修改Java中的最终字段 [英] Modifying final fields in Java

问题描述

让我们从一个简单的测试用例开始：

Let's start with a simple test case:

import java.lang.reflect.Field;

public class Test {
  private final int primitiveInt = 42;
  private final Integer wrappedInt = 42;
  private final String stringValue = "42";

  public int getPrimitiveInt()   { return this.primitiveInt; }
  public int getWrappedInt()     { return this.wrappedInt; }
  public String getStringValue() { return this.stringValue; }

  public void changeField(String name, Object value) throws IllegalAccessException, NoSuchFieldException {
    Field field = Test.class.getDeclaredField(name);
    field.setAccessible(true);
    field.set(this, value);
    System.out.println("reflection: " + name + " = " + field.get(this));
  }

  public static void main(String[] args) throws IllegalAccessException, NoSuchFieldException {
    Test test = new Test();

    test.changeField("primitiveInt", 84);
    System.out.println("direct: primitiveInt = " + test.getPrimitiveInt());

    test.changeField("wrappedInt", 84);
    System.out.println("direct: wrappedInt = " + test.getWrappedInt());

    test.changeField("stringValue", "84");
    System.out.println("direct: stringValue = " + test.getStringValue());
  }
}

任何人都在猜测将作为输出打印的内容（显示在底部，以免立即破坏惊喜。）

Anybody care to guess what will be printed as output (shown at the bottom as to not spoil the surprise immediately).

问题是：

1. 为什么原始和包装整数的行为有所不同？


2. 为什么反射与直接访问会返回不同的结果？


3. 困扰我的那个大多数 - 为什么String的行为类似于原始 int 而不是像整数？

1. Why do primitive and wrapped integer behave differently?
2. Why does reflective vs direct access return different results?
3. The one that plagues me most - why does String behave like primitive int and not like Integer?

结果（java 1.5）：

Results (java 1.5):

reflection: primitiveInt = 84
direct: primitiveInt = 42
reflection: wrappedInt = 84
direct: wrappedInt = 84
reflection: stringValue = 84
direct: stringValue = 42

推荐答案

编译时常量是内联的（在javac编译时）。参见JLS，特别是15.28定义了一个常量表达式，13.4.9讨论了二进制兼容性或最终字段和常量。

Compile-time constants are inlined (at javac compile-time). See the JLS, in particular 15.28 defines a constant expression and 13.4.9 discusses binary compatibility or final fields and constants.

如果你将字段设为非final或者指定一个非编译时间常量，该值不是内联的。例如：

If you make the field non-final or assign a non-compile time constant, the value is not inlined. For instance:

private final String stringValue = null！= null？：42;

private final String stringValue = null!=null?"": "42";

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