Can MethodHandle be used by frameworks/libraries (instead of traditional reflection)?

Question

In my framework I have a class like this:

public class Foo<B, V> {
    private final Method getterMethod;
    ...

    public V executeGetter(B bean) {
        try {
           return getterMethod.invoke(bean);
        } catch ...
    }
}

This class is used to call getters of classes created by users that aren't available at compile time of my framework. For example, B might be a class called Person.

Through profiling, I've discovered that this method is horribly slow. The Method.invoke() takes 40% of performance in sampling profiling (even with setAccessible(true)), while a non reflective implementation takes only a small fraction of that performance.

So I 'd like to replace is with a MethodHandle:

public class Foo<B, V> {
    private final MethodHandle getterMethodHandle;
    ...

    public V executeGetter(B bean) {
        try {
           return getterMethodHandle.invoke(bean);
        } catch ...
    }
}

But then I get this exception:

java.lang.ClassCastException: Cannot cast [Ljava.lang.Object; to Person
    at sun.invoke.util.ValueConversions.newClassCastException(ValueConversions.java:461)
    at sun.invoke.util.ValueConversions.castReference(ValueConversions.java:456)
    at ...Foo.executeGetter(Foo.java:123)

even though bean is an instance of Person. Now the misleading part is that it's trying to cast an Object[] (and not an Object) to Person. Note that wrapping it in an object array (which is a performance loss) doesn't help:

 return getterMethodHandle.invoke(new Object[]{bean}); // Same exception

Is it possible to get the MethodHandle to work in this situation?

handle constructred like this: `getterMethodHandle = lookup.unreflect(getterMethod);` — Geoffrey De Smet, Dec 02 '15 at 15:51

score 4 · Accepted Answer · answered Dec 03 '15 at 11:23

4

That ClassCastException only occurs if you compile with source/target level java 6.

Compile with source/target level 7 or higher to avoid that ClassCastException.

Answer found thanks to Tagir's answer. (vote up his answer too)

answered Dec 03 '15 at 11:23

Geoffrey De Smet

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score 3 · Answer 2 · answered Dec 02 '15 at 16:32

Using MethodHandles in framework/library code is perfectly fine and I see no problem in your code. This example works just fine:

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;

public class Foo<B, V> {
    private final MethodHandle getterMethodHandle;

    public Foo(MethodHandle mh) {
        this.getterMethodHandle = mh;
    }

    public V executeGetter(B bean) {
        try {
           return (V) getterMethodHandle.invoke(bean);
        } catch(RuntimeException | Error ex) {
            throw ex;
        } catch(Throwable t) {
            throw new RuntimeException(t);
        }
    }

    static class Pojo {
        String x;

        public Pojo(String x) {
            this.x = x;
        }

        public String getX() {
            return x;
        }
    }

    public static void main(String[] args) throws Exception {
        // I prefer explicit use of findXYZ
        Foo<Pojo, String> foo = new Foo<>(MethodHandles.lookup()
                .findVirtual(Pojo.class, "getX", MethodType.methodType(String.class)));
        // Though unreflect also works fine
        Foo<Pojo, String> foo2 = new Foo<>(MethodHandles.lookup()
                .unreflect(Pojo.class.getMethod("getX")));

        System.out.println(foo.executeGetter(new Pojo("foo")));
        System.out.println(foo2.executeGetter(new Pojo("bar")));
    }
}

The output is:

foo
bar

For even better performance consider using invokeExact, though it will not allow you automatic type conversions like unboxing.

Very interesting. You indeed don't compile time reference Pojo in the executeGetter method. I need to investigate why it doesn't work in my case. — Geoffrey De Smet, Dec 02 '15 at 16:37
In JDK 1.7 with source and target level Java 6, this exact code *throws the same exception*: `java.lang.RuntimeException: java.lang.ClassCastException: Cannot cast [Ljava.lang.Object; to org.optaplanner.core.impl.domain.common.Foo$Pojo at ...Foo.executeGetter(Foo.java:34)` — Geoffrey De Smet, Dec 03 '15 at 10:56
And **with the same JDK (1.7) but source and target level Java 7 it does work** (unlike with source/target java 6). — Geoffrey De Smet, Dec 03 '15 at 11:19
@GeoffreyDeSmet, that's actually expected. There's special annotation `@PolymorphicSignature` on `invoke/invokeExact` methods which is specially handled by java compiler since Java 7, but such annotation means nothing when target is Java 6 or lower which results in incorrect bytecode generation. — Tagir Valeev, Dec 03 '15 at 12:12
Makes sense now, although JDK 7 should have failed fast if it's compiling any code that uses MethodHandles with a target level of java 6. Thanks! PS: [I am baffled again for another reason with methodhandles](http://stackoverflow.com/questions/34069386/methodhandle-to-a-getter-setter-from-another-class-gvies-a-nosuchfielderror) — Geoffrey De Smet, Dec 03 '15 at 15:11

Can MethodHandle be used by frameworks/libraries (instead of traditional reflection)?

2 Answers2