Java8 从类中检索 lambda setter

Question

我正在尝试为私有(private)字段的 setter 获取 lambda 方法句柄，但由于某种原因，无法找到 setter。

这是我正在使用的功能接口(interface):

@FunctionalInterface
public interface ISetter<T, R> {
    void set(T object, R value);
}

这是用来获取 setter 的方法:

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;
}

按照惯例，所有 setter 的名称都相同:“setField(...)”。这是我的测试类:

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;
    }
}

但是当我为“id”和“Long.class”执行这个方法时，我得到了以下异常:

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

我不明白这个错误是从哪里来的。 “void setId(Long)”方法确实存在。

---

public String computeSetterName(String fieldName) {
    return "set" + fieldName.substring(0, 1).toUpperCase() + fieldName.substring(1);
}

调用:

ISetter setter = accessorFactory.getSetter(TestEntity.class, "id", Long.class);

Answer 1

您的代码中有一些错误。

首先，您正在使用 Void.class，您应该改用 void.class。

其次，func.generic返回(Object,Object)Object，真正要的是func.erase，所以返回类型被保留 ((Object,Object)void)。

第三，建议不要使用原始类型，我建议改为使用以下实现:

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;
}

哪个对我有用:http://ideone.com/KKx10r