Excuse me if this is has already been discussed, I could not find a satisfying answer.
I do not understand whats happening when i create a bean in Springframework and #Autowired it to a field in another bean. I understand the result of #Autowired and other annotations but i do not know how its done by Spring.
class Sample1{
//
}
class Sample2{
#Autowired
Sample1 sample1Bean;
}
<bean id="sample1Bean" class="...Sample1"/>
<bean id="sample2Bean" class="...Sample2"/>
My question is how does spring set the field sample1Bean in Sample2? i am not expecting a complete explanation, but a direction where i have to look would be great. Thanks.
The #Autowired, #Inject annotations are resolved by a BeanPostProcessor - specifically AutowiredAnnotationBeanPostProcessor. This bean post processor intercepts the creation(for cases where #Autowired is on constructors) of beans, setting of property on the beans to ensure that all the autowired fields are appropriately set.
Im no expert in Spring but I will answer what I know. When a spring powered web application starts up, Spring framework goes through bean instantiation process in the application context. While creating beans Spring checks the required dependencies for a given bean. It looks up a matching bean based on the required type of bean and autowires it when #Autowired annotation is specified.
In the above example, Spring will go through application context and create a bean(object) of type Sample1. When it will construct bean Sample2 it sees #Autowiredannotation and will look for instantiated bean of type Sample1. When it finds bean of type Sample1 it will inject that bean on Sample2 and finish creating Sample2. This is called dependency injection and is one of the very popular features of Spring framework.
Hope this helps.
Related
I have been given a task to assign a property from .properties file to a non Spring bean class using #Value annotation. To do this, I created a method on a #Component annotated class and set the property into it, then called that method from the non Spring bean class. I thought this would work, however, still showing as null.
I was told this is because the #Component annotated class I used is not spring loaded. Question, how can I tell if a class is Spring loaded bean? I have been searching on google but can't find anything helpful aside from examples with #Component or #Configuration annotations. Thanks.
Spring Container is responsible for creating or managing beans. It all satisfy the dependencies by injecting them either through constructor or setter method. But in your case you want the #Value injection in your non spring bean which is really not possible as per my understanding. Because here the spring does not creating the object then how it satisfy the dependencies of it.
You have two options for this situation.
Either annotate class using #Component
Either read property file using Properties
https://www.mkyong.com/java/java-properties-file-examples/
I claimed that:
Spring read bean definitions from java config
BeanFactory Create beans from defenitions
Then dependencies are injected by BeanPostProcessors
But it happened that it's not accurate:
#Configuration
#ImportResource("classpath:spring_config.xml")
public class JavaConfig {
#Autowired
MyBean bean;
#Bean
public Boolean isBeanAutowired(){
return bean != null;
}
}
The isBeanAutowired bean was initialized with true.
Question:
How does it happen that Autowired logic work before all beans in context were initialized?
Yes #Autowired is handled by a BeanPostProcessor. See org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor for more details and as an entrypoint if you try to find out more on this.
https://github.com/spring-projects/spring-framework/blob/master/spring-beans/src/main/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java
http://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.html
In the end Spring is able to analyze the dependencies of a bean (the other beans that need to be wired) and determine an order, in which the beans will be initialized. Thereby it is possible, to autowire directly after creation of a bean. There is one exception, which occurs when Spring tries to resolve circular dependencies. Then Spring will create both beans and autowire them to each other. This works only limited though.
I want to create a simple app built on Spring Boot. I don't have experience with the framework, and this issue is beyond me.
I want to use the Security module in my app, particularly the org.springframework.security.authentication.encoding.ShaPasswordEncoder class. I want to get this autowired as a bean.
How do I define that in Spring Boot? For instance, this answer explains how to use it in "regular" Spring - defining the bean in a XML file. However, when I was reading the Boot overview, it stated that there is no need for XML setup, so is it possible to somehow do this in code, not configuration? What is the Boot-ish way?
I have tried just using this code to get the encoder class.
#Autowired
private ShaPasswordEncoder passwordEncoder;
But I get an exception: org.springframework.beans.factory.NoSuchBeanDefinitionException -- it's not defined, but how do I define it?
Thanks
The beans can be defined in-code like this:
#Bean
public ShaPasswordEncoder passwordEncoder() {
return new ShaPasswordEncoder();
}
This method will create a bean named "passwordEncoder" which will be managed by Spring. It is the equivalent of the XML-styled
<bean class="org.springframework.security.authentication.encoding.ShaPasswordEncoder" id="passwordEncoder" />
You can put this method in the #SpringBootApplication-annotated class, or any class annotated with #Configuration.
More info here
I'm trying to get a better understanding of the #Autowired annotations component scanning, but all the examples I found so far use context.getBean(..) at some point to get at least one Bean to start with.
I also read that doing that is considered bad practice , but I can't seem to find any information on how to do it without context.getBean(..)
Could somebody please enlighten me with an example and information on how to do this ?
Define your bean in xml and use
<context:component-scan base-package="com" />
<mvc:annotation-driven />
Bean def
<bean id="processEngine" class="com.processEngine">
<property name="processEngineConfiguration" ref="processEngineConfiguration" />
</bean>
now you can get bean as following
#Autowired
private ProcessEngine processEngine;
how it works
spring scans the bean's recipes either from xml or java configuration. then spring creates a beanDefinitions which are 'loaded' into BeanFactory. BeanFactory triggers a set of BeanPostProcessors (BPP) which are scanning classes for particular annotations like Autowired/Resource/PostProcessor and etc. and do appropriate actions. in case when your class contains #Autowired annotation, AutowiredAnnotationBeanPostProcessor would auto wire required field (dependencies), and when creation of an object is done and all BPP worked out, object is ready to be used by the app, from this point your code can get 'ready' objects from container.
there are some cases when you would need to access this beans from the code which is out of spring's control and not managed by container. in order to do so, you would need to get the ApplicationContext (container) and call #getBean specifying either name or type. using applicationContext directly is not a good practice because there are some problems that you can come to, f.ex. id of a bean might be changed and if you refer to bean by id then NPE would be thrown.
configuration
there are several approaches to configure spring to scan the classes for finding bean recipes. one would be defining component-scan, in this case classes which are located in the path that you've set and having any of valid spring annotations like #Component, #Service, #Repository, #Controller (for web container) would be considered. another way would be specifying each bean separately using <bean> or #Bean.
examples.
if you want to create a web app then you should see DispatcherServlet with ContextLoaderListener classes. this classes would boot your app and load everything according to configuration. f.ex. here,
but if you want to create a desktop app, then you would end up with something like this
From time to time (usually when not using Spring Boot), I use something along the lines of the following code:
public static <T> T autowire(ApplicationContext ctx, T bean) {
ctx.getAutowireCapableBeanFactory().autowireBean(bean);
return bean;
}
In my main, I create an instance of the main application class that contains a few #Autowired annotations for the main services / entry points to my Spring application.
I was reading Spring 3.0 documentation and I came to the sentence -
Annotation injection is performed before XML injection, thus the latter configuration
will override the former for properties wired through both approaches.
Next the question came to my mind:
If I use an annotation in a bean (like #Service("myService")), now I am using the other bean and it uses "myService", and "myService" would be injected through XML configuration.
Would this work? I tried but it is giving me
BeanCreationException (Cannot resolve reference to bean 'myService' while setting bean property 'myService')
Later, I went through this question Wiring Spring bean through annotations and xml context, but in the solution it is told that "Just leave all your annotated fields unspecified, and they'll get injected auto-magically." (I didn't try out this solution)
But what if I want to specify all annotated fields, like I specified #Service annotation above?
Any suggestions??
I figured out the answer, it works very well. Actually I forgot to add tag in xml configuration. We need to add this tag in each config file i.e. if you have written config file for service layer beans, add tag for service layer annotated beans. Similar for dao and controller layers.
You need to autowire your constructor like below...
#Autowired(required=true)
public UserService(DataSource dataSource){
this.userDS = new UserDS(dataSource);
}
So, in the above code the DataSource will be injected in the UserService automatically.