I have a basic Auto-wiring Question. I see the following two implementations that are possible in Spring auto-wiring
Method1
public class SimpleMovieLister {
private MovieFinder movieFinder;
#Autowired
public void setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// ...
}
Method2
public class SimpleMovieLister {
#Autowired
private MovieFinder movieFinder;
}
My understanding is that both are the same and I use a lot of Method2 in my code. What are the situations in which Method1 is useful ? Or is it just a case of Spring evolution and we have both possible ways in which to implement.
Sorry, if the question is too basic, but I need to get this cleared up
Method 1 is Setter Injection.
Method 2 is Field Injection.
A 3rd method is Constructor Injection
Example:
public class SimpleMovieLister {
private MovieFinder movieFinder;
#Autowired
public SimpleMovieLister(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// ...
}
Method 3, Constructor Injection is preferred because it makes testing significantly easier since you can pass in the required dependency.
Also, if your Bean only has 1 Constructor, then you can omit the #Autowired annotation. Spring will automatically choose that constructor method when creating the bean.
A good snippet from the docs:
The Spring team generally advocates constructor injection as it
enables one to implement application components as immutable objects
and to ensure that required dependencies are not null. Furthermore
constructor-injected components are always returned to client
(calling) code in a fully initialized state. As a side note, a large
number of constructor arguments is a bad code smell, implying that the
class likely has too many responsibilities and should be refactored to
better address proper separation of concerns.
Setter injection should primarily only be used for optional
dependencies that can be assigned reasonable default values within the
class. Otherwise, not-null checks must be performed everywhere the
code uses the dependency. One benefit of setter injection is that
setter methods make objects of that class amenable to reconfiguration
or re-injection later. Management through JMX MBeans is therefore a
compelling use case for setter injection.
Use the DI style that makes the most sense for a particular class.
Sometimes, when dealing with third-party classes for which you do not
have the source, the choice is made for you. For example, if a
third-party class does not expose any setter methods, then constructor
injection may be the only available form of DI.
https://docs.spring.io/spring/docs/current/spring-framework-reference/html/beans.html
Method 1 is setter Injection
Setter injection should primarily only be used for optional
dependencies that can be assigned reasonable default values within the
class. Otherwise, not-null checks must be performed everywhere the
code uses the dependency. One benefit of setter injection is that
setter methods make objects of that class amenable to reconfiguration
or re-injection later.
Method 2 is field Injection
Related
My question is simple. Which approach is more efficient?
method 1:
#Autowired
private CustomerRepository customerRepo;
method 2:
private final CustomerRepository custormerRepo;
public StudentService(CustomerRepository customerRepo) {
this.customerRepo = customerRepo;
}
As I see the DI in the method2 is more recent. But I wanted to ask you which one should I use?
TL;DR: Method 2 is much more flexible.
Method 1 is an example of field injection and method 2 is an example of constructor injection.
Field injection has some drawbacks that constructor injection avoids. Here are some advantages of constructor injection:
Immutability:
You can't do this is plain Java:
#Autowired
private final CustomerRepository customerRepo;
// No constructor that sets "customerRepo".
So, Spring offers constructor injection:
private final CustomerRepository customerRepo;
#Autowired
public StudentService(final CustomerRepository customerRepo) {
this.customerRepo = customerRepo;
}
Immutability is sometimes preferred. One reason is that it helps with thread-safety. Another is security.
Personally, I follow the rule, "if it can be final, it should be final."
Testing:
You won't need reflection to set the dependencies. Yes, many mocking frameworks handle this for you, but with constructor injection, you have the option to call new on the constructor.
Nasty NullPointerExceptions:
An object is created by calling its constructor, right? We usually want our arguments to be non-null at the time they are passed in. With constructor injection, the Spring IoC container makes sure that all the arguments passed in the constructor are available before passing them into the constructor.
use constructor injection, Spring also recommends it
In your main code, you should use method 2 as field injection (method 1) is not recommended. (see here for reasons)
In your test code, it's okay to use method 1.
I suggest you suggest something better. By using the Lombok library's #RequiredArgConstructor you are thus avoiding the boilerplate code. and if you wonder why #Autowired not reccomended , because of when you want to write unit testing in your application and there will be problem , where if you use #Autowired.
In addition to what the other answers have said about immutability, another benefit of constructor injection is to be able to avoid NPE is the field is not initialized. Using autowired, from a test, you’d create the class and then must remember to set the field. Using constructor injection, you can’t not initialize the field. This is more prominent in Kotlin where autowired fields are declared as lateinit var and throw a runtime exception if used before initialized. But a constructor argument can be declared as not null type which prevents you from even explicitly passing null.
I am wondering what is the current best practice as to the use of factory pattern within the context of Spring framework in using dependency injection. My wonder arises about whether the factory pattern is still relevant nowadays in light of the use of Spring dependency injection. I did some searching and see some past discussion (Dependency Injection vs Factory Pattern) but seem there is different view.
I see in some real life project in using a Map to hold all the beans and rely on autowiring to create those beans. When the bean is needed, it get it via the map using the key.
public abstract class Service {
//some methods
}
#Component
public class serviceA extends Service {
//implementation
}
#Component
public class serviceB extends Service {
//implementation
}
Map<String, Service> services;
But I see there is some difference among the two approaches.
Using the above method, all beans are created on application start up and the creation of object is handled by the framework. It also implies there is only one bean for each type.
While for factory pattern, the factory class creates the object on request. And it can create a new object for each request.
I think a deeper question may be, when Spring framework is used in a project, should it be strived to not create any object inside a class, which means the factory pattern ( or any creational design patterns?) should not be used, as Spring is supposed to be the central handler of the objects dependency ?
The answer to this question can be really deep and broad, I'll try to provide some points that hopefully will help.
First off, spring stores its beans (singletons) in the ApplicationContext. Essentially this is the map you're talking about. In a nutshell, it allows getting the bean by name, type, etc.
ApplicationContext, while being a really important concept, is not the whole Spring, in fact Spring framework allows much more flexibility:
You say, using a map implies that all the beans will be created at the beginning of the application and there is one instance of the bean.
Spring has a concept of Lazy beans, basically supporting a concept of beans being actually created only when they're required for the first time, so Spring supports the "delayed" beans initialization
Spring also allows more than one instance of a bean per type. So this map is more "advanced". For example you can create more than one implementation of the interface and use declare both as beans. As long as you provide enough information about what bean should be injected to the class that might use them (for example with a help of qualifiers suppored in spring), you're good to go. In addition, there are features in spring IoC container that allow injecting all registered implementations of an interface into a list:
interface Foo {}
#Component
class FooImpl1 implements Foo {}
#Component
class FooImpl2 implements Foo {}
class Client {
#Autowired
List<Foo> allFoos;
}
Now you say:
While for factory pattern, the factory class creates the object on request. And it can create a new object for each request.
Actually Spring can create objects per request. Not all beans have to be singletons, in general spring has a concept of scopes for this purposes.
For example, scope prototype means that Spring will create a bean upon each usage. In particular one interesting usage that spring supports in variety of ways is Injecting prototype bean into singleton. Some solutions use exactly like a factory (read about annotation #Lookup others rely on auto-generated proxy in runtime (like javax.inject.Provider). Prototype scope beans are not held in the application context, so here again spring goes beyond a simple map abstraction.
Last feature that you haven't mentioned is that sometimes even for singletons the initialization can be a little bit more complicated then calling a constructor with Parameters. Spring can address that by using Java Configurations:
#Configuration
public class MyConfig {
public SomeComplicatedObject foo(#Value("...") config, Bar bar) {
SomeComplicatedObject obj = new SomeComplicatedObject() // lets pretend this object is from some thirdparty, it only has no-op constructor, and you can't place spring annotations on it (basically you can't change it):
obj.setConfig(config);
obj.setBar(bar);
return obj;
}
}
The method foo here initializes the object SomeComplicatedObject and returns it. This can be used instead of factories to integrate "legacy" code (well, java configurations go way beyond this, but its out of scope for this question).
So bottom line, you Spring as an IoC container can provide many different ways to deal with object creation, in particular it can do everything that factory design pattern offers.
Now, I would like to also refer to your last sentense:
I think a deeper question may be, when Spring framework is used in a project, should it be strived to not create any object inside a class, which means the factory pattern ( or any creational design patterns?) should not be used, as Spring is supposed to be the central handler of the objects dependency ?
Indeed you don't have to use Factory Pattern when using Spring, since (as I hopefully have convinced you) provides everything that factory can do and more.
Also I agree that spring is supposed to be the central handler of the objects dependency (unless there are also parts of the application which are written in a different manner so you have to support both :) )
I don't think we should avoid using "new" altogether, not everything should/can be a bean, but I do see (from my subjective experience, so this is arguable) that you use it much less leaving the creation of most of the objects to Spring.
Should we avoid a usage of any creation design pattern? I don't think so, sometimes you can opt for implementing "builder" design pattern for example, its also a creational pattern but spring doesn't provide a similar abstraction.
I think if your project uses Spring framework you should use it. Although it depends on your project design e.g. You may use creational patterns along side with Spring IoC. e.g when you have abstraction layers not framework dependant (agnostic code)
interface ServiceFactory {
Service create(String type);
}
#Component
class SpringServiceFactory implements ServiceFactory {
#Autowired private ApplicationContext context;
Service create(String type) {
return context.getBean(type)
}
}
I use Factory pattern as well when I refactor legacy not unit testable code which also uses Spring Framework in order to implement unit tests.
// legacy service impossible to mock
class LegacyApiClient implements Closeable {...}
#Component
class LegacyApiClientFactory {
LegacyApiClient create(String endpoint) {
return new LegacyApiClient(endpoint);
}
}
#Component
class OtherService {
private final String endpoint
private final LegacyApiClientFactory factory;
OtherService(#Value("${post.endpoint}") String endpoint,
LegacyApiClientFactory factory) {...}
void doCall {
try (LegacyApiClient client = factory.create(endpoint)) {
client.postSomething();
}
}
}
....
// a random unit test
LegacyApiClient client = mock(LegacyApiClient.class)
LegacyApiClientFactory factory = mock(LegacyApiClientFactory.class)
OtherService service = new OtherService("http://scxsc", factory);
when(factory.create(any())).thenReturn(client)
service.doCall()
....
This is the code that I have:
#Component
#Configuration
#PropertySource("application.properties")
public class Program {
#Value("${app.title}")
private String appTitle;
public Program() {
System.out.println(appTitle);
}
}
The application.properties has
app.title=The Program
The output is null insteaf of The Program.
So, what am I missing? I have tried several examples; none worked.
Since appTitle is an autowired field, it is not set until after the object is initially constructed. This is why the value is still null in your example. The bean construction process in this scenario is as follows:
The Program constructor is called, creating a new Program instance
The appTitle field is set on the newly constructed bean to ${app.title}
The ideal fix for this depends on your goals. If you truly need the value within the constructor, you can pass it in as an autowired constructor parameter. The value will then be available within the constructor:
#Component
#Configuration
#PropertySource("application.properties")
public class Program {
public Program(#Value("${app.title}") appTitle) {
System.out.println(appTitle);
}
}
If you don't need it in the constructor itself, but need it for the proper initialization of the bean, you could alternatively use the #javax.annotation.PostConstruct annotation to make use of it after the object's construction but before it is made available for use elsewhere:
#Component
#Configuration
#PropertySource("application.properties")
public class Program {
#Value("${app.title}")
private String appTitle;
#PostConstruct
public void printAppTitle() {
System.out.println(appTitle);
}
}
Finally, if you don't need the value at construction time, but need it during the life of the bean, what you have will work; it just won't be available within the body of the constructor itself:
#Component
#Configuration
#PropertySource("application.properties")
public class Program {
#Value("${app.title}")
private String appTitle;
}
Nothing wrong, just don't do it in a constructor...
Other answers on this question are written assuming the goal is creating a Spring-managed bean that uses the given property in its creation. However, based on your comments in another answer, it looks like the question you want answered is how to access an externalized property (one provided by #Value) within a no-argument constructor. This is based on your expectation that a Java inversion of control (IoC) container such as Spring should allow accessing externalized properties (and presumably other dependencies) within a no-argument constructor. That being the case, this answer will address the specific question of accessing the property within a no-argument constructor.
While there are certainly ways this goal could be achieved, none of them would be idiomatic usage of the Spring framework. As you discovered, autowired fields (i.e. fields initialized using setter injection) cannot be accessed within the constructor.
There are two parts to explaining why this is. First, why does it work the way it does, programmatically? Second, why was it designed the way it was?
The setter-based dependency injection section of the Spring docs addresses the first question:
Setter-based DI is accomplished by the container calling setter methods on your beans after invoking a no-argument constructor or a no-argument static factory method to instantiate your bean.
In this case, it means that first the object is created using the no-argument constructor. Second, once the object is constructed, the appTitle is initialized on the constructed bean. Since the field isn't initialized until after the object is constructed, it will have its default value of null within the constructor.
The second question is why Spring is designed this way, rather than somehow having access to the property within the constructor. The constructor-based or setter-based DI? sidebar within the Spring documentation makes it clear that constructor arguments are in fact the idiomatic approach when dealing with mandatory dependencies in general.
Since you can mix constructor-based and setter-based DI, it is a good rule of thumb to use constructors for mandatory dependencies and setter methods or configuration methods for optional dependencies. [...]
The Spring team generally advocates constructor injection, as it lets you implement application components as immutable objects and ensures that required dependencies are not null. Furthermore, constructor-injected components are always returned to the client (calling) code in a fully initialized state. [...]
Setter injection should primarily only be used for optional dependencies that can be assigned reasonable default values within the class. [...]
A property needed to construct the object certainly would be categorized as a mandatory dependency. Therefore, idiomatic Spring usage would be to pass in this required value in the constructor.
So in summary, trying to access an application property within a no-argument constructor is not supported by the Spring framework, and in fact runs contrary to the recommended use of the framework.
#Autowired
UserService userService;
What happens exactly inside `#Autowired annotation whether it uses Constructor Injection or Setter Injection. I know that it is field Injection.
I'm not asking How IOC or DI works, I'm asking How Field Injection in Spring Boot works internally?
Basically field inject is a type of injection (obviously), so Spring injects dependency based on field type and maybe some annotations (like #Qualifier).
How does it work?
When Spring creates a bean, there is a special Bean Post Processor org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor
Each field marked with #Autowired is considered by spring as a dependency, so it must analyze these dependencies (by using reflection under the hood) and find a match from the application context for each field (by Type, qualifier if specified, etc.). Then it sets the value, right into the field, again by reflection.
I don't intend to start "holly-wars" here, but I'll just mention that I personally try to avoid using this type of injection because it effectively breaks encapsulation of dependencies, making the class with autowired fields non-unit testable.
For example if you have something like this:
#Component
class Foo {
#Autowired
private Bar bar;
public Foo() {} // no-arg construction that exists by default
}
#Component
class Bar {
}
Then when you create an instance of Foo by yourself (e.g. in unit-test) you have no clear way to supply the Bar dependency to Foo instance without relying on spring.
Constructor Injection solves this for example.
I use #Autowired annotation like this:
#Autowired
private MyService1 myService1;
#Autowired
private MyService2 myService2;
But new Intellij IDE 2016(3) suggests and proposes to replace:
private final MyService1 myService1;
private final MyService2 myService2;;
#Autowired
public MyClass(MyService1 myService1, MyService2 myService2) {
this.myService1= myService1;
this.myService2= myService2;
}
Tell me what is the difference and what is right?
Both approaches are correct.
From docs
Spring included, provide a mechanism for ensuring that all dependencies are defined when you use Setter Injection, but by using Constructor Injection, you assert the requirement for the dependency in a container-agnostic manner"
#Autowire at constructor level guarantees that you will have all the required dependencies when your spring container finally creates your bean for that class.
It is suggecting to using constructor inject instead of Setter inject. For nomal use, there is no big different.
We usually advise people to use constructor injection for all mandatory collaborators and setter injection for all other properties. Again, constructor injection ensures all mandatory properties have been satisfied, and it is simply not possible to instantiate an object in an invalid state (not having passed its collaborators). In other words, when using constructor injection you do not have to use a dedicated mechanism to ensure required properties are set (other than normal Java mechanisms).
Here is an article to explain it Setter injection versus constructor injection and the use of #Required
Also you can get quite a lot question/answer in stackoverflow.
Setter DI vs. Constructor DI in Spring?
Yes, it is used correctly. This is called the Constructor Injection.
Constructor Injection allows you to use the final modifiers of your choice and to easily pass your own not managed by Spring objects (mocks, for example).
If you are not forced to using field injection, choose constructor injection.