can anyone explain his point?

Question

How does Java string being immutable increase security?

A very common practice in writing class libraries is storing the parameters passed into your API, say, in a constructor, like this:

public class MyApi {
final String myUrl;
public MyApi(String urlString) {
// Verify that urlString points to an approved server
if (!checkApprovedUrl(urlString)) throw new IllegalArgumentException();
myUrl = urlString;
}
}
Were String mutable, this would lead to a subtle exploit: an attacker would pass a good URL, wait for a few microseconds, and then set the URL to point to an attack site.

Since storing without copying is a reasonably common practice, and because strings are among the most commonly used data types, leaving strings mutable would open up many APIs that are not written yet open to a serious security problem. Making strings immutable closes this particular security hole for all APIs, including the ones that are not written yet.

Answer 1

Imagine as if it was a library performing security checks called in a multithreaded environment where you could time your change (the inner state) of urlString after the check has been performed. With Strings that's not possible because of immutability.