如何为字段创建功能接口实现？

Question

class Animal {
  int weight;
}

public static Supplier getter(Object obj, Class<?> cls, Field f) throws Exception {
  boolean isstatic = Modifier.isStatic(f.getModifiers());
  MethodType sSig = MethodType.methodType(f.getType());
  Class<?> dCls = Supplier.class;
  MethodType dSig = MethodType.methodType(Object.class);
  String dMthd = "get";
  MethodType dType = isstatic? MethodType.methodType(dCls) : MethodType.methodType(dCls, cls);
  MethodHandles.Lookup lookup = MethodHandles.lookup();
  MethodHandle fctry = LambdaMetafactory.metafactory(lookup, dMthd, dType, dSig, lookup.unreflectGetter(f), sSig).getTarget();
  fctry = !isstatic && obj!=null? fctry.bindTo(obj) : fctry;
  return (Supplier)fctry.invoke();
}

但这会产生以下错误：

java.lang.invoke.LambdaConversionException: Unsupported MethodHandle kind: getField x.Animal.weight:()int

更新

我试图创建一个类objectmap，实现interface map，它基本上试图将对象表示为map，其中对象可以是任何类型。当前正在使用field.get()和field.set()操作get()和put()方法中的字段，并使用上述方法创建supplier和consumer对象以调用getter和setter方法。我想知道是否可以将这两个单独的方法合并为一个。

可用作映射到对象映射的示例类：

public class ThisCanBeAnything {
  /* fields */
  public String normalField;
  private int hiddenFiled;
  private String hiddenReadonlyField;

  /* getters and setters */
  public int hiddenField() {
    return hiddenField;
  }
  public void hiddenField(int v) {
    System.out.println("set: hiddenField="+v);
    hiddenField = v;
  }

  public String hiddenReadonlyField() {
    return hiddenReadonlyField;
  }
}

下面是预期的用法：

Object o = new ThisCanBeAnything();
Map m = new ObjectMap(o);
m.put("normalField", "Normal");
System.out.println(m.get("normalField")); // Normal
m.put("hiddenField", 1); // set: hiddenField=1
System.out.println(m.get("hiddenField")); // 1
m.put("hiddenReadonlyField", 1); // does not do anything
System.out.println(m.get("hiddenReadonlyField")); // null

Answer 1

您可以直接编写lambda，根本不需要lambdametafactory:

public static Supplier getter(Object obj, Field f) {
    return () -> {
        try {
            return f.get(obj);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    };
}

或运行时类型安全版本：

public static <T> Supplier<T> getter(Object obj, Class<T> fieldClass, Field f) {

    if (!fieldClass.isAssignableFrom(f.getType()))
        throw new RuntimeException("Field is not of expected type");

    return () -> {
        try {
            return (T) f.get(obj);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    };
}

例如：

private class X {
    public int a;
}

@Test
public void supplier_getter_test() throws NoSuchFieldException {
    X a = new X();
    a.a = 5;

    Supplier<Integer> sup = getter(a, int.class, X.class.getField("a"));

    assertEquals(5, sup.get().intValue());
}