CODEWITHSUNDEEP
java
Riccardo
Riccardo

Reputation: 109

In Java what is the point of invokevirtual, if anyway the index of a method in a method table is known at compile-time?

I am reading about dynamic dispatch, as I have an exam tomorrow.

In C++ we have conforming subclasses, so through the static type of the identifier we know what index to access in the virtual method table of the runtime object.

From what I am reading, Java has conformance for subclasses as well, but instead of including the known index of a method in the virtual method table in the compiled code, it only includes a symbolic reference to the method, that needs to be resolved.

What is the point of this if the static type does not refer to an interface? It could be much faster to do it the C++ way.

Upvotes: 0

Views: 76

Answers (1)

Marko Topolnik
Marko Topolnik

Reputation: 200148

The Java platform defines linkage as a step taken at runtime. Virtual method tables aren't even involved in the JVM specification; they are just a typical way to implement linkage.

Note, however, that after the symbolic reference is resolved into a direct reference, there is nothing stopping the runtime from using very fast code paths for method invocation sites. That includes special-case optimizations such as monomorphic call sites, which have a hardwired direct pointer to the method code and are thus faster than vtable lookups. Monomorphic sites then become an easy target for method inlining, which opens a whole new field of applicable optimizations. Another option is an n-polymorphic site, accommodating up to n different target types in an inline cache.

As opposed to C++, all these optimizing decisions happen at runtime, subject to the specific conditions at work: the exact set of loaded classes, profiling data for each individual call site, etc. This gives managed-runtime platforms such as Java advantages of their own.

Upvotes: 2

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