A         System out println  creating bean A      this          Autowired   public void setBbb B bbb        System out println  setting A bbb with     bbb       this bbb   bbb         Autowired   public void setCcc C ccc        System out println  setting A ccc with     ccc       this ccc   ccc          While in the XML we only keep this    lt context annotation-config   gt    We load the context and the result is    Nothing  No beans are created  no beans are autowired  Nothing   That s because  as I said in the first paragraph  the  lt context annotation-config   gt  only works on beans registered within the application context  Because I removed the XML configuration for the three beans there is no bean created and  lt context annotation-config   gt  has no  targets  to work on   But that won t be a problem for  lt context component-scan gt  which can scan a package for  targets  to work on  Let s change the content of the XML config into the following entry    lt context component-scan base-package  com xxx    gt    When I load the context I get the following output   creating bean B  com xxx B 1be0f0a creating bean C  com xxx C 80d1ff   Hmmmm    something is missing  Why    If you look closelly at the classes  class A has package com yyy but I ve specified in the  lt context component-scan gt  to use package com xxx so this completely missed my A class and only picked up B and C which are on the com xxx package   To fix this  I add this other package also    lt context component-scan base-package  com xxx com yyy    gt    and now we get the expected result   creating bean B  com xxx B cd5f8b creating bean C  com xxx C 15ac3c9 creating bean A  com yyy A ec4a87 setting A bbb with com xxx B cd5f8b setting A ccc with com xxx C 15ac3c9   And that s it  Now you don t have XML definitions anymore  you have annotations   As a final example  keeping the annotated classes A  B and C and adding the following to the XML  what will we get after loading the context    lt context component-scan base-package  com xxx    gt   lt bean id  aBean  class  com yyy A    gt    We still get the correct result   creating bean B  com xxx B 157aa53 creating bean C  com xxx C ec4a87 creating bean A  com yyy A 1d64c37 setting A bbb with com xxx B 157aa53 setting A ccc with com xxx C ec4a87   Even if the bean for class A isn t obtained by scanning  the processing tools are still applied by  lt context component-scan gt  on all beans registered in the application context  even for A which was manually registered in the XML   But what if we have the following XML  will we get duplicated beans because we ve specified both  lt context annotation-config   gt  and  lt context component-scan gt     lt context annotation-config   gt   lt context component-scan base-package  com xxx    gt   lt bean id  aBean  class  com yyy A    gt    No  no duplications  We again get the expected result   creating bean B  com xxx B 157aa53 creating bean C  com xxx C ec4a87 creating bean A  com yyy A 1d64c37 setting A bbb with com xxx B 157aa53 setting A ccc with com xxx C ec4a87   That s because both tags register the same processing tools   lt context annotation-config   gt  can be omitted if  lt context component-scan gt  is specified  but Spring takes care of running them only once   Even if you register the processing tools yourself multiple times  Spring will still make sure they do their magic only once  this XML    lt context annotation-config   gt   lt context component-scan base-package  com xxx    gt   lt bean id  aBean  class  com yyy A    gt   lt bean id  bla  class  org springframework beans factory annotation AutowiredAnnotationBeanPostProcessor    gt   lt bean id  bla1  class  org springframework beans factory annotation AutowiredAnnotationBeanPostProcessor    gt   lt bean id  bla2  class  org springframework beans factory annotation AutowiredAnnotationBeanPostProcessor    gt   lt bean id  bla3  class  org springframework beans factory annotation AutowiredAnnotationBeanPostProcessor    gt    will still generate the following result   creating bean B  com xxx B 157aa53 creating bean C  com xxx C ec4a87 creating bean A  com yyy A 25d2b2 setting A bbb with com xxx B 157aa53 setting A ccc with com xxx C ec4a87   OK  that about raps it up    I hope this information along with the responses from  Tomasz Nurkiewicz and  Sean Patrick Floyd are all you need to understand how   lt context annotation-config gt  and  lt context component-scan gt  work

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As a complementary  you can use  ComponentScan to use  lt context component-scan gt  in annotation way     It s also described at spring io     Configures component scanning directives for use with    Configuration classes  Provides support parallel with Spring XML s    element    One thing to note  if you re using Spring Boot  the  Configuration and  ComponentScan can be implied by using  SpringBootApplication annotation

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lt context annotation-config gt    Only resolves the  Autowired and  Qualifer annotations  that s all  it about the Dependency Injection  There are other annotations that do the same job  I think how  Inject  but all about to resolve DI through annotations   Be aware  even when you have declared the  lt context annotation-config gt  element  you must declare your class how a Bean anyway  remember we have three available options   XML   lt bean gt    Annotations   Component   Service   Repository   Controller  JavaConfig   Configuration   Bean   Now with   lt context component-scan gt    It does two things    It scans all the classes annotated with  Component   Service   Repository   Controller and  Configuration and create a Bean It does the same job how  lt context annotation-config gt  does    Therefore if you declare  lt context component-scan gt   is not necessary anymore declare  lt context annotation-config gt  too   Thats all   A common scenario was for example declare only a bean through XML and resolve the DI through annotations  for example   lt bean id  serviceBeanA  class  com something CarServiceImpl    gt   lt bean id  serviceBeanB  class  com something PersonServiceImpl    gt   lt bean id  repositoryBeanA  class  com something CarRepository    gt   lt bean id  repositoryBeanB  class  com something PersonRepository    gt    We have only declared the beans  nothing about  lt constructor-arg gt  and  lt property gt   the DI is configured in their own classes through  Autowired  It means the Services use  Autowired for their Repositories components and the Repositories use  Autowired for the JdbcTemplate  DataSource etc  components

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lt context annotation-config  gt   lt  -- is used to activate the annotation for beans -- gt   lt context component-scan base-package  x y MyClass    gt   lt  -- is for the Spring IOC container to look for the beans in the base package  -- gt    The other important point to note is that context component-scan implicitly calls the context annotation-config to activate the annotations on beans  Well if you don t want context component-scan to implicitly activate annotations for you  you can go on setting the annotation-config element of the context component-scan to false   To summarize    lt context annotation-config  gt   lt  -- activates the annotations -- gt    lt context component-scan base-package  x y MyClass    gt   lt  -- activates the annotations   register the beans by looking inside the base-package -- gt

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lt context annotation-config gt    This tells Spring that I am going to use Annotated beans as spring bean and those would be wired through  Autowired annotation  instead of declaring in spring config xml file    lt context component-scan base-package  com test     gt      This tells Spring container  where to start searching those annotated beans  Here spring will search all sub packages of the base package

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you can find more information in spring context schema file  following is in spring-context-4 3 xsd   lt conxtext annotation-config   gt    Activates various annotations to be detected in bean classes  Spring s  Required and  Autowired  as well as JSR 250 s  PostConstruct   PreDestroy and  Resource  if available   JAX-WS s  WebServiceRef  if available   EJB 3 s  EJB  if available   and JPA s  PersistenceContext and  PersistenceUnit  if available   Alternatively  you may choose to activate the individual BeanPostProcessors for those annotations   Note  This tag does not activate processing of Spring s  Transactional or EJB 3 s  TransactionAttribute annotation  Consider the use of the  lt tx annotation-driven gt  tag for that purpose     lt context component-scan gt    Scans the classpath for annotated components that will be auto-registered as Spring beans  By default  the Spring-provided  Component   Repository   Service   Controller   RestController   ControllerAdvice  and  Configuration stereotypes    will be detected   Note  This tag implies the effects of the  annotation-config  tag  activating  Required   Autowired   PostConstruct   PreDestroy   Resource   PersistenceContext and  PersistenceUnit annotations in the component classes  which is usually desired for autodetected components  without external configuration   Turn off the  annotation-config  attribute to deactivate this default behavior  for example in order to use custom BeanPostProcessor definitions for handling those annotations   Note  You may use placeholders in package paths  but only resolved against system properties  analogous to resource paths   A component scan results in new bean definitions being registered  Spring s PropertySourcesPlaceholderConfigurer will apply to those bean definitions just like to regular bean definitions  but it won t apply to the component scan settings themselves

[java] Difference between <context:annotation-config> and <context:component-scan>

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