What in the world are Spring beans

Question

I am yet to find a high-level definition of Spring beans that I can understand  I see them referenced often in Grails documentation and books  but I think that understanding what they are would be beneficial  So what are Spring beans  How can they be used  Do they have something to do with Dependency Injection

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Well you understood it partially  You have to tailor the beans according to your need and inform Spring container to manage it when required  by using a methodology populalrly known as IoC  Inversion of Control  coined by Martin Fowler  also known as Dependency Injection  DI     You wire the beans in a way  so that you do not have to take care of the instantiating or evaluate any dependency on the bean  This is popularly known as Hollywood Principle   Google is the best tool to explore more on this in addition to the links you would get flooded with here in this question

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Spring have the IoC container  which carry the Bag of Bean   creation maintain and deletion are the responsibilities of Spring Container  We can put the bean in to Spring by Wiring and Auto Wiring  Wiring mean we manually configure it into the XML file and  Auto Wiring  mean we put the annotations in the Java file then Spring  automatically scan the root-context where java configuration file  make it and put into the bag of Spring   Here is the detail URI where you got more information about Beans

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First let us understand Spring     Spring is a lightweight and flexible framework     Analogy    Bean  is an object  which is created  managed and destroyed in Spring Container  We can inject an object into the Spring Container through the metadata either xml or annotation   which is called inversion of control   Analogy  Let us assume farmer is having a farmland cultivating by seeds or beans   Here  Farmer is Spring Framework  Farmland land is Spring Container  Beans are Spring Beans  Cultivating is Spring Processors         Like bean life-cycle  spring beans too having it s own life-cycle         img source  Following is sequence of a bean lifecycle in Spring    Instantiate  First the spring container finds the bean   s definition from the XML file and instantiates the bean  Populate properties  Using the dependency injection  spring populates all of the properties as specified in the bean definition  Set Bean Name  If the bean implements BeanNameAware interface  spring passes the bean   s id to setBeanName   method  Set Bean factory  If Bean implements BeanFactoryAware interface  spring passes the beanfactory to setBeanFactory   method  Pre-Initialization  Also called post process of bean  If there are any bean BeanPostProcessors associated with the bean  Spring calls postProcesserBeforeInitialization   method  Initialize beans  If the bean implements IntializingBean its afterPropertySet   method is called  If the bean has init method declaration  the specified initialization method is called  Post-Initialization      If there are any BeanPostProcessors associated with the bean  their postProcessAfterInitialization   methods will be called  Ready to use  Now the bean is ready to use by the application Destroy  If the bean implements DisposableBean  it will call the destroy   method

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The XML configuration of Spring is composed of Beans and Beans are basically classes  They re just POJOs that we use inside of our ApplicationContext  Defining Beans can be thought of as replacing the keyword new  So wherever you are using the keyword new in your application something like   MyRepository myRepository  new MyRepository       Where you re using that keyword new that s somewhere you can look at removing that configuration and placing it into an XML file  So we will code like this    lt bean name  myRepository          class  com demo repository MyRepository     gt    Now we can simply use Setter Injection  Constructor Injection  I m using Setter Injection   public class MyServiceImpl implements MyService       private MyRepository myRepository      public void setMyRepository MyRepository myRepository                this myRepository   myRepository             public List lt Customer gt  findAll             return myRepository findAll

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In Spring  those objects that form the backbone of your application and that are managed by the Spring IoC container are referred to as beans  A bean is simply an object that is instantiated  assembled and otherwise managed by a Spring IoC container

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In terms of a Spring boot application  a bean is simply a Java object which is created by Spring framework when the application starts  The purpose of the object can be pretty much anything - a configuration  a service  database connection factory etc  - Spring doesn t really care  Most beans depend on other beans to work  for example an entity manager might need a database connection  Spring framework is able to figure out how the beans should be wired together automatically  From your point of an application developer  you just have to declare the beans you need and they  quot magically quot  appear in your application ready to use

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The objects that form the backbone of your application and that are   managed by the Spring IoC  container are called beans  A bean is an   object that is instantiated  assembled  and otherwise managed by a   Spring IoC container  These beans are created with the configuration   metadata that you supply to the container  for example  in the form of   XML  lt bean  gt  definitions    More to learn about beans and scope from SpringSource      When you create a bean definition what you are actually creating is a   recipe for creating actual instances of the class defined by that bean   definition  The idea that a bean definition is a recipe is important    because it means that  just like a class  you can potentially have   many object instances created from a single recipe       You can control not only the various dependencies and configuration   values that are to be plugged into an object that is created from a   particular bean definition  but also the scope of the objects created   from a particular bean definition  This approach is very powerful and   gives you the flexibility to choose the scope of the objects you   create through configuration instead of having to  bake in  the scope   of an object at the Java class level  Beans can be defined to be   deployed in one of a number of scopes    IoC  Inversion of Control

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Spring beans are classes  Instead of instantiating a class  using new   you get an instance as a bean cast to your class type from the application context  where the bean is what you configured in the application context configuration  This way  the whole application maintains singleton-scope instance throughout the application  All beans are initialized following their configuration order right after the application context is instantiated  Even if you don t get any beans in your application  all beans instances are already created the moment after you created the application context

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For Spring  all objects are beans  The fundamental step in the Spring Framework is to define your objects as beans  Beans are nothing but object instances that would be created by the spring framework by looking at their class definitions  These definitions basically form the configuration metadata  The framework then creates a plan for which objects need to be instantiated  which dependencies need to be set and injected  the scope of the newly created instance  etc   based on this configuration metadata  The metadata can be supplied in a simple XML file  just like in the first chapter  Alternatively  one could provide the metadata as Annotation or Java Configuration   Book  Just Spring

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Spring beans are just object instances that are managed by the Spring IOC container  Spring IOC container carry the Bag of Bean Bean creation maintain and deletion    are the responsibilities of Spring Container   We can put the bean in to Spring by Wiring and Auto Wiring  Wiring mean we manually configure it into the XML file  Auto Wiring mean we put the annotations in the Java file then Spring automatically scan the root-context where java configuration file  make it and put into the bag of Spring

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A Bean is a POJO Plain Old Java Object   which is managed by the spring container   Spring containers create only one instance of the bean by default   This bean it is cached in memory so all requests for the bean will return a shared reference to the same bean   The  Bean annotation returns an object that spring registers as a bean in application context  The logic inside the method is responsible for creating the instance   When do we use  Bean annotation   When automatic configuration is not an option  For example when we want to wire components from a third party library  because the source code is not available so we cannot annotate the classes with  Component   A Real time scenario could be that someone wants to connect to Amazon S3 bucket  Because the source is not available he would have to create a  bean    Bean public AmazonS3 awsS3Client         BasicAWSCredentials awsCreds   new BasicAWSCredentials awsKeyId  accessKey       return AmazonS3ClientBuilder standard   withRegion Regions fromName region                withCredentials new AWSStaticCredentialsProvider awsCreds   build        Source for the code above -  https   www devglan com spring-mvc aws-s3-java  Because I mentioned  Component Annotation above    Component Indicates that an annotated class is a  component   Such classes are considered as candidates for auto-detection when using annotation-based configuration and class path scanning   Component annotation registers the class as a single bean

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Spring beans are just instance objects that are managed by the Spring container  namely  they are created and wired by the framework and put into a  bag of objects   the container  from where you can get them later   The  wiring  part there is what dependency injection is all about  what it means is that you can just say  I will need this thing  and the framework will follow some rules to get you the proper instance   For someone who isn t used to Spring  I think Wikipedia Spring s article has a nice description      Central to the Spring Framework is its inversion of control container    which provides a consistent means of configuring and managing Java   objects using reflection  The container is responsible for managing   object lifecycles of specific objects  creating these objects  calling   their initialization methods  and configuring these objects by wiring   them together       Objects created by the container are also called managed objects or   beans  The container can be configured by loading XML files or   detecting specific Java annotations on configuration classes  These   data sources contain the bean definitions which provide the   information required to create the beans       Objects can be obtained by means of either dependency lookup or   dependency injection  Dependency lookup is a pattern where a caller   asks the container object for an object with a specific name or of a   specific type  Dependency injection is a pattern where the container   passes objects by name to other objects  via either constructors    properties  or factory methods

[java] What in the world are Spring beans?

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