Spring - Transactional - What happens in background

Question

I want to know what actually happens when you annotate a method with  Transactional  Of course  I know that Spring will wrap that method in a Transaction   But  I have the following doubts    I heard that Spring creates a proxy class  Can someone explain this in more depth  What actually resides in that proxy class  What happens to the actual class  And how can I see Spring s created proxied class  I also read in Spring docs that        Note  Since this mechanism is based on proxies  only  external  method calls coming in through the proxy will be intercepted   This means that  self-invocation   i e  a method within the target object calling some other method of the target object  won t lead to an actual transaction at runtime even if the invoked method is marked with  Transactional    Source  http   static springsource org spring docs 2 0 x reference transaction html  Why only external method calls will be under Transaction and not the self-invocation methods

User · Answer

The simplest answer is    On whichever method you declare  Transactional the boundary of transaction starts and boundary ends when method completes    If you are using JPA call then all commits are with in this transaction boundary    Lets say you are saving entity1  entity2 and entity3  Now while saving entity3 an exception occur  then as enitiy1 and entity2 comes in same transaction so entity1 and entity2 will be rollback with entity3   Transaction          entity1 save   entity2 save   entity3 save      Any exception will result in rollback of all JPA transactions with DB Internally JPA transaction are used by Spring

User · Answer

When Spring loads your bean definitions  and has been configured to look for  Transactional annotations  it will create these proxy objects around your actual bean  These proxy objects are instances of classes that are auto-generated at runtime  The default behaviour of these proxy objects when a method is invoked is just to invoke the same method on the  target  bean  i e  your bean     However  the proxies can also be supplied with interceptors  and when present these interceptors will be invoked by the proxy before it invokes your target bean s method  For target beans annotated with  Transactional  Spring will create a TransactionInterceptor  and pass it to the generated proxy object  So when you call the method from client code  you re calling the method on the proxy object  which first invokes the TransactionInterceptor  which begins a transaction   which in turn invokes the method on your target bean  When the invocation finishes  the TransactionInterceptor commits rolls back the transaction  It s transparent to the client code   As for the  external method  thing  if your bean invokes one of its own methods  then it will not be doing so via the proxy  Remember  Spring wraps your bean in the proxy  your bean has no knowledge of it  Only calls from  outside  your bean go through the proxy   Does that help

User · Answer

This is a big topic  The Spring reference doc devotes multiple chapters to it  I recommend reading the ones on Aspect-Oriented Programming and Transactions  as Spring s declarative transaction support uses AOP at its foundation   But at a very high level  Spring creates proxies for classes that declare  Transactional on the class itself or on members  The proxy is mostly invisible at runtime  It provides a way for Spring to inject behaviors before  after  or around method calls into the object being proxied  Transaction management is just one example of the behaviors that can be hooked in  Security checks are another  And you can provide your own  too  for things like logging  So when you annotate a method with  Transactional  Spring dynamically creates a proxy that implements the same interface s  as the class you re annotating  And when clients make calls into your object  the calls are intercepted and the behaviors injected via the proxy mechanism    Transactions in EJB work similarly  by the way   As you observed  through  the proxy mechanism only works when calls come in from some external object  When you make an internal call within the object  you re really making a call through the  this  reference  which bypasses the proxy  There are ways of working around that problem  however  I explain one approach in this forum post in which I use a BeanFactoryPostProcessor to inject an instance of the proxy into  self-referencing  classes at runtime  I save this reference to a member variable called  me   Then if I need to make internal calls that require a change in the transaction status of the thread  I direct the call through the proxy  e g   me someMethod      The forum post explains in more detail  Note that the BeanFactoryPostProcessor code would be a little different now  as it was written back in the Spring 1 x timeframe  But hopefully it gives you an idea  I have an updated version that I could probably make available

User · Answer

All existing answers are correct  but I feel cannot give just this complex topic   For a comprehensive  practical explanation you might want to have a look at this Spring  Transactional In-Depth guide  which tries its best to cover transaction management in  4000 simple words  with a lot of code examples

User · Answer

It may be late but I came across something which explains your concern related to proxy  only  external  method calls coming in through the proxy will be intercepted  nicely   For example  you have a class that looks like this   Component  mySubordinate   public class CoreBusinessSubordinate        public void doSomethingBig             System out println  I did something small               public void doSomethingSmall int x           System out println  I also do something small but with an int                and you have an aspect  that looks like this    Component  Aspect public class CrossCuttingConcern         Before  execution   com intertech CoreBusinessSubordinate              public void doCrossCutStuff            System out println  Doing the cross cutting concern now              When you execute it like this     Service public class CoreBusinessKickOff         Autowired     CoreBusinessSubordinate subordinate          getter setters      public void kickOff            System out println  I do something big           subordinate doSomethingBig           subordinate doSomethingSmall 4             Results of calling kickOff above given code above   I do something big Doing the cross cutting concern now I did something small Doing the cross cutting concern now I also do something small but with an int   but when you change your code to   Component  mySubordinate   public class CoreBusinessSubordinate        public void doSomethingBig             System out println  I did something small            doSomethingSmall 4              public void doSomethingSmall int x          System out println  I also do something small but with an int                 public void kickOff       System out println  I do something big       subordinate doSomethingBig         subordinate doSomethingSmall 4       You see  the method internally calls another method so it won t be intercepted and the output would look like this   I do something big Doing the cross cutting concern now I did something small I also do something small but with an int   You can by-pass this by doing that  public void doSomethingBig         System out println  I did something small          doSomethingSmall 4         CoreBusinessSubordinate  AopContext currentProxy    doSomethingSmall 4       Code snippets taken from  https   www intertech com Blog secrets-of-the-spring-aop-proxy

User · Answer

As a visual person  I like to weigh in with a sequence diagram of the proxy pattern  If you don t know how to read the arrows  I read the first one like this  Client executes Proxy method      The client calls a method on the target from his perspective  and is silently intercepted by the proxy If a before aspect is defined  the proxy will execute it Then  the actual method  target  is executed  After-returning and after-throwing are optional aspects that are executed after the method returns and or if the method throws an exception After that  the proxy executes the after aspect  if defined  Finally the proxy returns to the calling client     I was allowed to post the photo on condition that I mentioned its origins  Author  Noel Vaes  website  www noelvaes eu

[java] Spring - @Transactional - What happens in background?

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