Multithreading

Thread/Process:

• Process: a program in execution, different process runs in different memory space

• Thread: interleave multiple threads in the context of a single process, runs in shared memory space. A lightweight sub-process(require fewer resources than process and run in shared memory space), a path of execution.

• Thread Problem: race condition(two or more threads have access to same object’s data), thread deadlock(two threads holding a key the other thread wants)

• Synchronized: only one thread can access an object’s method at one time

• Multiple-thread in single-CPU system: context-switching between threads

• Runnable: threads needs to implement runnable interface, override run() methods

•How to make Thread(both need to provide run() method):

• -> Provide Runnable object and pass into Thread constructor

• -> Inherited from Thread class

•Lock(interface): .lock(), try/finally(.unlock())

Multithreading is a Java feature that allows concurrent execution of two or more parts of a program for maximum utilization of CPU. Each part of such program is called a thread. So, threads are light-weight processes within a process.

Threads can be created by using two mechanisms : 1. Extending the Thread class 2. Implementing the Runnable Interface

Thread creation by extending the Thread class

We create a class that extends the java.lang.Thread class. This class overrides the run() method available in the Thread class. A thread begins its life inside run() method. We create an object of our new class and call start() method to start the execution of a thread. Start() invokes the run() method on the Thread object.

// Java code for thread creation by extending 
// the Thread class 
class MultithreadingDemo extends Thread { 
    public void run() { 
        try { 
            // Displaying the thread that is running 
            System.out.println ("Thread " + 
                Thread.currentThread().getId() + 
                " is running"); 

        } 
        catch (Exception e) { 
            // Throwing an exception 
            System.out.println ("Exception is caught"); 
        } 
    } 
} 

// Main Class 
public class Multithread { 
    public static void main(String[] args) { 
        int n = 8; // Number of threads 
        for (int i=0; i<8; i++) { 
            MultithreadingDemo object = new MultithreadingDemo(); 
            object.start(); 
        } 
    } 
}

Thread creation by implementing the Runnable Interface

We create a new class which implements java.lang.Runnable interface and override run() method. Then we instantiate a Thread object and call start() method on this object.

// Java code for thread creation by implementing 
// the Runnable Interface 
class MultithreadingDemo implements Runnable { 
    public void run() { 
        try { 
            // Displaying the thread that is running 
            System.out.println ("Thread " + 
                                Thread.currentThread().getId() + 
                                " is running"); 

        } 
        catch (Exception e) { 
            // Throwing an exception 
            System.out.println ("Exception is caught"); 
        } 
    } 
} 

// Main Class 
class Multithread { 
    public static void main(String[] args) { 
        int n = 8; // Number of threads 
        for (int i=0; i<8; i++) { 
            Thread object = new Thread(new MultithreadingDemo()); 
            object.start(); 
        } 
    } 
}

Thread Class vs Runnable Interface

1. If we extend the Thread class, our class cannot extend any other class because Java doesn’t support multiple inheritance. But, if we implement the Runnable interface, our class can still extend other base classes.

2. We can achieve basic functionality of a thread by extending Thread class because it provides some inbuilt methods like yield(), interrupt() etc. that are not available in Runnable interface.