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Exception Handling in Java: Multiple Exception Handlers


Java Exception Handling  Java 
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Exception Handling in Java: Multiple Exception Handlers 

 

Let’s now look at an example where the running code generates multiple types of exceptions. Rather than providing a generic exception handler to catch all types of exceptions, we provide an exception handler for each type of exception that may be generated by the running code. The application asks the user to enter a valid URL, opens it in the program using the built-in URL class, and dumps its contents (albeit in simple text format, for simplicity) to the user console. Now, what kinds of errors can we envision in this simple application?

At the simplest level,  

                           

  • ·         the user might not enter any URL
  • ·         the entered URL might be invalid
  • ·         the user might forget to specify the protocol
  • ·         there might be an error opening the URL
  • ·         the data reading might generate I/O errors


There could be any number of unforeseen errors. Our program will try to safeguard against all these errors, and if an error occurs, it will take corrective action to ensure against a program crash. Following program illustrates this use of multiple exception handlers.

import java.io.*;

import java.net.*;

 

public class MultipleExceptionsExample {

                     public static void main(String[] args) {

                             String urlStr = null;                                              

                             while (true) {

                                      try {

                                                System.out.print("Enter url: ");

                                                BufferedReader reader = new BufferedReader(

                                                                   new InputStreamReader(System.in));

                                                urlStr = reader.readLine();

                                                if (urlStr.length() == 0) {

                                                          System.out.println("No url specified:");

                                                          continue;

                                                }

                                                System.out.println("Opening " + urlStr);

                                                URL url = new URL(urlStr);

                                                reader = new BufferedReader(new InputStreamReader(

                                                                   url.openStream()));

                                                System.out.println(reader.readLine());

                                                reader.close();

                                      } catch (MalformedURLException e) {

                                                System.out.println("Invalid URL " + urlStr + ": "

                                                                   + e.getMessage());

                                      } catch (IOException e) {

                                                System.out.println("Unable to execute " + urlStr + ": "

                                                                   + e.getMessage());

                                      } catch (Exception e) {

                                                System.out.println(e.getMessage());

                                      }

                             }

                     }

}

 

Note that the application uses certain classes from the java.io and java.net packages. To understand the current application, we do not need a deep understanding of these classes. We will focus mainly on what happens when the execution of the code within these classes generates errors at runtime.

·       The main method defines an infinite loop to accept the user input endlessly. The user is asked to enter a desired URL. The program reads the input using BufferReader.

·       Then we read a line of input from the keyboard (until the enter key is pressed) and assigns it to the urlStr variable. The program then checks whether the user indeed input some string

·         Next, we try to establish a connection to this URL

·  At this time, if the URL is invalid, it will generate an error. This is caught in the MalformedURLException exception handler.


The following output shows what happens when an invalid URL is entered:

Enter url: mindstick.com

Opening mindstick.com

Invalid URL mindstick.com: no protocol: mindstick.com

 

Because we forgot to input the protocol, let’s try one more time by entering a protocol, as follows:

Enter url: ttp: // mindstick.com

Opening ttp: // mindstick.com

Invalid URL ttp: // mindstick.com: unknown protocol: ttp

 

 

Oops! This time we missed the h in http. The same exception handler has trapped the error, this time giving another message (unknown protocol), which again is an appropriate one. Now, let’s enter the URL one more time without any mistakes. The output is as follows:

Enter url: http://google.com

Opening http://google.com

<!doctype html><html><head><meta http-equiv="content-type" content="text/html;

charset=ISO-8859-1"><title>Google</title><script>window.google={kEI:"BK9TTbv

XJIjfcbD5sOEI",kEXPI:"28317,28600,28641,28722",kCSI:{e:"28317,28600,28641,28

722",ei:"BK9TTbvXJIjfcbD5sOEI",expi:"28317,28600,28641,28722"},ml:function()

{},kHL:"en",time:function(){return(new Date).getTime()},log:function(c,d,

 

Wow! We got the contents of the Google home page. Now, the question is, why did we include another exception handler, IOException?

catch (IOException e) {

System.out.println("Unable to execute " + urlStr + ": " + e.getMessage());

}

 

 

To understand this, try the following URL:

Enter url: http://google.com:81

You will get the following output:

Opening http://google.com:81

Unable to execute http://google.com:81: Connection timed out: connect

 

 

This time the program executed the code in the IOException handler. Finally, why do we have the most generic exception handler at the end?

catch (Exception e) {

System.out.println(e.getMessage());

}

 

This is to account for all remaining unforeseen errors. This takes care of all the errors mentioned earlier, thus ensuring the program runs without crashing.


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