Thursday, April 23, 2015

Performance Pitfalls: Inheriting JEE Singletons

In a previous blog post I wrote about the fact that JEE singleton EJBs have a very restrictive default lock mode - each method requires a WRITE lock which effectively allows for at most one method of the singleton to run at the same time.

Today I stumbled upon another gotcha: inherited methods are not affected by the default lock type set on the subclass. Consider the following code:

abstract class Parent {
  public void methodA() {}
}

@Singleton
@Lock(LockType.READ)
class Subclass extends Parent {
  public void methodB() {}
}


We have an abstract base class which implements some common functionality across multiple singletons (represented by the methodA()method). It is implemented as a POJO without any JEE annotations. Then we have the actual singleton which inherits from the base class and defines a default locking type READ. In our case, the developers were actually expecting that all methods, including those inherited from the Parent class, would have a READ lock type. Unfortunately, that was not the case. According to the EJB 3.1 specification, the methods inherited from the Parent class have a lock type as defined in the Parent class - if not specified then it falls back to the default WRITE lock type (page 113):
A concurrency locking attribute specified on a superclass S appplies to the business methods defined by S. If a class-level concurrency attribute is not specified on S, it is equivalent to specification of Lock(WRITE) on S.
In the example above we observed exactly that behavior. As a result, a thread running a method from the Parent class was effectively blocking any other thread from running any other method on the same singleton. In our case the situation was exacerbated by methods blocking on a slow external system.

The solution for us was easy as we did not really require any synchronization - switching to bean managed synchronization fixed the problem:

@Singleton
@ConcurrencyManagement(ConcurrencyManagementType.BEAN)
class Subclass extends Parent {
  public void methodB() {}
}


This time again we would have saved us some work if we switched to bean managed synchronization right away. I hope that this blog may help you work around similar issues in your applications.

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