Java Thread.start() vs Thread.run() Method
In Java's multi-threading concept, start() and run() are the two most important methods. In this article, we will learn the main differences between Thread.start() and Thread.run() in Java.
Thread.start() vs Thread.run() Method
| Thread.start() | Thread.run() |
|---|---|
| Creates a new thread and the run() method is executed on the newly created thread. | No new thread is created and the run() method is executed on the calling thread itself. |
| It can't be invoked more than one time otherwise throws java.lang.IllegalStateException | Multiple invocations are possible. |
| Defined in java.lang.Thread class. | Defined in java.lang.Runnable interface and must be overridden in the implementing class. |
New Thread Creation
When a program calls the start() method, a new thread is created and then the run() method is executed. But if we directly call the run() method then no new thread will be created and run() method will be executed as a normal method call on the current calling thread itself and no multi-threading will take place.
Example 1: Using start() method to create new thread
// Use of start() method
// Class extending thread
class MyThread extends Thread
{
// Overridng the run method
@Override
public void run()
{
System.out.println("Current thread name: "
+ Thread.currentThread().getName());
System.out.println("run() method called");
}
}
// Driver class
class Geeks
{
public static void main(String[] args)
{
// Creating new thread
MyThread t = new MyThread();
t.start();
}
}
Output
Current thread name: Thread-0 run() method called
Explanation: We can see in the above example, when we call the start() method of our thread class instance, a new thread is created with default name Thread-0 and then run() method is called and everything inside it is executed on the newly created thread.
Example 2: Let us try to directly call run() method directly instead of start() method.
// Use of run() method
// Class extending thread
class MyThread extends Thread
{
// Overriding the run method
@Override
public void run()
{
System.out.println("Current thread name: "
+ Thread.currentThread().getName());
System.out.println("run() method called");
}
}
// Driver class
class Geeks
{
public static void main(String[] args)
{
// Creating thread
MyThread t = new MyThread();
t.run();
}
}
Output
Current thread name: main run() method called
Explanation: As we can see in the above example, when we called the run() method of our MyThread class, no new thread is created and the run() method is executed on the current thread i.e. main thread. Hence, no multi-threading took place. The run() method is called as a normal function call.
Multiple Invocation
In Java's Multi-Threading Concept, another most important difference between start() and run() method is that we can't call the start() method twice otherwise it will throw an IllegalStateException whereas run() method can be called multiple times as it is just a normal method calling.
Example 1: Multiple invocation of start() method.
// Multiple invocation of start() method
// Class extending thread class
class MyThread extends Thread
{
// Overriding the run method
@Override
public void run()
{
System.out.println("Current thread name: "
+ Thread.currentThread().getName());
System.out.println("run() method called");
}
}
// Driver class
class Geeks
{
public static void main(String[] args)
{
// creating thread
MyThread t = new MyThread();
t.start();
t.start();
}
}
Output:
Current thread name: Thread-0
run() method called
Exception in thread "main" java.lang.IllegalThreadStateException
at java.lang.Thread.start(Thread.java:708)
at GeeksforGeeks.main(File.java:11)
Explanation: As we can see in the above example, calling start() method again raises java.lang.IllegalThreadStateException.
Example 2: Multiple invocation of run() method
// Multiple invocation of run() method
// Class extending thread
class MyThread extends Thread
{
// Overriding the run method
@Override
public void run()
{
System.out.println("Current thread name: "
+ Thread.currentThread().getName());
System.out.println("run() method called");
}
}
// Driver class
class Geeks
{
public static void main(String[] args)
{
// thread created
MyThread t = new MyThread();
t.run();
t.run();
}
}
Output
Current thread name: main run() method called Current thread name: main run() method called
Explanation: As we can see in the above example, calling run() method twice doesn't raise any exception and it is executed twice as expected but on the main thread itself.
