Introduction to java exceptions

Introduction to java exceptions

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  Introduction To JavaExceptions Sujit Kumar Zenolocity LLC © 2013-2023
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  Exceptions • Events thatdisrupt the normal flow of execution. • Exception Object 1) cause (can contain another exception), 2) detailed message. • Can be thrown by the programmer using the throw keyword. • Can be caught by the programmer using the catch keyword.
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  Benefits of JavaException Handling • Cleaner code by separating error handling code from normal or happy path code. • Propagate errors up the call stack where they can be handled meaningfully. • Helps to categorize exceptions using the exception hierarchy.
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  Types of Errors •JVM Errors => OutOfMemoryError, StackOverflowError, LinkageError • System Errors => FileNotFoundException, IOException, NullPointerException, ArrayIndexOutOfBoundsException • Application Errors => InsufficientFundsException (transfer balance from one account to another). • JVM & System Errors are built-in, Application Errors are user defined.
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  Exception Handling –try, catch, finally • Try block – normal flow of execution. • Catch blocks ( 1 or more) – exception handling code, are like if-else-if statements – only 1 can potentially execute. • Derived class exception should be caught before a base class exception in the catch blocks. • If an exception is thrown that is not handled, it is propagated up the call stack.
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  Finally Block • Finallyblock – clean up code, always gets executed, even when you have a return in try/catch or any exception is re-thrown. • When does finally not execute ? – It may not execute when JVM terminates (via System.exit()) or current thread is killed or system runs out of memory. • If an exception is thrown that is not handled, it is propagated up the call stack.
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  Checked & UncheckedExceptions • Checked – compiler forces you to either catch them or declare in the throws clause. Use it for recoverable errors. • Unchecked – RuntimeException and its subclasses are unchecked exceptions. Compiler does not force you to catch them. Unchecked exceptions do not need to be declared in a method’s throws clause.
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  Error • An Erroris a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. • Most such errors are abnormal conditions that should never occur. • A method is not required to declare in its throws clause any subclasses of Error that might be thrown during the execution of the method but not caught. • Error and its subclasses are regarded as unchecked exceptions for the purposes of compile-time checking of exceptions. • Examples: OutOfMemoryError, VirtualMachineError, AssertionError, IOError.
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  User Defined Exceptions •These are business or application exceptions. • Class Name ends with the string Exception. Examples: InsufficientFundsException, AccountClosedException. • If user defined exception is a checked exception, it should extend Exception, otherwise it should extend RuntimeException. • Add custom fields and getters/setters to store more information.
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  Nesting Exceptions • Anexception can nest or contain another exception. • Many APIs throw a generic exception which contain the implementation exception. • Example: Spring Framework DataAccessException nests the JDBC SQLException. • SomeApiException.getCause() retrieves the implementation exception.
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  Recommendations – part1 • Use exceptions only for exceptional conditions. • Close or release resources in the finally block. • Use checked exceptions for recoverable errors. • Runtime exceptions indicate programming errors - do not catch them. • Errors are unchecked exceptions – generally should not be caught. • Catch exceptions only where you can handle them meaningfully, otherwise re-throw them. • Use standard exceptions as far as possible. • Document all checked exceptions thrown by each method using the @throws tag or @exception tag in the method’s java doc.
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  Recommendation – part2 • Include appropriate details from the surrounding context when creating & throwing exceptions. • Log or print the root exception before you throw a new wrapped exception. • Avoid empty catch blocks – swallowing exceptions – hides errors and exceptions – leaves object or application in a corrupt or unusable state. • Exceptions are costly in terms of performance => minimize use of checked exceptions, try to convert checked exceptions into unchecked exceptions.