StringBuilder StringBuffer

StringBuffer

源码中定义如下：

A modifiable CharSequence for use in creating strings, where all
accesses are synchronized. This class has mostly been replaced by
StringBuilder because this synchronization is rarely useful.

StringBuffer可修改，所有可访问方法都是同步的但是此方法几乎已经被StringBuilder替代了，因为同步方法很少使用到。

StringBuilder

A modifiable CharSequence for use in creating strings. This class is intended as a direct replacement of StringBuffer for non-concurrent use; unlike StringBuffer this class is not synchronized.

如果只是单线程使用的话，建议使用StringBuilder，因为它的速度比StringBuffer快。若考虑线程安全，建议使用StringBuffer。