java - 通过反射更改私有(private)最终字段

Question

class WithPrivateFinalField {
    private final String s = "I’m totally safe";
    public String toString() {
        return "s = " + s;
    }
}
WithPrivateFinalField pf = new WithPrivateFinalField();
System.out.println(pf);
Field f = pf.getClass().getDeclaredField("s");
f.setAccessible(true);
System.out.println("f.get(pf): " + f.get(pf));
f.set(pf, "No, you’re not!");
System.out.println(pf);
System.out.println(f.get(pf));

输出:

s = I’m totally safe
f.get(pf): I’m totally safe
s = I’m totally safe
No, you’re not!

为什么会这样，你能解释一下吗？第一个打印告诉我们私有(private)“s”字段没有改变，正如我所料。但是，如果我们通过反射获得该字段，则第二个打印显示它已更新。

Answer 1

This answer在这个主题上非常详尽。

JLS 17.5.3 最终字段的后续修改

Even then, there are a number of complications. If a final field is initialized to a compile-time constant in the field declaration, changes to the final field may not be observed, since uses of that final field are replaced at compile time with the compile-time constant.

但是，如果您仔细阅读上面的段落，您可能会找到一种解决方法(在构造函数中而不是在字段定义中设置 private final 字段):

import java.lang.reflect.Field;


public class Test {

  public static void main(String[] args) throws Exception {
    WithPrivateFinalField pf = new WithPrivateFinalField();
    System.out.println(pf);
    Field f = pf.getClass().getDeclaredField("s");
    f.setAccessible(true);
    System.out.println("f.get(pf): " + f.get(pf));
    f.set(pf, "No, you’re not!");
    System.out.println(pf);
    System.out.println("f.get(pf): " + f.get(pf));
  }

  private class WithPrivateFinalField {
    private final String s;

    public WithPrivateFinalField() {
      this.s = "I’m totally safe";
    }
    public String toString() {
      return "s = " + s;
    }
  }

}

然后输出如下:

s = I’m totally safe
f.get(pf): I’m totally safe
s = No, you’re not!
f.get(pf): No, you’re not!

希望这会有所帮助。