Friday 4 March 2016

Java Error Handling Mechanism !!!


When a dynamic linking failure or some other “hard” failure in the virtual machine occurs, the virtual machine throws an Error. Typical Java programs should not catch Errors. In addition, it’s unlikely that typical Java programs will ever throw Errors either.


Most programs throw and catch objects that derive from the Exception class. Exceptions indicate that a problem occurred but that the problem is not a serious JVM problem. An Exception class has many subclasses. These descendants indicate various types of exceptions that can occur. For example, NegativeArraySizeException indicates that a program attempted to create an array with a negative size. One exception subclass has special meaning in the Java language: RuntimeException.

All the exceptions except RuntimeException are compiler checked exceptions. If a method is capable of throwing a checked exception it must declare it in its method header or handle it in a try/catch block. Failure to do so raises a compiler error. So checked exceptions can, at compile time, greatly reduce the occurrence of unhandled exceptions surfacing at runtime in a given application at the expense of requiring large throws declarations and encouraging use of poorly constructed try/catch blocks. Checked exceptions are present in other languages like C++, C#, and Python.

Runtime Exceptions (unchecked exception):

A RuntimeException class represents exceptions that occur within the Java virtual machine (during runtime). An example of a runtime exception is NullPointerException. The cost of checking for the runtime exception often outweighs the benefit of catching it. Attempting to catch or specify all of them all the time would make your code unreadable and unmaintainable. The compiler allows runtime exceptions to go uncaught and unspecified. If you like, you can catch these exceptions just like other exceptions. However, you do not have to declare it in your “throws" clause or catch it in your catch clause. In addition, you can create your own RuntimeException subclasses and this approach is probably preferred at times because checked exceptions can complicate method signatures and can be difficult to follow.

What are the exception handling best practices: 

Q. Why is it not advisable to catch type “Exception”?

Exception handling in Java is polymorphic in nature. For example if you catch type Exception in your code then it can catch or throw its descendent types like IOException as well. So if you catch the type Exception before the type IOException then the type Exception block will catch the entire exceptions and type IOException block is never reached. In order to catch the type IOException and handle it differently to type Exception, IOException should be caught first (remember that you can’t have a bigger basket above a smaller basket).

The diagram above is an example for illustration only. In practice it is not recommended to catch type
“Exception”. We should only catch specific subtypes of the Exception class. Having a bigger basket (i.e. Exception) will hide or cause problems. Since the RunTimeException is a subtype of Exception, catching the type Exception will catch all the run time exceptions (like NullPointerException, ArrayIndexOutOfBoundsException) aswell.

Example: The FileNotFoundException is extended (i.e. inherited) from the IOException. So (subclasses have to be caught first) FileNotFoundException (small basket) should be caught before IOException (big basket).

Q. Why should you throw an exception early?

The exception stack trace helps you pinpoint where an exception occurred by showing you the exact sequence of method calls that lead to the exception. By throwing your exception early, the exception becomes more accurate and more specific. Avoid suppressing or ignoring exceptions. Also avoid using exceptions just to get a flow control.

Instead of:
// assume this line throws an exception because filename == null.
InputStream in = new FileInputStream(fileName);

Use the following code because you get a more accurate stack trace:

if(filename == null) {
throw new IllegalArgumentException(“file name is null”);
InputStream in = new FileInputStream(fileName);

Q. Why should you catch a checked exception late in a catch {} block?

You should not try to catch the exception before your program can handle it in an appropriate manner. The natural tendency when a compiler complains about a checked exception is to catch it so that the compiler stops reporting errors. It is a bad practice to sweep the exceptions under the carpet by catching it and not doing anything with it.

The best practice is to catch the exception at the appropriate layer (e.g. an exception thrown at an integration layer can be caught at a presentation layer in a catch {} block), where your program can either meaningfully recover from the exception and continue to execute or log the exception only once in detail, so that user can identify the cause of the exception. 

Q. When should you use a checked exception and when should you use an unchecked exception?

Due to heavy use of checked exceptions and minimal use of unchecked exceptions, there has been a hot debate in the Java community regarding true value of checked exceptions. Use checked exceptions when the client code can take some useful recovery action based on information in exception. Use unchecked exception when client code cannot do anything. For example Convert your SQLException into another checked exception if the client code can recover from it. Convert your SQLException into an unchecked (i.e. RuntimeException) exception, if the client code can not recover from it. (Note: Hibernate 3 & Spring uses RuntimeExceptions prevalently).

Important: throw an exception early and catch an exception late but do not sweep an exception under the carpet by catching it and not doing anything with it. This will hide problems and it will be hard to debug and fix. 

A note on key words for error handling:

throw / throws – used to pass an exception to the method that called it.
try – block of code will be tried but may cause an exception.
catch – declares the block of code, which handles the exception.
finally – block of code, which is always executed (except System.exit(0) call) no matter what program flow, occurs when dealing with an exception.
assert – Evaluates a conditional expression to verify the programmer’s assumption.


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