Java – Exception propagation

February 24, 2017 SJ Exception Handling 0

In this article, we will discuss exception propagation in detail with example

1. Exception propagation:

  • Whenever exception is raised from method and if it isn’t handled in the same method, then it is propagated back to the caller-method
  • This step is repeated until handler code is found in one of the caller-method in the runtime stack or else it reaches bottom of the runtime-stack
  • This is known as Exception propagation

2. Rules for Exception propagation:

  • By default, unchecked-exception is propagated back to the runtime-stack one-by-one until it finds handler code or it reaches bottom of the stack
  • Checked-exception isn’t propagated, rather compiler forces the programmer to handle checked-exception in the same method by surrounding with try-catch block or declaring with throws keyword

3. Example for Exception propagation

  1. Exception propagation w.r.t Unchecked Exception
  2. Exception propagation w.r.t Checked Exception
  3. Explicit propagation w.r.t Unchecked Exception

3.1 Exception propagation w.r.t Unchecked exception:

In this example,

  • When arithmetic-exception is raised in methodThree(), then it isn’t handled. Therefore, it is propagated back to the caller-method i.e.; methodTwo()
  • Similarly, there is no handler-code available in methodTwo() also
  • Therefore, again it is propagated back to the caller method i.e.; methodOne()
  • Like this, it will be repeated until if finds a suitable handler-code or till it reached bottom of the runtime-stack
  • In this case, there is no handler code till it reaches the bottom of the stack
  • Finally JVM passes the control to Default-exception-handler along with exception object when no handler-code is found (i.e.; propagation reaches the main() method i.e.; last entry in the runtime-stack)
  • Default-exception-handler prints exception-information it has got from exception-object and terminates method abnormally

DefaultPropagationForUncheckedException.java

package in.bench.resources.exception.handling;

public class DefaultPropagationForUncheckedException {

	// main() method - start of JVM execution
	public static void main(String[] args) {
		callMethodOne();
	}

	// callMethodOne() method
	public static void callMethodOne() {
		callMethodTwo();
	}

	// callMethodTwo() method
	public static void callMethodTwo() {
		callMethodThree();
	}

	// callMethodThree() method
	public static void callMethodThree() {

		// performing arithmetic operation
		int result = 19/0;
		System.out.println("The result of division is : "
				+ result);
	}
}

Output:

Exception in thread "main" java.lang.ArithmeticException:/ by zero
	at in.bench.resources.exception.handling
	.DefaultPropagationForUncheckedException
.callMethodThree(DefaultPropagationForUncheckedException.java:24)
	at in.bench.resources.exception.handling
	.DefaultPropagationForUncheckedException
.callMethodTwo(DefaultPropagationForUncheckedException.java:17)
	at in.bench.resources.exception.handling
	.DefaultPropagationForUncheckedException
.callMethodOne(DefaultPropagationForUncheckedException.java:12)
	at in.bench.resources.exception.handling
	.DefaultPropagationForUncheckedException
.main(DefaultPropagationForUncheckedException.java:7)

3.2 Exception propagation w.r.t Checked exception:

Whenever checked exception is thrown, then compiler throws compile-time error stating “Unhandled exception type exception-class-name”

  • So default propagation like in earlier case for unchecked exception isn’t possible for this case (with checked exception)
  • Because, compiler forces/tells with an compile-time error to handle the checked exception either with a try-catch block combination or declaring throws clause
  • Therefore, it is must for checked exception to handle
  • Not providing handler-code leads to compile-time error
  • Note: default propagation isn’t possible for checked exception but programmer can manually propagate using throw keyword
  • Move over to next example for explanation

NoPropagationForCheckedException.java

3.3 Explicit propagation for Checked exception:

  • Explicitly, we can propagate checked exception too by declaring with throws clause
  • But it must be handled in one of the method in the runtime stack
  • Otherwise compile-time error will be thrown stating “Unhandled exception type exception-class-name”
  • In the below example for explicitly throwing checked exception, last entry in the runtime stack i.e.; main() method handled exception by surrounding the call with try-catch block

ExplicitPropagationForCheckedException.java

package in.bench.resources.exception.handling;

import java.io.FileNotFoundException;
import java.io.FileReader;

public class ExplicitPropagationForCheckedException {

	// main() method - start of JVM execution
	public static void main(String[] args) {

		try {
			callMethodOne();
		}
		catch (FileNotFoundException fnfex) {
			System.out.println("Exception handled successfully");
		}

		System.out.println("By handling for checked exception,"
				+ " program terminates gracefully");
	}

	// callMethodOne() method
	public static void callMethodOne()
			throws FileNotFoundException {
		callMethodTwo();
	}

	// callMethodTwo() method
	public static void callMethodTwo()
			throws FileNotFoundException {
		callMethodThree();
	}

	// callMethodThree() method
	public static void callMethodThree()
			throws FileNotFoundException {

		// performing IO operation assumed that,
		// we are trying to access file from remote location
		FileReader fileReader = new FileReader(
				"D:/Folder/test.txt");
	}
}

Output:

Exception handled successfully
By handling for checked exception, program terminates gracefully

Conclusion:

  • By default, unchecked exception are propagated back to the runtime stack until it reaches bottom of the stack or else it finds handler code
  • By default, checked exception aren’t propagated; because whenever there is possibility of raising checked exception then compiler forces/tells to handle it
  • But explicit propagation for checked exception is possible by use of throws keyword

Related Articles:

References:

Happy Coding !!
Happy Learning !!

Java - 5 important keywords in Exception handling
Java - Rules for Exception handling w.r.t Method Overriding

Related posts:

  1. Java – Checked Exception v/s Unchecked Exception
  2. Java – throw keyword or clause
  3. Java – User-defined Exception or Custom Exception
  4. Java – Top Exception and Error