I have the following bean / constructor definitions:
#Configuration
class Configuration {
#Bean
public List<Something> getSomethings(MyFancyStuff stuff, #Autowired Bar bar) {
//...
}
}
#Component
class SomeOtherThing {
public SomeOtherThing(MyFancyStuff stuff, #Autowired Bar bar) {
//...
}
}
Is it possible to extends dependency resolution to provide a custom resolver when a specific class or annotation is found for a given parameter? I looked at PropertyPlaceholderConfigurer and InstantiationAwareBeanPostProcessor but nothing seemed to help me write my own value provider.
As a context: I implemented a custom scope which creates many instances of a given bean for each configuration object it has. I want to pass this configuration object to the bean-creation-process of said scope without adding it to the application context. I don't want to add it to the application context because it is an object which no other object should be able to obtain through dependency injection. I need to extend the DI-process of spring because I want to support field injection, constructor injection and bean factory-methods like the shown getSomethings
note: this is not about automatic value conversion of SpringMVC request parameters.
You can use #Conditional Annotation
#Bean(name="dataSource")
#Conditional(value=DevCondition.class)
public Util getSource1() {
return new DevUtil();
}
#Bean(name="dataSource")
#Conditional(ProdCondition.class)
public Util getSource2() {
return new ProdUtil();
}
you can also create bean based on property value using #Profile
Right now i have a inherited project that is using annotation based spring dependency injection. So all classes are simply marked with #Component (or specific stereoTypes like #service, #Repository,#RestController, etc). This makes it a little hard to find where the dependency is located and i was thinking to change it so that each package has its own dependency configuration and then add each package to the #ComponentScan afterwards.
So for example if i had a package called com.mycoolpackage.login and mycoolpackage.networking
then i'd have a Spring configuration like this in first package:
#Configuration
public class LoginDIConfig {
#Bean
public LoginServiceImpl loginServiceImpl() {
return new LoginServiceImpl();
}
}
and in the second package i'd have the following:
#Configuration
public class NetworkDIConfig {
#Bean
public NetworkServiceImpl networkServiceImpl() {
return new NetworkServiceImpl();
}
}
and my#ComponentScan would look like this:
#ComponentScan(basePackages = {"com.mycoolpackage.login","com.mycoolpackage.network"})
So i have two questions about this approach.
How can i use a #Service annotation instead of bean here
Do you think this design is more easier as it tells you what your package dependencies are very easily instead of hunting them
down.
If you want to configure some been properties manually then you should go for above configuration else you should stick with exiting one.
This makes it a little hard to find where the dependency is located
#Autowire Or #Inject annotation will always lead you to dependency class.
I'm new in Spring and this is my first post here, I'm starting to use spring annotations, I was able to cofigure my project using XML configuration and now I'm able to configure it using only annotations and avoiding XML.
But my current need is to avoid the use of annotations in my java classes (beans) and use it only in my AppConfig.java class which I use to configure Spring.
This is my current working configuration:
AppConfig.java
#Configuration
#ComponentScan(basePackageClasses= {BlocBuilder.class,... all my classes go here})
public class AppConfig {
#Bean
#Scope("prototype")
public BlocBuilder blocBuilder(){
return new BlocBuilder();
}
}
And this is one of my java classes.
BlocBuilder.java
public class BlocBuilder {
#Autowired
#Qualifier("acServices")
private SomeInterface someInterface;
public SomeInterface getSomeInterface() {
return someInterface;
}
public void setSomeInterface(SomeInterface someInterface) {
this.someInterface = someInterface;
}
What I want to achieve is to avoid the use of annotations in my classes for example in my BlocBuilder.java class I don't want to have annotations and move them to my config class.
How can I manage it?
Any help whould be really appreciated.
Constructor injection is what you're looking for. Create a 1-arg constructor in BlocBuilder which takes SomeInterface type argument. And then in config class, pass it as argument:
#Configuration
#ComponentScan(basePackageClasses= {BlocBuilder.class,... all my classes go here})
public class AppConfig {
#Bean
public SomeInterface someInterface() {
return new SomeInterfaceImpl();
}
#Bean
#Scope("prototype")
public BlocBuilder blocBuilder(){
return new BlocBuilder(someInterface());
}
}
I wouldn't recommend a config class. It'll couple all your beans together.
Spring configuration doesn't need to be an all or nothing thing: annotations or XML. You can mix and match as you choose. Put annotations in the classes you want and use XML for the rest.
My goal is a framework where concrete types of beans can be easily changed by a properties file. I also prefer annotations to XML. Ideally I'd to use a combination of #Resource and SpEL like this:
#Resource(type="#{myProperties['enabled.subtype']}")
SomeInterface foo;
where I've loaded myProperties with a PropertiesFactoryBean or <util:properties> from a file that includes:
enabled.type = com.mycompany.SomeClassA; // which implements SomeInterface
This doesn't work because the argument of type must be a literal, i.e., no SpEL allowed. What's the best practice here?
Update: See my answer below.
This is exactly the use case for Spring Java Configuration.
http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/beans.html#beans-java
Or you can alternatively make a Factory.
Using: org.springframework.beans.factory.FactoryBean<SomeInterface>
The name of the bean that implements FactoryBean will be seen as a "SomeInterface" even though its not.
I think it is not possible, the solution I tend to adopt is to use a factory that creates the different objects depending on a configuration property (enabled.type in your example).
A second alternative could be to use injection by name:
#Resource(name="beanName")
And last, if you use Spring 3.1+ you can try to use profiles, and have different bean sets in different profiles, if that solves your problem.
Spring's Java Configuration and Bean Definition Profiles turn out to be exactly what I was looking for (thanks #Adam-Gent and #Guido-Garcia). The former seems necessary for the dynamic element, and the latter promotes a better practice.
Here's a solution with Java config and properties:
#Configuration
public class SomeClassConfig {
#Value("#{myProperties['enabled.subtype']}")
public Class enabledClass;
#Bean SomeInterface someBean()
throws InstantiationException, IllegalAccessException {
return (SomeInterface) enabledClass.newInstance();
}
}
Here's a slightly less dynamic solution with profiles.
#Configuration
#Profile("dev")
public class DevelopmentConfig {
#Bean SomeInterface someBean() {
return new DevSubtype();
}
}
#Configuration
#Profile("prod")
public class ProductionConfig {
#Bean SomeInterface someBean() {
return new ProdSubtype();
}
}
With profiles, the active profile(s) are declared using one of a variety of methods such as via system property, JVM property, web.xml, etc. For example, with a JVM property:
-Dspring.profiles.active="dev"
I understand that #Component annotation was introduced in spring 2.5 in order to get rid of xml bean definition by using classpath scanning.
#Bean was introduced in spring 3.0 and can be used with #Configuration in order to fully get rid of xml file and use java config instead.
Would it have been possible to re-use the #Component annotation instead of introducing #Bean annotation? My understanding is that the final goal is to create beans in both cases.
#Component
Preferable for component scanning and automatic wiring.
When should you use #Bean?
Sometimes automatic configuration is not an option. When? Let's imagine that you want to wire components from 3rd-party libraries (you don't have the source code so you can't annotate its classes with #Component), so automatic configuration is not possible.
The #Bean annotation returns an object that spring should register as bean in application context. The body of the method bears the logic responsible for creating the instance.
#Component and #Bean do two quite different things, and shouldn't be confused.
#Component (and #Service and #Repository) are used to auto-detect and auto-configure beans using classpath scanning. There's an implicit one-to-one mapping between the annotated class and the bean (i.e. one bean per class). Control of wiring is quite limited with this approach, since it's purely declarative.
#Bean is used to explicitly declare a single bean, rather than letting Spring do it automatically as above. It decouples the declaration of the bean from the class definition, and lets you create and configure beans exactly how you choose.
To answer your question...
would it have been possible to re-use the #Component annotation instead of introducing #Bean annotation?
Sure, probably; but they chose not to, since the two are quite different. Spring's already confusing enough without muddying the waters further.
#Component auto detects and configures the beans using classpath scanning whereas #Bean explicitly declares a single bean, rather than letting Spring do it automatically.
#Component does not decouple the declaration of the bean from the class definition where as #Bean decouples the declaration of the bean from the class definition.
#Component is a class level annotation whereas #Bean is a method level annotation and name of the method serves as the bean name.
#Component need not to be used with the #Configuration annotation where as #Bean annotation has to be used within the class which is annotated with #Configuration.
We cannot create a bean of a class using #Component, if the class is outside spring container whereas we can create a bean of a class using #Bean even if the class is present outside the spring container.
#Component has different specializations like #Controller, #Repository and #Service whereas #Bean has no specializations.
Let's consider I want specific implementation depending on some dynamic state.
#Bean is perfect for that case.
#Bean
#Scope("prototype")
public SomeService someService() {
switch (state) {
case 1:
return new Impl1();
case 2:
return new Impl2();
case 3:
return new Impl3();
default:
return new Impl();
}
}
However there is no way to do that with #Component.
Both approaches aim to register target type in Spring container.
The difference is that #Bean is applicable to methods, whereas #Component is applicable to types.
Therefore when you use #Bean annotation you control instance creation logic in method's body (see example above). With #Component annotation you cannot.
I see a lot of answers and almost everywhere it's mentioned #Component is for autowiring where component is scanned, and #Bean is exactly declaring that bean to be used differently. Let me show how it's different.
#Bean
First it's a method level annotation.
Second you generally use it to configure beans in Java code (if you are not using xml configuration) and then call it from a class using the
ApplicationContext.getBean method. Example:
#Configuration
class MyConfiguration{
#Bean
public User getUser() {
return new User();
}
}
class User{
}
// Getting Bean
User user = applicationContext.getBean("getUser");
#Component
It is the general way to annotate a bean and not a specialized bean.
It is a class level annotation and is used to avoid all that configuration stuff through java or xml configuration.
We get something like this.
#Component
class User {
}
// to get Bean
#Autowired
User user;
That's it. It was just introduced to avoid all the configuration steps to instantiate and use that bean.
You can use #Bean to make an existing third-party class available to your Spring framework application context.
#Bean
public ViewResolver viewResolver() {
InternalResourceViewResolver viewResolver = new InternalResourceViewResolver();
viewResolver.setPrefix("/WEB-INF/view/");
viewResolver.setSuffix(".jsp");
return viewResolver;
}
By using the #Bean annotation, you can wrap a third-party class (it may not have #Component and it may not use Spring), as a Spring bean. And then once it is wrapped using #Bean, it is as a singleton object and available in your Spring framework application context. You can now easily share/reuse this bean in your app using dependency injection and #Autowired.
So think of the #Bean annotation is a wrapper/adapter for third-party classes. You want to make the third-party classes available to your Spring framework application context.
By using #Bean in the code above, I'm explicitly declare a single bean because inside of the method, I'm explicitly creating the object using the new keyword. I'm also manually calling setter methods of the given class. So I can change the value of the prefix field. So this manual work is referred to as explicit creation. If I use the #Component for the same class, the bean registered in the Spring container will have default value for the prefix field.
On the other hand, when we annotate a class with #Component, no need for us to manually use the new keyword. It is handled automatically by Spring.
When you use the #Component tag, it's the same as having a POJO (Plain Old Java Object) with a vanilla bean declaration method (annotated with #Bean). For example, the following method 1 and 2 will give the same result.
Method 1
#Component
public class SomeClass {
private int number;
public SomeClass(Integer theNumber){
this.number = theNumber.intValue();
}
public int getNumber(){
return this.number;
}
}
with a bean for 'theNumber':
#Bean
Integer theNumber(){
return new Integer(3456);
}
Method 2
//Note: no #Component tag
public class SomeClass {
private int number;
public SomeClass(Integer theNumber){
this.number = theNumber.intValue();
}
public int getNumber(){
return this.number;
}
}
with the beans for both:
#Bean
Integer theNumber(){
return new Integer(3456);
}
#Bean
SomeClass someClass(Integer theNumber){
return new SomeClass(theNumber);
}
Method 2 allows you to keep bean declarations together, it's a bit more flexible etc. You may even want to add another non-vanilla SomeClass bean like the following:
#Bean
SomeClass strawberryClass(){
return new SomeClass(new Integer(1));
}
You have two ways to generate beans.
One is to create a class with an annotation #Component.
The other is to create a method and annotate it with #Bean. For those classes containing method with #Bean should be annotated with #Configuration
Once you run your spring project, the class with a #ComponentScan annotation would scan every class with #Component on it, and restore the instance of this class to the Ioc Container. Another thing the #ComponentScan would do is running the methods with #Bean on it and restore the return object to the Ioc Container as a bean.
So when you need to decide which kind of beans you want to create depending upon current states, you need to use #Bean. You can write the logic and return the object you want.
Another thing worth to mention is the name of the method with #Bean is the default name of bean.
Difference between Bean and Component:
#component and its specializations(#Controller, #service, #repository) allow for auto-detection
using classpath scanning. If we see component class like #Controller, #service, #repository will be scan automatically by the spring framework using the component scan.
#Bean on the other hand can only be used to explicitly declare a single bean in a configuration class.
#Bean used to explicitly declare a single bean, rather than letting spring do it automatically. Its make septate declaration of bean from the class definition.
In short #Controller, #service, #repository are for auto-detection and #Bean to create seprate bean from class
- #Controller
public class LoginController
{ --code-- }
- #Configuration
public class AppConfig {
#Bean
public SessionFactory sessionFactory()
{--code-- }
Spring supports multiple types annotations such as #Component, #Service, #Repository. All theses can be found under the org.springframework.stereotype package.
#Bean can be found under the org.springframework.context.annotation package.
When classes in our application are annotated with any of the above mentioned annotation then during project startup spring scan(using #ComponentScan) each class and inject the instance of the classes to the IOC container. Another thing the #ComponentScan would do is running the methods with #Bean on it and restore the return object to the Ioc Container as a bean.
#Component
If we mark a class with #Component or one of the other Stereotype annotations these classes will be auto-detected using classpath scanning. As long as these classes are in under our base package or Spring is aware of another package to scan, a new bean will be created for each of these classes.
package com.beanvscomponent.controller;
import org.springframework.stereotype.Controller;
#Controller
public class HomeController {
public String home(){
return "Hello, World!";
}
}
There's an implicit one-to-one mapping between the annotated class and the bean (i.e. one bean per class). Control of wiring is quite limited with this approach since it's purely declarative. It is also important to note that the stereotype annotations are class level annotations.
#Bean
#Bean is used to explicitly declare a single bean, rather than letting Spring do it automatically like we did with #Controller. It decouples the declaration of the bean from the class definition and lets you create and configure beans exactly how you choose. With #Bean you aren't placing this annotation at the class level. If you tried to do that you would get an invalid type error. The #Bean documentation defines it as:
Indicates that a method produces a bean to be managed by the Spring container.
Typically, #Bean methods are declared within #Configuration classes.We have a user class that we needed to instantiate and then create a bean using that instance. This is where I said earlier that we have a little more control over how the bean is defined.
package com.beanvscomponent;
public class User {
private String first;
private String last;
public User(String first, String last) {
this.first = first;
this.last = last;
}
}
As i mentioned earlier #Bean methods should be declared within #Configuration classes.
package com.beanvscomponent;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
#Configuration
public class ApplicationConfig {
#Bean
public User superUser() {
return new User("Partho","Bappy");
}
}
The name of the method is actually going to be the name of our bean. If we pull up the /beans endpoint in the actuator we can see the bean defined.
{
"beans": "superUser",
"aliases": [],
"scope": "singleton",
"type": "com.beanvscomponent.User",
"resource": "class path resource
[com/beanvscomponent/ApplicationConfig.class]",
"dependencies": []
}
#Component vs #Bean
I hope that cleared up some things on when to use #Component and when to use #Bean. It can be a little confusing but as you start to write more applications it will become pretty natural.
#Bean was created to avoid coupling Spring and your business rules in compile time. It means you can reuse your business rules in other frameworks like PlayFramework or JEE.
Moreover, you have total control on how create beans, where it is not enough the default Spring instantation.
I wrote a post talking about it.
https://coderstower.com/2019/04/23/factory-methods-decoupling-ioc-container-abstraction/
1. About #Component
#Component functs similarily to #Configuration.
They both indicate that the annotated class has one or more beans need to be registered to Spring-IOC-Container.
The class annotated by #Component, we call it Component of Spring. It is a concept that contains several beans.
Component class needs to be auto-scanned by Spring for registering those beans of the component class.
2. About #Bean
#Bean is used to annotate the method of component-class(as mentioned above). It indicate the instance retured by the annotated method needs to be registered to Spring-IOC-Container.
3. Conclusion
The difference between them two is relatively obivious, they are used in different circumstances.
The general usage is:
// #Configuration is implemented by #Component
#Configuration
public ComponentClass {
#Bean
public FirstBean FirstBeanMethod() {
return new FirstBean();
}
#Bean
public SecondBean SecondBeanMethod() {
return new SecondBean();
}
}
Additional Points from above answers
Let’s say we got a module which is shared in multiple apps and it contains a few services. Not all are needed for each app.
If use #Component on those service classes and the component scan in the application,
we might end up detecting more beans than necessary
In this case, you either had to adjust the filtering of the component scan or provide the configuration that even the unused beans can run. Otherwise, the application context won’t start.
In this case, it is better to work with #Bean annotation and only instantiate those beans,
which are required individually in each app
So, essentially, use #Bean for adding third-party classes to the context. And #Component if it is just inside your single application.
#Bean can be scoped and #component cannot
such as
#Scope(value = WebApplicationContext.SCOPE_REQUEST, proxyMode = ScopedProxyMode.TARGET_CLASS)