确定lambda表达式在Java中是无状态还是有状态 [英] Determine if a lambda expression is stateless or stateful in Java

问题描述

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，说明lambda表达式是否为无状态？如何确定lambda表达式的有状态？

Is there a function which accepts a reference to a lambda expression and returns a boolean saying whether the lambda expression is stateless or not? How can the statefulness of a lambda expression be determined?

推荐答案

嗯，lambda表达式只是一个特殊匿名类的实例只有一种方法。匿名类可以捕获周围范围内的变量。如果你对有状态类的定义是在其字段中携带可变内容的那个（否则它几乎只是一个常量），那么你很幸运，因为那是如何实现捕获。这是一个小实验：

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);
    }
}

输出如下所示：

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

StringBuilder 引用变成了匿名lambda类的字段（并且最终字符串内联常量被内联以提高效率，但这不是重点。所以这个函数在大多数情况下都应该这样做：

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;
}

编辑：正如@Federico所指出的，这是特定于实现的行为，可能不适用于某些异国情调的环境或未来版本的 Oracle / OpenJDK JVM 。

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.

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