A modifiable sequence of characters for use in creating strings, where all accesses are synchronized. This class has mostly been replaced by StringBuilder because this synchronization is rarely useful. This class is mainly used to interact with legacy APIs
that expose it.
一个可以修改的字符串序列用来创建字符串，这个类大多被StringBuilder替代，因为同步很少又有。这个类主要为以前的API相兼容。
For particularly complex string-building needs, consider Formatter.
The majority of the modification methods on this class return this so that method calls can be chained together. For example: new StringBuffer("a").append("b").append("c").toString().
See Also
CharSequence
Appendable
StringBuilder
String
format(String, Object...)
举例：
//创建一个StringBuffer
StringBuffer sb = new StringBuffer();
String line = null;
BufferedReader buffer = null;
try {
//根据给定字符串创建URL
url = new URL(urlstr);
//根据URL建立Http URL连接
HttpURLConnection urlConn = (HttpURLConnection)url.openConnection();
//根据Http URL的输入流先创建InputStreamReader，然后由InputStreamReader创建一个BufferedReader。
buffer = new BufferedReader(new InputStreamReader(urlConn.getInputStream()));
//有BufferedReader的对象循环读取 每一行数据，并将所有数据放到StringBuffer储存起来
while((line = buffer.readLine()) != null){
sb.append(line);
}
} catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
}finally{
try {
buffer.close();
} catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
}
//文件读取完毕，最后返回读到的内容
return sb.toString();
}