At the moment I'm Autowiring myService into various objects in my app and it works fine.
#Autowired
private MyService myService;
using this config:
<context:component-scan base-package="com.myapp.mypackage" />
However, I've added a third party library and want to Autowire myService into some objects in that package also but it's not working.
I made this change to my component scan but I'm getting a NullPointerException for myService when I try to access it in the third party package:
<context:component-scan
base-package="com.myapp.mypackage, com.thirdparty.thirdpartypackage" />
I thought this would work?
With 3rd party libraries the usual approach is to use xml:
<bean class="com.thirdparty.Foo">
<property name="somePropertyWithASetter" ref="myService" />
</bean>
You can't rely on annotations, because the 3rd party library most likely does not use #Autowired.
If the third party package is spring managed, Please ignore the rest of the answer
I think the question you should ask yourself before do dependency injections is, Is the object spring managed, To do that either you define them in spring context xml or annotate the object with (#component , #service spring stereo type)
Let's look at the case in hand
Do you have access to this third party source code?
Is the third party using dependency injection?
If the answer to above questions is negative, then It is not easy to do this.
Let's look at the example of (MyService), what if this service object has state (since spring objects are singleton), Also if this service has collaborating objects that are not being injected
public class MyService
{
//depending on how MyServiceOne is managed makes it hard to retrofit
//MyService as a spring bean
private MyServiceOne serviceone;
//this also would have an adverse affect if you try use this as a spring managed
//since all spring beans are singleton
private String someState;
}
Have you tried using the adapter pattern as an alternative? I believe that you can use #Autowired on the adapter.
Related
I am working on a Spring MVC project where I am dealing with different types of services,Repositories i.e classes annotated with #Service and #Repository. I am confused with a couple of questions:
When to use #AutoWired annotation?
I have seen various repositories using this:
CourseRepository crepo=new CourseRepository();
and I have seen this also
#AutoWired
private CourseRepository crepo;
Which one of the above options should be used to get an instance of
repository in Service class?
Can I use #AutoWired for classes which are not annotated with #Repository or
#Service?
I am a beginner in this java world.Any help will be highly appreciated.
Thanks
You use new for data objects, which in most modern architectures are passive (they're not "active records"). Everything else is a service object, and you should inject those. (The one place that you do use new is with an #Bean method, which is a "factory" that creates the service object; in this case you normally pass the dependencies as method parameters.)
Note that it is recommended to use constructor injection instead of field injection; it makes your code easier to test, and it eliminates the possibility of certain kinds of errors. In fact, if using constructor injection, it's not required to have any Spring annotations in your service classes at all; beans can be registered using #Import instructions or #Bean methods on a configuration class.
You should #Autowire the dependencies instead of instantiating it yourself. Doing so, service and repo layer will be loosely coupled. Moreover, a mock repository can be easily injected in service's JUnit test class if dependency is autowired. To conclude, use below:
#Autowired
private CourseRepository crepo;
A class not annotated with any of below stereotype annotations will not be in Spring's IoC (Inversion of Control) container. Hence, no point in autowiring in a class that is not annotated with any of below annotations.
#Component, #Controller, #Service, #Repository
Dependency injection means that the framework is the one who handles the classes instantiation and the object of that class is going to be injected (thanks to #Autowired annotation) in the class where you need it. In other words, you do not need to instantiate service and repository classes by yourself using new operator, you just need to tell the framework that those classes need to be injected and that's why you use #Autowired annotation.
I have a Spring 3 project which acts as a Rest API, and wanted to wire a spring bean I have into an unmanaged class for logging purposes.
After trying many different things, what worked was marking my unmanaged class with the annotation #Configurable.
Like:
#Configurable
public class ClassNotManagedBySpring {
#Autowired
#Qualifier("myBean")
private MyBean myBean;
}
#Service("myBean")
public class MyBean {
#Autowired
#Qualifier("someOtherBean")
private SomeOtherBean someOtherBean;
}
And then in my beans.xml:
<context:spring-configured/>
So now let's say that ClassNotManagedBySpring.java, is one of 6 classes that all do something similar, except 3 of them ARE managed by spring because they have the #Component annotation.
But all 6 of these classes need to #Autowire MyBean.java and only some need the #Configurable annotation.
To note, I was already previously using AspectJ in this app for multiple other purposes.
I want to know what is the risk in my spring application by all of a sudden wiring spring managed dependencies into un managed classes in this way?
Can there be performance issues? Risks of errors at runtime?
If this isn't the best way of wiring a spring managed bean into an unmanaged class, what is?
I've been using #Configurable for years without issues, it's a very easy solution if you need app instantiated beans configured by Spring. My use cases were all in the UI tier. I also used #Configurable(preConstruction = true) whenever I needed autowired values already in the constructor. Of course, if you make millions of #Configurable objects it might be a performance issue, otherwise I wouldn't worry too much about it. The only small aesthetic problem I had was Eclipse giving me some red underlines at class definitions extending #Configurable classes, complaining that the hierarchy of class SomeClass is inconsistent, but it compiled them nevertheless, no errors in the Problems view or at runtime whatsoever.
I am new to Spring framework.While reading dependency injection i found out two ways to inject beans anotationbased and xml based.
In xmlBased it is quite simple that you define one bean inside your application context xml file.
eg.
<bean id="wild" class="com.javapapers.spring.ioc.Wolf" />
<bean id="zoo" class="com.javapapers.spring.ioc.Zoo">
<property name="wild" ref="wild" />
</bean>
but in Annotation based configuration we just have to write
<context:component-scan base-package="com.javapapers.spring.ioc" />
I want to know how it will load "wild" and "zoo" .
Does it means it will load all beans or only specific which is written under #Service annotation..???
I also want to know how it is loaded ..??? is all beans gets initialized when application is loaded..??
Thanks ...!!
When you use <context:component-scan base-package="com.javapapers.spring.ioc" />, spring will instanciate all classes that are in the "com.javapapers.spring.ioc" package and have one of this annotation :
#Service
#Controller
#Repository
...
And yes, all beans gets initialized when you launch your application.
You can have more info in this page : here
Beans get initialized through an ApplicationContext, which is also a BeanFactory. With an XML configuration, you would need an implementation of that interface, ClassPathXmlApplicationContext. Your application needs to create such a class, register your XML file(s), and refresh the context. When that is done, Spring will start creating your beans by reading the configuration.
When it hits the <component-scan> element, Spring will scan your declared packages for any classes annotated with #Component or its specializations. From the docs:
In Spring 2.0 and later, the #Repository annotation is a marker for
any class that fulfills the role or stereotype (also known as Data
Access Object or DAO) of a repository. Among the uses of this marker
is the automatic translation of exceptions.
Spring 2.5 introduces further stereotype annotations: #Component,
#Service, and #Controller. #Component is a generic stereotype for any
Spring-managed component. #Repository, #Service, and #Controller are
specializations of #Component for more specific use cases, for
example, in the persistence, service, and presentation layers,
respectively.
Therefore, you can annotate your component classes with #Component,
but by annotating them with #Repository, #Service, or #Controller
instead, your classes are more properly suited for processing by tools
or associating with aspects. For example, these stereotype annotations
make ideal targets for pointcuts.
Thus, if you are choosing between using #Component or #Service for
your service layer, #Service is clearly the better choice. Similarly,
as stated above, #Repository is already supported as a marker for
automatic exception translation in your persistence layer.
When it finds those classes, it will create an instance of each of them.
As for how it does this, it's a little more complicated. The overall strategy is with reflection. However, because of your configuration, Spring will sometimes generate (java or cglib) proxies instead of clear instances so that it can add behavior.
All the steps are described in detail in the official documentation.
Will be loaded all beans annotated with
#Controller
#Component
#Service
#Repository
which are inside a packages com.javapapers.spring.ioc, and its subpackages.
I have a custom annotation that is declared as a Spring managed Service:
#Service
public #interface MyServiceAnnotation {
// my service url
String url();
}
The above declaration enables my services to be autowired as Spring Managed beans.
#MyServiceAnnotation(url="/path/serviceLocation")
public class SomeService {
doWork();
}
However, there are certain services that have their bean definitions in an applicationContext.xml. Adding the #MyServiceAnnotation to such beans makes them both Autowiring enabled, as well as inject dependecy through the xml file.
Due to issues related to legacy code, I don't want to remove the xml bean definitions and make them all autowired.
So, is there a way in which I could turn off autowiring in this case, and still use #MyServiceAnnotation? Ideally I would like to have the #Service annotation on MyServiceAnnotation, the existing services would still use the #MyServiceAnnotation but would get their dependencies injected based on the xml. All the new services would be autowired without the xml bean definitions.
One possible approach is to create NonSpringManagedMyServiceAnnotation that is same as MyServiceAnnotation, but without the #Service annotation on it. The downside of this is that I'd have to duplicate rest of the code from MyServiceAnnotation, which I don't want to.
This is one approach, may not be that optimal though. I am assuming you would have specified a component-scan tag in your xml to scan the classes having the Spring stereotype annotations, these tags support a exclude-filter sub-tag to ignore specific patterns. If the files that you specify follow a specific pattern(specific packages, specific names etc) then you may simply be able to specify this sub-tag to ignore classes holding your annotation.
<context:component-scan base-package="mypackage">
<context:exclude-filter type="regex" expression=".*ToBeIgnoredNaming"/>
</context:component-scan>
I think you should try to maintain separation of concern.
Adding #Service on your annotation makes it a de facto Spring service : since this is not the behavior you want, you may simply have a simple #MyAnnotation (with url property) on each services (legacy and new).
Then you add Spring's #Service annotation on each new service to enable bean registration by annotation.
I'm building a Google App Engine app using Spring 3.1 and am having a problem getting members in one of my jars wired.
I have three projects:
server
server.model
server.persistence
I have an ant build script so that when my workspace builds, it creates jars for server.model and server.persistence, and puts them in the correct lib directory for the server project.
In server, I can autowire things from both server.model and server.persistence, but in server.model my server.persistence beans aren't getting wired even though they're the exact same as in server.
snippet from my servlet application config:
<context:component-scan base-package="com.impersonal.server"/>
<bean autowire="byType" id="appEngineDataStore" class="com.impersonal.server.persistance.AppEngineDataStore"/>
<bean autowire="byType" id="userList" class="com.impersonal.server.model.UserList"/>
I have the following code in both the server project and the server.model project, and only the server one gets fulfilled. Here's the one failing:
package com.impersonal.server.model;
import java.util.ArrayList;
import java.util.UUID;
import org.springframework.beans.factory.annotation.Autowired;
import com.impersonal.server.persistance.AppEngineDataStore;
import com.impersonal.server.persistance.IDataStore;
public class UserList extends ArrayList<User>
{
private UserList(){}
//this is always null, but the same line in a class in the other project works
private #Autowired AppEngineDataStore _dataStore;
public UserList(UUID userId, String tempId)
{
String poo = "poo";
poo.concat("foo ");
int i = 3;
}
}
Edit:
Just did a test in the server.model project trying to #Autowired something that I don't have defined as a bean in my application config, and didn't get any errors. I should have got a 'no such bean found' error like I do if I do the same thing for the server project.
Any ideas why?
I was instantiating my objects incorrectly. For framework objects and such like MVC controllers, you don't need to do anything to get your #Autowired members wired.
For objects I was creating on the fly, I wasn't going through the IOC container, that's why their dependencies weren't being fulfilled.
<context:component-scan/> tag searches for annotated classes.
If you are going to autowire class using #Autowire annotation, Autowiring class should be annotated with one of stereotype annotation (#Component,#Controller,#Service,#Repository). Spring resolves first annotation configuration and then xml configuration. This is written in spring doc as
Annotation injection is performed before XML injection, thus the latter configuration will override the former for properties wired through both approaches.
Check proof on spring doc.
So what you need to do is add annotations for classes from server project as well as server.model. Same in case of your third project server.persistence. Add annotations according to layers or functionality.
Try:
#Autowired(required = true)
private AppEngineDataStore _dataStore;
Instead of:
private #Autowired AppEngineDataStore _dataStore;
EDIT 1:
While using the autowire above, in your spring xml, try:
<bean id="appEngineDataStore" class="com.impersonal.server.persistance.AppEngineDataStore" scope="prototype"></bean>
Instead of:
<bean autowire="byType" id="appEngineDataStore" class="com.impersonal.server.persistance.AppEngineDataStore"/>
autowire in xml means slightly different thing. Instead of defining your bean in xml, you can annotate it as #Service, it will be discovered by the component-scan and the #Autowired will work.
In your xml configuration, use autowire-candidate property
<bean autowire="byType" id="appEngineDataStore" class="com.impersonal.server.persistance.AppEngineDataStore" autowire-candidate="true" />