Java 反射常量时一道非常经典有名的面试题

public class Test {
    private static final Integer KEY_EXIT = 1024;
    private static void invok1() throws NoSuchFieldException, IllegalAccessException {
        System.out.println("invok1->"+Test.KEY_EXIT);
        Field field = Test.class.getDeclaredField("KEY_EXIT");
        field.setAccessible(true);
        field.set(null, 1000);
        System.out.println("invok1-<"+Test.KEY_EXIT);
    }
    private static void invok2() throws NoSuchFieldException, IllegalAccessException {
        System.out.println("invok2->"+Test.KEY_EXIT);
        Field field = Test.class.getField("KEY_EXIT");
        field.set(null, 512);
        System.out.println("invok2-<"+Test.KEY_EXIT);
    }
    private static void invok3() throws NoSuchFieldException, IllegalAccessException {
        System.out.println("invok3->"+Test.KEY_EXIT);
        Field field = Test.class.getDeclaredField("KEY_EXIT");
        field.setAccessible(true);
        Field modifiersField = Field.class.getDeclaredField("modifiers");
        modifiersField.setAccessible(true);
        modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
        field.set(null, 256);
        System.out.println("invok3-<"+Test.KEY_EXIT);
    }
    public static void main(String[] args) throws Exception {
        invok1();
        invok2();
        invok3();
    }
}

//invok1运行结果
invok1->1024
Exception in thread "main" java.lang.IllegalAccessException: Can not set static final java.lang.Integer field Test.KEY_EXIT to java.lang.Integer
    at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:76)
    at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:80)
    at sun.reflect.UnsafeQualifiedStaticObjectFieldAccessorImpl.set(UnsafeQualifiedStaticObjectFieldAccessorImpl.java:77)
    at java.lang.reflect.Field.set(Field.java:764)
    at Test.invok1(Test.java:14)
    at Test.main(Test.java:37)
//invok2运行结果
invok2->1024
Exception in thread "main" java.lang.NoSuchFieldException: KEY_EXIT
    at java.lang.Class.getField(Class.java:1703)
    at Test.invok2(Test.java:20)
    at Test.main(Test.java:38)
//invok3运行结果
invok3->1024
invok3-<256

public class Test {
    private static final int KEY_EXIT = 1024;
    private static int KEY_BACK = 1025;
    private static final String KEY_STR = "1001";
    private static void invok1() throws NoSuchFieldException, IllegalAccessException {
        System.out.println("invok1->"+Test.KEY_EXIT);
        Field field = Test.class.getDeclaredField("KEY_EXIT");
        field.setAccessible(true);
        Field modifiersField = Field.class.getDeclaredField("modifiers");
        modifiersField.setAccessible(true);
        modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
        field.set(null, 256);
        System.out.println("invok1-<"+Test.KEY_EXIT);
    }
    private static void invok2() throws NoSuchFieldException, IllegalAccessException {
        System.out.println("invok2->"+Test.KEY_BACK);
        Field field = Test.class.getDeclaredField("KEY_BACK");
        field.setAccessible(true);
        field.set(null, 1000);
        System.out.println("invok2-<"+Test.KEY_BACK);
    }
    private static void invok3() throws NoSuchFieldException, IllegalAccessException {
        System.out.println("invok3->"+Test.KEY_STR);
        Field field = Test.class.getDeclaredField("KEY_STR");
        field.setAccessible(true);
        Field modifiersField = Field.class.getDeclaredField("modifiers");
        modifiersField.setAccessible(true);
        modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
        field.set(null, "512");
        System.out.println("invok3-<"+Test.KEY_STR);
    }
    public static void main(String[] args) throws Exception {
        invok1();
        invok2();
        invok3();
    }
}

//invok1运行结果
invok1->1024
invok1-<1024
//invok2运行结果
invok2->1025
invok2-<1000
//invok3运行结果
invok3->1001
invok3-<1001