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If Spring can successfully intercept intra class function calls in a #Configuration class, why does it not support it in a regular bean?

I have recently noticed that Spring successfully intercepts intra class function calls in a #Configuration class but not in a regular bean.
A call like this
#Repository
public class CustomerDAO {
#Transactional(value=TxType.REQUIRED)
public void saveCustomer() {
// some DB stuff here...
saveCustomer2();
}
#Transactional(value=TxType.REQUIRES_NEW)
public void saveCustomer2() {
// more DB stuff here
}
}
fails to start a new transaction because while the code of saveCustomer() executes in the CustomerDAO proxy, the code of saveCustomer2() gets executed in the unwrapped CustomerDAO class, as I can see by looking at 'this' in the debugger, and so Spring has no chance to intercept the call to saveCustomer2.
However, in the following example, when transactionManager() calls createDataSource() it is correctly intercepted and calls createDataSource() of the proxy, not of the unwrapped class, as evidenced by looking at 'this' in the debugger.
#Configuration
public class PersistenceJPAConfig {
#Bean
public DriverManagerDataSource createDataSource() {
DriverManagerDataSource dataSource = new DriverManagerDataSource();
//dataSource.set ... DB stuff here
return dataSource;
}
#Bean
public PlatformTransactionManager transactionManager( ){
DataSourceTransactionManager transactionManager = new DataSourceTransactionManager(createDataSource());
return transactionManager;
}
}
So my question is, why can Spring correctly intercept the intra class function calls in the second example, but not in the first. Is it using different types of dynamic proxies?
Edit:
From the answers here and other sources I now understand the following:
#Transactional is implemented using Spring AOP, where the proxy pattern is carried out by wrapping/composition of the user class. The AOP proxy is generic enough so that many Aspects can be chained together, and may be a CGLib proxy or a Java Dynamic Proxy.
In the #Configuration class, Spring also uses CGLib to create an enhanced class which inherits from the user #Configuration class, and overrides the user's #Bean functions with ones that do some extra work before calling the user's/super function such as check if this is the first invocation of the function or not. Is this class a proxy? It depends on the definition. You may say that it is a proxy which uses inheritance from the real object instead of wrapping it using composition.
To sum up, from the answers given here I understand these are two entirely different mechanisms. Why these design choices were made is another, open question.

Is it using different types of dynamic proxies?
Almost exactly
Let's figure out what's the difference between #Configuration classes and AOP proxies answering the following questions:
Why self-invoked #Transactional method has no transactional semantics even though Spring is capable of intercepting self-invoked methods?
How #Configuration and AOP are related?
Why self-invoked #Transactional method has no transactional semantics?
Short answer:
This is how AOP made.
Long answer:
Declarative transaction management relies on AOP (for the majority of Spring applications on Spring AOP)
The Spring Framework’s declarative transaction management is made possible with Spring aspect-oriented programming (AOP)
It is proxy-based (§5.8.1. Understanding AOP Proxies)
Spring AOP is proxy-based.
From the same paragraph SimplePojo.java:
public class SimplePojo implements Pojo {
public void foo() {
// this next method invocation is a direct call on the 'this' reference
this.bar();
}
public void bar() {
// some logic...
}
}
And a snippet proxying it:
public class Main {
public static void main(String[] args) {
ProxyFactory factory = new ProxyFactory(new SimplePojo());
factory.addInterface(Pojo.class);
factory.addAdvice(new RetryAdvice());
Pojo pojo = (Pojo) factory.getProxy();
// this is a method call on the proxy!
pojo.foo();
}
}
The key thing to understand here is that the client code inside the main(..) method of the Main class has a reference to the proxy.
This means that method calls on that object reference are calls on the proxy.
As a result, the proxy can delegate to all of the interceptors (advice) that are relevant to that particular method call.
However, once the call has finally reached the target object (the SimplePojo, reference in this case), any method calls that it may make on itself, such as this.bar() or this.foo(), are going to be invoked against the this reference, and not the proxy.
This has important implications. It means that self-invocation is not going to result in the advice associated with a method invocation getting a chance to execute.
(Key parts are emphasized.)
You may think that aop works as follows:
Imagine we have a Foo class which we want to proxy:
Foo.java:
public class Foo {
public int getInt() {
return 42;
}
}
There is nothing special. Just getInt method returning 42
An interceptor:
Interceptor.java:
public interface Interceptor {
Object invoke(InterceptingFoo interceptingFoo);
}
LogInterceptor.java (for demonstration):
public class LogInterceptor implements Interceptor {
#Override
public Object invoke(InterceptingFoo interceptingFoo) {
System.out.println("log. before");
try {
return interceptingFoo.getInt();
} finally {
System.out.println("log. after");
}
}
}
InvokeTargetInterceptor.java:
public class InvokeTargetInterceptor implements Interceptor {
#Override
public Object invoke(InterceptingFoo interceptingFoo) {
try {
System.out.println("Invoking target");
Object targetRetVal = interceptingFoo.method.invoke(interceptingFoo.target);
System.out.println("Target returned " + targetRetVal);
return targetRetVal;
} catch (Throwable t) {
throw new RuntimeException(t);
} finally {
System.out.println("Invoked target");
}
}
}
Finally InterceptingFoo.java:
public class InterceptingFoo extends Foo {
public Foo target;
public List<Interceptor> interceptors = new ArrayList<>();
public int index = 0;
public Method method;
#Override
public int getInt() {
try {
Interceptor interceptor = interceptors.get(index++);
return (Integer) interceptor.invoke(this);
} finally {
index--;
}
}
}
Wiring everything together:
public static void main(String[] args) throws Throwable {
Foo target = new Foo();
InterceptingFoo interceptingFoo = new InterceptingFoo();
interceptingFoo.method = Foo.class.getDeclaredMethod("getInt");
interceptingFoo.target = target;
interceptingFoo.interceptors.add(new LogInterceptor());
interceptingFoo.interceptors.add(new InvokeTargetInterceptor());
interceptingFoo.getInt();
interceptingFoo.getInt();
}
Will print:
log. before
Invoking target
Target returned 42
Invoked target
log. after
log. before
Invoking target
Target returned 42
Invoked target
log. after
Now let's take a look at ReflectiveMethodInvocation.
Here is a part of its proceed method:
Object interceptorOrInterceptionAdvice = this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
++this.currentInterceptorIndex should look familiar now
Here is the target
And there are interceptors
the method
the index
You may try introducing several aspects into your application and see the stack growing at the proceed method when advised method is invoked
Finally everything ends up at MethodProxy.
From its invoke method javadoc:
Invoke the original method, on a different object of the same type.
And as I mentioned previously documentation:
once the call has finally reached the target object any method calls that it may make on itself are going to be invoked against the this reference, and not the proxy
I hope now, more or less, it's clear why.
How #Configuration and AOP are related?
The answer is they are not related.
So Spring here is free to do whatever it wants. Here it is not tied to the proxy AOP semantics.
It enhances such classes using ConfigurationClassEnhancer.
Take a look at:
CALLBACKS
BeanMethodInterceptor
BeanFactoryAwareMethodInterceptor
Returning to the question
If Spring can successfully intercept intra class function calls in a #Configuration class, why does it not support it in a regular bean?
I hope from technical point of view it is clear why.
Now my thoughts from non-technical side:
I think it is not done because Spring AOP is here long enough...
Since Spring Framework 5 the Spring WebFlux framework has been introduced.
Currently Spring Team is working hard towards enhancing reactive programming model
See some notable recent blog posts:
Reactive Transactions with Spring
Spring Data R2DBC 1.0 M2 and Spring Boot starter released
Going Reactive with Spring, Coroutines and Kotlin Flow
More and more features towards less-proxying approach of building Spring applications are introduced. (see this commit for example)
So I think that even though it might be possible to do what you've described it is far from Spring Team's #1 priority for now

Because AOP proxies and #Configuration class serve a different purpose, and are implemented in a significantly different ways (even though both involve using proxies).
Basically, AOP uses composition while #Configuration uses inheritance.
AOP proxies
The way these work is basically that they create proxies that do the relevant advice logic before/after delegating the call to the original (proxied) object. The container registers this proxy instead of the proxied object itself, so all dependencies are set to this proxy and all calls from one bean to another go through this proxy. However, the proxied object itself has no pointer to the proxy (it doesn't know it's proxied, only the proxy has a pointer to the target object). So any calls within that object to other methods don't go through the proxy.
(I'm only adding this here for contrast with #Configuration, since you seem to have correct understanding of this part.)
#Configuration
Now while the objects that you usually apply the AOP proxy to are a standard part of your application, the #Configuration class is different - for one, you probably never intend to create any instances of that class directly yourself. This class truly is just a way to write configuration of the bean container, has no meaning outside Spring and you know that it will be used by Spring in a special way and that it has some special semantics outside of just plain Java code - e.g. that #Bean-annotated methods actually define Spring beans.
Because of this, Spring can do much more radical things to this class without worrying that it will break something in your code (remember, you know that you only provide this class for Spring, and you aren't going to ever create or use its instance directly).
What it actually does is it creates a proxy that's subclass of the #Configuration class. This way, it can intercept invocation of every (non-final non-private) method of the #Configuration class, even within the same object (because the methods are effectively all overriden by the proxy, and Java has all the methods virtual). The proxy does exactly this to redirect any method calls that it recognizes to be (semantically) references to Spring beans to the actual bean instances instead of invoking the superclass method.

read a bit spring source code. I try to answer it.
the point is how spring deal with the #Configurationand #bean.
in the ConfigurationClassPostProcessor which is a BeanFactoryPostProcessor, it will enhance all ConfigurationClasses and creat a Enhancer as a subClass.
this Enhancer register two CALLBACKS(BeanMethodInterceptor,BeanFactoryAwareMethodInterceptor).
you call PersistenceJPAConfig method will go through the CALLBACKS. in BeanMethodInterceptor,it will get bean from spring container.
it may be not clearly. you can see the source code in ConfigurationClassEnhancer.java BeanMethodInterceptor.ConfigurationClassPostProcessor.java enhanceConfigurationClasses

You can't call #Transactional method in same class
It's a limitation of Spring AOP (dynamic objects and cglib).
If you configure Spring to use AspectJ to handle the transactions, your code will work.
The simple and probably best alternative is to refactor your code. For example one class that handles users and one that process each user. Then default transaction handling with Spring AOP will work.
Also #Transactional should be on Service layer and not on #Repository
transactions belong on the Service layer. It's the one that knows about units of work and use cases. It's the right answer if you have several DAOs injected into a Service that need to work together in a single transaction.
So you need to rethink your transaction approach, so your methods can be reuse in a flow including several other DAO operations that are roll-able

Spring uses proxying for method invocation and when you use this... it bypasses that proxy. For #Bean annotations Spring uses reflection to find them.

Pointcut or Aspect Around All Service Methods with Annotation #Transactional(readOnly = false)

Is it possible to use Spring AOP or AspectJ to intercept all Service methods (contained in classes in the com.app.service.* package) having the annotation
#Transactional(readOnly = false)
(other elements possible as well in Spring's #Transactional annotation, but we only care about readOnly = false).
I could only find examples pertaining to pointcuts with simple Annotations, or #Annotation(value).
My preference would be to use straight Spring, if possible.
It would probably be something like the below, but not sure about the syntax.
#Around("execution(* com.app.service..*.*(..))" && #Transactional[??])

You want to use a pointcut like this:
execution(#org.springframework.transaction.annotation.Transactional(readOnly = false) * com.app.service..*.*(..))

Unfortunately no easy way to do that. Even when we have an Annotation-based pointcut, e.g.
#Aspect
#Component
#EnableAspectJAutoProxy
public class WriteTransactionAspectBean {
#Before("#annotation(org.springframework.transaction.annotation.Transactional)")
public void test(org.springframework.transaction.annotation.Transactional t) {
System.out.println("TEST");
}
}
the issue is the Annotations aren't our own, they come from an external JAR (Hibernate). This would require Load-Time Weaving or some other difficult workaround.
Aspectj: intercept method from external jar
But to make things even worse, Annotations need RetentionPolicy=RUNTIME in order to be "discovered" by Pointcuts. And we would need to go thru every method and add this specification to every #Transactional. There's no way to automatically make all #Transactional's Runtime-retainable in the application.

Perf4j #profiled annotation for private method

I have added #Profiled annotation for public and private method. I can see the timing logs for public methods. But there is no timing logs for private methods.
I did the configurations correctly.
#Profiled
public BigInteger nextPrime() {
currentPrime = currentPrime.nextProbablePrime();
return currentPrime;
}
#Profiled
private void test()
{
try{
Thread.sleep(100);
}catch(Exception e)
{
e.printStackTrace();
}
}
Is #profiled annotation only applicable for only public methods?

Most likely you have configured perf4j with Spring AOP and since Spring AOP proxies are not invoked when a method is called from inside the same class (as in your case with the test method), no performance statistics will be recorded.
Check out this blog post which explains Spring AOP pitfalls
One solution to your problem would be to use AspectJ with perf4j, since AspectJ can handle this type of situation

Yes. It is limitation of the Spring AOP:
When using proxies, you should apply the you annotations only to
methods with public visibility. If you do annotate protected, private
or package-visible methods with these annotations, no error is raised,
but the annotated method does not exhibit the configured caching
settings. Consider the use of AspectJ (see below) if you need to
annotate non-public methods as it changes the bytecode itself.”

Writing interceptors for parameter annotations using Guice

Main References for My Question:
Writing Method Interceptors for Google Guice: http://code.google.com/p/google-guice/wiki/AOP
The JavaDoc for the MethodInterceptor interface: http://aopalliance.sourceforge.net/doc/org/aopalliance/intercept/MethodInterceptor.html
General references about Java annotations: http://docs.oracle.com/javase/tutorial/java/javaOO/annotations.html and http://docs.oracle.com/javase/1.5.0/docs/guide/language/annotations.html
Now My Question:
I am writing a Java application that heavily relies on Google Guice for creating objects and handling dependency injection. I am trying to use interceptors to run pre-processing code before certain annotated methods are executed. So far, I have successfully been able to execute interceptors (using the MethodInterceptor interface) on methods that have been annotated, using Guice's instructions. However, I want to now write interceptors that will execure on Parameter Annotations.
Here is an example scenario. First, I create my own annotation. For example::
#BindingAnnotation
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.PARAMETER)
public #interface MyParameterAnnotation {
}
Next, I write my own interceptor for this annotation:
public class MyParameterAnnotationInterceptor implements MethodInterceptor {
#Override
public Object invoke(MethodInvocation invocation) throws Throwable {
// Do some stuff
return invocation.proceed();
}
}
Here's an example of how I intend on using #MyParameterAnnotation:
public class ExampleObject {
public String foo(#MyParameterAnnotation String param) {
...
}
}
Finally, I need to create a Guice Injector and use it to create an instalce of ExampleObject, or else I cannot use a method interceptor in this project. I configure the Injector so that the MyParameterAnnotationInterceptor is bound to #MyParameterAnnotation, like so:
final MethodInterceptor interceptor = new MyParameterAnnotationInterceptor();
requestStaticInjection(MyParameterAnnotationInterceptor.class);
bindInterceptor(Matchers.any(), Matchers.annotatedWith(MyParameterAnnotation.class), interceptor);
When I follow the above steps and execute a call to ExampleObject.foo(), unfortunately the interceptor is not executed despite the parameter being marked by #MyParameterAnnotation. Note that these similar steps will work if the annotation was placed at the method level instead.
This leads me to come up with two possible conclusions: either Guice cannot support binding an interceptor to a parameter annotation, or I am doing something completely incorrect (perhaps I should use another AOP Alliance interface for the interceptor, like FieldInterceptor, but I highly doubt it because the JavaDoc for Guice's AbstractModule suggests that the bindInterceptor() method can only use a MethodInterceptor parameter).
Nonetheless, all help us much appreciated :)

The matcher is for method annotations not method parameter annotations.
There is no matcher provided by Guice for method parameter annotations--you either have to write one yourself or use some other scheme. Note that this is a bit of an odd use case--Generally you can get away with
public class ExampleObject {
#MyAnnotation
public String foo(String param) {
...
}
}
You have the right Guice interceptor config for the above example.

Intercepting Specific Annotations with Spring AOP

I'm looking to see whether or not the following is even possible, as all preliminary searches haven't turned back anything to indicate either way.
I'd like to use Hibernate's Validator annotations to validate bean methods, and I would like to use some AOP framework (Spring, AOP Alliance, AspectJ, etc.) to intercept methods annotated with a subset of the Hibernate Validator annotations (#NotNull, #NotEmpty, #Email, etc.); I then want AOP advice to run when they are encountered.
Is this possible to do? If so, I am having a tough time visualizing how the code would work. Using Spring AOP's MethodInterceptor interface as an example:
First, the bean using Hibernate Validator:
public class SomeBean
{
private String data;
// Hibernate Validator annotation specifying that "data" cannot be an empty
// string.
#NotEmpty
public String getData() { ... } // etc.
}
Then, some code using that bean:
public void someMethod()
{
SomeBean oBean = new SomeBean();
// Validation should fail because we specified that "data" cannot be empty.
oBean.setData("");
}
Next, the AOP advice to be ran when Hibernate Validator-annotated methods are encountered.
public class ValidationInterceptor implements MethodInterceptor
{
public Object invoke(MethodInvocation invocation)
{
// Here's where we would use Hibernate's validator classes.
// My code example here is wrong, but it gets the point across.
Class targetClass = invocation.getClass(); // Should give me SomeBean.class
ClassValidator<targetClass> oValidator= new ClassValidator<targetClass>();
// Here I need to get a reference to the instance of the offending
// SomeBean object whose data has been set to empty...not sure how!
SomeBean oOffendingBean = getTheBadBeanSomehow();
InvalidValue[] badVals = oValidator.getInvalidValues(oOffendingBean);
}
}
So, not only am I choking on what the Spring AOP (pointcut definitions, etc.) configuration would look like to intercept the Hibernate Validator annotations I want, and not only do I not fully grasp how to implement the actual advice (e.g. how to instantiate the offending SomeBean from inside the advice as I mention above in the comments), but I'm not even sure if this solution is possible, Spring or otherwise.
Thanks in advance for some gentle "nudges" in the right direction!

You might be interested in the method validation feature introduced with Hibernate Validator 4.2 which provides support for validating method parameters and return values.
You then might use Seam Validation which integrates this functionality with CDI. If you want to use method validation together with Spring you could have a look this project on GitHub which shows how to integrate the method validation functionality with Spring AOP (disclaimer: I'm the author of this project as well as of Seam Validation).
To make your example working you would have to annote the parameter of the setter method with #NotEmpty like this:
public class SomeBean {
private String data;
#NotEmpty
public String getData() { return data; }
public void setData(#NotEmpty String data) { this.data = data; }
}