metaprogramming2MethodHandles

1package pkg17metaprogramming;2 3import java.lang.invoke.MethodHandle;4import java.lang.invoke.MethodHandles;5import java.lang.invoke.MethodType;6 7/*8 * metaprogramming2MethodHandles.java9 * ------------------------------------10 * MethodHandles: faster, type-safe reflective invocation (Java 7+).11 *12 * DEFINITION:13 *   MethodHandles (java.lang.invoke) are the low-level building blocks behind14 *   lambdas and invokedynamic. They are more efficient than raw Reflection.15 *16 * KEY POINTS:17 *   - Lookup.unreflect() or findVirtual/findStatic obtain a MethodHandle.18 *   - invoke() / invokeExact() call the target with correct types.19 *   - Used by the JVM for lambda generation and by libraries needing speed.20 */21public class metaprogramming2MethodHandles {22 23    static class Calculator {24        int add(int a, int b) { return a + b; }25        static int multiply(int a, int b) { return a * b; }26    }27 28    public static void main(String[] args) throws Throwable {29        Calculator calc = new Calculator();30        MethodHandles.Lookup lookup = MethodHandles.lookup();31 32        // Virtual method handle33        MethodHandle add = lookup.findVirtual(Calculator.class, "add",34                MethodType.methodType(int.class, int.class, int.class));35        int sum = (int) add.invoke(calc, 10, 32);36        System.out.println("add(10,32) via MethodHandle = " + sum);37 38        // Static method handle39        MethodHandle mul = lookup.findStatic(Calculator.class, "multiply",40                MethodType.methodType(int.class, int.class, int.class));41        int product = (int) mul.invoke(6, 7);42        System.out.println("multiply(6,7) via MethodHandle = " + product);43 44        // Bound handle (receiver fixed)45        MethodHandle boundAdd = add.bindTo(calc);46        System.out.println("bound add(1,2) = " + boundAdd.invoke(1, 2));47    }48}