I think I have a good idea of how spring uses proxy concept with #Transactional annotation but I can't find anything about "default" implementation. Basically what I'm looking for is the code which wraps the invocation of original method (method from wrapped object).
There is no such thing as a default implementation because it all depends on the class implementing the method where you add the #Transactional annotation to.
If that class inherits from an interface then JDK Dynamic Proxy will be used.
If not, then an external library called CGLIB will be used to create the proxy.
Dynamic proxy will create a proxy that implements all interfaces your target class also implements while CGLIB will create a proxy that extends your target class.
Also make sure to read this SO question concerning the difference between Dynamic Proxy and CGLIB proxy as it contains valuable information as well.
Related
In spring why the interface is injected rather than the class implementing the interface?
In fact even if you use interface a class is injected. Spring looks for a bean - class which implements the interface.
In some cases like e.g. Spring Data dynamic proxy is created for the interfaces which in fact a class to listen interface's methods calls and do some logic on the calls.
If you set a breakpoint and check what is really injected you will find a class (either your class or some Proxy object)
I know what is Proxy in network community (server intermediary), but what is proxy in Spring ? Why spring beans are wrapped proxy ? I don't understand the idea of proxy in Spring. Thanks for response.
A proxy is a Spring generated class, that wraps your class for a given purpose, ie: adding transactional behaviour
Take a deeper look at the documentation here
It's a class that wraps your class. It is a proxy because all calls to methods of your class pass through it before actually getting to your class. The goal is to enhance your class with additional functionality, for example as #CristianMeneses said, to add transactional behavior to it, or inject some resources.
I want to know how spring change my method that has #transactional annotation on it?
for example I read here about how to run a transactional method without using #transactional.
What spring do exactly?
Spring effectively makes proxies for your objects. So if class "MyApplication" injects "DBService", and DBService has a #Transactional on it, spring will make a DBService Proxy. The Proxy will be injected into MyApplication, and all calls to methods of the DBService will instead call that Proxy. That Proxy would then be allowed to start transactions or do whatever it needs before calling the actual DBService method.
MyApplication -> DBService Proxy (intercepts calls) -> DBService
The details of how the proxy is made may change based on spring version and the way you've setup your code. You can use interfaces allowing spring to make a proxy of that, although, if you choose not to use interfaces, spring can extend the actual class too (using a library called CGLIB. Pre 4, this required a default constructor, although, in the latest Spring 4 the default constructor is not required and it does it via a slightly different mechanism)
I am reading the Spring Hibernate CRUD tutorial from this url
http://viralpatel.net/blogs/spring3-mvc-hibernate-maven-tutorial-eclipse-example/
Please can anyone tell me why in ContactController.java, ContactService interface is autowired instead of the class ContactServiceImpl.
Similarly in ContactServiceImpl ContactDAO interface is injected. Shouldn't we supposed to inject class instead of an interface?
When your code is dependent on interface and its implementation is injected by Spring, your code becomes decoupled with the implementation. This has an advantage that now you can swap in a different implementation without having to change the code that makes use of interface.
Spring is smart. It will find the implementation of the interface and inject it appropriately (or a proxy thereof.)
You should be programming to interfaces, not implementations.
Im using Jersey Rest and want a Jersey filter to have access to some spring beans.
however as I've discovered from other threads, Jersey does not obtain Spring beans if they are Java proxies as opposed to generated java proxies. I want to add the proxy-target-class="true"
What are the impacts of doing so and also can this just be set on a single bean or does it need to be set on all referenced beans?
By setting proxy-target-class="true" you will be using CGLIB2 for your proxies, instead of jdk proxys.
The implications are the following, as described in the documentation:
final methods cannot be advised, as they cannot be overriden.
You will need the CGLIB 2 binaries on your classpath, whereas dynamic proxies are available with the JDK. Spring will automatically
warn you when it needs CGLIB and the CGLIB library classes are not
found on the classpath.
The constructor of your proxied object will be called twice. This is a natural consequence of the CGLIB proxy model whereby a subclass
is generated for each proxied object. For each proxied instance, two
objects are created: the actual proxied object and an instance of the
subclass that implements the advice. This behavior is not exhibited
when using JDK proxies. Usually, calling the constructor of the
proxied type twice, is not an issue, as there are usually only
assignments taking place and no real logic is implemented in the
constructor.
Also, you should be able to make a "target-proxy" for a specific component by using
proxyMode=ScopedProxyMode.TARGET_CLASS
Forcing a CGLib-Proxy although the controller formally implements an interface (SpringBoot 1.2.3.RELEASE with Spring 4.1.6.RELEASE):
#Controller
#Scope( proxyMode = ScopedProxyMode.TARGET_CLASS )
public class ServiceImpl implements ServiceIntf
{ .... }
This enables valid and working #RequestMapping and #Transactional annotations
Use the following annotation in Java Spring Config class:
#EnableAspectJAutoProxy(proxyTargetClass = true)
This is the way I made my test working:
MyTarget target = new MyTarget();
AspectJProxyFactory factory = new AspectJProxyFactory(target);
factory.setProxyTargetClass(true);