Should We Cache Method References in Java 8?

Is Method Reference Caching a Valuable Technique in Java 8?

Understanding the Nature of Method References

Java 8's method references provide a convenient mechanism to refer to a specific method via its name and parameters. However, the execution of a method reference involves the creation of an anonymous class object known as a lambda. Caching this reference has been proposed as a potential optimization, raising questions about its validity.

Method Reference Caching Mechanics

Caching a method reference involves storing a placeholder that avoids the repeated creation of anonymous classes for each function call. However, it's crucial to distinguish between repeated executions of the same call-site (a specific point in the code where the method reference is invoked) and repeated uses of a method reference to the same method (invoked from different call-sites).

JIT Compilation and Method Reference Optimization

The Java Virtual Machine (JVM) employs Just-In-Time (JIT) compilation to analyze and optimize code at runtime. For stateless lambdas (those without parameters that encapsulate state), the JVM can create a constant lambda object caching the invokedynamic instruction. Similarly, for method references to static methods (also stateless), the JVM can optimize the caching.

State-Dependent Method References

However, for lambdas and method references with parameters that encapsulate state (known as "stateful lambdas"), the JVM has less incentive to cache the resulting lambda instances. This is because maintaining a map between parameter values and the corresponding lambda instances could introduce unnecessary overhead.

Special Cases and Best Practices

While method reference caching can be beneficial in certain scenarios, such as when references to the same method are used at numerous different call-sites, it's generally not recommended as a general best practice. The JVM's optimizations for stateless lambdas and the potential overhead associated with caching stateful lambdas should be taken into consideration.

In cases where caching is deemed necessary, such as for stateful lambdas created in constructors or called concurrently by multiple threads, careful profiling should be performed to verify the actual performance impact. Otherwise, it's advisable to rely on the JVM's optimization mechanisms for method references.

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