Does Method Reference Caching in Java 8 Offer Performance Gains?

Method Reference Caching in Java 8

Java 8 introduced method references, which provide a concise syntax for referencing instance methods and constructors. However, there are concerns regarding whether caching these references is beneficial. To understand this topic, it's essential to distinguish between caching method references for frequent executions of the same call-site and caching them for multiple uses of a method reference by different call-sites.

Method Reference Caching for Same Call-Site

Consider the following code where Foo.func() is called repeatedly through doSomethingWithAFunc():

class Foo {

   Y func(X x) {...} 

   void doSomethingWithAFunc(Function<X,Y> f){...}

   void hotFunction(){
        doSomethingWithAFunc(this::func);
   }

}

Caching this::func would supposedly eliminate the creation of new anonymous class instances each time hotFunction() is executed. However, the JVM already optimizes this scenario by reusing the call-site instance created during the first invocation of the lambda. Caching the method reference in this case is superfluous.

Method Reference Caching for Different Call-Sites

In contrast, consider the following example:

Runnable r1 = System::gc;
Runnable r2 = System::gc;

Here, two different call-sites produce method references to the same target method System.gc() in java.lang.System. The JVM is allowed to share a single lambda instance between them, but the current implementation in Java 8 does not do so. This is due to the uncertain performance benefits of maintaining a cache of lambda instances.

Best Practices

Given these considerations, it's generally not recommended to cache method references unless there are specific performance concerns that have been identified through measurement. Caching can potentially alter program execution behavior, and it should only be considered in the following cases:

• When there are numerous call-sites referencing the same method

• When the lambda is created in the constructor or class initialization and:

  • Will be called concurrently by multiple threads
  • Suffers from a performance penalty during the initial invocation

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