effect of changing String using reflection

user_572670 0
Viewed 6054

As we all know, String is immutable in java. however, one can change it using reflection, by getting the Field and setting access level. (I know it is unadvised, I am not planning to do so, this question is pure theoretical).

my question: assuming I know what I am doing (and modify all fields as needed), will the program run properly? or does the jvm makes some optimizations that rely on String being immutable? will I suffer performance loss? if so, what assumption does it make? what will go wrong in the program

p.s. String is just an example, I am interested actually in a general answer, in addition to the example.

thanks!

jvm immutability java
7 Answers

After compilation some strings may refer to the one instance, so, you will edit more than you want and never know what else are you editing.

public static void main(String args[]) throws Exception {
    String s1 = "Hello"; // I want to edit it
    String s2 = "Hello"; // It may be anywhere and must not be edited
    Field f = String.class.getDeclaredField("value");
    f.setAccessible(true);
    f.set(s1, "Doesn't say hello".toCharArray());
    System.out.println(s2);
}

Output:

Doesn't say hello
user_29335410 
public static void main(String args[]){
    String a = "test213";
    String s = new String("test213");
    try {
        System.out.println(s);
        System.out.println(a);
        char[] value = (char[])getFieldValue(s, "value");
        value[1] = 'a';
        System.out.println(s);
        System.out.println(a);

    } catch (Exception e) {
        e.printStackTrace();
    }
}

static Object getFieldValue(String s,String fieldName) throws SecurityException, NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
    Object chars = null;
    Field innerCharArray = String.class.getDeclaredField(fieldName);
    innerCharArray.setAccessible(true);
    chars = innerCharArray.get(s);
    return chars;
}

Changing value of S will change the literal of a as mentioned in all answers.

user_21823510
Related