Why is char[] preferred over String for passwords?

Question

In Swing, the password field has a getPassword() (returns char[]) method instead of the usual getText() (returns String)

Answer 1

To quote an official document, the Java Cryptography Architecture guide says this about char[] vs. String passwords (about password-based encryption, but this is more generally about passwords of course):

It would seem logical to collect and store the password in an object of type java.lang.String. However, here's the caveat: Objects of type String are immutable, i.e., there are no methods defined that allow you to change (overwrite) or zero out the contents of a String after usage. This feature makes String objects unsuitable for storing security sensitive information such as user passwords. You should always collect and store security sensitive information in a char array instead.

Guideline 2-2 of the Secure Coding Guidelines for the Java Programming Language, Version 4.0 also says something similar (although it is originally in the context of logging):

Guideline 2-2: Do not log highly sensitive information

Some information, such as Social Security numbers (SSNs) and passwords, is highly sensitive. This information should not be kept for longer than necessary nor where it may be seen, even by administrators. For instance, it should not be sent to log files and its presence should not be detectable through searches. Some transient data may be kept in mutable data structures, such as char arrays, and cleared immediately after use. Clearing data structures has reduced effectiveness on typical Java runtime systems as objects are moved in memory transparently to the programmer.

This guideline also has implications for implementation and use of lower-level libraries that do not have semantic knowledge of the data they are dealing with. As an example, a low-level string parsing library may log the text it works on. An application may parse an SSN with the library. This creates a situation where the SSNs are available to administrators with access to the log files.