java - 无法使用 java 反射更改静态最终字段？

Question

我最近偶然发现了 Change private static final field using Java reflection并测试了 polygenelubricants 的 EverythingIsTrue 类，工作正常，System.out.format("Everything is %s", false); 打印 Everything is true 确实。但是当我将代码更改为

public class EverythingIsTrue {

    public static final boolean FALSE = false;

    static void setFinalStatic(Field field, Object newValue) throws Exception {
        field.setAccessible(true);
        Field modifiersField = Field.class.getDeclaredField("modifiers");
        modifiersField.setAccessible(true);
        modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
        field.set(null, newValue);
    }

    public static void main(String[] args) throws Exception {
        setFinalStatic(EverythingIsTrue.class.getField("FALSE"), true);
        System.out.format("Everything is %s", FALSE);
    }
}

它打印

Everything is false

有人知道为什么吗？ setFinalStatic 是否真的有效？

Answer 1

您可以通过将值作为方法调用的结果来避免编译器内联，即使是虚拟调用也是如此。

public class Main {
    // value is not known at compile time, so not inlined
    public static final boolean FLAG = Boolean.parseBoolean("false");

    static void setFinalStatic(Class clazz, String fieldName, Object newValue) throws NoSuchFieldException, IllegalAccessException {
        Field field = clazz.getDeclaredField(fieldName);
        field.setAccessible(true);
        Field modifiers = field.getClass().getDeclaredField("modifiers");
        modifiers.setAccessible(true);
        modifiers.setInt(field, field.getModifiers() & ~Modifier.FINAL);
        field.set(null, newValue);
    }

    public static void main(String... args) throws Exception {
        System.out.printf("Everything is %s%n", FLAG);
        setFinalStatic(Main.class, "FLAG", true);
        System.out.printf("Everything is %s%n", FLAG);
    }
}

打印

Everything is false
Everything is true