I am working on a core java framework. I don't want to create instances directly inside the class which is why I want to use dependency injection.
I am thinking of declaring my custom annotations on the fields to be instantiated. And having a call back function which would create an instance and inject it into the field.
I had tried to create a custom annotation. But looks like there's no direct way to get a callback on the declared annotation. So, I was trying to scan the classes for that. But I ended up with this problem
Java Scanning Class for Annotation using Google Reflections
Please let me know if this is the right way of achieving this.
Since your question is tagged 'Spring', you can use Spring Framework's bean annotations (#Component / #Service / #Repository / ...), classpath scanning and #Autowired.
For example:
Setup classpath scanning on your spring config xml:
<context:component-scan base-package="com.mycompany.myapp" />
Create your bean to be scanned. Spring container will automatically create a singleton instance of this bean using default constructor:
#Repository
public class FooDAO {
...
}
Inject reference to above DAO instance using DI + autowiring
#Service
public class FooService {
#Autowired private FooDAO fooDAO;
...
}
Related
I know that there are questions similar to this one, but none of them have helped me. I'm following along this tutorial, and the part I can't wrap my mind around is:
#SpringBootApplication
public class Application {
private static final Logger log =
LoggerFactory.getLogger(Application.class);
public static void main(String[] args) {
SpringApplication.run(Application.class);
}
#Bean
public CommandLineRunner demo(CustomerRepository repository) {
return (args) -> {
// save a couple of customers
...
// more lines, etc...
What I don't understand is where the repository passed into demo comes from. I know that the Autowired annotation can do something like that, but it isn't used at all here.
The more specific reason I ask is because I'm trying to adapt what they do here to an application I'm working on. I have a class, separate from all of the persistence/repository stuff, and I want to call repository methods like save and findAll. The issue is that the repository is an interface, so I can't instantiate an object of it to call the methods. So do I have to make a new class that implements the interface and create an object of that? Or is there an easier way using annotations?
When creating a #Bean, adding the repository in the parameters of the bean is enough to wire the repos in your bean. This works pretty much like adding #Autowired annotation inside a class that is annotated as #Component or something similar.
Spring works mostly with interface, since that is simplier to wire vs wiring concrete classes.
Can you try #Repository before the declaration of class? Worked for me in a Spring MVC structure.
#Repository
public class EntityDAOImpl implements EntityDAO{
...
}
The thing to wrap your head around is a Spring Boot application at startup time aims to resolve its dependancy tree. This means discovering and instantiating Beans that the application defines, and those are classes annotated with #Service, #Repository, etc.
This means the default constructor (or the one marked with #Autowire) of all beans is invoked, and after all beans have been constructed the application starts to run.
Where the #Bean annotation comes into play is if you have a bean which does not know the values of it's constructor parameters at compile time (e.g. if you want to wire in a "started at" timestamp): then you would define a class with an #Configuration annotation on it, and expose an #Bean method in it, which would return your bean and have parameters that are the beans dependencies. In it you would invoke the beans constructor and return the bean.
Now, if you want a certain method of some class to be invoked after the application is resolved, you can implement the CommandLineRunner interface, or you can annotate a method with #PostConstruct.
Some useful links / references:
https://docs.spring.io/spring-javaconfig/docs/1.0.0.m3/reference/html/creating-bean-definitions.html
https://www.baeldung.com/spring-inject-prototype-bean-into-singleton
Running code after Spring Boot starts
Execute method on startup in Spring
I am developing a REST API using spring boot. Following is my package structure
Getting the following exception when I try to start my application
***************************
APPLICATION FAILED TO START
***************************
Description:
Field articleRepository in com.abc.service.ArticleService required a bean of type 'com.abc.dao.ArticleRepository' that could not be found.
Action:
Consider defining a bean of type 'com.abc.dao.ArticleRepository' in your configuration.
Following is my project structure-
com.abc
com.abc.bean
-Article.java
com.abc.controller
-ArticleController.java
com.abc.dao
-ArticleRepository.java (Interface)
com.abc.service
-ArticleService.java
com.abc.web
-AbcApplication.java (main Springboot class)
In AbcApplication.java as it is not in the root package, I have the below annotations
#SpringBootApplication
#ComponentScan(basePackages="com.abc.*")
I tried few ways -
I moved AbcApplication.java to root package com.abc but no success
Instead of interface(ArticleRepository.java) I made it a class, it is working
I keep it as interface but changed annotation from #Repository to #Service/Component still no success.
I am confused how it is working if I change it to class instead of interface.
#Repository
public interface ArticleRepository {
}
You don't have any bean for ArticleRepository. If you will use Spring Data Jpa you have to extend a type of Repository: https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#repositories.
If you will use your own repository, you must implement it.
If you are using any RDBMS and if ArticleRepository repository is responsible for interacting with your database, then you need to either extends CrudRepository or JpaRepository in your ArticleRepository, then only Spring will be able to create the bean of ArticleRepository,and you will be able to autowire your repository.
If you are not extending any of CrudRepository or JpaRepository,then at the time of bean creation,ArticleRepository is only plain java interface and a plain interface can not be instantiated.
And as for your question:
Instead of interface(ArticleRepository.java) I made it a class, it is working
Its because when you declare it as a class, then Spring does instantiate a concrete class, so actual object will be created at the bean creation time and everything will be working as it should be.
Ideally you should have one class that implements interface ArticleRepository . Annotate that class with #Repository, spring will take care of wiring.
I was also encountered with the same issue. What I missed was the spring-boot-starter-jpa dependency. It worked when I added this dependency in the pom.xml file.
MyBatis gives you two samples to access your databases with spring boot.# Component or #Mapper.
Sample link:
mybatis-spring-boot-sample-xml
Suggestion:
Show your ArticleRepository code fully.
interface does not require any annotation (neither #repository not #Service).
If its a DAO file add #Repository annotation to the class implementing the corresponding interface.
And, if its a service then add #Service annotation to the class implementing the corresponding interface.
I'm migrating code from JEE to SpringBoot. I was using cool dynamic injection in JEE with javax.enterprise.inject.Instance class:
Just annotating:
#Inject
private Instance<CCIntentHandler> allMycandidates;
Will make allMycandidates be filled with all classes inheriting CCIntentHandler interface in my classpath which then I can iterate simply with:
Iterator<CCIntentHandler> iterator = allMycandidates.iterator()
Nothing more needed. How can I achieve this in Spring Boot?
Thanks
Spring will inject all instances of Foo if you #Autowire a List<Foo>.
So, the Spring equivalent of ...
#Inject
private Instance<CCIntentHandler> allMycandidates;
... is:
#Autowire
private List<CCIntentHandler> allMycandidates;
Update 1 in response to this comment:
Do CCIntentHandler interface or classes implementing this interface need any Spring annotations?
Spring must be aware of any instances of CCIntentHandler, this could achieved as follows:
Annotate each class implementing CCIntentHandler with #Component and ensure that these classes are scanned by Spring Boot
Or
Provide a public method to return each class implementing CCIntentHandler and annotate each of these public methods with #Bean and ensure that the class which contains these public methods is annotated with #Configuration and that this configuration class is scanned by Spring Boot.
More details on bean declaration and dependency injection in the docs.
Unfortunately
#Autowire
private List<CCIntentHandler> allMycandidates;
is not
#Inject
private Instance<CCIntentHandler> allMycandidates;
because we can't select an instance from the list depending on type or annotation.
I've spent some time to find alternative in Spring, but looks like there is no equivalent...
We should defenetly bring that feature to Spring!
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