her0kings1ey
背景
最近在阅读一些框架代码中看到在代码中使用了System.getProperty来获取值，之前在想这个值是从哪里设置的呢？于是便写下这篇小文记录一下。
浅析
首先我们看看System.getProperties() 的注释。
/**
* Determines the current system properties.
* 得到当前系统属性
* <p>
* First, if there is a security manager, its
* 首先，如果存在安全管理器，
* <code>checkPropertiesAccess</code> method is called with no
* arguments. This may result in a security exception.
* 会调用其checkPropertiesAccess无参方法，有可能抛出安全异常
* <p>
* The current set of system properties for use by the
* 当前系统属性集会通过Property对象返回（对象有getProperty()方法）
* {@link #getProperty(String)} method is returned as a
* <code>Properties</code> object. If there is no current set of
* 如果当前没有系统属性集，会创建并初始化系统属性集。
* system properties, a set of system properties is first created and
* initialized. This set of system properties always includes values
* 系统属性通常包括了
* java.version、java.vendor等等
* for the following keys:
* <table summary="Shows property keys and associated values">
* <tr><th>Key</th>
* <th>Description of Associated Value</th></tr>
* <tr><td><code>java.version</code></td>
* <td>Java Runtime Environment version</td></tr>
* <tr><td><code>java.vendor</code></td>
* <td>Java Runtime Environment vendor</td></tr>
* <tr><td><code>java.vendor.url</code></td>
* <td>Java vendor URL</td></tr>
* <tr><td><code>java.home</code></td>
* <td>Java installation directory</td></tr>
* <tr><td><code>java.vm.specification.version</code></td>
* <td>Java Virtual Machine specification version</td></tr>
* <tr><td><code>java.vm.specification.vendor</code></td>
* <td>Java Virtual Machine specification vendor</td></tr>
* <tr><td><code>java.vm.specification.name</code></td>
* <td>Java Virtual Machine specification name</td></tr>
* <tr><td><code>java.vm.version</code></td>
* <td>Java Virtual Machine implementation version</td></tr>
* <tr><td><code>java.vm.vendor</code></td>
* <td>Java Virtual Machine implementation vendor</td></tr>
* <tr><td><code>java.vm.name</code></td>
* <td>Java Virtual Machine implementation name</td></tr>
* <tr><td><code>java.specification.version</code></td>
* <td>Java Runtime Environment specification version</td></tr>
* <tr><td><code>java.specification.vendor</code></td>
* <td>Java Runtime Environment specification vendor</td></tr>
* <tr><td><code>java.specification.name</code></td>
* <td>Java Runtime Environment specification name</td></tr>
* <tr><td><code>java.class.version</code></td>
* <td>Java class format version number</td></tr>
* <tr><td><code>java.class.path</code></td>
* <td>Java class path</td></tr>
* <tr><td><code>java.library.path</code></td>
* <td>List of paths to search when loading libraries</td></tr>
* <tr><td><code>java.io.tmpdir</code></td>
* <td>Default temp file path</td></tr>
* <tr><td><code>java.compiler</code></td>
* <td>Name of JIT compiler to use</td></tr>
* <tr><td><code>java.ext.dirs</code></td>
* <td>Path of extension directory or directories
* <b>Deprecated.</b> <i>This property, and the mechanism
* which implements it, may be removed in a future
* release.</i> </td></tr>
* <tr><td><code>os.name</code></td>
* <td>Operating system name</td></tr>
* <tr><td><code>os.arch</code></td>
* <td>Operating system architecture</td></tr>
* <tr><td><code>os.version</code></td>
* <td>Operating system version</td></tr>
* <tr><td><code>file.separator</code></td>
* <td>File separator ("/" on UNIX)</td></tr>
* <tr><td><code>path.separator</code></td>
* <td>Path separator (":" on UNIX)</td></tr>
* <tr><td><code>line.separator</code></td>
* <td>Line separator ("\n" on UNIX)</td></tr>
* <tr><td><code>user.name</code></td>
* <td>User's account name</td></tr>
* <tr><td><code>user.home</code></td>
* <td>User's home directory</td></tr>
* <tr><td><code>user.dir</code></td>
* <td>User's current working directory</td></tr>
* </table>
* <p>
* Multiple paths in a system property value are separated by the path
* separator character of the platform.
* <p>
* Note that even if the security manager does not permit the
* <code>getProperties</code> operation, it may choose to permit the
* {@link #getProperty(String)} operation.
*
* @return the system properties
* @exception SecurityException if a security manager exists and its
* <code>checkPropertiesAccess</code> method doesn't allow access
* to the system properties.
* @see #setProperties
* @see java.lang.SecurityException
* @see java.lang.SecurityManager#checkPropertiesAccess()
* @see java.util.Properties
*/
从这里可以看到这里主要能够拿到一些关于Java的系统属性，如：java版本，java的vendor，操作系统版本等等。另外，还能够运行java命令时，通过-D参数来指定系统属性，这也就是在一些框架内我们会看见使用System.getProperty来获取一些我们自定义的属性值。
关于java System能获取的属性值，请看这里
什么时候要使用System.getProperty
可以获取系统环境信息。
作为轻量级的配置文件使用，如：可以配置项目的运行路径、项目的配置文件路径等等
