<div class="output_wrapper" id="output_wrapper_id" style="font-size: 15px; color: rgb(62, 62, 62); line-height: 1.8; word-spacing: 2px; letter-spacing: 2px; font-family: 'Helvetica Neue', Helvetica, 'Hiragino Sans GB', 'Microsoft YaHei', Arial, sans-serif; background-image: linear-gradient(90deg, rgba(50, 0, 0, 0.05) 3%, rgba(0, 0, 0, 0) 3%), linear-gradient(360deg, rgba(50, 0, 0, 0.05) 3%, rgba(0, 0, 0, 0) 3%); background-size: 20px 20px; background-position: center center;"><h3 id="h64jvm32jvm" style="color: inherit; line-height: inherit; padding: 0px; margin: 1.6em 0px; font-weight: bold; border-bottom: 2px solid rgb(239, 112, 96); font-size: 1.3em;"><span style="font-size: inherit; line-height: inherit; margin: 0px; display: inline-block; font-weight: normal; background: rgb(239, 112, 96); color: rgb(255, 255, 255); padding: 3px 10px 1px; border-top-right-radius: 3px; border-top-left-radius: 3px; margin-right: 3px;">64位JVM和32位JVM</span><span style="display: inline-block; vertical-align: bottom; border-bottom: 36px solid rgb(239, 235, 233); border-right: 20px solid transparent;"> </span></h3> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 1.7em 0px;">最初的时候，JVM是32位的，但是随着64位系统的兴起，JVM也迎来了从32位到64位的转换，32位的JVM对比64位的内存容量比较有限，但是我们使用64位虚拟机的同时，也带来了一个问题，64位下的JVM中的对象会比32位中的对象多占用1.5倍的内存空间，这是我们不想看到的（又要马儿跑，又要马儿不吃草可还行？），但是机智的程序员不会屈服，所以在JDK 1.6的版本后，我们在64位中的JVM中可以开启指针压缩（UseCompressedOops）来压缩我们对象指针的大小来帮助我们节约内存空间，拿JDK 8来说，这个指令是默认开启的。</p> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 1.7em 0px;">如果我想要知道我们电脑上安装的JVM是32位的还是64位的，可以通过下面的这个命令去查看</p> <figure style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px;"><img src="http://viyoungblog.oss-cn-beijing.aliyuncs.com/2019-09-11-121750.png" alt="" title="" style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; display: block; margin: 0px auto; max-width: 100%;"><figcaption style="line-height: inherit; margin: 0px; padding: 0px; margin-top: 10px; text-align: center; color: rgb(153, 153, 153); font-size: 0.7em;"></figcaption></figure> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 1.7em 0px;">如果我们的系统是64位的，当我们输入<code style="font-size: inherit; line-height: inherit; overflow-wrap: break-word; padding: 2px 4px; border-radius: 4px; margin: 0px 2px; color: rgb(248, 35, 117); background: rgb(248, 248, 248);">java -d32</code>的时候会提示不支持32位的JVM，或者我们通过<code style="font-size: inherit; line-height: inherit; overflow-wrap: break-word; padding: 2px 4px; border-radius: 4px; margin: 0px 2px; color: rgb(248, 35, 117); background: rgb(248, 248, 248);">java -version</code>也可以看到：</p> <figure style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px;"><img src="http://viyoungblog.oss-cn-beijing.aliyuncs.com/2019-09-11-121938.png" alt="image-20190911201937600" title="image-20190911201937600" style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; display: block; margin: 0px auto; max-width: 100%;"><figcaption style="line-height: inherit; margin: 0px; padding: 0px; margin-top: 10px; text-align: center; color: rgb(153, 153, 153); font-size: 0.7em;">image-20190911201937600</figcaption></figure> <h3 id="h" style="color: inherit; line-height: inherit; padding: 0px; margin: 1.6em 0px; font-weight: bold; border-bottom: 2px solid rgb(239, 112, 96); font-size: 1.3em;"><span style="font-size: inherit; line-height: inherit; margin: 0px; display: inline-block; font-weight: normal; background: rgb(239, 112, 96); color: rgb(255, 255, 255); padding: 3px 10px 1px; border-top-right-radius: 3px; border-top-left-radius: 3px; margin-right: 3px;">指针压缩</span><span style="display: inline-block; vertical-align: bottom; border-bottom: 36px solid rgb(239, 235, 233); border-right: 20px solid transparent;"> </span></h3> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 1.7em 0px;">当我们启用了<code style="font-size: inherit; line-height: inherit; overflow-wrap: break-word; padding: 2px 4px; border-radius: 4px; margin: 0px 2px; color: rgb(248, 35, 117); background: rgb(248, 248, 248);">-XX:+UseCompressedOops</code>之后，我们原本的OOP（Ordinary Object Pointer，普通对象指针）就会被压缩，当然也不是所有的对象都会被压缩，只有 以下几种的对象才会被压缩</p> <blockquote style="line-height: inherit; display: block; padding: 15px 15px 15px 1rem; font-size: 0.9em; margin: 1em 0px; color: rgb(0, 0, 0); border-left: 5px solid rgb(239, 112, 96); background: rgb(239, 235, 233); overflow: auto; overflow-wrap: normal; word-break: normal;"> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 0px;">对象的<strong style="font-size: inherit; line-height: inherit; margin: 0px; padding: 0px; font-weight: bold; color: rgb(233, 105, 0);">全局静态变量</strong>（类属性）</p> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 0px;">对象头信息</p> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 0px;">对象的引用类型</p> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 0px;">对象数组类型</p> </blockquote> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 1.7em 0px;">而以下几种对象则不能被压缩：</p> <blockquote style="line-height: inherit; display: block; padding: 15px 15px 15px 1rem; font-size: 0.9em; margin: 1em 0px; color: rgb(0, 0, 0); border-left: 5px solid rgb(239, 112, 96); background: rgb(239, 235, 233); overflow: auto; overflow-wrap: normal; word-break: normal;"> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 0px;">指向PermGen的Class对象指针</p> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 0px;">局部变量</p> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 0px;">传参</p> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 0px;">返回值</p> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 0px;">NULL指针</p> </blockquote> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 1.7em 0px;">指针压缩的大概原理：</p> <ul style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; padding-left: 32px; list-style-type: disc;"> <li style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; margin-bottom: 0.5em;"><p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 1.7em 0px;">通过对齐，还有偏移量将64位指针压缩成32位。</p></li> <li style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; margin-bottom: 0.5em;"><p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 1.7em 0px;">零基压缩是针对压缩解压动作的进一步优化。 它通过<strong style="font-size: inherit; line-height: inherit; margin: 0px; padding: 0px; font-weight: bold; color: rgb(233, 105, 0);">改变正常指针的随机地址分配特性</strong>，<strong style="font-size: inherit; line-height: inherit; margin: 0px; padding: 0px; font-weight: bold; color: rgb(233, 105, 0);">强制堆地址从零开始分配</strong>。</p></li> </ul> <figure style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px;"><img src="http://viyoungblog.oss-cn-beijing.aliyuncs.com/2019-09-11-123526.png" alt="image-20190911203525671" title="image-20190911203525671" style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; display: block; margin: 0px auto; max-width: 100%;"><figcaption style="line-height: inherit; margin: 0px; padding: 0px; margin-top: 10px; text-align: center; color: rgb(153, 153, 153); font-size: 0.7em;">image-20190911203525671</figcaption></figure> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 1.7em 0px;">这里需要注意：32位HotSpot VM是不支持UseCompressedOops参数的，只有64位HotSpot VM才支持。</p> <h3 id="h32bit64bit" style="color: inherit; line-height: inherit; padding: 0px; margin: 1.6em 0px; font-weight: bold; border-bottom: 2px solid rgb(239, 112, 96); font-size: 1.3em;"><span style="font-size: inherit; line-height: inherit; margin: 0px; display: inline-block; font-weight: normal; background: rgb(239, 112, 96); color: rgb(255, 255, 255); padding: 3px 10px 1px; border-top-right-radius: 3px; border-top-left-radius: 3px; margin-right: 3px;">下面是32bit和64bit下的压缩或者不压缩所占的大小~</span><span style="display: inline-block; vertical-align: bottom; border-bottom: 36px solid rgb(239, 235, 233); border-right: 20px solid transparent;"> </span></h3> <figure style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px;"><img src="http://viyoungblog.oss-cn-beijing.aliyuncs.com/2019-09-11-141738.png" alt="image-20190911221738200" title="image-20190911221738200" style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; display: block; margin: 0px auto; max-width: 100%;"><figcaption style="line-height: inherit; margin: 0px; padding: 0px; margin-top: 10px; text-align: center; color: rgb(153, 153, 153); font-size: 0.7em;">image-20190911221738200</figcaption></figure> <h3 id="h-1" style="color: inherit; line-height: inherit; padding: 0px; margin: 1.6em 0px; font-weight: bold; border-bottom: 2px solid rgb(239, 112, 96); font-size: 1.3em;"><span style="font-size: inherit; line-height: inherit; margin: 0px; display: inline-block; font-weight: normal; background: rgb(239, 112, 96); color: rgb(255, 255, 255); padding: 3px 10px 1px; border-top-right-radius: 3px; border-top-left-radius: 3px; margin-right: 3px;">在内存结构中的不同</span><span style="display: inline-block; vertical-align: bottom; border-bottom: 36px solid rgb(239, 235, 233); border-right: 20px solid transparent;"> </span></h3> <figure style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px;"><img src="http://viyoungblog.oss-cn-beijing.aliyuncs.com/2019-08-23-151212.png" alt="image-20190823231212050" title="image-20190823231212050" style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; display: block; margin: 0px auto; max-width: 100%;"><figcaption style="line-height: inherit; margin: 0px; padding: 0px; margin-top: 10px; text-align: center; color: rgb(153, 153, 153); font-size: 0.7em;">image-20190823231212050</figcaption></figure> <h3 id="h-2" style="color: inherit; line-height: inherit; padding: 0px; margin: 1.6em 0px; font-weight: bold; border-bottom: 2px solid rgb(239, 112, 96); font-size: 1.3em;"><span style="font-size: inherit; line-height: inherit; margin: 0px; display: inline-block; font-weight: normal; background: rgb(239, 112, 96); color: rgb(255, 255, 255); padding: 3px 10px 1px; border-top-right-radius: 3px; border-top-left-radius: 3px; margin-right: 3px;">公众号</span><span style="display: inline-block; vertical-align: bottom; border-bottom: 36px solid rgb(239, 235, 233); border-right: 20px solid transparent;"> </span></h3> <p style="font-size: inherit; color: inherit; line-height: inherit; padding: 0px; margin: 1.7em 0px;"></p></div> 
原文出处：https://www.cnblogs.com/viyoung/p/12159433.html
来源：oschina
链接：https://my.oschina.net/u/4407134/blog/3242626