This question already has answers here:
What exactly is Field Injection and how to avoid it?
(4 answers)
Closed 2 years ago.
can anyone tell me why #autowired is saying field injection is not recommended and the TextWriter object 'text' also says it could not autowire because there is more than one bean of textwriter type.
#RestController
public class HelloWorld{
#Autowired
TextWriter text
public HelloWorld(TextWriter text){
this.text = text;
}
#RequestMapping("/")
public String index(){
return "Hello World";
}
}
can anyone tell me why #autowired is saying field injection is not
recommended ?
For a design reason. Injecting beans directly into fields makes your dependencies "hidden" and encourage bad design :
the class API (public/protected member) doesn't specify them while they exist.
no way to unit test without reflection or a Spring container (the most important part for me)
you may finish by declaring potentially many injected fields. Which may make your class with a strong coupling to other classes without that you are "really" aware of that.
Generally constructor injection should be favored (no need to annotate the constructor with #Autowired since Spring 4) if few fields, otherwise setters should be the way.
Both ways don't have all drawbacks mentioned above.
I assume Spring does not see your TextWriter class as a bean. Probably TextWriter is in another package. For example, in the “models” package. If you use Spring Boot, then this will help you:
#SpringBootApplication(scanBasePackages={"com.programwithwaqas.restservice.models"})
I recommend you read the #SpringBootApplication annotation. Pay attention to the annotation #ComponentScan.
This is in case you are not using Spring Boot
#ComponentScan(basePackages={"com.programwithwaqas.restservice.models"})
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
This question already has answers here:
Difference between #Bean and #Autowired
(6 answers)
Closed 4 years ago.
What is the difference between #Bean and #Autowired in Spring?
As far as I understood we can use both to inject dependencies, #Autowired when the interface was implemented just in one class, and #Bean, when the interface was implemented in more than one class, in the last case #configuration, is required.
thanks in advance
In short #Bean is producer and #Autowired is consumer, #Bean annotation let spring know instance of this class and it hold for it and #Autowired annotation ask for please give me instance of class which we created with #Bean annotation.
more detailed answer find here
When you use #Bean you are telling to Spring that:
this is the object that I want you to put within your stack and later I will ask
about it from you
And When you use #Autowired you are telling to Spring that:
Now please give me the object that I already told you to keep it in your stack (means the #Bean object)
When you use #Bean you are responsible for providing an Id and calling that Id when you wish to use that particular object using getBean() method.
Autowired helps avoid the calling part and returns an object everytime it is needed. Spring handles the job of returning the appropriate object and helps reduce additional syntax for referring to a particular bean.
Spring provides a very nice documentation about Autowired and Bean API
#BEAN
#Target(value={METHOD,ANNOTATION_TYPE})
#Retention(value=RUNTIME)
#Documented
public #interface Bean
Indicates that a method produces a bean to be managed by the Spring
container.
On Bean #Target annotation confirms that it can be applied over a METHOD.
#AUTOWIRED
#Target(value={CONSTRUCTOR,METHOD,PARAMETER,FIELD,ANNOTATION_TYPE})
#Retention(value=RUNTIME)
#Documented
public #interface Autowired
Marks a constructor, field, setter method or config method as to be autowired by Spring's dependency injection facilities. This is an alternative to the JSR-330 Inject annotation.
On Autowired #Target confirms that it can be applied over a CONSTRUCTOR,METHOD,PARAMETER,FIELD.
IoC is also known as dependency injection (DI). It is a process whereby objects define their dependencies, that is, the other objects they work with, only through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method. The container then injects those dependencies when it creates the bean.
I'm totally lost on this topic . I've researched Spring Autowiring on internet trying to figure out what it does and everything I find is mostly saying "Spring Autowiring helps you autowire collaborating Beans " and thats it . I would greatly appreciate if someone can explain to me from scratch what is Spring Bean, What is Autowiring and how these two work together with examples and explanations of this examples. Don't just say that "Spring Autowiring is autowiring two beans " because I don't get it what it means .
What I understand right now is that lets say if have a simple class :
Public class Car(){
public int numberOfWheels;
}
and we declare this class as a Bean, we than can create the instance of the object of this class without saying "new" keyword. So we can just declare it in a bean and insert value of this "numberOfWheels" property outside the java class . I might be super wrong here or using very bad example , because I've been trying to learn Spring Framework and it's just been very hard for me , so any help would be greatly appreciated.
Well, Spring is a Dependency Injection framework, so you should start learning about this.
DEPENDENCY INJECTION
This is a technique whereby one object supplies the dependencies of another object. Such as, for example: object a of type A need is dependent of an object b of type B. In a code example:
Customer it's dependent of Person:
public class Person {
private String name;
private String lastName;
}
public class Customer {
private Person person;
}
In this case, any Customer object will have a Person object dependency.
Now, in Spring scope, all objects are named beans, so this is a bean, an Object that it's injected into Spring Context.
Spring provides a Dependency Injection mechanism that it's very easy. For our example, you can put put some of the following annotation: #Component, #Bean, #Service on class Person, and Spring will create an object of type Person with calling default constructor. Afer that, in class Customer, you can put #Autowired annotation on top of the Person person attribute. This annotation, will tell Spring to search for a specific bean of type Person that was injected into the Spring Context, and Spring using Reflection(searching by name) will find a Person object type that was creating by using one of the #Component, #Bean, #Service annotations. After that, Spring will pass the reference of the Person object that was found to the Customer object that requires it.
Spring Bean is an object which is instantiated and maintained automatically by Spring Framework. They usually depend on each other in some way, and the dependencies therefore must be resolved.
Autowiring is one of the ways how to resolve these dependencies. You don't manually specify which concrete dependency should be provided to the class, you just choose the way how Spring should automatically find the correct dependency (by matching the class's property name and name of the desired bean, for instance).
Spring's documentation is very extensive and you can find a lot of useful information there. For more info about Spring Beans, you can read this, if you want to know more about autowiring, try this.
To basically, Spring has a bean pool which means Spring creates java objects and put them into a pool. Whenever you want to use an object, you fetch object from pool using #Autowired annotation.
Spring basically uses Reflection to create objects by its own, instead of passing objects from class to class. This is what basically Autowire is.
This concept is called Dependency Injection. Spring injects components, services, repositories whenever you want.
This question already has answers here:
Why is my Spring #Autowired field null?
(21 answers)
Closed 5 years ago.
Lets say that I have a model for an object X, this object implements all the CRUD operations with the help of Spring Boot.
Now, I need to be able to edit this object using an standard POJO. The POJO looks like this:
public class Foo {
#Autowired
private XRepository xDAO;
/*
Do whatever I want with X and then save it again in the DB using xDAO
*/
}
So far I've tried using #Configurable, #Component and even #Service, but neither of those can #Autowire my XRepository.
What can I do?
What you describe is not possible. Components can be wired only if the object is managed by Spring. In your case it's not and as such it's not possible to autowire in any dependency. You have various options. Here are some:
Use repository outside of Foo class. Orchestrate the operations in another
class that is managed by Spring
Pass repository as dependency to
Foo in the constructor by class that is managed by Spring
This is somewhat hacky and probably not recommended but you can make repository a static variable in Foo and set it up by Spring managed component in something like #PostConstruct
In my opinion the best is to use option 1.
I think that I didn't express myself sufficiently, either way I found a solution to my problem.
The solution is here.
I only declared Foo as a #Service, then I just #Autowired Foo:
#Autowired
public class Foo {
#Autowired
private XRepository xDAO;
//some code
}
Then I call this class using the #Autowired annotation
#Autowired Foo foo
foo.doThings();
In my ApplicationContext I have several Beans being created the same style. So I have a lot of dublicated code writing a FactoryBean for each of this beans. Those beans have a common ground, implementing all one special interface.
I would like to move all that bean creation to one factory. That one would have to provide a methode like this
<T extends CommonInterface> T createInstance(Class<T> clazz);
There I could implement all the instantiation necessary to create one of my special beans.
My implementation would be called by spring for
#Autowired
private MyCommonInterfaceImplementation impl;
in that way
createInstance(MyCommonInterfaceImplementation.class)
So far I looked at BeanFactory and FactoryBean, both seem not to be I'm searching for.
Any suggestions?
why not use #bean
#Bean
public MyCommonInterfaceImplementation getMyCommonInterfaceImplementation(){
return MyBeanFactory.createInstance(MyCommonInterfaceImplementation.class);
}
//should autowire here
#Autowired
private MyCommonInterfaceImplementation impl;
Basically you need the #Bean annotation on a "factory" only if you need some special handling during the creation of a bean.
If everything can be #Autowired, either by setters, fields, or one constructor, and nothing else needs to be done on a bean during initialization, you can simply declare the annotation #Component on each implementation of your interface. This works as long as you have component scanning active inside your application. The result will be that for each component spring will create a bean which you can use.
I'm writing this on a mobile so showing code is not the best. Just follow some tutorial on #ComponentScan, or if you need, let me know and I can augment this answer with an example.
As of Spring 4.3 you no longer have to annotate your bean classes and you can let them be instantiated via a componentscan.
#Configuration
#ComponentScan(
value = "some.package.path",
includeFilters = {
#Filter(type = ASSIGNABLE_TYPE, value = {
MyClass1.class,
MyClass2.class,
MyClass3.class
})
})
This actually creates beans for the three classes listed there. The example should work without filters as well (everything in the package becomes a bean). This works as long as the classes have a single constructor that can be used for autowiring. I don't think it is possible to filter for all implementations of a particular interface and then register a bean.
To do that, you might do something with a ContextListener and e.g. use reflection to find out what classes to instantiate and then use context.autowire(..) to inject any dependencies from your context. A bit hacky but it might work.
#Override
public void onApplicationEvent(ContextRefreshedEvent event) {
ApplicationContext context = event.getApplicationContext();
MyClass bean
= context
.getAutowireCapableBeanFactory()
.autowire(MyClass.class, Autowire.BY_NAME.value(), true);
...
}
That still leaves the problem of how to get the bean registered in the context of course.
You might also be able to adapt the answer to this SO question on how to add beans programmatically.
Finally the best approach I've found is using a ConfigurationClassPostProcessor. As example I've used https://github.com/rinoto/spring-auto-mock
But, since it is quite complicated and "too much magic" to create beans from nothing, we decided to explicitly create those beans via #Bean.
Thanks for your answers.