I'm just starting to pick up Spring, coming from a purely Java EE background, so stuff like IOC and dependency injection is all kinda new to me. As per the Spring docs, I understand that for any class to call on instances of its dependencies, I can choose to autowire those dependencies as a form of dependency injection. And that this is actually aside from the usual instance declaration we always do in java like below: Animal animal1 = new Animal()
In my one of my little test Spring boot project services, I noticed that I end up doing both dependency injection and normal class instantiation. I use JPA Repository to craft my repo layer, and autowire the repo classes so I can use them like eg.
#Autowired
customerAccountRepo
This is all fine. I also have a DAO customerMembershipValidity whose attributes are other declared POJOs and has some public helper functions within to set the DAO's attributes. In order to use this DAO, however, I find myself creating multiple instances of the DAO the traditional way, instantiating
CustomerMembershipValidity customerMembershipValidity1 = new CustomerMembershipValidity() multiple times throughout the service to call on public helper methods like customerMembershipValidity.setNewExpiry(). I didnt think there would be a need to autowire this since I'm dealing with a DAO or POJO, not another service... or should I?
For now, the code seems functional when I do my unit-testing, but I would like to know if this would harm the overall longevity and sensibility of the code, or if it's forseeable to end up breaking in E2E when I run the Spring Boot application.
I have a few recommendations:
DAO should be a singleton Spring Bean that you inject into service bean(s) that need it. Make sure there is no mutable shared state in your DAO - just database operations.
The DAOs I write might use mapper functions to map ResultSet to objects/collections, but I don't see the need for helpers.
Spring preference is to use constructor injection. You should not use setter or autowired attribute to inject dependencies.
If you use new, that means the object is not under Spring's control. It's appropriate to call new for objects in method scope, but not a Spring Bean like a DAO.
Unit testing and production are two different things. I prefer to leave Spring out of unit testing. I call new for JUnit tests, but not production code. Once I've tested the DAO, I can use mocks for services that depend on it.
You're smart to take up Spring and leave Java EE behind. It's a twenty year old dead standard.
I asked a general Spring question: Auto-cast Spring Beans and had multiple people respond that calling Spring's ApplicationContext.getBean() should be avoided as much as possible. Why is that?
How else should I gain access to the beans I configured Spring to create?
I'm using Spring in a non-web application and had planned on accessing a shared ApplicationContext object as described by LiorH.
Amendment
I accept the answer below, but here's an alternate take by Martin Fowler who discusses the merits of Dependency Injection vs. using a Service Locator (which is essentially the same as calling a wrapped ApplicationContext.getBean()).
In part, Fowler states, "With service locator the application class asks for it [the service] explicitly by a message to the locator. With injection there is no explicit request, the service appears in the application class - hence the inversion of control.
Inversion of control is a common feature of frameworks, but it's something that comes at a price. It tends to be hard to understand and leads to problems when you are trying to debug. So on the whole I prefer to avoid it [Inversion of Control] unless I need it. This isn't to say it's a bad thing, just that I think it needs to justify itself over the more straightforward alternative."
I mentioned this in a comment on the other question, but the whole idea of Inversion of Control is to have none of your classes know or care how they get the objects they depend on. This makes it easy to change what type of implementation of a given dependency you use at any time. It also makes the classes easy to test, as you can provide mock implementations of dependencies. Finally, it makes the classes simpler and more focused on their core responsibility.
Calling ApplicationContext.getBean() is not Inversion of Control! While it's still easy to change what implemenation is configured for the given bean name, the class now relies directly on Spring to provide that dependency and can't get it any other way. You can't just make your own mock implementation in a test class and pass that to it yourself. This basically defeats Spring's purpose as a dependency injection container.
Everywhere you want to say:
MyClass myClass = applicationContext.getBean("myClass");
you should instead, for example, declare a method:
public void setMyClass(MyClass myClass) {
this.myClass = myClass;
}
And then in your configuration:
<bean id="myClass" class="MyClass">...</bean>
<bean id="myOtherClass" class="MyOtherClass">
<property name="myClass" ref="myClass"/>
</bean>
Spring will then automatically inject myClass into myOtherClass.
Declare everything in this way, and at the root of it all have something like:
<bean id="myApplication" class="MyApplication">
<property name="myCentralClass" ref="myCentralClass"/>
<property name="myOtherCentralClass" ref="myOtherCentralClass"/>
</bean>
MyApplication is the most central class, and depends at least indirectly on every other service in your program. When bootstrapping, in your main method, you can call applicationContext.getBean("myApplication") but you should not need to call getBean() anywhere else!
Reasons to prefer Service Locator over Inversion of Control (IoC) are:
Service Locator is much, much easier for other people to following in your code. IoC is 'magic' but maintenance programmers must understand your convoluted Spring configurations and all the myriad of locations to figure out how you wired your objects.
IoC is terrible for debugging configuration problems. In certain classes of applications the application will not start when misconfigured and you may not get a chance to step through what is going on with a debugger.
IoC is primarily XML based (Annotations improve things but there is still a lot of XML out there). That means developers can't work on your program unless they know all the magic tags defined by Spring. It is not good enough to know Java anymore. This hinders less experience programmers (ie. it is actually poor design to use a more complicated solution when a simpler solution, such as Service Locator, will fulfill the same requirements). Plus, support for diagnosing XML problems is far weaker than support for Java problems.
Dependency injection is more suited to larger programs. Most of the time the additional complexity is not worth it.
Often Spring is used in case you "might want to change the implementation later". There are other ways of achieving this without the complexity of Spring IoC.
For web applications (Java EE WARs) the Spring context is effectively bound at compile time (unless you want operators to grub around the context in the exploded war). You can make Spring use property files, but with servlets property files will need to be at a pre-determined location, which means you can't deploy multiple servlets of the same time on the same box. You can use Spring with JNDI to change properties at servlet startup time, but if you are using JNDI for administrator-modifiable parameters the need for Spring itself lessens (since JNDI is effectively a Service Locator).
With Spring you can lose program Control if Spring is dispatching to your methods. This is convenient and works for many types of applications, but not all. You may need to control program flow when you need to create tasks (threads etc) during initialization or need modifiable resources that Spring didn't know about when the content was bound to your WAR.
Spring is very good for transaction management and has some advantages. It is just that IoC can be over-engineering in many situations and introduce unwarranted complexity for maintainers. Do not automatically use IoC without thinking of ways of not using it first.
It's true that including the class in application-context.xml avoids the need to use getBean. However, even that is actually unnecessary. If you are writing a standalone application and you DON'T want to include your driver class in application-context.xml, you can use the following code to have Spring autowire the driver's dependencies:
public class AutowireThisDriver {
private MySpringBean mySpringBean;
public static void main(String[] args) {
AutowireThisDriver atd = new AutowireThisDriver(); //get instance
ClassPathXmlApplicationContext ctx = new ClassPathXmlApplicationContext(
"/WEB-INF/applicationContext.xml"); //get Spring context
//the magic: auto-wire the instance with all its dependencies:
ctx.getAutowireCapableBeanFactory().autowireBeanProperties(atd,
AutowireCapableBeanFactory.AUTOWIRE_BY_TYPE, true);
// code that uses mySpringBean ...
mySpringBean.doStuff() // no need to instantiate - thanks to Spring
}
public void setMySpringBean(MySpringBean bean) {
this.mySpringBean = bean;
}
}
I've needed to do this a couple of times when I have some sort of standalone class that needs to use some aspect of my app (eg for testing) but I don't want to include it in application-context because it is not actually part of the app. Note also that this avoids the need to look up the bean using a String name, which I've always thought was ugly.
One of the coolest benefits of using something like Spring is that you don't have to wire your objects together. Zeus's head splits open and your classes appear, fully formed with all of their dependencies created and wired-in, as needed. It's magical and fantastic.
The more you say ClassINeed classINeed = (ClassINeed)ApplicationContext.getBean("classINeed");, the less magic you're getting. Less code is almost always better. If your class really needed a ClassINeed bean, why didn't you just wire it in?
That said, something obviously needs to create the first object. There's nothing wrong with your main method acquiring a bean or two via getBean(), but you should avoid it because whenever you're using it, you're not really using all of the magic of Spring.
The motivation is to write code that doesn't depend explicitly on Spring. That way, if you choose to switch containers, you don't have to rewrite any code.
Think of the container as something is invisible to your code, magically providing for its needs, without being asked.
Dependency injection is a counterpoint to the "service locator" pattern. If you are going to lookup dependencies by name, you might as well get rid of the DI container and use something like JNDI.
Using #Autowired or ApplicationContext.getBean() is really the same thing. In both ways you get the bean that is configured in your context and in both ways your code depends on spring.
The only thing you should avoid is instantiating your ApplicationContext. Do this only once! In other words, a line like
ApplicationContext context = new ClassPathXmlApplicationContext("AppContext.xml");
should only be used once in your application.
One of Spring premises is avoid coupling. Define and use Interfaces, DI, AOP and avoid using ApplicationContext.getBean() :-)
One of the reasons is testability. Say you have this class:
interface HttpLoader {
String load(String url);
}
interface StringOutput {
void print(String txt);
}
#Component
class MyBean {
#Autowired
MyBean(HttpLoader loader, StringOutput out) {
out.print(loader.load("http://stackoverflow.com"));
}
}
How can you test this bean? E.g. like this:
class MyBeanTest {
public void creatingMyBean_writesStackoverflowPageToOutput() {
// setup
String stackOverflowHtml = "dummy";
StringBuilder result = new StringBuilder();
// execution
new MyBean(Collections.singletonMap("https://stackoverflow.com", stackOverflowHtml)::get, result::append);
// evaluation
assertEquals(result.toString(), stackOverflowHtml);
}
}
Easy, right?
While you still depend on Spring (due to the annotations) you can remove you dependency on spring without changing any code (only the annotation definitions) and the test developer does not need to know anything about how spring works (maybe he should anyway, but it allows to review and test the code separately from what spring does).
It is still possible to do the same when using the ApplicationContext. However then you need to mock ApplicationContext which is a huge interface. You either need a dummy implementation or you can use a mocking framework such as Mockito:
#Component
class MyBean {
#Autowired
MyBean(ApplicationContext context) {
HttpLoader loader = context.getBean(HttpLoader.class);
StringOutput out = context.getBean(StringOutput.class);
out.print(loader.load("http://stackoverflow.com"));
}
}
class MyBeanTest {
public void creatingMyBean_writesStackoverflowPageToOutput() {
// setup
String stackOverflowHtml = "dummy";
StringBuilder result = new StringBuilder();
ApplicationContext context = Mockito.mock(ApplicationContext.class);
Mockito.when(context.getBean(HttpLoader.class))
.thenReturn(Collections.singletonMap("https://stackoverflow.com", stackOverflowHtml)::get);
Mockito.when(context.getBean(StringOutput.class)).thenReturn(result::append);
// execution
new MyBean(context);
// evaluation
assertEquals(result.toString(), stackOverflowHtml);
}
}
This is quite a possibility, but I think most people would agree that the first option is more elegant and makes the test simpler.
The only option that is really a problem is this one:
#Component
class MyBean {
#Autowired
MyBean(StringOutput out) {
out.print(new HttpLoader().load("http://stackoverflow.com"));
}
}
Testing this requires huge efforts or your bean is going to attempt to connect to stackoverflow on each test. And as soon as you have a network failure (or the admins at stackoverflow block you due to excessive access rate) you will have randomly failing tests.
So as a conclusion I would not say that using the ApplicationContext directly is automatically wrong and should be avoided at all costs. However if there are better options (and there are in most cases), then use the better options.
The idea is that you rely on dependency injection (inversion of control, or IoC). That is, your components are configured with the components they need. These dependencies are injected (via the constructor or setters) - you don't get then yourself.
ApplicationContext.getBean() requires you to name a bean explicitly within your component. Instead, by using IoC, your configuration can determine what component will be used.
This allows you to rewire your application with different component implementations easily, or configure objects for testing in a straightforward fashion by providing mocked variants (e.g. a mocked DAO so you don't hit a database during testing)
Others have pointed to the general problem (and are valid answers), but I'll just offer one additional comment: it's not that you should NEVER do it, but rather that do it as little as possible.
Usually this means that it is done exactly once: during bootstrapping. And then it's just to access the "root" bean, through which other dependencies can be resolved. This can be reusable code, like base servlet (if developing web apps).
There is another time when using getBean makes sense. If you're reconfiguring a system that already exists, where the dependencies are not explicitly called out in spring context files. You can start the process by putting in calls to getBean, so that you don't have to wire it all up at once. This way you can slowly build up your spring configuration putting each piece in place over time and getting the bits lined up properly. The calls to getBean will eventually be replaced, but as you understand the structure of the code, or lack there of, you can start the process of wiring more and more beans and using fewer and fewer calls to getBean.
I've only found two situations where getBean() was required:
Others have mentioned using getBean() in main() to fetch the "main" bean for a standalone program.
Another use I have made of getBean() are in situations where an interactive user configuration determines the bean makeup for a particular situation. So that, for instance, part of the boot system loops through a database table using getBean() with a scope='prototype' bean definition and then setting additional properties. Presumably, there is a UI that adjusts the database table that would be friendlier than attempting to (re)write the application context XML.
however, there are still cases where you need the service locator pattern.
for example, i have a controller bean, this controller might have some default service beans, which can be dependency injected by configuration.
while there could also be many additional or new services this controller can invoke now or later, which then need the service locator to retrieve the service beans.
You should to use: ConfigurableApplicationContext instead of for ApplicationContext
Currently, my application architecture flows like this:
View → Presenter → Some asynchronous executor → DAOFactory → DAO (interface) → DAO (Impl)
For the time being, this kind of architecture works; mainly because I've only been needing one kind of DAO at the moment. But as the requirement grows, I'd need to expand to multiple DAOs, each with their own implementation on how to get the data.
Here's an illustration to my case:
The main headache comes from FooCloudDao which loads data from an API. This API needs some kind of authentication method - a string token that was stored during login (say, a Session object - yes, this too has its own DAO).
It's tempting to just pass a Session instance through FooDaoFactory, just in case there's no connection, but it seems hackish and counter-intuitive. The next thing I could imagine is to access SessionDAOFactory from within FooDaoFactory to gain instance of a Session (and then pass that when I need a FooCloudDAO instance).
But as I said, I'm not sure whether or not I could do a thing like this - well, may be I could, but is it this really the correct way of doing it?
I presume your problem is actually that FooCloudDao has different "dependencies" than other components, and you want to avoid passing the dependencies through every class on the way.
Altough there are quite some design patterns which would kind of solve your problem, I would suggesting taking a look on Dependency Injection / Inversion of Control principles and frameworks. What you would do with this is:
You would create an interface for what your FooCloudDao needs, for example:
interface ApiTokenProvider {
string GetToken();
}
You would create and implementation of that interface which would get it from the session or wherever that thing comes from:
class SessionBasedApiTokenPrivider implements ApiTokenProvider {
public string GetToken() {
// get it from the session here
}
}
The defined class above would need to be registered with IoC container of your choice as the implementation of ApiTokenProvider
interface (so that whoever asks for ApiTokenProvider will be decoupled
from the actual implementation -> the container would give him the
proper implementation).
You would have something called constructor injection on your FooCloudDao class (this is later used by the container to "inject"
your dependency):
public FooCloudDao(ApiTokenProvider tokenProvider) {
// store the provider so that the class can use it later where needed
}
Your FooDaoFactory would use the IoC container to resolve the FooCloudDao with all its dependencies (so you would not
instantiate the FooCloudDao with new)
When following these steps you will make sure that:
FooDaoFactory remains clean of passing dependecies through
you make your code much more testable because you could test your FooCloudDao without the real session (you could only give in the fake interface implementation)
and all other benefits which come with Inversion of Control...
Note on the session: if you encounter the problem of getting the session in the SessionBasedApiTokenProvider, most of the time the session itself is also registered with the IoC controller, and injected where needed.
I've seen two different strategies of injecting objects in Seam (in my case a DAO)
1.
#In(create="true")
private WeirdDao weirdDao;
...
#Name("weirdDao")
public class WeirdConcreteDao implements WeirdDao
2.
#In
private WeirdDao weirdDao;
...
#Name("weirdConcreteDao")
public class WeirdConcreteDao implements WeirdDao
...
components.xml
<factory auto-create="true" name="weirdDao" value="#{weirdConcreteDao}"/>
My theory is that in the second example Seam is taking care of the creation of the object and (hopefully) controls something like a pool of its instances.
Is there any oficial explanation of the pro/con of the usage of these two?
Thanks in advance!
The advantage in terms of performance is not that much concern in this case. But the problem is rather philosophical.
Factory Pattern is a well-known Creational design pattern. In Seam, we generally use factory method to created the instances of Entity or Domain type Classes. They generally holds data and does not has any processing or business logic.
For injecting the beans like Service or DAO, we generally don't use factory.
The main cause is, creation of Service or DAO type beans do not hold states or depend on any states. So why would I take the headache of writing factory method or configuration for that, where I can easily transfer the responsibility to the container.
But in case of Entity or Domain type beans may differ depending on states. For example, you need to create spouse bean depending on the user logged in. To do that you write a factory method to create spouse bean depending on the user. You generally do not depend on container to do that.
Hope this helps!
How would you extract something prior 2.5 version from .xml config? It bothers me because if #Autowired is removed from my arsenal I would not really know what to do.
Say I want to use some DAO implementation.
In service class I usually write:
#Autowired
someDaoInterface generalDao;
Then I typically call
generalDao.someInterfaceMethod(someParam param);
How would I extract implementation from config in Spring 2.0 to use this method?
Is it as dumb as just: new ApplicationContext(pathToXml) and then use .getBean or there is other way?
Why do I ask for taking bean out from configuration file?
Because in Spring MVC how can you perform your logic without getting beans out from the application context.
If you have #Controller handler then you need to make calls to the service classes' methods? So they should be somehow retrieved from the context and the only way so far is using #Autowired? Then I would also want to populate Service classes as I stated in previous example with DAO classes and they also need to be retrieved from the application context, so I would be able to write logic for service classes themself. How would people do it in the past?
I see the #Autowired as the only mean of taking something out, not because it is convenient to wire automatically - I am perfectly ok with XML.
You still have option to wire it explicitely via property or constructor parameter. (Anyway, autowired is not going to work if there is ambiguity in your container )
Of course, you can use application context and getBean() in your java code, but it violates DI pattern and makes all the spring stuff useless. Purpose of DI is to decouple your business loginc from implementation details - it's not business logic it's how and where it dependencies come from. Dependencies are just there.
By using ApplicationContext.getBean() you are breaking this pattern, and introduce dependency to:
spring itself
your configuration names
After you done this, you can as well drop use of DI and spring because you just voided all the advandages DI is providing to you. (BTW, #Autowired also introduces dependency to spring, and violates DI pattern, it also implies that there is only one instance available)
Also, answer is: in ideal case there shall be no reference to spring in your code at all.
No imports, no annotations - just interfaces of collaborating entities.