问一个Java字符串真的是不可变的？

String s1 = "Hello World";  

String s2 = "Hello World";  

String s3 = s1.substring(6);  

System.out.println(s1); // Hello World  

System.out.println(s2); // Hello World  

System.out.println(s3); // World  

Field field = String.class.getDeclaredField("value");  

field.setAccessible(true);  

char[] value = (char[])field.get(s1);  

value[6] = 'J';  

value[7] = 'a';  

value[8] = 'v';  

value[9] = 'a';  

value[10] = '!';  

System.out.println(s1); // Hello Java!  

System.out.println(s2); // Hello Java!  

System.out.println(s3); // World