RandomAccessFile 类支持“随机访问”的方式，程序可以直接跳到文件的任意地方来读、写文件（其实我更愿意叫它“随意访问”）----支持只访问文件的部分内容----可以向已存在的文件后追加内容
RandomAccessFile 对象包含一个记录指针，用以标示当前读写处的位置。RandomAccessFile 类对象可以自由移动记录指针：long getFilePointer()：获取文件记录指针的当前位置void seek(long pos)：将文件记录指针定位到 pos位置
创建 RandomAccessFile 类实例需要指定一个 mode参数，该参数指定 RandomAccessFile 的访问模式：r：以只读方式打开rw：打开以便读取和写入rwd：打开以便读取和写入，同步文件内容的更新rws：打开以便读取和写入，同步文件内容和元数据的更新
JDK 1.4后很多 RandomAccessFile 的功能被转到“内存映射文件”里了，可以考虑使用 memory-mapped files
下面举例说明（注意：直接在指针后 write 将会是覆盖操作，这里只演示插入操作）
<ol class="linenums"><li class="L0"><code><span class="com"> // 文本文件的复制（实现读写）</span></code></li><li class="L1"><code><span class="pln"> </span><span class="lit">@Test</span></code></li><li class="L2"><code><span class="pln"> </span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> test1</span><span class="pun">()</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L3"><code><span class="pln"> </span><span class="typ">RandomAccessFile</span><span class="pln"> rafin </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">null</span><span class="pun">;</span></code></li><li class="L4"><code><span class="pln"> </span><span class="typ">RandomAccessFile</span><span class="pln"> rafout </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">null</span><span class="pun">;</span></code></li><li class="L5"><code><span class="pln"> </span><span class="kwd">try</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L6"><code><span class="pln"> rafin </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">RandomAccessFile</span><span class="pun">(</span><span class="str">"C:/Users/59929/Desktop/test1.txt"</span><span class="pun">,</span><span class="pln"> </span><span class="str">"r"</span><span class="pun">);</span></code></li><li class="L7"><code><span class="pln"> rafout </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">RandomAccessFile</span><span class="pun">(</span><span class="str">"C:/Users/59929/Documents/test.txt"</span><span class="pun">,</span><span class="pln"> </span><span class="str">"rw"</span><span class="pun">);</span></code></li><li class="L8"><code><span class="pln"> </span><span class="kwd">int</span><span class="pln"> len</span><span class="pun">;</span></code></li><li class="L9"><code><span class="pln"> </span><span class="kwd">byte</span><span class="pun">[]</span><span class="pln"> b </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="kwd">byte</span><span class="pun">[</span><span class="lit">1024</span><span class="pun">];</span></code></li><li class="L0"><code><span class="pln"> </span><span class="kwd">while</span><span class="pln"> </span><span class="pun">((</span><span class="pln">len </span><span class="pun">=</span><span class="pln"> rafin</span><span class="pun">.</span><span class="pln">read</span><span class="pun">(</span><span class="pln">b</span><span class="pun">))</span><span class="pln"> </span><span class="pun">!=</span><span class="pln"> </span><span class="pun">-</span><span class="lit">1</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L1"><code><span class="pln"> rafout</span><span class="pun">.</span><span class="pln">write</span><span class="pun">(</span><span class="pln">b</span><span class="pun">,</span><span class="pln"> </span><span class="lit">0</span><span class="pun">,</span><span class="pln"> len</span><span class="pun">);</span></code></li><li class="L2"><code><span class="pln"> </span><span class="pun">}</span></code></li><li class="L3"><code><span class="pln"> </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">catch</span><span class="pln"> </span><span class="pun">(</span><span class="typ">FileNotFoundException</span><span class="pln"> e</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L4"><code><span class="pln"> e</span><span class="pun">.</span><span class="pln">printStackTrace</span><span class="pun">();</span></code></li><li class="L5"><code><span class="pln"> </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">catch</span><span class="pln"> </span><span class="pun">(</span><span class="typ">IOException</span><span class="pln"> e</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L6"><code><span class="pln"> e</span><span class="pun">.</span><span class="pln">printStackTrace</span><span class="pun">();</span></code></li><li class="L7"><code><span class="pln"> </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">finally</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L8"><code><span class="pln"> </span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="pln">rafout </span><span class="pun">!=</span><span class="pln"> </span><span class="kwd">null</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L9"><code><span class="pln"> </span><span class="kwd">try</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L0"><code><span class="pln"> rafout</span><span class="pun">.</span><span class="pln">close</span><span class="pun">();</span></code></li><li class="L1"><code><span class="pln"> </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">catch</span><span class="pln"> </span><span class="pun">(</span><span class="typ">IOException</span><span class="pln"> e</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L2"><code><span class="pln"> e</span><span class="pun">.</span><span class="pln">printStackTrace</span><span class="pun">();</span></code></li><li class="L3"><code><span class="pln"> </span><span class="pun">}</span></code></li><li class="L4"><code><span class="pln"> </span><span class="pun">}</span></code></li><li class="L5"><code><span class="pln"> </span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="pln">rafin </span><span class="pun">!=</span><span class="pln"> </span><span class="kwd">null</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L6"><code><span class="pln"> </span><span class="kwd">try</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L7"><code><span class="pln"> rafin</span><span class="pun">.</span><span class="pln">close</span><span class="pun">();</span></code></li><li class="L8"><code><span class="pln"> </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">catch</span><span class="pln"> </span><span class="pun">(</span><span class="typ">IOException</span><span class="pln"> e</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L9"><code><span class="pln"> e</span><span class="pun">.</span><span class="pln">printStackTrace</span><span class="pun">();</span></code></li><li class="L0"><code><span class="pln"> </span><span class="pun">}</span></code></li><li class="L1"><code><span class="pln"> </span><span class="pun">}</span></code></li><li class="L2"><code><span class="pln"> </span><span class="pun">}</span></code></li><li class="L3"><code><span class="pln"> </span><span class="pun">}</span></code></li><li class="L4"><code></code></li><li class="L5"><code><span class="pln"> </span><span class="com">// 实现文本文件的插入操作</span></code></li><li class="L6"><code><span class="pln"> </span><span class="lit">@Test</span></code></li><li class="L7"><code><span class="pln"> </span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> test2</span><span class="pun">()</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L8"><code><span class="pln"> </span><span class="typ">RandomAccessFile</span><span class="pln"> raf </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">null</span><span class="pun">;</span></code></li><li class="L9"><code><span class="pln"> </span><span class="kwd">try</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L0"><code><span class="pln"> raf </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">RandomAccessFile</span><span class="pun">(</span><span class="str">"C:/Users/59929/Desktop/test2.txt"</span><span class="pun">,</span><span class="pln"> </span><span class="str">"rw"</span><span class="pun">);</span></code></li><li class="L1"><code><span class="pln"> </span><span class="typ">System</span><span class="pun">.</span><span class="pln">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"现在指针位置为："</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> raf</span><span class="pun">.</span><span class="pln">getFilePointer</span><span class="pun">());</span></code></li><li class="L2"><code><span class="pln"> raf</span><span class="pun">.</span><span class="pln">seek</span><span class="pun">(</span><span class="lit">16</span><span class="pun">);</span></code></li><li class="L3"><code><span class="pln"> </span><span class="typ">System</span><span class="pun">.</span><span class="pln">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"移到要修改的位置，现在指针位置为："</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> raf</span><span class="pun">.</span><span class="pln">getFilePointer</span><span class="pun">());</span></code></li><li class="L4"><code><span class="pln"> </span><span class="kwd">int</span><span class="pln"> len</span><span class="pun">;</span></code></li><li class="L5"><code><span class="pln"> </span><span class="kwd">byte</span><span class="pun">[]</span><span class="pln"> b </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="kwd">byte</span><span class="pun">[</span><span class="lit">10</span><span class="pun">];</span></code></li><li class="L6"><code><span class="pln"> </span><span class="typ">StringBuffer</span><span class="pln"> sb </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">StringBuffer</span><span class="pun">();</span></code></li><li class="L7"><code><span class="pln"> </span><span class="kwd">while</span><span class="pln"> </span><span class="pun">((</span><span class="pln">len </span><span class="pun">=</span><span class="pln"> raf</span><span class="pun">.</span><span class="pln">read</span><span class="pun">(</span><span class="pln">b</span><span class="pun">))</span><span class="pln"> </span><span class="pun">!=</span><span class="pln"> </span><span class="pun">-</span><span class="lit">1</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L8"><code><span class="pln"> sb</span><span class="pun">.</span><span class="pln">append</span><span class="pun">(</span><span class="kwd">new</span><span class="pln"> </span><span class="typ">String</span><span class="pun">(</span><span class="pln">b</span><span class="pun">,</span><span class="pln"> </span><span class="lit">0</span><span class="pun">,</span><span class="pln"> len</span><span class="pun">));</span></code></li><li class="L9"><code><span class="pln"> </span><span class="pun">}</span></code></li><li class="L0"><code><span class="pln"> </span><span class="typ">System</span><span class="pun">.</span><span class="pln">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"读完后现在指针位置为："</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> raf</span><span class="pun">.</span><span class="pln">getFilePointer</span><span class="pun">());</span></code></li><li class="L1"><code><span class="pln"> raf</span><span class="pun">.</span><span class="pln">seek</span><span class="pun">(</span><span class="lit">16</span><span class="pun">);</span></code></li><li class="L2"><code><span class="pln"> </span><span class="typ">System</span><span class="pun">.</span><span class="pln">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"再次移到要修改的位置，现在指针位置为："</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> raf</span><span class="pun">.</span><span class="pln">getFilePointer</span><span class="pun">());</span></code></li><li class="L3"><code><span class="pln"> raf</span><span class="pun">.</span><span class="pln">write</span><span class="pun">(</span><span class="str">"c"</span><span class="pun">.</span><span class="pln">getBytes</span><span class="pun">());</span></code></li><li class="L4"><code><span class="pln"> raf</span><span class="pun">.</span><span class="pln">write</span><span class="pun">(</span><span class="pln">sb</span><span class="pun">.</span><span class="pln">toString</span><span class="pun">().</span><span class="pln">getBytes</span><span class="pun">());</span></code></li><li class="L5"><code><span class="pln"> </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">catch</span><span class="pln"> </span><span class="pun">(</span><span class="typ">FileNotFoundException</span><span class="pln"> e</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L6"><code><span class="pln"> e</span><span class="pun">.</span><span class="pln">printStackTrace</span><span class="pun">();</span></code></li><li class="L7"><code><span class="pln"> </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">catch</span><span class="pln"> </span><span class="pun">(</span><span class="typ">IOException</span><span class="pln"> e</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L8"><code><span class="pln"> e</span><span class="pun">.</span><span class="pln">printStackTrace</span><span class="pun">();</span></code></li><li class="L9"><code><span class="pln"> </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">finally</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L0"><code><span class="pln"> </span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="pln">raf </span><span class="pun">!=</span><span class="pln"> </span><span class="kwd">null</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L1"><code><span class="pln"> </span><span class="kwd">try</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L2"><code><span class="pln"> raf</span><span class="pun">.</span><span class="pln">close</span><span class="pun">();</span></code></li><li class="L3"><code><span class="pln"> </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">catch</span><span class="pln"> </span><span class="pun">(</span><span class="typ">IOException</span><span class="pln"> e</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span></code></li><li class="L4"><code><span class="pln"> e</span><span class="pun">.</span><span class="pln">printStackTrace</span><span class="pun">();</span></code></li><li class="L5"><code><span class="pln"> </span><span class="pun">}</span></code></li><li class="L6"><code><span class="pln"> </span><span class="pun">}</span></code></li><li class="L7"><code><span class="pln"> </span><span class="pun">}</span></code></li><li class="L8"><code><span class="pln"> </span><span class="pun">}</span></code></li></ol>RandomAccessFile
标签：[] 覆盖 地方 sys 同步 ack top 映射文件 演示
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