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.

Performance Pitfalls: JEE Singletons

JEE singletons are session beans which are guaranteed to exist at most once per application/JVM combination. They were introduced with EJB 3.1 as a replacement for the "normal" java singletons in a JEE context.

The java pattern for singletons relies on a private static field holding the single instance of the class. The static field is accessible through a static method. The field is either eagerly initialized in a static block or lazily initialized upon first invocation of the method. In order to guarantee that there is only one single instance, the method is usually synchronized. In high concurrency scenario you might use double checked locking and not class level synchronization.

Due to the fact that static variables are stored in the context of a class, such singletons are only guaranteed to be unique within the same class-loader. If you have a JEE application consisting of multiple modules containing the same singleton class, you are in trouble. Each module can (and probably will) have its own class-loader and thus its own static instance of the singleton resulting in multiple instances within the same JVM.

One very important aspect of JEE singletons is the fact that all of their methods are per default synchronized! If not otherwise configured, each singleton gets one read-write lock (check out this for details on read-write locks) from the container. Each method acquires the WRITE lock upon invocation. This effectively means that you can only have only one method of the class being executed at any point in time. If that is what you want, you are fine.

In order to achieve the same behaviour as with a "normal" java singleton (no synchronization of the singleton's methods) you will need to  explicitly switch to bean managed concurrency using the @ConcurrencyManagement(ConcurrencyManagementType.BEAN) annotation. If you need the read-write lock semantics, e.g. you have methods that change the state of the singleton and others that only read it, you can set the default lock mode of all methods to READ (check out section 4.8.5 of the specification) and only annotate the state-changing methods with the WRITE lock mode.

If you just migrate from java to JEE singletons without considering synchronization, you risk getting contention on the singleton's instance methods which would result in poor performance.