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Memory and the Java Runtime.exec Process |
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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.
The very most important part of this exercise is found on line #111
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 }
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