I found many examples about setter dependency injection for class member variables, but found it difficult to find any examples about setter dependency injection for local variables inside methods using spring.
How can I do this?
Your given example of a SpellChecker being used by a TextEditor class could look like this before it is capable of Dependency Injection. You are creating a new instance of the class SpellChecker inside the method where you are using it.
public class TextEditor {
public void doSomething() {
new SpellChecker().checkSpelling();
}
}
If you want to decouple the code and be able to replace the SpellChecker by a mock for unit testing purposes or by another implementation provided by your Dependency Injection container you have to change the class and provide a field where this instance can be injected.
public class TextEditor {
#Autowired
private SpellChecker spellChecker;
public void doSomething() {
spellChecker.checkSpelling();
}
}
Since this is still not easy to test because you would need to use Reflection to inject a mock into that private field you can provide a Setter or make the field package visible by removing the modifier private from the field. So a unit test in the same package can access this field and provide the mock.
It doesn't seem to be possible to inject dependencies into local variables of methods or any local scope even static local scopes.
The reason to that is simply because there is no way to inject this via reflection, reflection doesn't provide any API allowing access to local variables of a method.
you can read more here: https://stackoverflow.com/a/24249500/11035655.
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've learned a lot recently about Spring and one thing i think i might be misunderstanding is the #Autowired annotation, especially when using it in constructors. You see, the app i'm developing is a service so basically EVERYTHING is initialized within a constructor. The only actual user-driven events that happen are buttons that restart certain modules of the service. This is my main method :
ConfigurableApplicationContext ctx = new SpringApplicationBuilder(MDHIS_Service.class)
.headless(false).web(false).run(args);
java.awt.EventQueue.invokeLater(() ->
{
MDHIS_Service frame = ctx.getBean(MDHIS_Service.class);
frame.setSize(1024, 768);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
});
This is the constructor of my main class, where basically everything happens. I have omitted the calls to the methods initializing each module to shorten it :
#Autowired
public MDHIS_Service(GlobalParamService globalParamService, LogEntryService logentryService, InterfaceService interfaceService,
ConnectionService connectionService, OutboundMessageService outboundMessageService, OutboundMessageHistoryService outboundMessageHistoryService,
InboundMessageService inboundMessageService, FacilityService facilityService, ModuleStatusService moduleStatusService,
SequenceService sequenceService)
{
this.globalParamService = globalParamService;
this.logEntryService = logentryService;
this.interfaceService = interfaceService;
this.connectionService = connectionService;
this.outboundMessageService = outboundMessageService;
this.outboundMessageHistoryService = outboundMessageHistoryService;
this.inboundMessageService = inboundMessageService;
this.facilityService = facilityService;
this.moduleStatusService = moduleStatusService;
this.sequenceService = sequenceService;
}
My main class has a private final global variable for each service. Each module is a separate thread and i'm finding myself having to pass those variables to the constructor of each module which in term stores them into it own private final variables. The way i'm doing things right now #Autowired is pretty much useless since i'm having to pass the instance around. Is there a way to better use #Autowired? This service is used as the backend for a large web app and i find myself making much better use of the annotation in there. I did a lot of research on this topic and i did try the #PostContruct annotation but all i ever got was null services.
Any help would be appreciated.
Thanks!
I figured out my problem, and it was a pretty dumb one. First off, i had not annotated my main class with #Component so Spring never bothered to inject the dependencies in it. Secondly, I did not realize that a method annotated with #PostContruct would run by itself after the constructor runs WITHOUT NEEDING TO EXPLICITELY BE CALLED!
I moved all my initialization code to an init method annotated with #PostConstruct and annotated my main class with #Component, everything is working now!
You typicall don't have to use a constructor + #Autorwired, you can directly use autowired on fields and spring would fill the dependencies for you:
#Component
public class MDHIS_Service {
#Autowired
private GlobalParamService globalParamService;
}
What is important to understand is that for spring to work, you must let it create the objects for you, and not calling the constructors explicitely. It would then fill the dependencies as needed. This is done by declaring the service as a component (for example with the #Component annotation) and never create the service yourself but getting them from dependency injection.
The first object you start with has to have been created by spring and returned by the application context.
What you gain in exchange is that you don't have to forwared everything explicitely. A sub-sub-sub service quite distant from the root of the application can depend on anything it has visibility without you having to forward the reference all the way.
I would advise to take a look a the spring reference documentation, it quite detailled and complete:
https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#spring-core
Edit: I'll try to clarify a bit with an example... What do the init code of the various service actually does ?
Maybe it set the dependencies. Then just autowire them:
#Component
MySubService {
#Autowired MySubSubService mySubSubService;
}
Maybe it does some more thing than setting fields so you can add on top an init method that do it and this init method can eventually call the other services.
#Component
MySubService {
#Autowired MySubSubService mySubSubService;
#PostConstruct
public void init() {
//Init code that may use mySubSubService.
}
}
You don't have to declare a constructor and forward dependencies yourself, sprint does it for you.
The only case where you'd have problem is if finally you need some parameters that are not dependency to the init method. But even in that case you could do it from you main code. That's actually what you did with the main service calling the various setters rather than messing with the constructor to set theses values.
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
Why can't we autowire the static instance variable in the Spring bean. I know there is another way to achieve this but just want to know why cant we do it in below way.
e.g.
#Autowired
public static Test test;
We can't autowire static fields in spring because the Spring context might not be loaded when the java class loader loads the static values.
In that case the class loader won't properly inject the static fields in the bean and will fail.
Because using static fields encourages the usage of static methods. And static methods are evil. The main purpose of dependency injection is to let the container create objects for you and wire them. Also it makes testing easier.
Once you start to use static methods, you no longer need to create an instance of object and testing is much harder. Also you cannot create several instances of a given class, each with a different dependency being injected (because the field is implicitly shared and creates global state - also evil).
According to OOP concept, it will be bad design if static variables are autowired.
Static variable is not a property of Object, but it is a property of a Class.
Spring auto wiring is done on objects, and that makes the design clean in my opinion.
You can deploy the auto wired bean object as singleton, and achieve the same as defining it static.
By this solution you can autowired static fields in spring.
#Component
public class TestClass {
private static Test test;
#Autowired
public void setTest(Test test) {
TestClass.test = test;
}
}
The short question:
Does Castle Windsor have something similar to Spring.Net's "Lookup Method Injection" that can be configured from XML, which provides the ability to fetch transient instances from the container without the class being aware of the IoC container?
The long question:
I'm a long time Spring/Spring.Net user and I have been experimenting with Castle Windsor, by trying to port a project over to it. Spring.Net has a concept of "Lookup Method Injection" which (from the Spring docs)...
Lookup method injection is the ability of the container to override methods on container managed objects, to return the result of looking up another named object in the container. The lookup typically involves a prototype object as in the scenario described in the preceding section. The Spring framework implements this method injection by a dynamically generating a subclass overriding the method using the classes in the System.Reflection.Emit namespace.
What this means is, If I had the following...
public class SomeTransient
{
// ... I have dependencies that need to be filled by IoC container
}
public class SomeClass
{
public virtual void Work()
{
var o = CreateTransient();
}
public virtual SomeTransient CreateTransient() { }
}
I can instruct Spring to override the CreateTransient method, and have that method return a new container created transient instance (with it's dependencies initialized) each time the method is called.
The unique part of this is, it doesn't require direct links to the Spring Framework (eg. SomeClass doesn't have to implement a specific interface).
Is there something similar in Castle Windsor to accomplish this via XML?
(I will eventually move away from XML config, but at the moment I'm just trying to get it running)
Castle has something better; Typed Factories.
You can also inject even a delegate!
http://stw.castleproject.org/Windsor.Typed-Factory-Facility-delegate-based-factories.ashx
It is better because it does not depend on dynamically generation code, and it looks much more cleaner.
It looks much more cleaner because the class doesn't depend on someone overriding that method. It is impossible to test this class without subclassing.
If you really want to do something like this, i would expect:
public abstract class SomeClass
{
public abstract SomeTransient CreateTransient();
}
but... again it doesn't feel right.
Edit 2
Unity 2 support these kind of delegate factories; you can read more here:
http://www.truewill.net/myblog/index.php/2010/05/06/unity_2_0_combining_injectionfactory_and
thanks to #eiximenis