Method handles

A typed, directly executable reference to a method, resolved once through a Lookup that captures the caller's access rights. Faster than reflection and the basis of invokedynamic.

MethodHandles.Lookup lookup = MethodHandles.lookup();
MethodType type = MethodType.methodType(String.class, int.class, int.class);
MethodHandle mh = lookup.findVirtual(String.class, "substring", type);

String s = (String) mh.invokeExact("hello world", 0, 5);

Against reflection

Access is checked once, when the handle is looked up, not on every call.
The handle carries an exact MethodType, so the JIT can inline through it.
Arguments are not boxed into an array.
It can be transformed: bindTo, asType, insertArguments, filterArguments build new handles from existing ones.

invokeExact vs invoke

invokeExact demands that the call site’s static signature match the handle’s type character for character — including the cast on the return value. invoke inserts the conversions for you and costs a little more. A WrongMethodTypeException from invokeExact almost always means a missing cast.

Where you have already used it

Every lambda you write compiles to an invokedynamic instruction whose bootstrap method builds a MethodHandle. That is why lambdas do not generate a class file per lambda the way anonymous classes do.

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