I've a newb to Java coming from C++ / C#.
My project is currently set up like this:
org.blah.config
HibernateConfig.java
org.blah.customer
Customer.java
CustomerController.java
CustomerService.java
HibernateConfig sets up the hibernate stuff and exposes the LocalSessionFactoryBean bean.
CustomerController is the REST entry point, it doesn't really have much logic, it sort of just wraps the CustomerService.java (or should I call it CustomerRepository?).
CustomerService.java (or CustomerRepository?) wraps the DB stuff. This is where I have:
#Autowired
private SessionFactory sessionFactory;
In my CustomerController, if I do:
private CustomerService customerService = new CustomerService();
it doesn't inject the sessionFactory.
From some samples I've found, people seem to create a CustomerService bean inside of HibernateConfig and then inject that into the constructor of the controller. Is that a good practice? Would I then rename my HibernateConfig to something more specific like CustomerHibernateConfig, etc.
Just trying to get an understanding of how/where to put the config & create the service / repo instance so it can be injected into the controller.
This is quite a hard question to answer in a few words. You are asking for a whole architecture that will depend a lot on the chosen design pattern, but considering you have controllers and services, let me put it this way. MVS is just a simplification and it's not a perfect approach. This will raise so many more questions like.
How much logic should go into a controller?
Should a model contain any logic?
Should a view contain logic?
Questions like these are very hard to answer, since these are very subjective. Still, I have been using Spring for a while and will give you my advise.
The controller's function is to validate inputs and a redirect to views when needed or to respond to clients' requests. If you got domain logic (aka business logic, business rules, and domain knowledge), the logic that makes business-critical decisions, then it shouldn't be on the controller, it should be on the services.
Now, I would say that you are missing a layer on your architecture, I would definitely add a DAO, which is a design pattern that defines a way to decouple the persistence layer of your application. Which is what you would understand as a Repository.
With that said, you only need one HibernateConfiguration and do the corresponding mappings in each model or XML file related to that model. I would advise to use annotations instead of XML files.
If you want to learn how to implement a Spring project that uses Hibernate, you can see how easy it is by following this Baeldung's tutorial. That page does have the best guides regarding the usage of Spring.
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
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.
As per my understanding both Factory class and Spring DI follows the Dependency injection. I mean in both the cases external entity is used to push the dependency. Right?
My question is which one i should go for between factory classes and Spring DI when my intention is just to get the objects . Assume i don't want any other features like aop, dao support etc. Only purpose is to get the objects either from Factory class or Spring DI. Which one is preferable.
on some site read this statement
DI loosely coupled and less intrusive in comparison to Factory classes
But could not get how spring DI loosely coupled and less intrusive than factory classes?
in both the cases we have to insert some kind of get object code in our core program .
Spring DI promotes loosely coupled code because the Spring container injects your dependencies based on configuration. If you are injecting interface implementations, you don't have to change code to change which specific implementation gets injected, unless you consider your configuration code, which many do.
If you use a Factory to create configured objects that are used by the rest of your code, you are writing code to create the objects, configure them, etc. If you want to change what the factory returns, you have to change actual code, which some would argue is a more intrusive change.
Typically Spring is used to configure how the various layers of your application are wired together. X service takes such and such DAO implementations, for example. That's application level organization. Lets say you have a scenario where want to create a button for every row in a list -- in that case you could use a factory to create the buttons. This scenario is based on a runtime situation where the GUI has different elements that you couldn't configure up front (because its based on the data), so DI makes less sense here.
EDIT - based on your comment questions, I think the primary point here is that you have to consider is that Spring is also an Inversion of Control container. That means you don't program in which components in your application go where. Without IoC, you might do something like
MyServiceImpl extends MyService {
Dao1 = new Dao1Impl(); // you programmatically configure which components go in here
Dao2 = new Dao2Impl();
....
}
instead you do something like
MyServiceImpl extends MyService {
public Dao1; // you haven't specified which components, only interfaces
public Dao2;
....
}
In the second code sample, Spring (or whatever you use) will inject the appropriate DAO instances for you. You have moved control of which components to use to a higher level. So IoC and DI go hand and hand, IoC promotes loose coupling because in your component definitions (i.e. interfaces) you only specify behavior.
In other words, IoC and DI are not necessary for loose coupling; you can have loose coupling with a Factory too
MyServiceImpl extends MyService {
public dao1
public dao2;
MyServiceImpl(){
dao1 = DaoFactory.getDao1();
...
}
....
}
here your service still only depends on DAO definitions and you use the factory to get implementations. The caveat is that your service is now coupled to the factory. You can make it more loose by passing a Factory into your constructor if you want....
Also, dont forget that Spring provides other useful functionalities, like its transaction management. That's incredibly helpful, even though you said for your app you don't need it.
But could not get how spring DI loosely coupled and less intrusive
than factory classes? in both the cases we have to insert some kind of
get object code in our core program .
Spring makes it less intrusive because it uses reflection to automatically "inject/create" the dependencies. Thus your code does not need a reference to a the factory.
Spring is generally used for "Singleton-like" object creation. People generally use custom factories for transient throw away object creation (like request objects).
In fact often times you will make Spring create and inject your custom factories (ie factory of a factory).
I am wiring my UserService type classes using spring's IOC.
But what about my User class?
I have a interface User, then a UserImpl class.
In my controller action's do I just do:
User u = new UserImpl();
Or would it sometimes make sense to use IOC for this also?
Sometimes I use a different constructor also when instantiating a class, based on some conditions. I guess your stuck in these situations?
It will not make sense to use dependency injection or IOC for your business objects like User because business objects are not the dependencies of the class they are the part of the class using them.
Spring IOC, by default, will create Singletons for you. Which means all user threads using your app will share that single instance of class. This is generally fine for Service type classes. If needed this singleton behavior can be changed to object per request (prototype), but this will cause you to change this setting for the users of the non-sigleton object as well.
Domain/business classes are state-full, it's easiest to create such objects once-per-request in order to avoid concurrency issues.