Does Java 8 Cache Method References and When Should You Consider Caching Them?

Method Reference Caching in Java 8: A Detailed Examination

Introduction

When working with Java 8's method references, a question arises regarding the potential benefits of caching. This article explores the implications of caching method references and provides guidance on when it can be advantageous.

Distinguishing Call-site and Method-reference Execution

It's crucial to differentiate between frequent executions of the same call-site with stateless or stateful lambdas and frequent uses of method references to the same method by different call-sites.

Example Analysis

Consider the following examples:

Runnable r1 = null;
for (int i = 0; i < 2; i++) {
    Runnable r2 = System::gc;
    if (r1 == null) {
        r1 = r2;
    } else {
        System.out.println(r1 == r2 ? "shared" : "unshared");
    }
}

Here, the same call-site is executed twice, producing a stateless lambda, and the implementation will print "shared."

Runnable r1 = null;
for (int i = 0; i < 2; i++) {
    Runnable r2 = Runtime.getRuntime()::gc;
    if (r1 == null) {
        r1 = r2;
    } else {
        System.out.println(r1 == r2 ? "shared" : "unshared");
        System.out.println(r1.getClass() == r2.getClass() ? "shared class" : "unshared class");
    }
}

With this example, the same call-site is executed twice, producing a lambda containing a reference to a Runtime instance, and the implementation will print "unshared" but "shared class."

Runnable r1 = System::gc, r2 = System::gc;
System.out.println(r1 == r2 ? "shared" : "unshared");
System.out.println(r1.getClass() == r2.getClass() ? "shared class" : "unshared class");

In contrast, the last example includes two separate call-sites producing an equivalent method reference, but as of Java 8.0.05, it will print "unshared" and "unshared class."

JVM Behavior

The Java Virtual Machine (JVM) plays a significant role in handling method references. It employs an invokedynamic instruction, which refers to a JRE bootstrap method in LambdaMetafactory. The compiler provides arguments necessary to generate the lambda implementation class.

The JVM has the flexibility to remember and reuse the CallSite instance formed during the first invocation. For stateless lambdas and single call-sites, the JVM typically creates a ConstantCallSite comprising a MethodHandle to a constant object.

On the other hand, for lambdas with parameters (e.g., this::func), the JVM may cache them, but it involves additional overhead in maintaining a map between parameters and the lambda instances. Currently, the JVM does not cache such lambdas. This behavior applies similarly to method references to the same target method created by different call-sites.

Caching Considerations

Based on the aforementioned points, caching method references may yield different results, but not necessarily better performance. Performance impact should be measured before implementing any caching mechanisms. There are specific cases where caching might be beneficial:

• When multiple call-sites refer to the same method
• When the lambda is created in the constructor/class initialize
• When the use-site will be executed concurrently by multiple threads
• When the lower performance of the first lambda invocation is a concern

Conclusion

Caching method references can be an optimization technique in certain scenarios. However, the decision to cache should be based on careful analysis of the code and specific performance requirements. The JVM's handling of method references provides a solid foundation for optimizing lambda and method reference usage in Java 8.

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