Understanding Spring Autowired usage

Question

I am reading the spring 3 0 x reference documentation to understand Spring Autowired annotation   3 9 2  Autowired and  Inject  I am not able to understand the below examples  Do we need to do something in the XML for it to work    EXAMPLE 1  public class SimpleMovieLister        private MovieFinder movieFinder        Autowired     public void setMovieFinder MovieFinder movieFinder            this movieFinder   movieFinder                        EXAMPLE 2   public class MovieRecommender        private MovieCatalog movieCatalog       private CustomerPreferenceDao customerPreferenceDao        Autowired     public void prepare MovieCatalog movieCatalog                      CustomerPreferenceDao customerPreferenceDao            this movieCatalog   movieCatalog          this customerPreferenceDao   customerPreferenceDao                        How can the two classes be autowired implementing the same interface and using the same class    Example    class Red implements Color class Blue implements Color  class myMainClass       Autowired      private Color color       draw            color design                Which design method will be called  How do I make sure the design method of Red class will be called and not Blue

User · Answer

Yes  you can configure the Spring servlet context xml file to define your beans  i e   classes   so that it can do the automatic injection for you   However  do note  that you have to do other configurations to have Spring up and running and the best way to do that  is to follow a tutorial ground up   Once you have your Spring configured probably  you can do the following in your Spring servlet context xml file for Example 1 above to work  please replace the package name of com movies to what the true package name is and if this is a 3rd party class  then be sure that the appropriate jar file is on the classpath      lt beans bean id  movieFinder  class  com movies MovieFinder    gt    or if the MovieFinder class has a constructor with a primitive value  then you could something like this    lt beans bean id  movieFinder  class  com movies MovieFinder   gt       lt beans constructor-arg value  100    gt   lt  beans bean gt    or if the MovieFinder class has a constructor expecting another class  then you could do something like this    lt beans bean id  movieFinder  class  com movies MovieFinder   gt       lt beans constructor-arg ref  otherBeanRef    gt   lt  beans bean gt       where  otherBeanRef  is another bean that has a reference to the expected class

User · Answer

Nothing in the example says that the  classes implementing the same interface   MovieCatalog is a type and CustomerPreferenceDao is another type  Spring can easily tell them apart   In Spring 2 x  wiring of beans mostly happened via bean IDs or names  This is still supported by Spring 3 x but often  you will have one instance of a bean with a certain type - most services are singletons  Creating names for those is tedious  So Spring started to support  autowire by type    What the examples show is various ways that you can use to inject beans into fields  methods and constructors   The XML already contains all the information that Spring needs since you have to specify the fully qualified class name in each bean  You need to be a bit careful with interfaces  though   This autowiring will fail     Autowired  public void prepare  Interface1 bean1  Interface1 bean2             Since Java doesn t keep the parameter names in the byte code  Spring can t distinguish between the two beans anymore  The fix is to use  Qualifier     Autowired  public void prepare   Qualifier  bean1   Interface1 bean1        Qualifier  bean2    Interface1 bean2

User · Answer

TL DR The  Autowired annotation spares you the need to do the wiring by yourself in the XML file  or any other way  and just finds for you what needs to be injected where and does that for you  Full explanation The  Autowired annotation allows you to skip configurations elsewhere of what to inject and just does it for you  Assuming your package is com mycompany movies you have to put this tag in your XML  application context file    lt context component-scan base-package  quot com mycompany movies quot    gt   This tag will do an auto-scanning  Assuming each class that has to become a bean is annotated with a correct annotation like  Component  for simple bean  or  Controller  for a servlet control  or  Repository  for DAO classes  and these classes are somewhere under the package com mycompany movies  Spring will find all of these and create a bean for each one  This is done in 2 scans of the classes - the first time it just searches for classes that need to become a bean and maps the injections it needs to be doing  and on the second scan it injects the beans  Of course  you can define your beans in the more traditional XML file or with an  Configuration class  or any combination of the three   The  Autowired annotation tells Spring where an injection needs to occur  If you put it on a method setMovieFinder it understands  by the prefix set   the  Autowired annotation  that a bean needs to be injected  In the second scan  Spring searches for a bean of type MovieFinder  and if it finds such bean  it injects it to this method  If it finds two such beans you will get an Exception  To avoid the Exception  you can use the  Qualifier annotation and tell it which of the two beans to inject in the following manner   Qualifier  quot redBean quot   class Red implements Color         Class code here     Qualifier  quot blueBean quot   class Blue implements Color         Class code here    Or if you prefer to declare the beans in your XML  it would look something like this   lt bean id  quot redBean quot  class  quot com mycompany movies Red quot   gt    lt bean id  quot blueBean quot  class  quot com mycompany movies Blue quot   gt   In the  Autowired declaration  you need to also add the  Qualifier to tell which of the two color beans to inject   Autowired  Qualifier  quot redBean quot   public void setColor Color color      this color   color     If you don t want to use two annotations  the  Autowired and  Qualifier  you can use  Resource to combine these two   Resource name  quot redBean quot   public void setColor Color color      this color   color     The  Resource  you can read some extra data about it in the first comment on this answer  spares you the use of two annotations and instead  you only use one  I ll just add two more comments   Good practice would be to use  Inject instead of  Autowired because it is not Spring-specific and is part of the JSR-330 standard  Another good practice would be to put the  Inject    Autowired on a constructor instead of a method  If you put it on a constructor  you can validate that the injected beans are not null and fail fast when you try to start the application and avoid a NullPointerException when you need to actually use the bean   Update  To complete the picture  I created a new question about the  Configuration class

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