获取指定键的系统属性，如：System.getProperty("user.dir")获取当前项目的根目录

今天写一个小程序，需要获取当前项目的根目录。百度搜索得知System.getProperty("user.dir")该方法可以做到。看到这行代码不由让我思考：“user.dir”是一个什么玩意？System.getProperty("key")该方法到底有多少个key？于是经过对api的研究，发现了System.getProperties()这个方法可以获取到所有的key值;代码如下：

public void getSystemKey(){

    File sysFile = new File("d:/study","sysFile.properties");
    Properties sysPro = System.getProperties();
    try {
        sysPro.store(new FileOutputStream(sysFile), "Properties ClASS");
    } catch (FileNotFoundException e1) {
        e1.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    
}

得到的sysFile.properties文件内容如下：

#Properties ClASS
#Wed Mar 18 16:04:29 CST 2015
java.runtime.name=Java(TM) SE Runtime Environment
sun.boot.library.path=D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\jre\\bin
java.vm.version=11.3-b02
java.vm.vendor=Sun Microsystems Inc.
java.vendor.url=http\://java.sun.com/
path.separator=;
java.vm.name=Java HotSpot(TM) Client VM
file.encoding.pkg=sun.io
sun.java.launcher=SUN_STANDARD
user.country=CN
sun.os.patch.level=
java.vm.specification.name=Java Virtual Machine Specification
user.dir=D\:\\workspace\\study\\2015-3-17Java-FileUpDown
java.runtime.version=1.6.0_13-b03
java.awt.graphicsenv=sun.awt.Win32GraphicsEnvironment
java.endorsed.dirs=D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\jre\\lib\\endorsed
os.arch=x86
java.io.tmpdir=C\:\\Users\\ADMINI~1\\AppData\\Local\\Temp\\
line.separator=\r\n
java.vm.specification.vendor=Sun Microsystems Inc.
user.variant=
os.name=Windows Vista
sun.jnu.encoding=GBK
java.library.path=D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\bin;.;C\:\\Windows\\Sun\\Java\\bin;C\:\\Windows\\system32;C\:\\Windows;D\:/InstallFiles/Genuitec/Common/binary/com.sun.java.jdk.win32.x86_1.6.0.013/jre/bin/client;D\:/InstallFiles/Genuitec/Common/binary/com.sun.java.jdk.win32.x86_1.6.0.013/jre/bin;C\:\\ProgramData\\Oracle\\Java\\javapath;C\:\\Program Files (x86)\\NVIDIA Corporation\\PhysX\\Common;C\:\\Windows\\system32;C\:\\Windows;C\:\\Windows\\System32\\Wbem;C\:\\Windows\\System32\\WindowsPowerShell\\v1.0\\;D\:\\InstallFiles\\dzpk\\Java\\jdk1.6.0_24\\lib
java.specification.name=Java Platform API Specification
java.class.version=50.0
sun.management.compiler=HotSpot Client Compiler
os.version=6.2
user.home=C\:\\Users\\Administrator
user.timezone=Asia/Shanghai
java.awt.printerjob=sun.awt.windows.WPrinterJob
file.encoding=GBK
java.specification.version=1.6
java.class.path=D\:\\workspace\\study\\2015-3-17Java-FileUpDown\\WebRoot\\WEB-INF\\classes;D\:\\workspace\\study\\2015-3-17Java-FileUpDown\\WebRoot\\WEB-INF\\lib\\poi-3.11-20141221.jar;D\:\\workspace\\study\\2015-3-17Java-FileUpDown\\WebRoot\\WEB-INF\\lib\\commons-logging-1.1.3.jar;D\:\\workspace\\study\\2015-3-17Java-FileUpDown\\WebRoot\\WEB-INF\\lib\\junit-4.11.jar;D\:\\workspace\\study\\2015-3-17Java-FileUpDown\\WebRoot\\WEB-INF\\lib\\log4j-1.2.17.jar
user.name=Administrator
java.vm.specification.version=1.0
java.home=D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\jre
sun.arch.data.model=32
user.language=zh
java.specification.vendor=Sun Microsystems Inc.
awt.toolkit=sun.awt.windows.WToolkit
java.vm.info=mixed mode
java.version=1.6.0_13
java.ext.dirs=D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\jre\\lib\\ext;C\:\\Windows\\Sun\\Java\\lib\\ext
sun.boot.class.path=D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\jre\\lib\\resources.jar;D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\jre\\lib\\rt.jar;D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\jre\\lib\\sunrsasign.jar;D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\jre\\lib\\jsse.jar;D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\jre\\lib\\jce.jar;D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\jre\\lib\\charsets.jar;D\:\\InstallFiles\\Genuitec\\Common\\binary\\com.sun.java.jdk.win32.x86_1.6.0.013\\jre\\classes;D\:\\InstallFiles\\Genuitec\\Common\\plugins\\com.genuitec.eclipse.j2eedt.core_8.6.1.me201010211225\\data\\libraryset\\EE_5\\javaee.jar;D\:\\InstallFiles\\Genuitec\\Common\\plugins\\com.genuitec.eclipse.j2eedt.core_8.6.1.me201010211225\\data\\libraryset\\EE_5\\jsf-impl.jar;D\:\\InstallFiles\\Genuitec\\Common\\plugins\\com.genuitec.eclipse.j2eedt.core_8.6.1.me201010211225\\data\\libraryset\\EE_5\\jsf-api.jar;D\:\\InstallFiles\\Genuitec\\Common\\plugins\\com.genuitec.eclipse.j2eedt.core_8.6.1.me201010211225\\data\\libraryset\\EE_5\\jstl-1.2.jar
java.vendor=Sun Microsystems Inc.
file.separator=\\
java.vendor.url.bug=http\://java.sun.com/cgi-bin/bugreport.cgi
sun.io.unicode.encoding=UnicodeLittle
sun.cpu.endian=little
sun.desktop=windows
sun.cpu.isalist=pentium_pro+mmx pentium_pro pentium+mmx pentium i486 i386 i86