Java8 retrieving lambda setter from class

java, java-8, lambda, reflection

Solution

There are a few mistakes in your code.

First, you are using `Void.class`, you should use `void.class` instead.

Second, `func.generic` returns `(Object,Object)Object`, what you really want is `func.erase`, so the return type is retained (`(Object,Object)void`).

Third, it advisable not to use raw types, I would propose the following implementation instead:

public static <T, R> ISetter<T, R> getSetter(Class<T> clazz, String fieldName,
        Class<R> fieldType) throws Throwable {

    MethodHandles.Lookup caller = MethodHandles.lookup();
    MethodType setter = MethodType.methodType(void.class, fieldType);
    MethodHandle target = caller.findVirtual(clazz, computeSetterName(fieldName), setter);
    MethodType func = target.type();

    CallSite site = LambdaMetafactory.metafactory(
            caller,
            "set",
            MethodType.methodType(ISetter.class),
            func.erase(),
            target,
            func
    );

    MethodHandle factory = site.getTarget();
    ISetter<T, R> r = (ISetter<T, R>) factory.invoke();

    return r;
}

Which works for me: http://ideone.com/KKx10r

Problem

I'm trying to get a lambda method handle for the setter of a private field, but for some reason, the setter cannot be found. This is the functional interface I am using: ``` @FunctionalInterface public interface ISetter<T, R> { void set(T object, R value); } ``` And this the method used to fetch the setter: ``` public ISetter getSetter(Class clazz, String fieldName, Class fieldType) throws Throwable { MethodHandles.Lookup caller = MethodHandles.lookup(); MethodType setter = MethodType.methodType(Void.class, fieldType); MethodHandle target = caller.findVirtual(clazz, computeSetterName(fieldName), setter); MethodType func = target.type(); CallSite site = LambdaMetafactory.metafactory( caller, "set", MethodType.methodType(ISetter.class), func.generic(), target, func ); MethodHandle factory = site.getTarget(); ISetter r = (ISetter) factory.invoke(); return r; } ``` All setters are, by convention, named the same: "setField(...)". This is my test class: ``` public class TestEntity { private Long id; public TestEntity(Long id) { this.id = id; } public Long getId() { return id; } public void setId(Long id) { this.id = id; } } ``` But when I execute this method for "id" and "Long.class" I get the follow exception: ``` Exception in thread "main" java.lang.NoSuchMethodException: no such method: de.cyclonit.exercise.TestEntity.setId(Long)Void/invokeVirtual at java.lang.invoke.MemberName.makeAccessException(MemberName.java:871) at java.lang.invoke.MemberName$Factory.resolveOrFail(MemberName.java:1003) at java.lang.invoke.MethodHandles$Lookup.resolveOrFail(MethodHandles.java:1381) at java.lang.invoke.MethodHandles$Lookup.findVirtual(MethodHandles.java:859) at de.cyclonit.exercise.AccessorFactory.getSetter(AccessorFactory.java:41) at de.cyclonit.exercise.Main.main(Main.java:21) Caused by: java.lang.NoSuchFieldError: method resolution failed at java.lang.invoke.MethodHandleNatives.resolve(Native Method) at java.lang.invoke.MemberName$Factory.resolve(MemberName.java:975) at java.lang.invoke.MemberName$Factory.resolveOrFail(MemberName.java:1000) ... 4 more ``` I don't understand where this error is coming from. The method "void setId(Long)" does exist. ``` public String computeSetterName(String fieldName) { return "set" + fieldName.substring(0, 1).toUpperCase() + fieldName.substring(1); } ``` And the call: ``` ISetter setter = accessorFactory.getSetter(TestEntity.class, "id", Long.class); ```

Original source