Java8从类中检索lambda setter

Question

我正在尝试为私有字段的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界面:

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

这是用于获取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,所有的setter都是相同的:"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"执行此方法时,我得到以下异常:

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

我不明白这个错误来自哪里.方法"void setId(Long)"确实存在.

---

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

电话:

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

Answer 1

您的代码中存在一些错误.

首先,你正在使用Void.class,你应该使用void.class.

第二,func.generic返回(Object,Object)Object,你真正想要的是func.erase,所以保留返回类型((Object,Object)void).

第三,建议不使用原始类型,我建议改为:

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;
}

哪个适合我:http://ideone.com/KKx10r