Any application can have multiple processes (instances). Each of this process can be assigned either as a single thread or multiple threads. We will see in this tutorial how to perform multiple tasks at the same time and also learn more about threads and synchronization between threads.


What is Single Thread?

A single thread is basically a lightweight and the smallest unit of processing. Java uses threads by using a "Thread Class".

There are two types of thread – user thread and daemon thread (daemon threads are used when we want to clean the application and are used in the background).

When an application first begins, user thread is created. Post that, we can create many user threads and daemon threads.

Single Thread Example:

package demotest;
 
public class Sample
{
public static void main(String[] args) {
System.out.println("Single Thread");
}
}


Advantages of single thread:

  • Reduces overhead in the application as single thread execute in the system
  • Also, it reduces the maintenance cost of the application.

What is Multithreading in Java?

MULTITHREADING in Java is a process of executing two or more threads simultaneously to maximum utilization of CPU. Multithreaded applications execute two or more threads run concurrently. Hence, it is also known as Concurrency in Java. Each thread runs parallel to each other. Mulitple threads don't allocate separate memory area, hence they save memory. Also, context switching between threads takes less time.

Example of Multi thread:

package demotest;
 
public class Sample implements Runnable
{
public static void main(String[] args) {
Thread sampleThread1 = new Thread("sample1");
Thread sampleThread2 = new Thread("sample2");
sampleThread1.start();
sampleThread2.start();
System.out.println("Thread names are following:");
System.out.println(sampleThread1.getName());
System.out.println(sampleThread2.getName());
}
@Override
public void run() {
}
 
}


Advantages of multithread:

  • The users are not blocked because threads are independent, and we can perform multiple operations at times
  • As such the threads are independent, the other threads won't get affected if one thread meets an exception.

Thread Life Cycle in Java

The Lifecycle of a thread:

Thread Life Cycle

There are various stages of life cycle of thread as shown in above diagram:

  1. New
  2. Runnable
  3. Running
  4. Waiting
  5. Dead
  1. New: In this phase, the thread is created using class "Thread class".It remains in this state till the program starts the thread. It is also known as born thread.
  2. Runnable: In this page, the instance of the thread is invoked with a start method. The thread control is given to scheduler to finish the execution. It depends on the scheduler, whether to run the thread.
  3. Running: When the thread starts executing, then the state is changed to "running" state. The scheduler selects one thread from the thread pool, and it starts executing in the application.
  4. Waiting: This is the state when a thread has to wait. As there multiple threads are running in the application, there is a need for synchronization between threads. Hence, one thread has to wait, till the other thread gets executed. Therefore, this state is referred as waiting state.
  5. Dead: This is the state when the thread is terminated. The thread is in running state and as soon as it completed processing it is in "dead state".

Some of the commonly used methods for threads are:

MethodDescription
start()This method starts the execution of the thread and JVM calls the run() method on the thread.
Sleep(int milliseconds)This method makes the thread sleep hence the thread's execution will pause for milliseconds provided and after that, again the thread starts executing. This help in synchronization of the threads.
getName()It returns the name of the thread.
setPriority(int newpriority)It changes the priority of the thread.
yield ()It causes current thread on halt and other threads to execute.

Example: In this example we are going to create a thread and explore built-in methods available for threads.

package demotest;
public class thread_example1 implements Runnable {
@Override
public void run() {
}
public static void main(String[] args) {
Thread samplethread1 = new Thread();
samplethread1.start();
try {
samplethread1.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
samplethread1.setPriority(1);
int samplepriority = samplethread1.getPriority();
System.out.println(samplepriority);
System.out.println("Thread Running");
}
}


When you execute the above code, you get the following output:

Multithreading in Java

Output:

5 is the Thread priority, and Thread Running is the text which is the output of our code.

Java Thread Synchronization

In multithreading, there is the asynchronous behavior of the programs. If one thread is writing some data and another thread which is reading data at the same time, might create inconsistency in the application.

When there is a need to access the shared resources by two or more threads, then synchronization approach is utilized.

Java has provided synchronized methods to implement synchronized behavior.

In this approach, once the thread reaches inside the synchronized block, then no other thread can call that method on the same object. All threads have to wait till that thread finishes the synchronized block and comes out of that.

In this way, the synchronization helps in a multithreaded application. One thread has to wait till other thread finishes its execution only then the other threads are allowed for execution.

It can be written in the following form:

Synchronized(object)
{
//Block of statements to be synchronized
}

Java Multithreading Example

In this example, we will take two threads and fetch the names of the thread.

Example1:

sampleThread1.java
package demotest;
public class sampleThread1 implements Runnable{
 
/**
* @param args
*/
public static void main(String[] args) {
Thread sampleThread1 = new Thread("sample1");
Thread sampleThread2 = new Thread("sample2");
sampleThread1.start();
sampleThread2.start();
System.out.println("Thread names are following:");
System.out.println(sampleThread1.getName());
System.out.println(sampleThread2.getName());
}
@Override
public void run() {
}
 
}



Output:

Thread names are being outputted here as

  • sample1
  • sample2

Example 2:

In this example, we will learn about overriding methods run() and start() method of a runnable interface and create two threads of that class and run them accordingly.

Also, we are taking two classes,

  • One which will implement the runnable interface and
  • Another one which will have the main method and execute accordingly.
package demotest;
public class sampleThread2 {
 
public static void main(String[] args) {
// TODO Auto-generated method stub
sampleThread3 threadsample1 = new sampleThread3("sample1");
threadsample1.start();
sampleThread3 threadsample2 = new sampleThread3("sample2");
threadsample2.start();
}
}
class sampleThread3 implements Runnable {
Thread samplethread;
private String samplename;
sampleThread3(String name) {
samplename = name;
}
@Override
public void run() {
System.out.println("Thread running" + samplename);
for (int i = 0; i < 4; i++) {
System.out.println(i);
System.out.println(samplename);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
System.out.println("Thread has been interrupted");
}
}
}
public void start() {
System.out.println("Thread started");
if (samplethread == null) {
samplethread = new Thread(this, samplename);
samplethread.start();
}
 
}
}
 
 

When you execute the above code you get the following output:


Output:

There are two threads hence, we get two times message "Thread started".

We get the names of the thread as we have outputted them.

It goes into for loop where we are printing the counter and thread name and counter starts with 0.

The loop executes three times and in between the thread is slept for 1000 milliseconds.

Hence, first, we get sample1 then sample2 then again sample2 because the thread sleeps here for 1000 milliseconds and then next sample1 and again sample1, thread sleeps for 1000 milliseconds, so we get sample2 and then sample1.

Summary:

In this tutorial, we saw multithreaded applications in Java and how to use single and multi threads.

  • In multithreading, users are not blocked as threads are independent and can perform multiple operations at time
  • Various stages of life cycle of the thread are,
    • New
    • Runnable
    • Running
    • Waiting
    • Dead
  • We also learned about synchronization between threads, which help the application to run smoothly.
  • Multithreading makes many more application tasks easier.


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