System.getEnv（）和System.getProperties（）的差别
和环境变量 都是名称与值之间的映射。两种机制都能用来将用户定义的信息传递给 Java 进程。环境变量产生很多其它的全局效应，由于它们不仅对Java 子进程可见，并且对于定义它们的进程的全部子进程都是可见的。在不同的操作系统上，它们的语义有细微的区别，比方，不区分大写和小写。由于这些原因，环境变量更可能有意料不到的副作用。最好在可能的地方使用系统属性。环境变量应该在须要全局效应的时候使用，或者在外部系统接口要求使用环境变量时使用（比方 PATH）。
代码例如以下：
public static void main(String [] args)
{
Map m = System.getenv();
for ( Iterator it = m.keySet().iterator(); it.hasNext(); )
{
String key = (String ) it.next();
String value = (String ) m.get(key);
System.out.println(key +":" +value);
}
System.out.println( "--------------------------------------" );
Properties p = System.getProperties();
for ( Iterator it = p.keySet().iterator(); it.hasNext(); )
{
String key = (String ) it.next();
String value = (String ) p.get(key);
System.out.println(key +":" +value);
}
}
输入例如以下：
ANT_HOME:D:/program/devel/ant
PROCESSOR_ARCHITECTURE:x86
LOGONSERVER://RJ-WEIJIANJUN
HOMEDRIVE:C:
CATALINA_HOME:D:/program/server/Tomcat5.5
DXSDK_DIR:d:/Program Files/Microsoft DirectX SDK (August 2008)/
VS80COMNTOOLS:C:/Program Files/Microsoft Visual Studio 8/Common7/Tools/
SESSIONNAME:Console
HOMEPATH:/Documents and Settings/Administrator
TMP:C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp
windir:C:/WINDOWS
PROCESSOR_IDENTIFIER:x86 Family 6 Model 15 Stepping 13, GenuineIntel
VS90COMNTOOLS:e:/Program Files/Microsoft Visual Studio 9.0/Common7/Tools/
SystemDrive:C:
USERPROFILE:C:/Documents and Settings/Administrator
PATHEXT:.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH
CommonProgramFiles:C:/Program Files/Common Files
NUMBER_OF_PROCESSORS:2
ComSpec:C:/WINDOWS/system32/cmd.exe
COMPUTERNAME:RJ-WEIJIANJUN
OS:Windows_NT
USERNAME:Administrator
CLIENTNAME:Console
TEMP:C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp
USERDOMAIN:RJ-WEIJIANJUN
ALLUSERSPROFILE:C:/Documents and Settings/All Users
lib:C:/Program Files/SQLXML 4.0/bin/
PROCESSOR_LEVEL:6
SystemRoot:C:/WINDOWS
ClusterLog:C:/WINDOWS/Cluster/cluster.log
APPDATA:C:/Documents and Settings/Administrator/Application Data
Path:C:/WINDOWS/system32;C:/WINDOWS;C:/WINDOWS/System32/Wbem;C:/Program Files/Microsoft SQL Server/80/Tools/Binn/;C:/Program Files/Microsoft SQL Server/90/DTS/Binn/;C:/Program Files/Microsoft SQL Server/90/Tools/binn/;C:/Program Files/Microsoft SQL Server/90/Tools/Binn/VSShell/Common7/IDE/;C:/Program Files/Microsoft Visual Studio 8/Common7/IDE/PrivateAssemblies/;D:/program/devel/flex_sdk2/bin;D:/program/devel/ant/bin;C:/Program Files/Java/jdk1.6.0_07/bin;%JONAS_ROOT%/bin/nt;d:/program/devel/ant/bin
JAVA_HOME:C:/Program Files/Java/jdk1.6.0_07
FP_NO_HOST_CHECK:NO
PROCESSOR_REVISION:0f0d
ProgramFiles:C:/Program Files
下面是property的输出：
--------------------------------------
java.runtime.name:Java(TM) 2 Runtime Environment, Standard Edition
sun.boot.library.path:D:/Program Files/MyEclipse 6.5/jre/bin
java.vm.version:1.5.0_11-b03
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:Service Pack 2
java.vm.specification.name:Java Virtual Machine Specification
user.dir:D:/dev/eclipse/mye65/workspace/jmx
java.runtime.version:1.5.0_11-b03
java.awt.graphicsenv:sun.awt.Win32GraphicsEnvironment
java.endorsed.dirs:D:/Program Files/MyEclipse 6.5/jre/lib/endorsed
os.arch:x86
java.io.tmpdir:C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/
line.separator:
java.vm.specification.vendor:Sun Microsystems Inc.
user.variant:
os.name:Windows 2003
sun.jnu.encoding:GBK
java.library.path:D:/Program Files/MyEclipse 6.5/jre/bin;.;C:/WINDOWS/system32;C:/WINDOWS;C:/WINDOWS/system32;C:/WINDOWS;C:/WINDOWS/System32/Wbem;C:/Program Files/Microsoft SQL Server/80/Tools/Binn/;C:/Program Files/Microsoft SQL Server/90/DTS/Binn/;C:/Program Files/Microsoft SQL Server/90/Tools/binn/;C:/Program Files/Microsoft SQL Server/90/Tools/Binn/VSShell/Common7/IDE/;C:/Program Files/Microsoft Visual Studio 8/Common7/IDE/PrivateAssemblies/;D:/program/devel/flex_sdk2/bin;D:/program/devel/ant/bin;C:/Program Files/Java/jdk1.6.0_07/bin;%JONAS_ROOT%/bin/nt;d:/program/devel/ant/bin
java.specification.name:Java Platform API Specification
java.class.version:49.0
sun.management.compiler:HotSpot Client Compiler
os.version:5.2
user.home:C:/Documents and Settings/Administrator
user.timezone:Asia/Shanghai
java.awt.printerjob:sun.awt.windows.WPrinterJob
file.encoding:GBK
java.specification.version:1.5
java.class.path:D:/dev/eclipse/mye65/workspace/jmx/bin;D:/program/lib/jmx/jmxtools.jar;D:/program/lib/log/commons-logging-1.1.1.jar;D:/program/lib/log/log4j-1.2.15.jar;D:/program/lib/registry/registry.jar
user.name:Administrator
java.vm.specification.version:1.0
java.home:D:/Program Files/MyEclipse 6.5/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:1.5.0_11
java.ext.dirs:D:/Program Files/MyEclipse 6.5/jre/lib/ext
sun.boot.class.path:D:/Program Files/MyEclipse 6.5/jre/lib/rt.jar;D:/Program Files/MyEclipse 6.5/jre/lib/i18n.jar;D:/Program Files/MyEclipse 6.5/jre/lib/sunrsasign.jar;D:/Program Files/MyEclipse 6.5/jre/lib/jsse.jar;D:/Program Files/MyEclipse 6.5/jre/lib/jce.jar;D:/Program Files/MyEclipse 6.5/jre/lib/charsets.jar;D:/Program Files/MyEclipse 6.5/jre/classes
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
个人觉得比較重要的系统属性：
java.home：Java 安装文件夹 (比方D:/JavaStudio/jdk15/jre) java.class.path：Java 类路径 java.vm.version：Java虚拟机版本(比方：1.5.0-b64） java.version：Java版本(比方：1.5.0)
os.name：操作系统的名称(比方：Windows XP) os.version：操作系统的版本号(比方：Windows XP 的版本号为5.1)
user.home：用户的主文件夹 (比方：C:/Documents and Settings/zhangsan) user.name：当前username user.country：当前用户所属国家 user.dir：用户的当前工作文件夹比較重要的环境变量（Windows操作系统）： %COMPUTERNAME% 返回计算机的名称。 %COMSPEC% 返回命令行解释器可运行程序的准确路径。
%WINDIR% 返回操作系统文件夹的位置。 %OS% 返回操作系统的名称。Windows 2000 将操作系统显示为 Windows_NT。 %PATH% 指定可运行文件的搜索路径。
%USERDOMAIN% 返回包括用户帐户的域的名称。 %USERNAME% 返回当前登录的用户的名称。
以下是一篇在jdk1.4中得到环境变量的方法：
In JDK 1.4 System.getenv() is deprecated. (In JDK 1.5 it is un-deprected again.) The getProperty method is now the correspoding method to get the variables. For example: System.getProperty("java.class.path",""). But it doesn't work for user defined environment variable. This problem can be solved by using the following code: import java.io.BufferedReader; import java.io.InputStreamReader; /** * Environment class simulates the System.getenv() method which is deprecated * on java 1.4.2 * * @author v-josp */ public class TestRoot { //result of all enviornment variables private static BufferedReader commandResult; static { String cmd = null; String os = null; //getting the OS name os = System.getProperty("os.name").toLowerCase(); // according to OS set the command to execute if(os.startsWith("windows")) { cmd = "cmd /c SET"; } else { cmd="env"; } try { //execute the command and get the result in the form of InputStream Process p = Runtime.getRuntime().exec(cmd); //parse the InputStream data InputStreamReader isr = new InputStreamReader(p.getInputStream()); commandResult= new BufferedReader(isr); } catch (Exception e) { System.out.println("OSEnvironment.class error: " + cmd + ":" + e); } } /** * This method is used to get the path of the given enviornment variable. This * method tries to simulates the System.getenv() which is deprecated on java 1.4.2 * * @param String - name of the environment variable * @param String - default value * @return */ public static String getenv(String envName,String defaultValue) { String line = null; try { while ((line = commandResult.readLine()) != null) { if(line.indexOf(envName)>-1) { return line.substring(line.indexOf(envName)+envName.length()+1); } } return defaultValue; } catch (Exception e) { return defaultValue; } }
public static void main(String args[]) { System.out.println(TestRoot.getenv("CLASSPATH","")); } }