Use Finally For Guaranteed Code Execution

Java’s exception handling mechanism provides developers with an elegant, easy-to-use way to handle exceptional situations in their programs. For example, if there is a file operation to be performed, one might write code like:

 String fileToRead = "someFileToRead"; String fileToWrite = "someFileToWrite"; try {     FileInputStream fis = new FileInputStream(fileToRead);     FileOutputStream fos = new FileOutputStream(fileToWrite);     byte[] readBuffer = new byte[2152];     int bytesIn = 0;     while((bytesIn = fis.read(readBuffer)) != -1)     {         fos.write(readBuffer, 0, bytesIn);                }     //close the streams     fos.close();     fis.close(); } catch (Exception e) {     //handle exception } 

Now, if any exception happens before we get to close the streams, the execution of the program will go to our exception handler, and we will leave the streams open. This is a very potential source of memory leaks. Java’s finally clause provides us with a mechanism to prevent such situations from happening.

The code inside a finally block will be executed through both normal execution of the program as well as situations where exceptions take place. Thus, finally blocks guarantee code execution. Let’s re-arrange our code above to take advantage of a finally block when your code runs normally but also when an exception occurs in the try block.

 String fileToRead = "someFileToRead"; String fileToWrite = "someFileToWrite"; FileInputStream fis = null; FileOutputStream fos = null; try {     fis = new FileInputStream(fileToRead);     fos = new FileOutputStream(fileToWrite);     byte[] readBuffer = new byte[2152];     int bytesIn = 0;     while((bytesIn = fis.read(readBuffer)) != -1)     {         fos.write(readBuffer, 0, bytesIn);                } } catch (Exception e) {     //handle exception } finally {    //close the streams    //the close operation may throw exceptions    try    {         fos.close();     }     catch(Exception e)     {     }     finally     {         try         {              fis.close         }         catch(Exception e)         {         }     } } 

This way, we make sure that we close our streams regardless stream operations being successful or exceptional.

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