Threads in Java

like labour

Introduction

A Thread is a small set of instructions designed to be scheduled and executed by the CPU independently of the parent process. To run the process in another thread, you the support of both the operating system and the process. When you create another thread, JVM communicates OS to create a new thread which can then run on the "multi-threaded CPU". Well this article isn't about explain threads, it's about how you can play with them in Java

Threads are everywhere by default

• When write and run a simple java program, JVM is creating the main thread for you to print hello world

public class Solution {

	public static void main(String[] args) throws Exception {
		System.out.println("hello world");
		throw new Exception();
	}
}

// output : 
hello world
Exception in thread "main" java.lang.Exception
        at Solution.main(Solution.java:5)

If you read the error message carefully, it says Exception in thread "main" , hence proved ...

How to create a thread ?

Well, cuz Java is an Object-Oriented Language, everything is wrapped in OOP concepts. To run a method of some class in-parallel

• Extend the class from the Thread class

• Create a method overriding the run() method

This run method is what is executed separately in a thread.

Here is an example :

class Hi extends Thread {
	@Override
	public void run() {
		for (int i = 1; i < 5; i++) {
			System.out.println("hi");
			
			// just sleeping for some time
			try {Thread.sleep(400);} catch (Exception e) {}
		}
	}
}

public class App {
	public static void main(String[] args) {
		Hi obj1 = new Hi();
		obj1.start();
	}
}

To run two thread of seperate class

class Hi extends Thread {
	@Override
	public void run() {
		for (int i = 1; i < 5; i++) {
			System.out.println("hi");

			try { Thread.sleep(400); } catch (Exception e) {}
		}

	}
}

class Hello extends Thread {
	public void run() {
		for (int i = 1; i < 5; i++) {
			System.out.println("hello");
			
			try { Thread.sleep(400); } catch (Exception e) {}
		}
	}
}

public class App {
	public static void main(String[] args) {
		Hi obj1 = new Hi();
		Thread obj2 = new Hello();

		obj1.start();
		obj2.start();
	}
}

When you call the .start() method, it will execute the run() method in a seperate thread.

Well becuase java doesnt support mulitple inheritanc, if you ever have to inherit from another class, you wont be able to do it. To overcome this limitation/feature we can implement the Runnable interface

class Hi implements Runnable {
	public void run() {
		for (int i = 1; i < 5; i++) {
			System.out.println("hi");

			try { Thread.sleep(400); } catch (Exception e) {}
		}
	}
}

class Hello implements Runnable {
	public void run() {
		for (int i = 1; i < 5; i++) {
			System.out.println("hello");
            
			try { Thread.sleep(400); } catch (Exception e) {}
		}
	}
}

public class App {
	public static void main(String[] args) {
		Runnable obj1 = new Hi();
		Runnable obj2 = new Hello();

		Thread t1 = new Thread(obj1);
		Thread t2 = new Thread(obj2);

		t1.start();
		t2.start();
	}
}

The only difference here is you implement the Runnable interface, create objects of the Thread class passing the objects implementing Runnable, and call the start method on the thread object, which in--return executes the .run() methods.

Properties of a Thread Class

• getName() : String gets you the name of the thread

  • setName(String name) : set name of a thread

• isAlive() : Boolean whether the thread is active or not

• getId() : Long get the execution id of the thread

• getPriority() : get the priority of thread

  • setPriority(int num) : on a scale of 1-10

  • Predefined ENUMS

    • Thread.MAX_PRIORITY = 10

    • Thread.MIN_PRIORITY = 1

    • Thread.NORM_PRIORITY = 5

For more, refer to documentation : https://download.java.net/java/early_access/loom/docs/api/java.base/java/lang/Thread.html

class Hi extends Thread {
	public void run() {
		for (int i = 1; i < 5; i++) {
			try { Thread.sleep(400); } catch (Exception e) { }
		}
	}
}

public class App {
	public static void main(String[] args) throws InterruptedException {
		Hi obj1 = new Hi();
		obj1.start();

		System.out.println("Name : " + obj1.getName());
		var threadName = obj1.getName();
		System.out.println(threadName + " isAlive  : " + obj1.isAlive());
		System.out.println(threadName + " threadId : " + obj1.getId());
		System.out.println(threadName + " priority : " + obj1.getPriority());
		

		System.out.println(Thread.MAX_PRIORITY);
		System.out.println(Thread.MIN_PRIORITY);
		System.out.println(Thread.NORM_PRIORITY);

		obj1.join();
	}
}

Joining Threads

When you spin off a thread, you also have to wait for it to finish its task before moving on to avoid errors/exception

class App {
	public static void main(String[] args) {

		Thread obj1 = new Thread(() -> {
			for (int i = 1; i < 5; i++) {
				System.out.println(Thread.currentThread().getName());
				try { Thread.sleep(400); } catch (Exception e) {}
			}
		}, "Thread 1");

		Thread obj2 = new Thread(() -> {
			for (int i = 1; i < 5; i++) {
				System.out.println(Thread.currentThread().getName());
				try { Thread.sleep(400); } catch (Exception e) {}
			}
		}, "Thread 2");

		obj1.start();
		obj2.start();

		try {
			obj1.join();
			obj2.join();
		} catch (Exception e) {
			System.out.println("execption");
		}

		System.out.println("bye");
	}
}

Synchronizing Threads

• thread sync happens at object level, not at method level (even tho marked at method), applicable for methods marked with sync

• use synchronized on blocks to avoid wait times

  • by synchronized(this) to lock the current object

  • by synchronized(x) to pass an object and lock it

  • by synchronized(X.class) to get class level lock

class DataClass {
    int num;
    boolean valueSet = false;

    public synchronized void getNum() {
        while (!valueSet) {
            try {
                wait();
            } catch (Exception e) {
            }
        }
        System.out.println("get : " + num);
        valueSet = false;
        notify();
    }

    public synchronized void setNum(int num) {
        while (valueSet) {
            try {
                wait();
            } catch (Exception e) {
            }
        }

        this.num = num;
        System.out.println("set : " + num);
        valueSet = true;
        notify();
    }
}

class Producer implements Runnable {

    DataClass dataClass;

    public Producer(DataClass data) {
        this.dataClass = data;
    }

    @Override
    public void run() {
        int i = 0;
        while (true) {
            dataClass.setNum(i++);
            try {
                Thread.sleep(800);
            } catch (Exception e) {
                System.out.println("exception");
            }
        }
    }
}

class Consumer implements Runnable {

    DataClass dataClass;

    public Consumer(DataClass data) {
        this.dataClass = data;
    }

    @Override
    public void run() {
        while (true) {
            dataClass.getNum();
            try {
                Thread.sleep(1000);
            } catch (Exception e) {
                System.out.println("exception");
            }
        }
    }
}

public class App {
    public static void main(String[] args) {
        DataClass dataClass = new DataClass();
        var producer = new Producer(dataClass);
        var consumer = new Consumer(dataClass);

        Thread t1 = new Thread(producer);
        Thread t2 = new Thread(consumer);

        t1.start();
        t2.start();
    }
}

Thread interrupt

public class App {
    public static void main(String[] args) {

        MyThread thClass = new MyThread();
        thClass.start();
        thClass.interrupt();
    }
}

/**
 * MyThread
 */
public class MyThread extends Thread {

    @Override
    public void run() {
        try {
            for (int i = 0; i < 20; i++) {
                System.out.println("child thread");
                Thread.sleep(1000);
            }
        } catch (InterruptedException e) {
            System.out.println("got interrupted");
        }       
    }
}

Thread Yield

Java Thread yield() method : The yield() method of thread class causes the currently executing thread object to temporarily pause and allow other threads to execute.

public class MyThread extends Thread {

    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            System.out.println("child thread");
            Thread.yield();
        }      
    }
}

Inter thread communication methods

• wait

• notify

• notifyAll()

Important Points

• using runnable is better because when you extend from thread you cannot inherit from other class

Limitations of using thread and runnable

• time consuming : creation and management of thread

• poor resource management

• not robust : cannot handle scale

ExecutorService

example :

```java
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class App {
    public static void main(String[] args) throws InterruptedException{

        MyThread[] myThreadsArr = { new MyThread("ATM"), 
                                    new MyThread("Bank"),
                                    new MyThread("Mobile"),
                                    new MyThread("ATM2"), 
                                    new MyThread("Bank2"),
                                    new MyThread("Mobile2"),
                                    new MyThread("ATM3"), 
                                    new MyThread("Bank3"),
                                    new MyThread("Mobile3")};

        ExecutorService es = Executors.newFixedThreadPool(2);
        for (MyThread mThreadObj : myThreadsArr) {
            es.submit(mThreadObj);
        }

        es.shutdown();
    }
}
```

Future & Callable

example

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class App {
    public static void main(String[] args) throws InterruptedException{

        MyCallable[] myCallables = { new MyCallable(20), 
                                    new MyCallable(30),
                                    new MyCallable(40),
                                    new MyCallable(50), 
                                    new MyCallable(60)};

        ExecutorService es = Executors.newFixedThreadPool(2);
        for (MyCallable mThreadObj : myCallables) {
            Future<Integer> future = es.submit(mThreadObj);
            try {
                System.out.println(future.get());
            } catch (Exception e) {
                System.out.println("exception");
            }
        }
        es.shutdown();
    }
}

# MyCallable class
import java.util.concurrent.Callable;

public class MyCallable implements Callable<Integer> {
    int number;
    MyCallable(int number) { this.number = number;  }

    @Override
    public Integer call() throws Exception {
        System.out.println(Thread.currentThread().getName()+" upto "+ number);
        int sum=0;
        for(int i=0; i<=number; i++) { sum+=i; }
        return sum;
    }
    
}