I have a POJO class with a method annotated with #Transactional
public class Pojo {
#Transactional
public void doInTransaction() {
...
}
}
Spring declarative transaction management is based on AOP but I don't have any experience with that. My question is:
Is it possible that when invoking the (new Pojo).doInTransaction() alone, Spring will start a Transaction.
Spring declarative transaction
management is based on APO but I don't
have any experience with that.
I would recommend to start working with it and you will get the experience of using transaction advices using AOP. A good starting point is here.
Is it possible that when invoking the
(new Pojo).doInTransaction() alone,
Spring will start a Transaction.
No, you can't expect Spring to be aware of a bean that you manually invoked. However, it sounds like that you are wanting to avoid declarative transaction management and do programmatic transaction management. There is a way to do that with Spring using the Transaction Template. Is that what you were looking for?
It is somewhat possible, but in a cumbersome way: You must use the AutowireCapableBeanFactory mechanism.
Here is a transactional class as example
public interface FooBar{
void fooIze(Object foo);
}
public class FooBarImpl implements FooBar{
#Transactional
#Override
public void fooIze(final Object foo){
// do stuff here
}
}
And here is how we can use it:
public class FooService implements ApplicationContextAware{
private ApplicationContext applicationContext;
#Override
public void setApplicationContext(
final ApplicationContext applicationContext){
this.applicationContext = applicationContext;
}
public void serviceMethod(){
//declare variable as interface, initialize to implementation
FooBar fooBar = new FooBarImpl();
// try to use it, won't work, as it's not a proxy yet
Object target = new Object[0];
fooBar.fooIze(target); // no transaction
// now let spring create the proxy and re-assign the variable
// to the proxy:
fooBar = // this is no longer an instance of FooBarImpl!!!
(FooBar) applicationContext
.getAutowireCapableBeanFactory()
.applyBeanPostProcessorsAfterInitialization(fooBar,
"someBeanName");
fooBar.fooIze(fooBar); // this time it should work
}
}
This is not a best practice. For one thing, it makes your application highly aware of the Spring Framework and also, it violates the dependency injection principles. So use this only if there is no other way!
Yes, it is possible. Spring does not require the use of dynamic proxies for #Transactional to work. Instead, you can use "true AOP", as provided by AspectJ.
For the details, see http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/transaction.html#transaction-declarative-aspectj
The way Spring handle the transaction through Annotation is using AOP as you've said.
The AOP bit is implemented using Dynamic Proxies (see doc)
So in order to do so you'll need to retrieve an instance of your class (Pojo here) through the spring container since to make it work, Spring will return you a Dynamic Proxy over your Pojo that will automatically surround any annotated method with the transaction management code.
If you simply do a
Pojo p = new Pojo();
p.doInTransaction();
Spring doesn't have any role to play here and your method call won't be inside a transaction.
so what you need to do is something like this
ApplicationContext springContext = ...;
Pojo p = (Pojo) springContext.getBean("your.pojo.id");
p.doInTransaction();
Note: this is an example, you should prefer dependency injection instead of retrieving your bean manually from the context
By doing so, and with a properly configured Spring Context, Spring should have lookout your classes to scan for transactional annotation and automatically wrapped your beans into annotation aware dynamic proxies instances. From your point of view that doesn't change anything, you'll still cast your object to your own Classes, but if you try to print out the class name of your spring context Pojo bean, you'll get something as Proxy$... and not your original class name.
Have a look at this link anyway : link text
Related
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.
Anyone can explain how Spring defines the bean creation mechanism when create a bean which depends on a list of other beans? It would be good to show the part of Spring specification on how it's defined.
Code like:
public interface Test {
}
#Service
public class TestImpl1 implements Test{
}
#Service
public class TestImpl2 implements Test{
}
public class TestContainer {
List<Test> testList;
TestContainer() {
testList = new ArrayList<>();
}
public void addTest(Test test) {
testList.add(test);
}
}
then
#Bean
public TestContainer testContainer(List<Test> testList) {
TestContainer testContainer = new TestContainer();
for (Test test : testList) {
testContainer.addTest(test);
}
return testContainer;
}
Question is really: when creating bean for TestContainer, how does Spring figure out what should be in List testList?
This looks like what you are looking for:
https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#beans-factory-class
You haven't specified what exactly do you want to know about bean creation but here is the minimum you should know. By default all beans are singleton scoped (only created once during the container life-cycle and for all the subsequent request the same instance is returned). All singleton scoped beans are created eagerly. If the singleton bean is dependent on some other beans (needs them for instantiation) then those other beans will be instantiated with it/right before it, doesn't matter whether they are singletons or not, marked as lazy or not.
This is part of the spring documentation I think on their website. Look for it. The whole spring documentation is worth to read, even if it is a long read (https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#spring-core)
But mostly, spring would first read all the XML, anotations and other source of config data and get a list of beans to init. Then it would get a list of dependencies between beans constructing kind of a tree. As the dependencies need to be initialized first, there an obvious orderining for initialization.
On top, you can set your own priorities for beans so that you can get some beans initialized first or last for example.
For your specific case, spring will inspect the code source as well as use the Java reflection API to figure out you want a collection of interface Test implementations. So spring would look for ALL the defined bean that match and return them that not more complex than that.
It's mostly using the concepts of graph data structure where the beans become the nodes of the graph and they are resolved using topological sort.
I'm working on Spring Boot Rest API, and I did end up using the new keyword here and there.
I'm wondering, did I do something wrong when I used the new keyword for my program. And if it is absolutely forbidden to use new keyword on a real project.
If the answer is yes should i annotate each class i wrote with #component annotation so i can instantiate an object using #autowired.
If the answer is no when can we break that rule ?
You can create objects using the new keyword in a spring application.
But these objects would be outside the scope of the Spring Application Context and hence are not spring managed.
Since these are not spring managed, any nested levels of dependency (such as your Service class having a reference to your Repository class etc)
will not be resolved.
So if you try to invoke a method in your service class, you might end up getting a NullPointer for the repository.
#Service
public class GreetingService {
#Autowired
private GreetingRepository greetingRepository;
public String greet(String userid) {
return greetingRepository.greet(userid);
}
}
#RestController
public class GreetingController {
#Autowired
private GreetingService greetingService;
#RequestMapping("/greeting")
public String greeting(#RequestParam(value = "name", defaultValue = "World") String name) {
return String.format("Hello %s", greetingService.greet(name));
}
#RequestMapping("/greeting2")
public String greeting2(#RequestParam(value = "name", defaultValue = "World") String name) {
GreetingService newGreetingService = new GreetingService();
return String.format("Hello %s", newGreetingService.greet(name));
}
}
In the above example /greeting will work but /greeting2 will fail because the nested dependencies are not resolved.
So if you want your object to be spring managed, then you have to Autowire them.
Generally speaking, for view layer pojos and custom bean configurations, you will use the new keyword.
There is no rule for using or not using new.
It's up to you if you want Spring to manage your objects or want to take care of them on your own.
Spring eases object creation, dependency management, and auto wiring; however, you can instantiate it using new if you don't want that.
I think its fine to use new keyword, but you should learn the difference between different stereotype (Controller, Service, Repository)
You can follow this question to get some clarity:
What's the difference between #Component, #Repository & #Service annotations in Spring?
Using appropriate annotation will allow you to correctly use DI (dependency injection), that will help in writing sliced tests for your spring boot application. Also the Service,Controller and Repository components are created as Singleton, so lesser GC overhead. Moreover components that you create using new keyword are not managed by Spring, and by default Spring will never inject dependencies in a object created using new.
Spring official documentation:
https://docs.spring.io/spring-boot/docs/current/reference/html/using-boot-spring-beans-and-dependency-injection.html
You will need new on Spring mock tests when you will have to create an object as service and inject mock object as dao.
Look at the following code; here as you see, based on a condition it's necessary to dynamically load advertisements on demand. so here you can not #autowire this group of items because all the information are loaded from DB or an external system, so you just need to fill you model accordingly.
if (customer.getType() == CustomerType.INTERNET) {
List < Advertisement > adList = new ArrayList < Advertisement > ();
for (Product product: internetProductList) {
Advertisement advertisement = new Advertisement();
advertisement.setProduct(product);
adList.add(advertisement);
}
}
Note it's appropriate to use Spring for managing external dependencies
like plugging a JDBC connection into a DAO or configurations like
specifying which database type to use.
I have a standard Magnolia module that I've implemented as a Spring MVC REST client. In this module, I am trying to retrieve a JCR node and use Node2BeanProcessor to transform the Node object into my custom bean. Code below:
#Repository
public class JcrRepo() {
#Inject
public Node2BeanProcessor node2Bean;
public MagicWord getMagicWord(String key) {
Session session = LifeTimeJCRSessionUtil.getSession("magic");
Node theNode = session.getNode("/magicWords/" + key);
return node2Bean.toBean(theNode, MagicWord.class);
}
}
When I run this, I encounter a NullPointerException for the variable node2Bean. Which means it wasn't injected properly. However, I am able to do this:
node2Bean = Components.getComponent(Node2BeanProcessor.class);
The Components.getComponent() javadoc states: "Returns a component from the currently set ComponentProvider. Consider using IoC to inject the component instead." Which is what I'm trying to figure out.
Note that I have not done any Guice configuration as I'm looking for a way to leverage on Magnolia's already initialized Guice context to grab my objects.
Is there a better way to do injection than this, or have I done anything wrong or skipped a step?
Appreciate the help.
P.S. For now I've implemented a hacky way to use this in Spring IoC:
#Bean
public Node2BeanProcessor node2Bean() {
return Components.getComponent(Node2BeanProcessor.class);
}
(Working with Magnolia 4.5) I use #Inject for Node2BeanProcessor in a class implementing info.magnolia.module.ModuleLifecycle:
public class MyModule implements ModuleLifecycle {
#Inject
private Node2BeanProcessor node2BeanProcessor;
#Override
public void start(ModuleLifecycleContext moduleLifecycleContext) {
...
getNode2BeanProcessor().toBean(someNode);
...
}
}
Maybe your NullPointerException comes from theNode? Have you verified that theNode is not null?
Another guess is that it could be a lifecycle issue. From what I remember, Components.getComponent() works in situations where #Inject does not (in Magnolia).
Finally: Your instance variable should definitely be private.
If JcrRepo is not instantiated by Guice then Guice also won't be available to inject the Node2BeanProcessor field. Mixing Spring and Guice IoC containers can get confusing, so I tend to stick with Guice as that's what comes with Magnolia.
Is it possible to use #Configurable on a class that's weaved using AspectJ and get Spring to load in values on fields/methods which are annotated with #Value?
I know its possible with #Autowired and #Resource etc... Are there any others.
e.g.
#Configurable
public Class MyObj{
#Value("$(my.prop)")
private String aField;
public String getAField(){
return aField;
}
}
And then have something like
public aMethodSomewhereElse(){
MyObj obj = new MyObj()
assertNotNull(obj.getAField());
}
Are there any alternatives to being able to create MyObj with the new operator and still get spring to handle the annotations?
--EDIT:--
It IS possible to do this using new when using #Autowired, have a look at some Hibernate and JPA stuff with Spring and AOP... I've used this in the past to do some profiling of Java code. But I really want to use SPEL and #Value before I mock up a full example I was hoping to find the answer here. FYI - if you don't belive me the Spring Manual even says it is possible to do this, what I want to know is if its possible to use #Value annotations in the same scope...
The Spring container instantiates and configures beans defined in your
application context. It is also possible to ask a bean factory to
configure a pre-existing object given the name of a bean definition
containing the configuration to be applied. The spring-aspects.jar
contains an annotation-driven aspect that exploits this capability to
allow dependency injection of any object.
And...
Using the annotation on its own does nothing of course. It is the
AnnotationBeanConfigurerAspect in spring-aspects.jar that acts on the
presence of the annotation. In essence the aspect says "after
returning from the initialization of a new object of a type annotated
with #Configurable, configure the newly created object using Spring in
accordance with the properties of the annotation". In this context,
initialization refers to newly instantiated objects (e.g., objects
instantiated with the 'new' operator) as well as to Serializable
objects that are undergoing deserialization (e.g., via readResolve()).
http://static.springsource.org/spring/docs/3.0.0.RC2/reference/html/ch07s08.html
Cheers.
You are absolutely right - #Autowired fields will be wired in an #Configurable annotated class even outside of a Spring container, assuming that you have a AspectJ infrastructure in place.
You have noted a good catch though, #Value fields are processed by a Spring bean post processor(AutowiredAnnotationBeanPostProcessor), which resolves the #Value annotated fields. It does not act on objects instantiated outside of the container though - so in short, the #Autowired fields should get wired in, but #Value properties will not.
Doing
MyObj obj = new MyObj()
means that obj is not managed by spring, so it will not do autowiring.
Only way to do that is to obtain instance from an application context. For example:
ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
MyObj obj = context.getBean("myBean");
I don't think it is possible to use new operator and ask spring to autowire properties. I think 1 way to solve this is to get a static reference to applicationContext and create a prototype scoped bean.
#Component
public class ApplicationContextLocator {
private static ApplicationContext applicationContext;
public static ApplicationContext getApplicationContext() {
return applicationContext;
}
public ApplicationContextLocator() {
super();
}
#Autowired
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
ApplicationContextLocator.applicationContext = applicationContext;
}
}
#Scope(BeanDefinition.SCOPE_PROTOTYPE)
#Component
public class MyObj {
.....
}
public aMethodSomewhereElse(){
MyObj obj = ApplicationContextLocator.getApplicationContext().getBean(MyObj.class)
assertNotNull(obj.getAField());
}