Java中的String，StringBuilder，StringBuffer三者的区别

https://www.cnblogs.com/su-feng/p/6659064.html

jdk8源码中对StringBuilder的描述：

Instances of {@code StringBuilder} are not safe for
* use by multiple threads. If such synchronization is required then it is
* recommended that {@link java.lang.StringBuffer} be used.

jdk8源码中对StringBuffer的描述：

A thread-safe, mutable sequence of characters.
String buffers are safe for use by multiple threads.

运行速度比较为：StringBuilder > StringBuffer > String
因为StringBuilder 是线程不安全的，StringBuffer 是线程安全的，StringBuffer 中的方法大都采用了 synchronized 关键字进行修饰。