Read environment variables from an application[转]

从应用程序获取环境变量Read environment variables from an application

 

NOTE: JDK1.5 or better provides a simpler way to achieve this, see this HowTo .

JDK up to 1.4
Start the JVM with the "-D" switch to pass properties to the application and read them with the System.getProperty() method.

SET myvar=Hello world
SET myothervar=nothing
java -Dmyvar="%myvar%" -Dmyothervar="%myothervar%" myClass

then in myClass

String myvar = System.getProperty("myvar");
String myothervar = System.getProperty("myothervar");

If you don't know in advance, the name of the variable to be passed to the JVM, then there is no 100% Java way to retrieve them.

---

One approach (not the easiest one), is to use a JNI call to fetch the variables, see this HowTo .

---

A more low-tech way, is to launch the appropriate call to the operating system and capture the output. The following snippet puts all environment variables in a Properties class and display the value the TEMP variable.

import java.io.*;
import java.util.*;

public class ReadEnv {
 public static Properties getEnvVars() throws Throwable {
  Process p = null;
  Properties envVars = new Properties();
  Runtime r = Runtime.getRuntime();
  String OS = System.getProperty("os.name").toLowerCase();
  // System.out.println(OS);
  if (OS.indexOf("windows 9") > -1) {
    p = r.exec( "command.com /c set" );
    }
  else if ( (OS.indexOf("nt") > -1)
         || (OS.indexOf("windows 2000") > -1 )
         || (OS.indexOf("windows xp") > -1) ) {
    // thanks to JuanFran for the xp fix!
    p = r.exec( "cmd.exe /c set" );
    }
  else {
    // our last hope, we assume Unix (thanks to H. Ware for the fix)
    p = r.exec( "env" );
    }
  BufferedReader br = new BufferedReader
     ( new InputStreamReader( p.getInputStream() ) );
  String line;
  while( (line = br.readLine()) != null ) {
   int idx = line.indexOf( '=' );
   String key = line.substring( 0, idx );
   String value = line.substring( idx+1 );
   envVars.setProperty( key, value );
   // System.out.println( key + " = " + value );
   }
  return envVars;
  }

  public static void main(String args[]) {
   try {
     Properties p = ReadEnv.getEnvVars();
     System.out.println("the current value of TEMP is : " +
        p.getProperty("TEMP"));
     }
   catch (Throwable e) {
     e.printStackTrace();
     }
   }
}

thanks to w.rijnders for the w2k fix.

---

An update from Van Ly :

I found that, on Windows 2003 server, the property value for "os.name" is actually "windows 2003." So either that has to be added to the bunch of tests or just relax the comparison strings a bit:

  
else if ( (OS.indexOf("nt") > -1)
  || (OS.indexOf("windows 2000") > -1 )
  || (OS.indexOf("windows 2003") > -1 )  // ok 
                                         // but specific to 2003
  || (OS.indexOf("windows xp") > -1) ) {

 

  
else if ( (OS.indexOf("nt") > -1)
  || (OS.indexOf("windows 20") > -1 )  // better, 
                                       // since no other OS would 
                                       // return "windows"
  || (OS.indexOf("windows xp") > -1) ) {

I started with "windows 200" but thought "what the hell" and made it "windows 20" to lengthen its longivity. You could push it further and use "windows 2," I suppose. The only thing to watch out for is to not overlap with "windows 9."

---

On Windows, pre-JDK 1.2 JVM has trouble reading the Output stream directly from the SET command, it never returns. Here 2 ways to bypass this behaviour.

First, instead of calling directly the SET command, we use a BAT file, after the SET command we print a known string. Then, in Java, when we read this known string, we exit from loop.

[env.bat]
@set
@echo **end

[java]
...
  if (OS.indexOf("windows") > -1) {
    p = r.exec( "env.bat" );
    }
...

  while( (line = br.readLine()) != null ) {
   if (line.indexOf("**end")>-1) break;
   int idx = line.indexOf( '=' );
   String key = line.substring( 0, idx );
   String value = line.substring( idx+1 );
   hash.put( key, value );
   System.out.println( key + " = " + value );
   }

The other solution is to send the result of the SET command to file and then read the file from Java.

...
if (OS.indexOf("windows 9") > -1) {
    p = r.exec( "command.com /c set > envvar.txt" );
    }
else if ( (OS.indexOf("nt") > -1)
       || (OS.indexOf("windows 2000") > -1
       || (OS.indexOf("windows xp") > -1) ) {
    // thanks to JuanFran for the xp fix!
    p = r.exec( "cmd.exe /c set > envvar.txt" );
    }
...

// then read back the file
Properties p = new Properties();
      p.load(new FileInputStream("envvar.txt"));

Thanks to JP Daviau

---

 

// UNIX
public Properties getEnvironment() throws java.io.IOException {
    Properties env = new Properties();
    env.load(Runtime.getRuntime().exec("env").getInputStream());
    return env;
    }

Properties env = getEnvironment();
String myEnvVar = env.get("MYENV_VAR");

---

To read only one variable :

// NT version , adaptation for other OS is left as an exercise...
Process p = Runtime.getRuntime().exec("cmd.exe /c echo %MYVAR%");
BufferedReader br = new BufferedReader
     ( new InputStreamReader( p.getInputStream() ) );
String myvar = br.readLine();
System.out.println(myvar);

---

Java's System properties contains some useful informations about the environment, for example, the TEMP and PATH environment variables (on Windows).

public class ShowSome {
  public static void main(String args[]){
    System.out.println("TEMP : " 
      + System.getProperty("java.io.tmpdir"));
    System.out.println("PATH : " 
      + System.getProperty("java.library.path"));
    System.out.println("CLASSPATH : " 
      + System.getProperty("java.class.path"));
    System.out.println("SYSTEM DIR : " +
       System.getProperty("user.home")); // ex. c:\windows on Win9x
    System.out.println("CURRENT DIR: "  
      + System.getProperty("user.dir"));
    }
  }

Here some tips from H. Ware about the PATH on different OS.

PATH is not quite the same as library path. In unixes, they are completely different---the libraries typically have their own directories.

System.out.println("the current value of PATH is:  {" +
    p.getProperty("PATH")+"}");

System.out.println("LIBPATH:  {" +
   System.getProperty("java.library.path")+"}");

gives

the current value of PATH is:
{/home/hware/bin:/usr/local/bin:/usr/xpg4/bin:/opt/SUNWspro/bin:
 /usr/ucb:/bin:/usr/bin:/home/hware/linux-bin:/usr/openwin/bin/:
 /usr/local/games:/usr/ccs/lib/:/usr/new:/usr/sbin/:/sbin/:
 /usr/openwin/lib:/usr/X11/bin:/usr/bin/X11/:/usr/local/bin/X11:
 /usr/bin/pbmplus:/usr/etc/:/usr/dt/bin/:/usr/lib:
 /usr/lib/nis:/usr/share/bin:/usr/share/bin/X11:
 /home/hware/work/cdk/main/cdk/../bin:.}
LIBPATH:
{/usr/lib/j2re1.3/lib/i386:/usr/lib/j2re1.3/lib/i386/native_threads:
/usr/lib/j2re1.3/lib/i386/client:/usr/lib/j2sdk1.3/lib/i386:/usr/lib:/lib}

on my linux workstation. (java added all those except /lib and /usr/lib). But these two lines aren't the same on window either:

This system is windows nt

the current value of PATH is:
{d:\OrbixWeb3.2\bin;D:\jdk1.3\bin;c:\depot\cdk\main\cdk\bin;c:\depot\
cdk\main\cdk\..\bin;d:\OrbixWeb3.2\bin;D:\Program
Files\IBM\GSK\lib;H:\pvcs65\VM\win32\bin;c:\cygnus
\cygwin-b20\H-i586-cygwin32\bin;d:\cfn\bin;D:\orant\bin;
C:\WINNT\system32;C:\WINNT;
d:\Program Files\Symantec\pcAnywhere;
C:\Program Files\Executive Software\DiskeeperServer\;}
LIBPATH:
{D:\jdk1.3\bin;.;C:\WINNT\System32;C:\WINNT;D:\jdk1.3\bin;
c:\depot\cdk\main\cdk\bin;c:\depot\cdk\main\cdk\..\bin;
d:\OrbixWeb3.2\bin;D:\Program Files\IBM\GSK\lib;
H:\pvcs65\VM\win32\bin;c:\cygnus\cygwin-b20\H-i586-cygwin32\bin;d:\cfn\bin;
D:\orant\bin;C:\WINNT\system32;
C:\WINNT;C:\Program Files\Dell\OpenManage\ResolutionAssistant\Common\bin;
d:\Program Files\Symantec\pcAnywhere;
C:\Program Files\Executive Software\DiskeeperServer\;}

Java is prepending itself! That confused me--- and broke my exec from ant.