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Recreating Linux traceroute for an Android app

I am extremely new the Android app development and Stack Overflow. I am trying to recreate traceroute in an Android app since Android devices do not come with traceroute by default. I've encountered a couple stack overflow posts talking about solutions to this, but I have still run into challenges.
Traceroute on android - the top post on this thread links an Android Studio project that implements traceroute using ping. If I understand the algorithm correctly, it continually pings the destination IP, incrementing the time-to-live field to obtain information about intermediary routers. I've tried to recreate this behavior, but for certain values of TTL, the ping stalls and doesn't retrieve any router information. I'm not really sure why this happens. Here's a quick demo function I spun up... at some point in the loop the pings stall.
public static void smallTracerouteDemoShowingThatTheProgramStallsAtCertainTTLs() {
try {
String host = "google.com";
int maxTTL = 20;
for (int i = 1; i < maxTTL; i++) {
// Create a process that executes the ping command
Process p = Runtime.getRuntime().exec("ping -c 1 -t " + i + " " + host);
// Get a buffered reader with the information returned by the ping
BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()));
// Convert the BufferedReader to a string
String dataReturnedByPing = "";
for (String line; (line = br.readLine()) != null; dataReturnedByPing += "\n"+line);
// Print out information about each TTL
System.out.println("TTL = " + i + " out of " + maxTTL);
System.out.println(dataReturnedByPing);
System.out.println("========================================");
}
} catch (Exception e) {
e.printStackTrace();
}
}
how to run traceroute command through your application? - The solution on this thread suggests using BusyBox. I've not used BusyBox as yet, but it seems like I would have to embed BusyBox into my app to get things to work. After doing some research it looks like BusyBox provides numerous Linux commands through one executable. I'm a bit hesitant to explore this option because I really only need the traceroute command. In addition, I know that Android targets a few different CPU architectures, and I'm not sure if one executable will support them all.
I've also run into a github repository that takes another approach to running traceroute:
https://github.com/wangjing53406/traceroute-for-android - In this repository the author embeds the traceroute source code into the project and uses the NDK to build the source code along with the rest of his app. I really like this approach because it feels the most "correct." It uses a built traceroute instead of a Java-based implementation, so you can't find yourself in a situation where the Java implementation gives you one thing and the actual traceroute gives you another. When I open this project to experiment with it, my build fails. The top line says:
org.gradle.initialization.ReportedException: org.gradle.internal.exceptions.LocationAwareException: A problem occurred configuring root project 'traceroute-for-android-master'.
Any help on why this happens or ways to troubleshoot it would be fantastic.
For reference, the minimum SDK I am targeting is API 21 and I am running on Android Studio 3.3.0.
So, at this point I'm stumped. If you were trying to make an app that would let you execute traceroute commands, how would you do it? I really like the NDK approach because it guarantees you're getting true traceroute behavior. If you have any guides to getting that set up for my Android version/SDK, I would appreciate if you would post them. If you'd take another approach I'd to hear about it as well.
Thank you in advance.

AS400 jt400 get RAM usage

I am just experimenting with the jt400.jar to receive system information from an AS400.
I figured out how to connect and how to receive values by using the class SystemStatus and how to read SystemValues. (only need to find an explanation for those values, any hints for me?)
Can anyone tell me, which of the functions in SystemStatus delivers me the usage of RAM or a poper way of receiving this information?
private static void getSystemStatus() throws AS400SecurityException, ErrorCompletingRequestException,
InterruptedException, IOException, ObjectDoesNotExistException, RequestNotSupportedException {
//Connect to AS400
AS400 as400 = new AS400("myAs400", "myUser", "myPassword");
//Reading SystemStatus like CPU usage and hdd usage
SystemStatus systemStatus = new SystemStatus(as400);
System.out.println(systemStatus.getPercentProcessingUnitUsed());
System.out.println(systemStatus.getActiveJobsInSystem());
//Reading SystemValues
SystemValueList sysValList = new SystemValueList(as400);
Vector<SystemValue> sysValVec = new Vector<SystemValue>();
sysValVec = sysValList.getGroup(SystemValueList.GROUP_ALL);
System.out.println("<<<< SystemValues >>>>");
for (SystemValue systemValue : sysValVec) {
String sysValName = systemValue.getName();
systemValue.getValue();
System.out.println("Value: " + sysValName + " - " + systemValue.getValue());
}
System.out.println("<<<< SystemValues >>>>");
}
I already read a lot of documentation but was not able to find anything.
http://www-01.ibm.com/support/knowledgecenter/ssw_ibm_i_71/rzahh/as400obj.htm
http://itknowledgeexchange.techtarget.com/itanswers/system-information-into-a-file/
http://www.ibm.com/developerworks/ibmi/library/i-javatoolbox/
Thanks in advance

I don't think you're going to find that information.
IBM i Operating System, (aka OS/400) doesn't see RAM & HDD separately. Instead, it sees a single large address space known as the single level store.
http://db2fori.blogspot.com/2012/11/one-of-crown-jewels-single-level-storage.html
Certainly the low level Technology Independent Machine Interface (TIMI) knows about RAM/SSD/HDD. But that's buried deep. IBM surfaces some of that information via it's own command such as
Work Disk Status (WRKDSKSTS)
Size %
Unit Type (M) Used
1 4327 52923 68.9
2 4327 52923 68.9
But memory is basically always 100% used. The system basically treats all RAM as a cache for objects from auxiliary (SSD/HDD) storage.

#Charles answered the question about RAM usage.
With respect to System Values, a System Value is a... configuration item for the host system. An example is QDATFMT which describes the way a data is displayed, 03-31-2016 or 31.03.6, etc. Generally, the System Administrator is most interested in System Values. The Knowledge Center explains System Values: http://www-01.ibm.com/support/knowledgecenter/ssw_ibm_i_72/rzakz/rzakz1.htm?lang=en

Is Windows 7 Bug Preventing Java getHostName() from Retrieving the Computer Name? [duplicate]

Which of the following is the best and most portable way to get the hostname of the current computer in Java?
Runtime.getRuntime().exec("hostname")
vs
InetAddress.getLocalHost().getHostName()

Strictly speaking - you have no choice but calling either hostname(1) or - on Unix gethostname(2). This is the name of your computer. Any attempt to determine the hostname by an IP address like this
InetAddress.getLocalHost().getHostName()
is bound to fail in some circumstances:
The IP address might not resolve into any name. Bad DNS setup, bad system setup or bad provider setup may be the reason for this.
A name in DNS can have many aliases called CNAMEs. These can only be resolved in one direction properly: name to address. The reverse direction is ambiguous. Which one is the "official" name?
A host can have many different IP addresses - and each address can have many different names. Two common cases are: One ethernet port has several "logical" IP addresses or the computer has several ethernet ports. It is configurable whether they share an IP or have different IPs. This is called "multihomed".
One Name in DNS can resolve to several IP Addresses. And not all of those addresses must be located on the same computer! (Usecase: A simple form of load-balancing)
Let's not even start talking about dynamic IP addresses.
Also don't confuse the name of an IP-address with the name of the host (hostname). A metaphor might make it clearer:
There is a large city (server) called "London". Inside the city walls much business happens. The city has several gates (IP addresses). Each gate has a name ("North Gate", "River Gate", "Southampton Gate"...) but the name of the gate is not the name of the city. Also you cannot deduce the name of the city by using the name of a gate - "North Gate" would catch half of the bigger cities and not just one city. However - a stranger (IP packet) walks along the river and asks a local: "I have a strange address: 'Rivergate, second left, third house'. Can you help me?" The local says: "Of course, you are on the right road, simply go ahead and you will arrive at your destination within half an hour."
This illustrates it pretty much I think.
The good news is: The real hostname is usually not necessary. In most cases any name which resolves into an IP address on this host will do. (The stranger might enter the city by Northgate, but helpful locals translate the "2nd left" part.)
In the remaining corner cases you must use the definitive source of this configuration setting - which is the C function gethostname(2). That function is also called by the program hostname.

InetAddress.getLocalHost().getHostName() is the more portable way.
exec("hostname") actually calls out to the operating system to execute the hostname command.
Here are a couple other related answers on SO:
Java current machine name and logged in user?
Get DNS name of local machine as seen by a remote machine
EDIT: You should take a look at A.H.'s answer or Arnout Engelen's answer for details on why this might not work as expected, depending on your situation. As an answer for this person who specifically requested portable, I still think getHostName() is fine, but they bring up some good points that should be considered.

As others have noted, getting the hostname based on DNS resolution is unreliable.
Since this question is unfortunately still relevant in 2018, I'd like to share with you my network-independent solution, with some test runs on different systems.
The following code tries to do the following:
On Windows
Read the COMPUTERNAME environment variable through System.getenv().
Execute hostname.exe and read the response
On Linux
Read the HOSTNAME environment variable through System.getenv()
Execute hostname and read the response
Read /etc/hostname (to do this I'm executing cat since the snippet already contains code to execute and read. Simply reading the file would be better, though).
The code:
public static void main(String[] args) throws IOException {
String os = System.getProperty("os.name").toLowerCase();
if (os.contains("win")) {
System.out.println("Windows computer name through env:\"" + System.getenv("COMPUTERNAME") + "\"");
System.out.println("Windows computer name through exec:\"" + execReadToString("hostname") + "\"");
} else if (os.contains("nix") || os.contains("nux") || os.contains("mac os x")) {
System.out.println("Unix-like computer name through env:\"" + System.getenv("HOSTNAME") + "\"");
System.out.println("Unix-like computer name through exec:\"" + execReadToString("hostname") + "\"");
System.out.println("Unix-like computer name through /etc/hostname:\"" + execReadToString("cat /etc/hostname") + "\"");
}
}
public static String execReadToString(String execCommand) throws IOException {
try (Scanner s = new Scanner(Runtime.getRuntime().exec(execCommand).getInputStream()).useDelimiter("\\A")) {
return s.hasNext() ? s.next() : "";
}
}
Results for different operating systems:
macOS 10.13.2
Unix-like computer name through env:"null"
Unix-like computer name through exec:"machinename
"
Unix-like computer name through /etc/hostname:""
OpenSuse 13.1
Unix-like computer name through env:"machinename"
Unix-like computer name through exec:"machinename
"
Unix-like computer name through /etc/hostname:""
Ubuntu 14.04 LTS
This one is kinda strange since echo $HOSTNAME returns the correct hostname, but System.getenv("HOSTNAME") does not:
Unix-like computer name through env:"null"
Unix-like computer name through exec:"machinename
"
Unix-like computer name through /etc/hostname:"machinename
"
EDIT: According to legolas108, System.getenv("HOSTNAME") works on Ubuntu 14.04 if you run export HOSTNAME before executing the Java code.
Windows 7
Windows computer name through env:"MACHINENAME"
Windows computer name through exec:"machinename
"
Windows 10
Windows computer name through env:"MACHINENAME"
Windows computer name through exec:"machinename
"
The machine names have been replaced but I kept the capitalization and structure. Note the extra newline when executing hostname, you might have to take it into account in some cases.

InetAddress.getLocalHost().getHostName() is better (as explained by Nick), but still not very good
One host can be known under many different hostnames. Usually you'll be looking for the hostname your host has in a specific context.
For example, in a web application, you might be looking for the hostname used by whoever issued the request you're currently handling. How to best find that one depends on which framework you're using for your web application.
In some kind of other internet-facing service, you'll want the hostname your service is available through from the 'outside'. Due to proxies, firewalls etc this might not even be a hostname on the machine your service is installed on - you might try to come up with a reasonable default, but you should definitely make this configurable for whoever installs this.

Although this topic has already been answered there's more to say.
First of all: Clearly we need some definitions here. The InetAddress.getLocalHost().getHostName() gives you the name of the host as seen from a network perspective. The problems with this approach are well documented in the other answers: it often requires a DNS lookup, it's ambiguous if the host has multiple network interfaces and it just plain fails sometimes (see below).
But on any OS there's another name as well. A name of the host that gets defined very early in the boot process, long before the network is initialized. Windows refers to this as computername, Linux calls it kernel hostname and Solaris uses the word nodename. I like best the word computername, so I'll use that word from now on.
Finding the computername
On Linux/Unix the computername is what you get from the C function gethostname(), or hostname command from shell or HOSTNAME environment variable in Bash-like shells.
On Windows the computername is what you get from environment variable COMPUTERNAME or Win32 GetComputerName function.
Java has no way of obtaining what I've defined as 'computername'. Sure, there are workarounds as described in other answers, like for Windows calling System.getenv("COMPUTERNAME"), but on Unix/Linux there's no good workaround without resorting to JNI/JNA or Runtime.exec(). If you don't mind a JNI/JNA solution then there's gethostname4j which is dead simple and very easy to use.
Let's move on with two examples, one from Linux and one from Solaris, which demonstrate how you can easily get into a situation where you cannot obtain the computername using standard Java methods.
Linux example
On a newly created system, where the host during installation has been named as 'chicago', we now change the so-called kernel hostname:
$ hostnamectl --static set-hostname dallas
Now the kernel hostname is 'dallas', as evident from the hostname command:
$ hostname
dallas
But we still have
$ cat /etc/hosts
127.0.0.1 localhost
127.0.0.1 chicago
There's no misconfiguration in this. It just means the host's networked name (or rather the name of the loopback interface) is different from the host's computername.
Now, try executing InetAddress.getLocalHost().getHostName() and it will throw java.net.UnknownHostException. You are basically stuck. There's no way to retrieve neither the value 'dallas' nor the value 'chicago'.
Solaris example
The example below is based on Solaris 11.3.
The host has deliberately been configured so that the loopback name <> nodename.
In other words we have:
$ svccfg -s system/identity:node listprop config
...
...
config/loopback astring chicago
config/nodename astring dallas
and the contents of /etc/hosts :
:1 chicago localhost
127.0.0.1 chicago localhost loghost
and the result of the hostname command would be:
$ hostname
dallas
Just like in the Linux example a call to InetAddress.getLocalHost().getHostName() will fail with
java.net.UnknownHostException: dallas: dallas: node name or service name not known
Just like the Linux example you are now stuck. There's no way to retrieve neither the value 'dallas' nor the value 'chicago'.
When will you really struggle with this?
Very often you'll find that InetAddress.getLocalHost().getHostName() will indeed return a value which is equal to the computername. So there's no problem (except for the added overhead of name resolution).
The problem arises typically within PaaS environments where there's a difference between computername and the name of the loopback interface. For example people report problems in Amazon EC2.
Bug/RFE reports
A bit of searching reveals this RFE report : link1, link2. However, judging from the comments on that report the issue seems to have been largely misunderstood by the JDK team, so it is unlikely it will be addressed.
I like the comparison in the RFE to other programming languages.

Just one-liner ... cross platform (Windows-Linux-Unix-Mac(Unix)) [Always works, No DNS required]:
String hostname = new BufferedReader(
new InputStreamReader(Runtime.getRuntime().exec("hostname").getInputStream()))
.readLine();
You're done !!

Environment variables may also provide a useful means -- COMPUTERNAME on Windows, HOSTNAME on most modern Unix/Linux shells.
See: https://stackoverflow.com/a/17956000/768795
I'm using these as "supplementary" methods to InetAddress.getLocalHost().getHostName(), since as several people point out, that function doesn't work in all environments.
Runtime.getRuntime().exec("hostname") is another possible supplement. At this stage, I haven't used it.
import java.net.InetAddress;
import java.net.UnknownHostException;
// try InetAddress.LocalHost first;
// NOTE -- InetAddress.getLocalHost().getHostName() will not work in certain environments.
try {
String result = InetAddress.getLocalHost().getHostName();
if (StringUtils.isNotEmpty( result))
return result;
} catch (UnknownHostException e) {
// failed; try alternate means.
}
// try environment properties.
//
String host = System.getenv("COMPUTERNAME");
if (host != null)
return host;
host = System.getenv("HOSTNAME");
if (host != null)
return host;
// undetermined.
return null;

The most portable way to get the hostname of the current computer in Java is as follows:
import java.net.InetAddress;
import java.net.UnknownHostException;
public class getHostName {
public static void main(String[] args) throws UnknownHostException {
InetAddress iAddress = InetAddress.getLocalHost();
String hostName = iAddress.getHostName();
//To get the Canonical host name
String canonicalHostName = iAddress.getCanonicalHostName();
System.out.println("HostName:" + hostName);
System.out.println("Canonical Host Name:" + canonicalHostName);
}
}

If you're not against using an external dependency from maven central, I wrote gethostname4j to solve this problem for myself. It just uses JNA to call libc's gethostname function (or gets the ComputerName on Windows) and returns it to you as a string.
https://github.com/mattsheppard/gethostname4j

hostName == null;
Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces();
{
while (interfaces.hasMoreElements()) {
NetworkInterface nic = interfaces.nextElement();
Enumeration<InetAddress> addresses = nic.getInetAddresses();
while (hostName == null && addresses.hasMoreElements()) {
InetAddress address = addresses.nextElement();
if (!address.isLoopbackAddress()) {
hostName = address.getHostName();
}
}
}
}

Building off of Dan Ortega's answer, I created a generic executeCommand(String) method that takes a command as a paramater.
import java.io.*;
public class SystemUtil {
public static void main(String[] args) throws IOException {
System.out.println(retrieveHostName());
}
public static String retrieveHostName() throws IOException {
return executeCommand("hostname");
}
private static String executeCommand(String command) throws IOException {
return new BufferedReader(
new InputStreamReader(Runtime.getRuntime().exec(command).getInputStream()))
.readLine();
}
}

InetAddress.getLocalHost().getHostName() is the best way out of the two as this is the best abstraction at the developer level.

Java & Windows 7: Reliably getting IPv4 netmask?

I've run into a known bug with Java 6 on Windows. My understanding is that the normal way to get the netmask is to look up the network prefix length and do some bit shifts. The problem is that on Windows the prefix length is often returned incorrectly, so we get a 128 when we should get a 24 or 20.
In this solution, it is suggested to put -Djava.net.preferIPv4Stack=true on the Java command line. Unfortunately, on Windows 7, adding that as either a VM parameter or on the Java command line seems to have no effect.
(a) Does anyone know any OTHER work-arounds for this problem that might still work on Windows 7?
(b) Alternatively, is there an entirely different way to get the netmask that is reliable?
Thanks!
P.S. Here is the bug report that pertains to this.

The -Djava.net.preferIPv4Stack=true VM option should work under any OS. Alternatively, it can be put into Java code as System.setProperty("java.net.preferIPv4Stack","true");. Unless, something (library or whatever) is resetting its true state.

The code below displays the subnet mask. On a computer with more than one network connection (like a laptop with a wireless and Cat-5 Ethernet connection) it may write the subnet mask twice because there can be two different IP addresses for the client.
String os = System.getProperty("os.name");
try {
if(os.indexOf("Windows 7")>=0) {
Process process = Runtime.getRuntime().exec("ipconfig");
process.waitFor();
InputStream commandOut= process.getInputStream();
BufferedReader in = new BufferedReader(new InputStreamReader(commandOut));
String line;
while((line = in.readLine()) !=null) {
if(line.indexOf("Subnet Mask")>=0) {
int colon = line.indexOf(":");
System.out.println(line.substring(colon+2));
}
}
}
catch(IOException ioe) { }
catch(java.lang.InterruptedException utoh) { }
On my laptop with both a wired and wireless connection active, I get this output:
255.255.254.0
255.255.254.0
When I turn off my wireless connection, I only see one line of output for the wired Ethernet link, as expected.

Since the problem us just in Windows 7, why not look for an OS specific solution?
I know we can launch windows programs from Java, including the windows command line or bat files. There must be a way to re-direct the output of ipconfig to a text file in windows. Your program should be able to get the subnet mask by calling ipconfig and then reading the output.

How to get a unique computer identifier in Java (like disk ID or motherboard ID)?

I'd like to get an id unique to a computer with Java, on Windows, MacOS and, if possible, Linux. It could be a disk UUID, motherboard S/N...
Runtime.getRuntime().exec can be used (it is not an applet).
Ideas?

The problem with MAC address is that there can be many network adapters connected to the computer. Most of the newest ones have two by default (wi-fi + cable). In such situation one would have to know which adapter's MAC address should be used. I tested MAC solution on my system, but I have 4 adapters (cable, WiFi, TAP adapter for Virtual Box and one for Bluetooth) and I was not able to decide which MAC I should take... If one would decide to use adapter which is currently in use (has addresses assigned) then new problem appears since someone can take his/her laptop and switch from cable adapter to wi-fi. With such condition MAC stored when laptop was connected through cable will now be invalid.
For example those are adapters I found in my system:
lo MS TCP Loopback interface
eth0 Intel(R) Centrino(R) Advanced-N 6205
eth1 Intel(R) 82579LM Gigabit Network Connection
eth2 VirtualBox Host-Only Ethernet Adapter
eth3 Sterownik serwera dostepu do sieci LAN Bluetooth
Code I've used to list them:
Enumeration<NetworkInterface> nis = NetworkInterface.getNetworkInterfaces();
while (nis.hasMoreElements()) {
NetworkInterface ni = nis.nextElement();
System.out.println(ni.getName() + " " + ni.getDisplayName());
}
From the options listen on this page, the most acceptable for me, and the one I've used in my solution is the one by #Ozhan Duz, the other one, similar to #finnw answer where he used JACOB, and worth mentioning is com4j - sample which makes use of WMI is available here:
ISWbemLocator wbemLocator = ClassFactory.createSWbemLocator();
ISWbemServices wbemServices = wbemLocator.connectServer("localhost","Root\\CIMv2","","","","",0,null);
ISWbemObjectSet result = wbemServices.execQuery("Select * from Win32_SystemEnclosure","WQL",16,null);
for(Com4jObject obj : result) {
ISWbemObject wo = obj.queryInterface(ISWbemObject.class);
System.out.println(wo.getObjectText_(0));
}
This will print some computer information together with computer Serial Number. Please note that all classes required by this example has to be generated by maven-com4j-plugin. Example configuration for maven-com4j-plugin:
<plugin>
<groupId>org.jvnet.com4j</groupId>
<artifactId>maven-com4j-plugin</artifactId>
<version>1.0</version>
<configuration>
<libId>565783C6-CB41-11D1-8B02-00600806D9B6</libId>
<package>win.wmi</package>
<outputDirectory>${project.build.directory}/generated-sources/com4j</outputDirectory>
</configuration>
<executions>
<execution>
<id>generate-wmi-bridge</id>
<goals>
<goal>gen</goal>
</goals>
</execution>
</executions>
</plugin>
Above's configuration will tell plugin to generate classes in target/generated-sources/com4j directory in the project folder.
For those who would like to see ready-to-use solution, I'm including links to the three classes I wrote to get machine SN on Windows, Linux and Mac OS:
Java code to get computer SN on Windows
Java code to get computer SN on Linux
Java code to get computer SN on Mac OS

The OSHI project provides platform-independent hardware utilities.
Maven dependency:
<dependency>
<groupId>com.github.oshi</groupId>
<artifactId>oshi-core</artifactId>
<version>LATEST</version>
</dependency>
For instance, you could use something like the following code to identify a machine uniquely:
import oshi.SystemInfo;
import oshi.hardware.CentralProcessor;
import oshi.hardware.ComputerSystem;
import oshi.hardware.HardwareAbstractionLayer;
import oshi.software.os.OperatingSystem;
class ComputerIdentifier
{
static String generateLicenseKey()
{
SystemInfo systemInfo = new SystemInfo();
OperatingSystem operatingSystem = systemInfo.getOperatingSystem();
HardwareAbstractionLayer hardwareAbstractionLayer = systemInfo.getHardware();
CentralProcessor centralProcessor = hardwareAbstractionLayer.getProcessor();
ComputerSystem computerSystem = hardwareAbstractionLayer.getComputerSystem();
String vendor = operatingSystem.getManufacturer();
String processorSerialNumber = computerSystem.getSerialNumber();
String processorIdentifier = centralProcessor.getIdentifier();
int processors = centralProcessor.getLogicalProcessorCount();
String delimiter = "#";
return vendor +
delimiter +
processorSerialNumber +
delimiter +
processorIdentifier +
delimiter +
processors;
}
public static void main(String[] arguments)
{
String identifier = generateLicenseKey();
System.out.println(identifier);
}
}
Output for my machine:
Microsoft#57YRD12#Intel64 Family 6 Model 60 Stepping 3#8
Your output will be different since at least the processor serial number will differ.

It is common to use the MAC address is associated with the network card.
The address is available in Java 6 through through the following API:
Java 6 Docs for Hardware Address
I haven't used it in Java, but for other network identification applications it has been helpful.

What do you want to do with this unique ID? Maybe you can do what you want without this ID.
The MAC address maybe is one option but this is not an trusted unique ID because the user can change the MAC address of a computer.
To get the motherboard or processor ID check on this link.

On Windows only, you can get the motherboard ID using WMI, through a COM bridge such as JACOB.
Example:
import java.util.Enumeration;
import com.jacob.activeX.ActiveXComponent;
import com.jacob.com.ComThread;
import com.jacob.com.EnumVariant;
import com.jacob.com.Variant;
public class Test {
public static void main(String[] args) {
ComThread.InitMTA();
try {
ActiveXComponent wmi = new ActiveXComponent("winmgmts:\\\\.");
Variant instances = wmi.invoke("InstancesOf", "Win32_BaseBoard");
Enumeration<Variant> en = new EnumVariant(instances.getDispatch());
while (en.hasMoreElements())
{
ActiveXComponent bb = new ActiveXComponent(en.nextElement().getDispatch());
System.out.println(bb.getPropertyAsString("SerialNumber"));
break;
}
} finally {
ComThread.Release();
}
}
}
And if you choose to use the MAC address to identify the machine, you can use WMI to determine whether an interface is connected via USB (if you want to exclude USB adapters.)
It's also possible to get a hard drive ID via WMI but this is unreliable.

Not Knowing all of your requirements. For example, are you trying to uniquely identify a computer from all of the computers in the world, or are you just trying to uniquely identify a computer from a set of users of your application. Also, can you create files on the system?
If you are able to create a file. You could create a file and use the creation time of the file as your unique id. If you create it in user space then it would uniquely identify a user of your application on a particular machine. If you created it somewhere global then it could uniquely identify the machine.
Again, as most things, How fast is fast enough.. or in this case, how unique is unique enough.

Be careful when using the MAC address as an identifier. I've experienced several gotchas:
On OS X, ethernet ports that are not active/up do not show up in the NetworkInterface.getNetworkInterfaces() Enumeration.
It's insanely easy to change a MAC address on cards if you've got appropriate OS privileges.
Java has a habit of not correctly identifying "virtual" interfaces. Even using the NetworkInterface.isVirtual() won't always tell you the truth.
Even with the above issues, I still think it's the best pure Java approach to hardware locking a license.

I think you should look at this link ... you can make a mixed key using several
identifiers such as mac+os+hostname+cpu id+motherboard serial number.

The usage of MAC id is most easier way if the task is about logging the unique id a system.
the change of mac id is though possible, even the change of other ids of a system are also possible is that respective device is replaced.
so, unless what for a unique id is required is not known, we may not be able to find an appropriate solution.
However, the below link is helpful extracting mac addresses.
http://www.stratos.me/2008/07/find-mac-address-using-java/

For identifying a windows machine uniquely.
Make sure when you use wmic to have a strategy of alternative methods. Since "wmic bios get serialnumber" might not work on all machines, you might need to have additional methods:
# Get serial number from bios
wmic bios get serialnumber
# If previous fails, get UUID
wmic csproduct get UUID
# If previous fails, get diskdrive serialnumber
wmic DISKDRIVE get SerialNumber
Resources:
The Best Way To Uniquely Identify A Windows Machine
http://www.nextofwindows.com/the-best-way-to-uniquely-identify-a-windows-machine/

In the java programs I have written for release I used the motherboard serial number (which is what I beleive windows use); however, this only works on windows as my function creates a temporary VB script which uses the WMI to retrieve the value.
public static String getMotherboardSerial() {
String result = "";
try {
File file = File.createTempFile("GetMBSerial",".vbs");
file.deleteOnExit();
FileWriter fw = new FileWriter(file);
String vbs =
"Set objWMIService = GetObject(\"winmgmts:\\\\.\\root\\cimv2\")\n"
+ "Set colItems = objWMIService.ExecQuery _ \n"
+ " (\"Select * from Win32_ComputerSystemProduct\") \n"
+ "For Each objItem in colItems \n"
+ " Wscript.Echo objItem.IdentifyingNumber \n"
+ "Next \n";
fw.write(vbs);
fw.close();
Process gWMI = Runtime.getRuntime().exec("cscript //NoLogo " + file.getPath());
BufferedReader input = new BufferedReader(new InputStreamReader(gWMI.getInputStream()));
String line;
while ((line = input.readLine()) != null) {
result += line;
System.out.println(line);
}
input.close();
}
catch(Exception e){
e.printStackTrace();
}
result = result.trim();
return result;
}