为何自定义函数式接口下LambdaMetafactory失效，Function却正常？

下面这段Java代码使用LambdaMetafactory动态生成函数式接口实例，运行时runFunction和runBuilder2能正常执行，但启用runBuilder1会抛出异常：

import java.lang.invoke.LambdaMetafactory;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.util.function.Function;

class Testcase
{
    @FunctionalInterface
    public interface MyBuilder1<R>
    {
        R apply(String message);
    }

    @FunctionalInterface
    public interface MyBuilder2<R>
    {
        R apply(Object message);
    }

    public static void main(String[] args) throws Throwable
    {
        Class<?> clazz = IllegalArgumentException.class;

        MethodHandles.Lookup lookup = MethodHandles.lookup();
        MethodHandle mh = lookup.findConstructor(clazz, MethodType.methodType(void.class, String.class));
        MethodHandle myFunctionConstructor = LambdaMetafactory.metafactory(
            lookup,
            "apply",
            MethodType.methodType(Function.class),
            mh.type().erase(),
            mh,
            mh.type()
        ).getTarget();

        MethodHandle myBuilderConstructor1 = LambdaMetafactory.metafactory(
            lookup,
            "apply",
            MethodType.methodType(MyBuilder1.class),
            mh.type().erase(),
            mh,
            mh.type()
        ).getTarget();

        MethodHandle myBuilderConstructor2 = LambdaMetafactory.metafactory(
            lookup,
            "apply",
            MethodType.methodType(MyBuilder2.class),
            mh.type().erase(),
            mh,
            mh.type()
        ).getTarget();

        @SuppressWarnings("unchecked")
        Function<String, IllegalArgumentException> functionFactory =
            (Function<String, IllegalArgumentException>) myFunctionConstructor.invokeExact();

        @SuppressWarnings("unchecked")
        MyBuilder1<IllegalArgumentException> myBuilder1Factory =
            (MyBuilder1<IllegalArgumentException>) myBuilderConstructor1.invokeExact();

        @SuppressWarnings("unchecked")
        MyBuilder2<IllegalArgumentException> myBuilder2Factory =
            (MyBuilder2<IllegalArgumentException>) myBuilderConstructor2.invokeExact();

        IllegalArgumentException runFunction = functionFactory.apply("test");
//      IllegalArgumentException runBuilder1 = myBuilder1Factory.apply("test");
        IllegalArgumentException runBuilder2 = myBuilder2Factory.apply("test");

    }
}

抛出的异常信息：

java.lang.AbstractMethodError: Receiver class Testcase$$Lambda$233/0x0000000800d21d88 does not define or inherit an implementation of the resolved method 'abstract java.lang.Object apply(java.lang.String)' of interface MyBuilder1.

问题原因解析 #

核心问题出在LambdaMetafactory.metafactory方法的**第四个参数（samMethodType，即函数式接口方法的擦除类型）**与目标方法句柄的匹配逻辑上，逐个拆解三个案例：

• Function<String, IllegalArgumentException> 案例
  Function的泛型方法签名擦除后是Object apply(Object)，我们传入的mh.type().erase()是构造器擦除后的void(Object)。LambdaMetafactory会自动处理类型适配：将传入的String向上转型为Object传给构造器，再把构造器返回的实例向上转型为Object，最后强制转回Function要求的IllegalArgumentException，完全符合JVM类型规则，因此可以正常运行。

• MyBuilder2 案例
  MyBuilder2的方法签名擦除后同样是Object apply(Object)，和Function的场景一致，String作为Object的子类可以合法向上转型适配构造器参数，所以执行正常。

• MyBuilder1 案例
  MyBuilder1的方法签名擦除后是Object apply(String)，但我们传入的samMethodType是构造器的擦除类型void(Object)。此时LambdaMetafactory生成的Lambda实例，实际实现的是擦除后的Object apply(Object)方法，而非MyBuilder1要求的Object apply(String)。当调用apply("test")时，JVM找不到对应方法的实现，就抛出了AbstractMethodError。

修复方案 #

给MyBuilder1对应的metafactory调用传入正确的samMethodType——也就是MyBuilder1接口方法擦除后的类型：

MethodHandle myBuilderConstructor1 = LambdaMetafactory.metafactory(
    lookup,
    "apply",
    MethodType.methodType(MyBuilder1.class),
    // 替换为MyBuilder1接口方法的擦除类型
    MethodType.methodType(Object.class, String.class),
    mh,
    mh.type()
).getTarget();

修改后生成的Lambda会正确实现MyBuilder1的apply(String)方法，即可正常执行。

内容的提问来源于stack exchange，提问作者Gili