In last post I wrote about how can we speed up our processing using multiple threads in Java, basically doing parallel processing using threads. Using multithreading definitely helps speed up processing, but it can also result in corrupting data if not used properly. Let’s look at this example

//A simple Runnable class
public class RunnableTest implements Runnable{
 int count=0;
 public void test()
 {
  try {
   count++;
   Thread.sleep(1000); //This will just introduce some delay
   System.out.println("Hello from Runnable:"+count);
   } catch (Exception e) {
    e.printStackTrace();
  }
 }

@Override
public void run() {
test();
}
}

//and call it from somewhere like
Runnable r=new RunnableTest();
for(int i=0;i<20;i++)
{
Thread t1=new Thread(r);
t1.start();
}

Output:
Hello from Runnable:20
Hello from Runnable:20
Hello from Runnable:20
Hello from Runnable:20
Hello from Runnable:20
..
..

We are calling run method 20 times which in turn calls test. We have deliberately introduced some delay in test to give it a feel of real method which is taking time to execute some statements. The output depicts that instead of count being printed as 1, 2, 3 and so on, it is 20 for all the calls. Which means that all the threads are trying to execute same block of code simultaneously. Though parallel processing of code is desirable behavior from multithreading, it can cause issues in scenarios mentioned above.

Let’s say in actual code we are trying to write data into a database. Instead of count we have ID. So first thread creates and ID as X. Then it does some processing and creates an insert statement. While it is doing the processing, a second thread starts executing the same code and creates another ID Y which overrides the ID X (same as count is being modified). So when our thread 1 reaches to the code where it is inserting the data, it will try inserting ID as Y (as X is overridden by Y). Then thread 2 comes and again try to insert ID Y, causing an error.

So in real world we can have blocks of code (or complete methods) which we want to make sure does not get executed by multiple threads at the same time. Fortunately Java provides a simple way to do this. The keyword is synchronized.

public void test()
{
 synchronized(this){
  try {
count++;
Thread.sleep(1000);
System.out.println("Hello from Runnable:"+count);
} catch (Exception e) {
e.printStackTrace();
}
}
}

Now if we run the same code, we will get

Hello from Runnable:1
Hello from Runnable:2
Hello from Runnable:3
Hello from Runnable:4
Hello from Runnable:5
Hello from Runnable:6

Synchronized block makes sure that only one thread is executing blocked code at a time. Another way to synchronize is define method itself as synchronized.

public synchronized void test()

This will make sure only one thread enters this method at a time.

What is multithreading? First of all let us understand what is thread? A thread is simply flow of control in a program/ process.

Now Let’s understand what is multithreading? Let’s take an example, there is a code which reads data from a file and uploads it to a database. Let’s say there are 10 such files and each file takes 1 hr to finish data upload. In a single threaded environment, it will simply translate to 10 hrs job as our code will read and upload files one by one.  Whereas when we move to multithreading, that is multiple threads are executing at the same time asynchronously, it will result in parallel processing of these files and the total time taken might be close to time taken by one file only.

Before getting into details lets understand how to implement multithreading in Java? There are 2 ways to achieve this.

//1. Extend Thread class
public class ThreadTest extends Thread{
 public void test()
 {
  System.out.println("Hello from Thread");
 }
 public void run()
 {
  test();
 }
}
 //2. Implement Runnable interface 
public class RunnableTest implements Runnable{
 public void test()
 {
  System.out.println("Hello from Runnable");
 }
@Override
 public void run() {
  test();
 }
}

//And somewhere in the main method 
public static void main(String s[])
{
 Thread t1=new ThreadTest();
 t1.start();
 Thread t2=new Thread(new RunnableTest());
 t2.start();
}

As you can see the important method while using threads is “run”. This method gets called automatically by JVM whenever a thread starts.

Note that we do not call the run method directly, as that will be like calling a normal method and will wait for control to come back to calling class. Whereas using start will make sure the control flow continues without waiting for method call return. To understand what I am referring here, lets change the main method slightly

for(int i=0;i<20;i++)
{
 Thread t1=new ThreadTest();
 t1.start();
 Thread t2=new Thread(new RunnableTest());
 t2.start();
}

We will something like (no order)

Hello from Thread
Hello from Runnable
Hello from Thread
Hello from Runnable
Hello from Runnable
Hello from Thread
Hello from Thread
Hello from Thread
Hello from Runnable
Hello from Runnable

Now let’s change it to run

for(int i=0;i<20;i++)
{
 Thread t1=new ThreadTest();
 t1.run();
 Thread t2=new Thread(new RunnableTest());
 t2.run();
}

Output

Hello from Thread
Hello from Runnable
Hello from Thread
Hello from Runnable
Hello from Thread
Hello from Runnable

So not actually using multithreading here.

Coming back to original example. Now you can see if we write the code for uploading the data from file to run method, and execute it in multithreaded fashion, we will be able to upload files simultaneously.

We have seen 2 ways to create threads in Java, one by extending thread class and other by implementing runnable. Which is better?

Usually we go for implementing runnable. When implementing runnable we can only override run method, and that is the only thing which we need in most of the cases. Whereas when we extent Thread, we can override other methods like start, sleep, stop etc, which is usually not required. Additionally, extending Thread has the obvious implementation that now our class cannot extend any other class (as Java allows only one class to be extended).