Today, we will be learning how to get a Mac address off of your machine and that Java's memory management
does not work on external processes run by the operating system even if they were invoked by a call from
within a running Java class.

If we do not destroy the process that we started by calling the operating system it will
remain open in memory even after we finish running our program in the JVM.

For this example we will assume that we could be dealing with
the Windows operating system
or
the Linux operating system.

1   import java.io.*;
2   import java.util.regex.*;
3 
4   public class MyMac {
5 
6     // main method
7     public static void main(String[] args) {
8 
9       // create an instance
10      MyMac mac1 = new MyMac();
11 
12      // call the getMacAddress() method and print the value returned
13      System.out.println(mac1.getMacAddress());
14    }
15 
16    // getMacAddress() method
17    private StringBuffer getMacAddress() {
18 
19      // StringBuffer to be used to return info about the Mac address
20      StringBuffer myMacAddress = new StringBuffer();
21 
22      // The Runtime.exec() method returns an instance of a subclass of Process
23      Process myProc = null;
24  
25      // surround with a try catch block
26      try {
27  
28        // we will be walking through each line of
29        // output produced by invoking a command line process
30        // so we will store them one at a time in this String
31        String currentLine = "";
32 
33        // the operating systems name as referenced by the System
34        String osName = System.getProperty("os.name");
35  
36        // a regular expression used to match the area of text we want
37        String macRegExp = "";
38  
39        if(osName.startsWith("Windows")) { // Windows operating system will run this
40  
41          // the regular expression we will be matching for the Mac address on Windows
42          macRegExp = "[\\da-zA-Z]{1,2}\\-[\\da-zA-Z]{1,2}\\-[\\da-zA-Z]" + 
43                      "{1,2}\\-[\\da-zA-Z]{1,2}\\-[\\da-zA-Z]{1,2}\\-[\\da-zA-Z]{1,2}";
44          // get the live Process returned through the call to the
45          // Runtime.getRuntime().exec method and invoking
46          // the Windows command ipconfig /all
47          myProc = Runtime.getRuntime().exec("ipconfig /all");
48        }
49        else if(osName.startsWith("Linux")) { // Linux operating system runs this
50  
51          // the regular expression we will be matching for the Mac address on Linux
52          macRegExp = "[\\da-zA-Z]{1,2}\\:[\\da-zA-Z]{1,2}\\:[\\da-zA-Z]" + 
53                      "{1,2}\\:[\\da-zA-Z]{1,2}\\:[\\da-zA-Z]{1,2}\\:[\\da-zA-Z]{1,2}";
54          // get the live Process returned through the call to the
55          // Runtime.getRuntime().exec method and invoking
56          // the Linux command /sbin/ifconfig -a
57          myProc = Runtime.getRuntime().exec("/sbin/ifconfig -a");
58        }
59        else{
60        // other operating systems are not supported
61          // we are done here
62          myMacAddress.append("Only works on Windows or Linux systems");
63   
64          // exiting method now
65          return myMacAddress;
66        }
67   
68        myMacAddress.append("Mac address(es)");
69   
70        // we'll wrap a buffer around the InputStream we get from the "myProc" Process
71        BufferedReader in = new BufferedReader(
72                      new InputStreamReader(myProc.getInputStream()));
73        // compile the macRegExp string into a Pattern
74        Pattern macPattern = Pattern.compile(".*" + macRegExp + ".*");
75   
76        // a Matcher object for matching the regular expression to the string
77        Matcher macMtch = null;
78   
79        while((currentLine = in.readLine()) != null) { 
80         
81          // walk through each line and try to match the pattern
82          macMtch = macPattern.matcher(currentLine);
83   
84          if(macMtch.matches()) {
85   
86            // it matched so we split the line
87            String[] splitLine = currentLine.split(macRegExp);
88   
89            for(int a = 0; a < splitLine.length; a++) {
90              // REPLACE ALL PORTIONS of the currentLine 
91              // that do not match the expression 
92              // with an empty string
93              currentLine = currentLine.replaceAll(splitLine[a].toString(), "");
94            }
95             
96            // mac address(es) returned in the StringBuffer
97            myMacAddress.append(System.getProperty("line.separator") + currentLine);
98            
99            // reset the matcher just in case we have more than one mac address 
100           macMtch.reset();
101           // after all there could more than one NIC card!
102         }
103       }
104        
105       // last but 
106       // MOST IMPORTANT !!!!!!!!!!!!
107       // we must destroy the process 
108       // or the "operating system" 
109       // won't "close the process" or
110       // clean up "it's memory allocation"
111       myProc.destroy();
112     }
113     catch(java.io.IOException e) {
114       e.printStackTrace();
115     }
116     return myMacAddress;
117   }
118 }