I read that Spring annotations like #Autowired, #Transactional & #PostConstruct are a form of / use BeanPostProcessors. So from what I understand about BeanPostProcessors, they are used to manage the lifecycle of a Spring Bean. Meaning you can specify any code which should be run before or after the intialization of a bean.
Now annotations like #Component or #Bean specify to Spring that it should create beans of this type. For example,
#Component
public Class Foo {
}
will tell spring to create a Bean of type "Foo" and then Spring will manage it's lifecycle. So does that make #Component and #Bean BeanPostProcessors?
I read that Spring annotations like #Autowired, #Transactional & #PostConstruct are a form of / use BeanPostProcessors.
Neither, it's actually the opposite. It is BeanPostProcessors that look for the annotations and react accordingly.
AutowiredAnnotationBeanPostProcessor - BeanPostProcessor implementation that autowires annotated fields, setter methods, and arbitrary config methods. Such members to be injected are detected through annotations: by default, Spring's #Autowired and #Value annotations.
Also supports JSR-330's #Inject annotation, if available, as a direct alternative to Spring's own #Autowired.
CommonAnnotationBeanPostProcessor - BeanPostProcessor implementation that supports common Java annotations out of the box, in particular the JSR-250 annotations in the javax.annotation package. These common Java annotations are supported in many Java EE 5 technologies (e.g. JSF 1.2), as well as in Java 6's JAX-WS.
This post-processor includes support for the PostConstruct and PreDestroy annotations - as init annotation and destroy annotation, respectively - through inheriting from InitDestroyAnnotationBeanPostProcessor with pre-configured annotation types.
Note that #Transactional is not handled by a BeanPostProcessor.
Now annotations like #Component or #Bean specify to Spring that it should create beans of this type. [...] So does that make #Component and #Bean BeanPostProcessors?
No.
#Component is an annotation that the component-scanning framework looks for to find beans to register. The component-scanning framework also looks for annotations that are themselves annotated with #Component, such as the #Configuration, #Controller, #Service, and #Repositiory annotations.
The #Bean annotation is then handled by a BeanDefinitionRegistryPostProcessor (not a BeanPostProcessor).
ConfigurationClassPostProcessor - BeanFactoryPostProcessor used for bootstrapping processing of #Configuration classes.
This post processor is priority-ordered as it is important that any Bean methods declared in #Configuration classes have their corresponding bean definitions registered before any other BeanFactoryPostProcessor executes.
No! They are different.
BeanPostProcessor is used to perform some operations before and after creating a bean,this allows us to add some code before and after creating the bean.
BeanPostProcessor class has two methods.
postProcessBeforeInitialization : It's used to make sure required actions are taken before initialization. e.g. you want to load certain property file/read data from the remote source/service.
postProcessAfterInitialization : It takes care of tasks that you want to do after initialization before bean reference is given to application.
Refer to : spring-doc-beans-customizing-using-BeanPostProcessor
interface for details.
#Component : It's the most generic Spring annotation.
A Java class decorated with #Component is found during classpath scanning and registered in the context as a Spring bean.
Refer to : spring-doc-component-annotation for details.
Related
I'm trying to understand the transition from using xml annotation to java based annotation in Spring. I have these definitions
<context:annotation-config>: Scanning and activating annotations for already registered beans in spring config xml.
<context:component-scan>: Bean registration + <context:annotation-config>
is #Configuration and is #ComponentScan.
If lets say I declare all my beans with #Component (disregard first the more specific ones like #Repository, #Service etc) annotation and make sure that the packages are getting scanned by the #ComponentScan annotation, what is a particular use case where I will still annotate my class with both #Configuration together with #ComponentScan?
I ask this question because sometimes I see classes annotated with both #Configuration and #ComponentScan at the same time.
First read the following carefully:
Difference between <context:annotation-config> vs <context:component-scan>
Thus <context:component-scan> does the scan job and the same job than <context:annotation-config> does, it means work around with the DI annotations
Then now consider:
<context:component-scan> equivalent to #ComponentScan
<context:annotation-config> no equivalent for annotation.
what is a particular use case where I will still annotate my class
with both #Configuration together with #ComponentScan?
#Configuration is used to define beans about Infastructure such as Database, JMS etc...
Yes, a class can use both, It could be used for example for MVC Infrastructure #Configuration such as:
#EnableWebMvc
#Configuration
#ComponentScan("com.manuel.jordan.controller", "com.manuel.jordan.rest")
public class WebMvcConfig extends WebMvcConfigurerAdapter {
Thus from that class your are configuring MVC and indicating only to scan the MVC classes created by you for the "web side", such as: #Controller, #RestController.
This really depends on your likings and coding style. Documentation states:
Either basePackageClasses() or basePackages() (or its alias value()) may be specified to define specific packages to scan. If specific packages are not defined, scanning will occur from the package of the class that declares this annotation.
So every time you see just #ComponentScan annotation this means that all sub-packages should be scanned. This is a reasonable approach to take with package per feature layout: when you have a #Configuration class for your feature and all #Components related to the feature in sub-packages.
I am trying to understand how beans that we make using #Configuration tends to override the beans that are generated by SpringBoot by default. I have been working on a project where in many cases we create beans for things like ZuulConfigs and the assumption is, whatever we are making shall take precedence over the default generated bean. I have been trying to figure this out but can't. Basically,
Is Spring achieving this via some custom class loader
If not how is this precedence working. Can I give some precedence in similar manner to my beans
Can I generate similar hierarchy in my project,if so how
The help is highly appreciated
Spring AutoConfiguration is used to provide a basic configuration if certain classes are in the classpath or not.
If you want to configure the order in which beans are instantiated by spring you can use
#DependsOn("A")
public class B {
...
}
This would create bean "A", then "B". Hence you can order the configuration depending upon the beans need first to be done. Anyways Spring automatically detects the dependencies by analyzing the bean classes.
for more help check this question
Spring Boot AutoConfiguration Order
Alternative :
There is also "#AutoConfigureOrder" annotation(where you can prioritise the configuration), you can have a look in the code for deeper understanding.
Documentation of AutoConfiguration is here
First of all, class loading and bean creation are two different things. We don't need to create a bean to load a class, however, a class has to be loaded in order to create a bean.
Now, coming back to Spring's example, Spring looks into all the packages configured by #componentScan and creates beans of all the classes annotated with #Bean, #Configuration and/or #Component. Spring's container keeps track of all the beans created and hence, when it encounters user defined bean with same name and class type as default bean, it replaces the original definition with user defined one (e.g. we can create our custom #ObjectMapper to override Spring boot's own instance). You can also use #Primary annotation to make you bean take precedence if another definition with same class exists (documentation here).
Below are the answers for your questions:
Spring uses reflection to load the classes and create instances. Although you can load the classes with your custom class loader (more on that here), you don't need to worry about it for #Configuration.
Yes, you can use #Primary annotation to give your bean a precedence. You can also use #Order(here) to define the creation order for your beans.
With #Primary, #Order and #Qualifier annotation you can define your own hierarchy for bean creation.
Can I give some precedence in similar manner to my beans
Yes.
A) To define a specific order your Configuration classes will be handled (by the way, a Configuration class does not have to be annotated with #Configuration (so-called full definition), but it's enough to be annotated with #Component, #ComponentScan, #Import, #ImportResource or just have a method annotated with #Bean - so-called lite definition), you should
1) add your Configuration Candidates to your SpringApplication's primarySource, for example, in your main method like that
SpringApplication.run(
new Class[]{YourSpringBootApplication.class, Config1.class, Config2.class, ...},
args);
2) and annotate each of your Configuration Candidates with #Order annotation, any other ordering means like Ordered interface, #DependsOn etc will be ignored by ConfigurationClassPostProcessor, the order in the primarySource array will also be ignored.
Then ConfigurationClassPostProcessor will sort your Configuration Candidates and handle them according the #Order annotation value you specified.
B) The precedence can also be achieved by defining your own AutoConfiguration classes. Although both Configuration and AutoConfiguration are handled by the same ConfigurationClassPostProcessor, they are essentially distinctive machineries. To do so
1) define in your classpath /META-INF/spring.factories file and put in the EnableAutoConfiguration section of it your AutoConfiguration classes like that
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
your.package.AutoConfig1,your.package.AutoConfig2
2) and annotate your AutoConfiguration classes with #AutoConfigureOrder, #AutoConfigureAfter, or #AutoConfigureAfter annotations, any other ordering means again will be ignored.
Like #Strelok pointed out, AutoConfiguration classes, your own and provided e.g. by spring-boot-autoconfigure library alike, will be added to the end of the list of Configuration Candidates.
Remember, however, that the order the Configuration Candidates will be handled by ConfigurationClassPostProcessor does not necessarily coincide with the order the beans defined by the Configuration classes will be created. For example, you might define your Configuration class that overrides TomcatServletWebServerFactory to make your own customization of Tomcat web server like
#Configuration
public class EmbeddedTomcatConfig {
#Bean
public TomcatServletWebServerFactory containerFactory() {
...
return customizedTomcatWebServerFactory;
}
but this method will be called right at the moment when your Spring Boot application decides to create a Web server, regardless of how you defined the precedence for your EmbeddedTomcatConfig Configuration class.
Is Spring achieving this via some custom class loader
There is no need to. Although you could, as always with Spring, define your own ClassLoader for BeanFactory, standard ClassLoader is good enough if everything you need for Configuration in your application is available in the classpath. Please notice, that at first phase ConfigurationClassPostProcessor does not load (i.e. does not resolve) the Configuration candidates classes (otherwise, most of the classes in spring-boot-autoconfigure library will fail to load). Instead it analyzes their annotations with bytecode analyzer, ASM by default. For that purpose, it is just enough to get a binary form, a byte array, of a class to feed it to bytecode analyzer.
Just know this: Spring Boot (specifically) auto configuration classes are always configured last. After all user beans have been created. Spring Boot auto configuration classes almost always use the #ConditionalXXXX annotations to make sure that any beans of the same type/name and other conditions that are configured in your application will take precedence over the Spring Boot auto-configured beans.
If you want your #Component to take precedence over other #Component while scanning all the components by spring, use #Order(Ordered.LOWEST_PRECEDENCE) i.e. the max value to load your component over other.
#Primary is used to give your bean a default preference, we can override the default preference using #Qualifier
I have a bean with #RequestScope in it, and when I inject it in one of my Singletons, it is injected as a singleton and not as a request scope. However, if I change the #RequestScope to #Scope( value = "request", scopeName = "request", proxyMode = ScopedProxyMode.TARGET_CLASS), Spring creates the bean as request scope and injects them to the singleton correctly.
I read the documentation of Spring regarding this:
The JSR-330 default scope is like Spring’s prototype. However, in order to keep it consistent with Spring’s general defaults, a JSR-330 bean declared in the Spring container is a singleton by default. In order to use a scope other than singleton, you should use Spring’s #Scope annotation. javax.inject also provides a #Scope annotation. Nevertheless, this one is only intended to be used for creating your own annotations.
Does this mean also that #RequestScoped is really being ignored by Spring? Is there any Provider/Resolver that resolves this issue with Spring? As much as possible I want to use the #RequestScoped annotation instead of #Scope annotation of spring as we are required to use JSR annotations only
http://docs.spring.io/spring/docs/4.2.5.RELEASE/spring-framework-reference/html/beans.html#beans-standard-annotations-limitations
As mentioned by by M. Deinum, Spring doesn't support #RequestScoped out of the box. I had to create a ScopeMetadataResolver to convert #RequestScoped into Spring #Scope
I referred here for my custom resolver:
https://github.com/matzew/spring-cdi-bridge/blob/master/src/main/java/net/wessendorf/spring/scopes/CdiScopeMetadataResolver.java
I am quite new in Spring framework and I have some questions about the use of #Autowired annotation and interface declaration.
Referring to this example:
http://viralpatel.net/blogs/spring3-mvc-hibernate-maven-tutorial-eclipse-example/
I know that #Autowired annotation can be used to automatically link a bean on a property.
In the previous example I have the following situation:
I have a ContactDAO interface and it's implementation class named ContactDAOImpl
Next in the class ContactServiceImpl there is this variable annoted using #Autowired:
#Autowired
private ContactDAO contactDAO;
My first doubt is related to the fact that ContactDAO is an interface so what am I wiring? The concrete type: ContactDAOImpl ? If yes, is the Spring Framework that do it?
The second doubt is related to the fact that in the spring-servlet.xml configuration file there is not a bean definizion for the ContactDAO orf ContactAOImpl class...why? Is it because ContactDAOImpl class is annoted using #Repository annotation?
Thanks
Andrea
My first doubt is related to the fact that ContactDAO is an interface
so what am I wiring? The concrete type: ContactDAOImpl ? If yes, is
the Spring Framework that do it?
Spring will autowire an implementation of the interface for you, as long as there's only one matching implementation. There's also a way to match a single implementation from multiple candidates to your autowiring by using #Qualifier with #Autowired and naming the implementation.
The second doubt is related to the fact that in the spring-servlet.xml
configuration file there is not a bean definizion for the ContactDAO
orf ContactAOImpl class...why? Is it because ContactDAOImpl class is
annoted using #Repository annotation?
If you're using annotations (#Component, #Repository, #Service, #Controller) in your implementations for configuration, you don't need to explicitly define the bean in the xml (although you can do that also).
Edit: this old answer of mine might shed some more light about using annotations in Spring.
The answers to your two questions are Yes, and Yes.
In fact, you might not have an instance of ContactDAOImpl autowired in the service, but an instance of a proxy, which deletages to an instance of ContactDAOImpl. The proxy will typically handle transactions, translate exceptions, etc.
And the #Repository annotation is an alternative (simpler) way to declare a Spring bean. It works only if you have an element in the Spring xml file telling it to discover annotated beans.
Spring will autoscan all your classes and find all annotated classes and register them, this in your spring config will tell it to do that:
<context:component-scan base-package="my.base.package" />
Therefore you do not need to declare your #Repository in your configuration file.
Onto the first part of your question, this is the unpinning of the IOC pattern; your Service class is only aware of the interface of the DAO, this means that it is not dependent on the implementation.
During scanning Spring will find all of your annotated classes and when you ask for an #Autowired then it will attempt to find a class that your have annotated that is an implentor of the interface you have asked to have Autowired.
Have a look at the Spring documentation on Annotation Configuration.
I am learning java for 3 months and sometimes
i can not understand the usage purpose of something.
one topic was dependency injection and spring beans i figured out the finally =)
now i confused with the two annotations #Autowired and #Repository.
First What does Autowiring mean? then
Why should i use them and what is the difference between using them and not using?
Also today i tried to use hibernate in a spring mvc project and i had to search for about 15(cause of class not found errors) jar files beacuse of the dependencies of other jar files used in the project.
is this had to be this way? this makes learning java very hard for the beginners
thanks...
#Repository is an annotation that marks the specific class as a Data Access Object, thus clarifying it's role. Other markers of the same category are #Service and #Controller
#Autowired is an annotation with a completely different meaning: it basically tells the DI container to inject a dependency. More info at http://apollo89.com/java/spring-framework-2.5.3/api/org/springframework/beans/factory/annotation/Autowired.html
Edit
More info at tutorialpoint
or docs.spring.io
Both the annotations have different purposes to be used.
#Autowired: This is same as <bean="xyz" autowire="byType"> you define in the configuration file. The reference variable (dependency) that is annotated with #Autowired, will be injected by Spring container as any matching #Bean found in #Configuration class.
Plus the classes annotated with #Component, #Service, #Repository are too considered as beans so their objects are injected into the matching dependencies.
Spring container scans the beans in the classes you mentioned for "component-scan" or #ComponentScan("xyz").
#Repository: This is also a spring-framework's annotation. When you annotate a class #Repository, spring container understands it's a DAO class and translates all unchecked exceptions (thrown from DAO methods) into Spring DataAccessException.
DAO class is the class where you write methods to perform operations over db.
#Autowired and #Repository are very 2 different concepts.
1.# Repository: This define a class to be a repository, In general term you can use simply #Component but to define specifically, there are 3 more annotations like Controller,service and repository.Mainly 2 advantages:
1.If you have defined(context:component-scan)in servlet.xml to scan the defined package and find its own by spring.
2. More advantages you get from spring like database access error translation, so it is mainly defined to use with class in which you are connecting with database either with hibernate or jdbc.
#Autowired: to inject dependency at run-time by spring, means in a class, autowire a object ,and use it ,so this bean will automatically be made without defining in xml file