确定lambda表达式在Java中是无状态还是有状态

Question

是否有一个函数接受对lambda表达式的引用并返回一个布尔值,表示lambda表达式是否为无状态？如何确定lambda表达式的有状态？

Answer 1

Well, a lambda expression is just an instance of a special anonymous class that only has one method. Anonymous classes can "capture" variables that are in the surrounding scope. If your definition of a stateful class is one that carries mutable stuff in its fields (otherwise it's pretty much just a constant), then you're in luck, because that's how capture seems to be implemented. Here is a little experiment :

import java.lang.reflect.Field;
import java.util.function.Function;

public class Test {
    public static void main(String[] args) {
        final StringBuilder captured = new StringBuilder("foo");
        final String inlined = "bar";
        Function<String, String> lambda = x -> {
            captured.append(x);
            captured.append(inlined);

            return captured.toString();
        };

        for (Field field : lambda.getClass().getDeclaredFields())
            System.out.println(field);
    }
}

The output looks something like this :

private final java.lang.StringBuilder Test$$Lambda$1/424058530.arg$1

The StringBuilder reference got turned into a field of the anonymous lambda class (and the final String inlined constant was inlined for efficiency, but that's beside the point). So this function should do in most cases :

public static boolean hasState(Function<?,?> lambda) {
    return lambda.getClass().getDeclaredFields().length > 0;
}

EDIT : as pointed out by @Federico this is implementation-specific behavior and might not work on some exotic environments or future versions of the Oracle / OpenJDK JVM.

Answer 2

这是一个简单而愚蠢的想法。只需检查您的 lambda 是否有字段即可。

例如，考虑以下有状态 lambda。

  List<Integer> serialStorage = new ArrayList<>();
  Function<? super Integer, ? extends Integer> statefulLambda =
      e -> { serialStorage.add(e); return e; };

这有一个明显引用的statefulLambda私有最终内部字段。所以arg$1serialStorage

  statefulLambda.getClass().getDeclaredFields().length > 0

可以用作 lambda 是有状态的指示器。

但是我不知道这通常是否有效。