Java 8：将lambda转换为包含clousure的Method实例 [英] Java 8: convert lambda to a Method instance with clousure included

问题描述

（这很难搜索，因为结果都是关于方法参考）

(This is difficult to search because results are all about "method reference")

我想要一个方法 lambda表达式的实例，用于基于遗留反射的API。应该包含clousure，因此调用 thatMethod.invoke（null，...）应该与调用lambda具有相同的效果。

I want to get a Method instance for a lambda expression for use with a legacy reflection-based API. The clousure should be included, so calling thatMethod.invoke(null, ...) should have the same effect as calling the lambda.

我查看了 MethodHandles.Lookup ，但它似乎只与逆转换相关。但我想 bind 方法可能有助于包含clousure？

I have looked at MethodHandles.Lookup, but it only seems to be relevant for the reverse transform. But I guess the bind method may help to include the clousure?

编辑：

说我有lambda experssion：

Say I have am lambda experssion:

Function<String, String> sayHello = name -> "Hello, " + name;

我有一个遗留框架（Spel ），其API类似

and I have a legacy framework (SpEL) that has an API like

registerFunction(String name, Method method)

调用给定的方法，不带这个参数（即假定为静态的方法）。所以我需要一个特殊的方法实例，其中包括lambda逻辑+ clousure数据。

which will call the given Method with no this argument (i.e. Method assumed to be static). So I'll need to get a special Method instance that includes the lambda logic + the clousure data.

推荐答案

如果你找不到优雅的方式，这就是丑陋的方式（ Ideone ）。涉及反射时的常用警告：可能在将来的版本中中断等。

In case you don't find an elegant way, here is the ugly way (Ideone). Usual warning when reflection is involved: may break in future releases etc.

public static void main(String[] args) throws Exception {
  Function<String, String> sayHello = name -> "Hello, " + name;
  Method m = getMethodFromLambda(sayHello);
  registerFunction("World", m);
}

static void registerFunction(String name, Method method) throws Exception {
  String result = (String) method.invoke(null, name);
  System.out.println("result = " + result);
}

private static Method getMethodFromLambda(Function<String, String> lambda) throws Exception {
  Constructor<?> c = Method.class.getDeclaredConstructors()[0];
  c.setAccessible(true);
  Method m = (Method) c.newInstance(null, null, null, null, null, 0, 0, null, null, null, null);
  m.setAccessible(true); //sets override field to true

  //m.methodAccessor = new LambdaAccessor(...)
  Field ma = Method.class.getDeclaredField("methodAccessor");
  ma.setAccessible(true);
  ma.set(m, new LambdaAccessor(array -> lambda.apply((String) array[0])));

  return m;
}

static class LambdaAccessor implements MethodAccessor {
  private final Function<Object[], Object> lambda;
  public LambdaAccessor(Function<Object[], Object> lambda) {
    this.lambda = lambda;
  }

  @Override public Object invoke(Object o, Object[] os) {
    return lambda.apply(os);
  }
}

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