Fast Java Reflection

Node.java

package demo;

public interface Node {}

Util.java

package demo;

public class Util
{
    /**
     * Casts the receiver to type T (unsafe). T can be automatically inferred.
     */
    @SuppressWarnings("unchecked")
    public static <T> T cast (Object object)
    {
        return (T) object;
    }
}

WalkerDemoFast.java

package demo;

import java.lang.invoke.CallSite;
import java.lang.invoke.LambdaMetafactory;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodHandles.Lookup;
import java.lang.invoke.MethodType;
import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.function.Consumer;

public class WalkerDemoFast
{
    @FunctionalInterface
    interface NodeGetter {
        Node apply (Node node);
    }

    @FunctionalInterface
    interface NodesGetter {
        Collection<? extends Node> apply (Node node);
    }

    class ClassData
    {
        ArrayList<NodeGetter>  simple_getters   = new ArrayList<>();
        ArrayList<NodesGetter> compound_getters = new ArrayList<>();

        ClassData(Lookup lookup, Class<?> klass) throws Throwable
        {
            for (Method m: klass.getMethods())
            {
                // small performance boost
                m.setAccessible(true);

                // we only want getters
                if (m.getParameterCount() != 0) continue;

                // this method returns a subtype of `Node`
                if (Node.class.isAssignableFrom(m.getReturnType())) {
                    simple_getters.add(compile_node_getter(lookup, m));
                    continue;
                }

                // this method returns a subtype of `Collection`
                if (Collection.class.isAssignableFrom(m.getReturnType())) {
                    ParameterizedType ret = ((ParameterizedType) m.getGenericReturnType());
                    Type param = ret.getActualTypeArguments()[0];

                    // this method returns a subtype of `Collection<? extends Node>`
                    if (param instanceof Class<?>
                        && Node.class.isAssignableFrom((Class<?>) param))
                        compound_getters.add(compile_nodes_getter(lookup, m));

                }
            }
        }
    }

    private static NodeGetter compile_node_getter(Lookup lookup, Method method) throws Throwable
    {
        MethodHandle handle = lookup.unreflect(method);

        CallSite site = LambdaMetafactory.metafactory(
            lookup, "apply",
            MethodType.methodType(NodeGetter.class),
            MethodType.methodType(Node.class, Node.class),
            handle, handle.type());

        return (NodeGetter) site.getTarget().invoke();
    }

    private static NodesGetter compile_nodes_getter(Lookup lookup, Method method) throws Throwable
    {
        MethodHandle handle = lookup.unreflect(method);

        CallSite site = LambdaMetafactory.metafactory(
            lookup, "apply",
            MethodType.methodType(NodesGetter.class),
            MethodType.methodType(Collection.class, Node.class), // TODO
            handle, handle.type());

        return (NodesGetter) site.getTarget().invoke();
    }

    private Lookup lookup = MethodHandles.lookup();
    private HashMap<Class<?>, ClassData> class_data = new HashMap<>();

    public void walk (Node node, Consumer<Node> visitor) throws Throwable
    {
        visitor.accept(node);

        Class<?> klass = node.getClass();

        ClassData data = class_data.computeIfAbsent(klass, k ->  {
            try { return new ClassData(lookup, k); }
            catch (Throwable e) { throw new RuntimeException(e); }
        });

        for (NodeGetter getter: data.simple_getters)
        {
            Node child = getter.apply(node);
            if (child == null) continue;
            walk(child, visitor);
        }

        for (NodesGetter getter: data.compound_getters)
        {
            Collection<Node> children = Util.cast(getter.apply(node));
            if (children == null) continue;
            for (Node child: children)
                walk(child, visitor);
        }
    }
}

WalkerDemoSlow.java

package demo;

import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.function.Consumer;

public class WalkerDemoSlow
{
    private static class ClassData
    {
        ArrayList<Method> simple_getters = new ArrayList<>();
        ArrayList<Method> compound_getters = new ArrayList<>();

        ClassData(Class<?> klass)
        {
            for (Method m : klass.getMethods()) {
                // small performance boost
                m.setAccessible(true);

                // we only want getters
                if (m.getParameterCount() != 0) continue;

                // this method returns a subtype of `Node`
                if (Node.class.isAssignableFrom(m.getReturnType())) {
                    simple_getters.add(m);
                    continue;
                }

                // this method returns a subtype of `Collection`
                if (Collection.class.isAssignableFrom(m.getReturnType())) {
                    ParameterizedType ret = ((ParameterizedType) m.getGenericReturnType());
                    Type param = ret.getActualTypeArguments()[0];

                    // this method returns a subtype of `Collection<? extends Node>`
                    if (param instanceof Class<?>
                        && Node.class.isAssignableFrom((Class<?>) param))
                        compound_getters.add(m);

                }
            }
        }
    }

    private HashMap<Class<?>, ClassData> class_data = new HashMap<>();

    public void walk(Node node, Consumer<Node> visitor) throws Exception
    {
        visitor.accept(node);

        Class<?> klass = node.getClass();
        ClassData data = class_data.computeIfAbsent(klass, ClassData::new);

        for (Method m: data.simple_getters)
        {
            Node child = (Node) m.invoke(node);
            if (child == null) continue;
            walk(child, visitor);
        }

        for (Method m: data.compound_getters)
        {
            Collection<Node> children = Util.cast(m.invoke(node));
            if (children == null) continue;
            for (Node child: children)
                walk(child, visitor);
        }
    }
}

WalkerDemoSlowest.java

package demo;

import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.Collection;
import java.util.function.Consumer;

public class WalkerDemoSlowest
{
    public void walk (Node node, Consumer<Node> visitor) throws Exception
    {
        visitor.accept(node);
        Method[] methods = node.getClass().getMethods();

        for (Method m : methods) {
            // we only want getters
            if (m.getParameterCount() != 0) continue;

            // this method returns a subtype of `Node`
            if (Node.class.isAssignableFrom(m.getReturnType()))
            {
                Node child = (Node) m.invoke(node);
                if (child == null) continue;
                walk(child, visitor);
            }

            // this method returns a subtype of `Collection`
            if (Collection.class.isAssignableFrom(m.getReturnType()))
            {
                ParameterizedType ret = ((ParameterizedType)m.getGenericReturnType());
                Type param = ret.getActualTypeArguments()[0];

                // this method returns a subtype of `Collection<? extends Node>`
                if (param instanceof Class<?>
                && Node.class.isAssignableFrom((Class<?>) param))
                {
                    Collection<Node> children = Util.cast(m.invoke(node));
                    if (children == null) continue;
                    for (Node child: children)
                        walk(child, visitor);
                }
            }
        }
    }
}