Java 中使用内存映射文件需要考虑的 10 个问题

清疚 2022-08-06 02:11 109阅读 0赞

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      <h2 style="margin:0px 0px 10pt; padding:0px 0px 5px; font-size:14pt; border-bottom-width:1px; border-bottom-style:solid; border-bottom-color:rgb(204,204,204)"> java中的内存映射IO和内存映射文件是什么？</h2> 
      <p style="margin-top:0px; margin-bottom:5pt; padding-top:0px; padding-bottom:0px"> 内存映射文件非常特别，它允许Java程序直接从内存中读取文件内容，通过将整个或部分文件映射到内存，由操作系统来处理加载请求和写入文件，应用只需要和内存打交道，这使得IO操作非常快。加载内存映射文件所使用的内存在<a href="http://javarevisited.blogspot.com/2011/05/java-heap-space-memory-size-jvm.html" style="margin:0px; padding:0px; color:rgb(62,98,166); outline:0px" rel="nofollow">Java堆区</a>之外。Java编程语言支持内存映射文件，通过java.nio包和MappedByteBuffer 可以从内存直接读写文件。</p> 
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     <h2 style="margin:0px 0px 10pt; padding:0px 0px 5px; font-size:14pt; border-bottom-width:1px; border-bottom-style:solid; border-bottom-color:rgb(204,204,204)"> 内存映射的优缺点</h2> 
     <p style="margin-top:0px; margin-bottom:5pt; padding-top:0px; padding-bottom:0px"> 内存映射IO最大的优点可能在于性能，这对于建立高频电子交易系统尤其重要。内存映射文件通常比标准通过正常IO访问文件要快。另一个巨大的优势是内存映 射IO允许加载不能直接访问的潜在巨大文件 。经验表明，内存映射IO在大文件处理方面性能更加优异。尽管它也有不足——增加了页面错误的数目。由于操作系统只将一部分文件加载到内存，如果一个请求 页面没有在内存中，它将导致页面错误。同样它可以被用来在两个进程中共享数据。</p> 
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     <h2 style="margin:0px 0px 10pt; padding:0px 0px 5px; font-size:14pt; border-bottom-width:1px; border-bottom-style:solid; border-bottom-color:rgb(204,204,204)"> 支持内存映射IO的操作系统</h2> 
     <p style="margin-top:0px; margin-bottom:5pt; padding-top:0px; padding-bottom:0px"> 大多数主流操作系统比如Windows平台，<a href="http://javarevisited.blogspot.com/2011/04/unix-commands-tutorial-and-tips-for.html" style="margin:0px; padding:0px; color:rgb(62,98,166); outline:0px" rel="nofollow">UNIX</a>，Solaris和其他类UNIX操作系统都支持内存映射IO和<span style="margin:0px; padding:0px">64位架构</span>，你几乎可以将所有文件映射到内存并通过JAVA编程语言直接访问。</p> 
     <h2 style="margin:0px 0px 10pt; padding:0px 0px 5px; font-size:14pt; border-bottom-width:1px; border-bottom-style:solid; border-bottom-color:rgb(204,204,204)"> <span style="margin:0px; padding:0px"><span style="margin:0px; padding:0px">Java的</span><span style="margin:0px; padding:0px">内存映射</span><span style="margin:0px; padding:0px">IO</span><span style="margin:0px; padding:0px">的要点</span></span></h2> 
     <p style="margin-top:0px; margin-bottom:5pt; padding-top:0px; padding-bottom:0px"> 如下为一些你需要了解的java内存映射要点:</p> 
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     <h6 style="margin:0px; padding:0px; font-weight:normal; font-size:10pt; color:rgb(68,68,68)"> <a href="http://my.oschina.net/bigtiger" style="margin:0px; padding:0px; color:rgb(62,98,166); outline:0px; text-decoration:none" rel="nofollow">bigtiger02</a><br style="margin:0px; padding:0px"> 翻译于 1年前</h6> 
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      <li style="margin:0px; padding:0px">java通过java.nio包来支持内存映射IO。</li> 
      <li style="margin:0px; padding:0px">内存映射文件主要用于<span style="margin:0px; padding:0px">性能敏感的应用</span>，例如高频电子交易平台。</li> 
      <li style="margin:0px; padding:0px">通过使用内存映射IO，你可以将大<span style="margin:0px; padding:0px">文件加载到内存</span>。</li> 
      <li style="margin:0px; padding:0px">内存映射文件可能导致<span style="margin:0px; padding:0px">页面请求错误</span>，如果请求页面不在内存中的话。</li> 
      <li style="margin:0px; padding:0px">映射文件区域的能力取决于于内存寻址的大小。在32位机器中，你不能访问超过4GB或2 ^ 32（以上的文件）。</li> 
      <li style="margin:0px; padding:0px">内存映射IO比起Java中的IO流要快的多。</li> 
      <li style="margin:0px; padding:0px">加载文件所使用的内存是<a href="http://javarevisited.blogspot.com/2011/09/javalangoutofmemoryerror-permgen-space.html" style="margin:0px; padding:0px; color:rgb(62,98,166); outline:0px" rel="nofollow">Java堆区</a>之外，并驻留共享内存，允许两个不同进程共享文件。</li> 
      <li style="margin:0px; padding:0px">内存映射文件读写由操作系统完成，所以即使在将内容写入内存后java程序崩溃了，它将仍然会将它写入文件直到操作系统恢复。</li> 
      <li style="margin:0px; padding:0px">出于性能考虑，推荐使用直接字节缓冲而不是非直接缓冲。</li> 
      <li style="margin:0px; padding:0px">不要频繁调用MappedByteBuffer.force()方法，这个方法意味着强制操作系统将内存中的内容写入磁盘，所以如果你每次写入内存映射文件都调用force()方法，你将不会体会到使用映射字节缓冲的好处，相反，它(的性能)将类似于磁盘IO的性能。</li> 
      <li style="margin:0px; padding:0px">万一发生了电源故障或主机故障，将会有很小的机率发生内存映射文件没有写入到磁盘，这意味着你可能会丢失关键数据。</li> 
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     <p style="margin-top:0px; margin-bottom:5pt; padding-top:0px; padding-bottom:0px"> <span style="margin:0px; padding:0px">好</span><span style="margin:0px; padding:0px">吧</span><span style="margin:0px; padding:0px">，</span><span style="margin:0px; padding:0px">小伙伴们，就是</span><span style="margin:0px; padding:0px">这些</span>。内存映射IO是高级程序员特别是对于用Java编写高性能应用的程序员来说是一个重要的概念。 如果你喜欢这些建议的话，你可以到我的博客<a href="http://javarevisited.blogspot.com/" style="margin:0px; padding:0px; color:rgb(62,98,166); outline:0px" rel="nofollow">http://javarevisited.blogspot.com</a>&nbsp;来浏览更多类似文章。</p> 
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