关于这三者的区别已经是老话题了，上stackoverflow看到目前最中肯简洁的回答

http://stackoverflow.com/a/2971343

Mutability Difference:

String is immutable, if you try to alter their values, another object gets created, whereas StringBuffer and StringBuilder are mutable so they can change their values.

Thread-Safety Difference:

The difference between StringBuffer and StringBuilder is that StringBuffer is thread-safe. So when the application needs to be run only in a single thread then it is better to use StringBuilder. StringBuilder is more efficient than StringBuffer.

Situations:

• If your string is not going to change use a String class because a String object is immutable.
• If your string can change (example: lots of logic and operations in the construction of the string) and will only be accessed from a single thread, using a StringBuilder is good enough.
• If your string can change, and will be accessed from multiple threads, use a StringBufferbecause StringBuffer is synchronous so you have thread-safety.

String, StringBuffer, and StringBuilder

原文：http://blog.csdn.net/chziroy/article/details/44975477