怎麼理解Stringbuffer是線程安全的stringbuilder是線程不安全的

http://www.w3c.com.cn/%E6%80%8E%E4%B9%88%E7%90%86%E8%A7%A3stringbuffer%E6%98%AF%E7%BA%BF%E7%A8%8B%E5%AE%89%E5%85%A8%E7%9A%84stringbuilder%E6%98%AF%E7%BA%BF%E7%A8%8B%E4%B8%8D%E5%AE%89%E5%85%A8%E7%9A%84

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