Java ByteBuffer用法总结

最近用SocketChannel进行网络编程比较多，中间也遇到了几个问题，出现的bug也主要来自于对于ByteBuffer的使用不当。现在终于调通了，对ByteBuffer及Socket网络编程也有了更深的认识，特此总结一下。

对于ByteBuffer主要需要注意的是几个标志的含义：position,limit,capability,mark.几个操作的影响：flip(),clear(),rewind().还有就是在读取或者写入时，标志的变化,比如get()方法导致position加1.

SocketChannel采用的是非阻塞异步读取流数据，在读取的时候，通常是

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如果流中有数据，就会把数据从position开始读到ByteBuffer中，在读取之前ByteBuffer的clear操作会把position置为0,limit置为capability,也就是相当于清空了之前的内容，但是ByteBuffer中数组的内容在read之前是没有改变的.

read之后，通常就是开始从ByteBuffer中提取读到的数据，如果你的数据是以自己定义的数据包的格式进行发送的，那你还需要判断是否读到了数据包的结尾，因为对流数据本身来说是没有结尾这一说的。在提取数据之前，要先把position放到开始读取时的位置,把limit放到当前位置，所以要flip一下,表示从position到limit的位置都是需要的数据。

<ol class="linenums" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px 0px 0px 26px; vertical-align: baseline; list-style-position: initial;"><li value="1" class="L0" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; list-style-type: decimal;"><span class="typ" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 0, 102);">ByteBuffer</span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">.</span><span class="pln" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 0);">flip</span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">();</span></li><li class="L1" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="kwd" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 136);">while</span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">(</span><span class="typ" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 0, 102);">ByteBuffer</span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">.</span><span class="pln" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 0);">hasRemaining</span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">()){</span></li><li class="L2" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; list-style-type: decimal;"><span class="pln" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 0);">	</span><span class="kwd" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 136);">byte</span><span class="pln" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 0);"> c</span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">=</span><span class="typ" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 0, 102);">ByteBuffer</span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">.</span><span class="kwd" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 136);">get</span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">();</span></li><li class="L3" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 0);">	</span><span class="kwd" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 136);">if</span><span class="pln" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 0);"> </span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">(</span><span class="pln" style="border: 0px; 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color: rgb(102, 102, 0);">{</span></li><li class="L4" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; list-style-type: decimal;"><span class="pln" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 0);">		</span><span class="com" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(136, 0, 0);">//you can return the package here</span></li><li class="L5" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 0);">	</span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">}</span><span class="kwd" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 136);">else</span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">{</span></li><li class="L6" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; list-style-type: decimal;"><span class="pln" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 0);">		</span><span class="com" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(136, 0, 0);">//you can append the byte here.like StringBuilder.append().</span></li><li class="L7" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(0, 0, 0);">	</span><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">}</span></li><li class="L8" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; list-style-type: decimal;"><span class="pun" style="border: 0px; font-family: inherit; font-style: inherit; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline; color: rgb(102, 102, 0);">}</span></li></ol>

这样以来也存在一个问题，当一次读到的ByteBuffer不包含完整的数据包或者包含多个数据包.那么就需要在下一次继续把这些包分拆出来.那么在读取数据的代码处就可以改为,这样就把之前读取到的未完整的包保留了下来:

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另外一个可能会用到的操作就是ByteBuffer.rewind(),他会把position置为0，limit保持不变，可以用于重复读取一段数据.

ByteBuffer是nio中一个非常方便的工具.设计思想也非常值得借鉴.