The Thread class provides methods to change and get the name of a thread. By default, each thread has a name i.e. thread-0, thread-1 and so on. By we can change the name of the thread by using setName() method. The syntax of setName() and getName() methods are given below:
1. public String getName(): is used to return the name of a thread.
2. public void setName(String name): is used to change the name of a thread.

Example of naming a thread

1. class TestMultiNaming1 extends Thread{
2. public void run(){
3. System.out.println(“running…”);
4. }
5. public static void main(String args[]){
6. TestMultiNaming1 t1=new TestMultiNaming1();
7. TestMultiNaming1 t2=new TestMultiNaming1();
8. System.out.println(“Name of t1:”+t1.getName());
9. System.out.println(“Name of t2:”+t2.getName());
10.
11. t1.start();
12. t2.start();
13.
14. t1.setName(“Sonoo Jaiswal”);
15. System.out.println(“After changing name of t1:”+t1.getName());
16. }
17. }

Output:Name of t1:Thread-0
Name of t2:Thread-1
id of t1:8
running…
After changeling name of t1:Sonoo Jaiswal
running…

Current Thread

The currentThread() method returns a reference of currently executing thread.
1. public static Thread currentThread()

Example of currentThread() method

1. class TestMultiNaming2 extends Thread{
2. public void run(){
3. System.out.println(Thread.currentThread().getName());
4. }
5. public static void main(String args[]){
6. TestMultiNaming2 t1=new TestMultiNaming2();
7. TestMultiNaming2 t2=new TestMultiNaming2();
8.
9. t1.start();
10. t2.start();
11. }
12. }

Output:Thread-0
Thread-1

Priority of a Thread (Thread Priority):

Each thread have a priority. Priorities are represented by a number between 1 and 10. In most cases, thread schedular schedules the threads according to their priority (known as preemptivescheduling). But it is not guaranteed because it depends on JVM specification that which scheduling it chooses.

3 constants defined in Thread class:

1. public static int MIN_PRIORITY
2. public static int NORM_PRIORITY
3. public static int MAX_PRIORITY
Default priority of a thread is 5 (NORM_PRIORITY). The value of MIN_PRIORITY is 1 and the value of MAX_PRIORITY is 10.

Example of priority of a Thread:

1. class TestMultiPriority1 extends Thread{
2. public void run(){
3. System.out.println(“running thread name is:”+Thread.currentThread().getName());
4. System.out.println(“running thread priority is:”+Thread.currentThread().getPriority());
5.
6. }
7. public static void main(String args[]){
8. TestMultiPriority1 m1=new TestMultiPriority1();
9. TestMultiPriority1 m2=new TestMultiPriority1();
10. m1.setPriority(Thread.MIN_PRIORITY);
11. m2.setPriority(Thread.MAX_PRIORITY);
12. m1.start();
13. m2.start();
14.
15. }
16. }

Output:running thread name is:Thread-0
running thread priority is:10
running thread name is:Thread-1
running thread priority is:1

Next TopicDaemon Thread

Daemon Thread in Java

Daemon thread in java is a service provider thread that provides services to the user thread. Its life depend on the mercy of user threads i.e. when all the user threads dies, JVM terminates this thread automatically.
There are many java daemon threads running automatically e.g. gc, finalizer etc.
You can see all the detail by typing the jconsole in the command prompt. The jconsole tool provides information about the loaded classes, memory usage, running threads etc.

Points to remember for Daemon Thread in Java

• It provides services to user threads for background supporting tasks. It has no role in life than to serve user threads.
• Its life depends on user threads.
• It is a low priority thread.

Why JVM terminates the daemon thread if there is no user thread?

The sole purpose of the daemon thread is that it provides services to user thread for background supporting task. If there is no user thread, why should JVM keep running this thread. That is why JVM terminates the daemon thread if there is no user thread.

Methods for Java Daemon thread by Thread class

The java.lang.Thread class provides two methods for java daemon thread.

No.	Method	Description
1)	public void setDaemon(boolean status)	is used to mark the current thread as daemon thread or user thread.
2)	public booleanisDaemon()	is used to check that current is daemon.

Simple example of Daemon thread in java

File: MyThread.java
1. public class TestDaemonThread1 extends Thread{
2. public void run(){
3. if(Thread.currentThread().isDaemon()){//checking for daemon thread
4. System.out.println(“daemon thread work”);
5. }
6. else{
7. System.out.println(“user thread work”);
8. }
9. }
10. public static void main(String[] args){
11. TestDaemonThread1 t1=new TestDaemonThread1();//creating thread
12. TestDaemonThread1 t2=new TestDaemonThread1();
13. TestDaemonThread1 t3=new TestDaemonThread1();
14.
15. t1.setDaemon(true);//now t1 is daemon thread
16.
17. t1.start();//starting threads
18. t2.start();
19. t3.start();
20. }
21. }

Output
daemon thread work
user thread work
user thread work
Note: If you want to make a user thread as Daemon, it must not be started otherwise it will throw IllegalThreadStateException.
File: MyThread.java
1. class TestDaemonThread2 extends Thread{
2. public void run(){
3. System.out.println(“Name: “+Thread.currentThread().getName());
4. System.out.println(“Daemon: “+Thread.currentThread().isDaemon());
5. }
6.
7. public static void main(String[] args){
8. TestDaemonThread2 t1=new TestDaemonThread2();
9. TestDaemonThread2 t2=new TestDaemonThread2();
10. t1.start();
11. t1.setDaemon(true);//will throw exception here
12. t2.start();
13. }
14. }

Output:exception in thread main: java.lang.IllegalThreadStateException

Java Thread Pool

Java Thread pool represents a group of worker threads that are waiting for the job and reuse many times.
In case of thread pool, a group of fixed size threads are created. A thread from the thread pool is pulled out and assigned a job by the service provider. After completion of the job, thread is contained in the thread pool again.

Advantage of Java Thread Pool

Better performance It saves time because there is no need to create new thread.

Real time usage

It is used in Servlet and JSP where container creates a thread pool to process the request.

Example of Java Thread Pool

Let’s see a simple example of java thread pool using ExecutorService and Executors.
File: WorkerThread.java
1. import java.util.concurrent.ExecutorService;
2. import java.util.concurrent.Executors;
3. class WorkerThread implements Runnable {
4. private String message;
5. public WorkerThread(String s){
6. this.message=s;
7. }
8. public void run() {
9. System.out.println(Thread.currentThread().getName()+” (Start) message = “+message);
10. processmessage();//call processmessage method that sleeps the thread for 2 seconds
11. System.out.println(Thread.currentThread().getName()+” (End)”);//prints thread name
12. }
13. private void processmessage() {
14. try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); }
15. }
16. }
File: JavaThreadPoolExample.java
1. public class TestThreadPool {
2. public static void main(String[] args) {
3. ExecutorService executor = Executors.newFixedThreadPool(5);//creating a pool of 5 threads
4. for (int i = 0; i < 10; i++) {
5. Runnable worker = new WorkerThread(“” + i);
6. executor.execute(worker);//calling execute method of ExecutorService
7. }
8. executor.shutdown();
9. while (!executor.isTerminated()) { }
10.
11. System.out.println(“Finished all threads”);
12. }
13. }
Output:
pool-1-thread-1 (Start) message = 0
pool-1-thread-2 (Start) message = 1
pool-1-thread-3 (Start) message = 2
pool-1-thread-5 (Start) message = 4
pool-1-thread-4 (Start) message = 3
pool-1-thread-2 (End)
pool-1-thread-2 (Start) message = 5
pool-1-thread-1 (End)
pool-1-thread-1 (Start) message = 6
pool-1-thread-3 (End)
pool-1-thread-3 (Start) message = 7
pool-1-thread-4 (End)
pool-1-thread-4 (Start) message = 8
pool-1-thread-5 (End)
pool-1-thread-5 (Start) message = 9
pool-1-thread-2 (End)
pool-1-thread-1 (End)
pool-1-thread-4 (End)
pool-1-thread-3 (End)
pool-1-thread-5 (End)
Finished all threads

ThreadGroup in Java

Java provides a convenient way to group multiple threads in a single object. In such way, we can suspend, resume or interrupt group of threads by a single method call.
Note: Now suspend(), resume() and stop() methods are deprecated.
Java thread group is implemented by java.lang.ThreadGroup class.
A ThreadGroup represents a set of threads. A thread group can also include the other thread group. The thread group creates a tree in which every thread group except the initial thread group has a parent.
A thread is allowed to access information about its own thread group, but it cannot access the information about its thread group’s parent thread group or any other thread groups.

Constructors of ThreadGroup class

There are only two constructors of ThreadGroup class.

No.	Constructor	Description
1)	ThreadGroup(String name)	creates a thread group with given name.
2)	ThreadGroup(ThreadGroup parent, String name)	creates a thread group with given parent group and name.

Methods of ThreadGroup class

There are many methods in ThreadGroup class. A list of ThreadGroup methods are given below.

S.N.	Modifier and Type	Method	Description
1)	void	checkAccess()	This method determines if the currently running thread has permission to modify the thread group.
2)	int	activeCount()	This method returns an estimate of the number of active threads in the thread group and its subgroups.
3)	int	activeGroupCount()	This method returns an estimate of the number of active groups in the thread group and its subgroups.
4)	void	destroy()	This method destroys the thread group and all of its subgroups.
5)	int	enumerate(Thread[] list)	This method copies into the specified array every active thread in the thread group and its subgroups.
6)	int	getMaxPriority()	This method returns the maximum priority of the thread group.
7)	String	getName()	This method returns the name of the thread group.
8)	ThreadGroup	getParent()	This method returns the parent of the thread group.
9)	void	interrupt()	This method interrupts all threads in the thread group.
10)	boolean	isDaemon()	This method tests if the thread group is a daemon thread group.
11)	void	setDaemon(boolean daemon)	This method changes the daemon status of the thread group.
12)	boolean	isDestroyed()	This method tests if this thread group has been destroyed.
13)	void	list()	This method prints information about the thread group to the standard output.
14)	boolean	parentOf(ThreadGroup g	This method tests if the thread group is either the thread group argument or one of its ancestor thread groups.
15)	void	suspend()	This method is used to suspend all threads in the thread group.
16)	void	resume()	This method is used to resume all threads in the thread group which was suspended using suspend() method.
17)	void	setMaxPriority(int pri)	This method sets the maximum priority of the group.
18)	void	stop()	This method is used to stop all threads in the thread group.
19)	String	toString()	This method returns a string representation of the Thread group.

Let’s see a code to group multiple threads.
1. ThreadGroup tg1 = new ThreadGroup(“Group A”);
2. Thread t1 = new Thread(tg1,new MyRunnable(),”one”);
3. Thread t2 = new Thread(tg1,new MyRunnable(),”two”);
4. Thread t3 = new Thread(tg1,new MyRunnable(),”three”);
Now all 3 threads belong to one group. Here, tg1 is the thread group name, MyRunnable is the class that implements Runnable interface and “one”, “two” and “three” are the thread names.
Now we can interrupt all threads by a single line of code only.
1. Thread.currentThread().getThreadGroup().interrupt();

ThreadGroup Example

File: ThreadGroupDemo.java
1. public class ThreadGroupDemo implements Runnable{
2. public void run() {
3. System.out.println(Thread.currentThread().getName());
4. }
5. public static void main(String[] args) {
6. ThreadGroupDemo runnable = new ThreadGroupDemo();
7. ThreadGroup tg1 = new ThreadGroup(“Parent ThreadGroup”);
8.
9. Thread t1 = new Thread(tg1, runnable,”one”);
10. t1.start();
11. Thread t2 = new Thread(tg1, runnable,”two”);
12. t2.start();
13. Thread t3 = new Thread(tg1, runnable,”three”);
14. t3.start();
15.
16. System.out.println(“Thread Group Name: “+tg1.getName());
17. tg1.list();
18.
19. }
20. }
Output:
one
two
three
Thread Group Name: Parent ThreadGroup
java.lang.ThreadGroup[name=Parent ThreadGroup,maxpri=10]
Thread[one,5,Parent ThreadGroup]
Thread[two,5,Parent ThreadGroup]
Thread[three,5,Parent ThreadGroup]

So, this brings us to the end of blog. This Tecklearn ‘Deep Dive into Threads in Java’ blog helps you with commonly asked questions if you are looking out for a job in Java Programming. If you wish to learn Java and build a career Java Programming domain, then check out our interactive, Java and JEE Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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