This presentation introduces the java exception handling mechanisms. In detail, the main focus of the slides is to show how the language implements its exception handling polices, such as: - Checked and uncheked exception - Try / catch blocks - Assertions - Logging The presentation is took from the Java course I run in the bachelor-level informatics curriculum at the University of Padova.

1. EXCEPTION HANDLING,
ASSERTIONS AND LOGGING
PROGRAMMAZIONE CONCORRENTE E DISTR.
Università degli Studi di Padova
Dipartimento di Matematica
Corso di Laurea in Informatica, A.A. 2015 – 2016
rcardin@math.unipd.it

2. Programmazione concorrente e distribuita
SUMMARY
 Exception handling
 Throwing an exception
 Catching exceptions
 Chaining
 Assertions
 Using assertions
 Logging
 Using loggers
 Handlers
 Appendix: Checked or Unchecked?
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3. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 Mechanism for transferring the control from the
point of failure to a component handler
 Dealing with the unexpected is more complex than
implementing the "happy path"!
 Pitfalls using error codes
 The caller is obliged to check for errors
 Programmers have to actively check and propagate
these error codes
 Violation of the Single Responsibility Principle
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4. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 Throwing exceptions
 A method can signal a problem by throwing an
exception
 Decoupling of the process of detecting and handling errors
 Trying to fix in loco is not a good idea...
 ...let’s rise an exception, instead!
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private static Random generator = new Random();
public static int readInt(int low, int high) {
return low + (int) (generator.nextDouble() * (high – low + 1));
}
What if low > high ?
if (low > high)
throw new IllegalArgumentException(
String.format("%d is greater than %d!", low, high);

5. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 Throwing exceptions
 The normal flow of execution is interrupted
 No value is returned to the caller
 The control is transferred to a handler
 The handler is searched in the call stack
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methodA(arg1, arg2)
methodB(arg1)
methodC(arg1, arg2, arg3)
methodD(arg1, arg2)
Exception

6. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 The Exception hierarchy
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7. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 The Exception hierarchy
 Error
 Thrown when something exceptional happens that the program
cannot be expected to handle
 OutOfMemoryError, StackOverflowError, ...
 RuntimeException
 Unchecked exceptions indicate logic errors caused by
programmers, not by unavoidable external risks
 NullPointerException, IllegalArgumentException, ...
 Exception
 Checked exceptions (by the compiler), that must be either
catched or declared in the method signature
 IOException
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8. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 Declaring checked exception
 Method that might give rise to a checked exception,
must declare it in its header with a throws clause
 Superclass combination (not a good idea)
 An overriding method can throw at most the same checked
exceptions of the overriden method
 Use javadoc @throws tag to document when a
method throws and exception
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public void write(String str) throws IOException
/**
* @throws NullPointerException if filename is null
*/
public void write(String str) throws IOException

9. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 It’s possible to create your own exception
 Extend Exception, RuntimeException or another
existing exception class
 Supply different ctors, such as a default ctor, a ctor with a
string message and a ctor with a Throwable
 Supply every method you need to give information on the ex.
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public class MyException extends Exception {
public MyException() {}
public MyException(String message) {
// The error message shown in stack trace
super(message);
}
public MyException(Throwable cause) {
// Cause is the exception that generate this exception
super(cause);
}
}

10. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 Catching exceptions
 The handling of an exception is accomplished with a
try block
 Sharing one handler among multiple exception classes
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try {
// Statments that could throw an exception
} catch (ExceptionClass1 ex) {
// Handling of the exception of type ExceptionClass1
} catch (ExceptionClass2 ex) {
// Handling of the exception of type ExceptionClass2
}
The catch
clauses are
matched top
to bottom
and they
respect type
hierarchies
try {
// Statments that could throw an exception
} catch (ExceptionClass1 | ExceptionClass2 ex) {
// Handling of the exception of type ExceptionClass1 and 2
}
Java 7 and
above

11. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 Resource management
 When try block exits, exception or not, the close
methods of all resources objects are invoked
 Resources must implement AutoClosable interface
 Resources are closed in reverse order of their initialization
 If a close method throws an exception, it is normally
propagated
 If both a statement in the try block and a close
method throw an exception, the former is propagated
 The latter is attached as «suppressed»
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try (ResourceType1 res1 = init1; ResourceType2 res2 = init2) {
// Statments that use res1 and res2 and
// that could throw and exception
} catch (Exception ex) { /* ... */ }

12. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 The finally clause
 It is executed when the try block comes to an end,
either normally or due to an exeption
 Avoid throwing exception in the finally block
 Shadowing of the original exception
 finally block should not contain a return
statement
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try {
// Statments
} catch (Exception ex) {
// Handle exception
} finally {
// Do some cleanup (release locks, close db connection, ...)
}

13. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 Rethrowing and Chaining Exception
 It is possible in a catch block to rethrow and
exception
 Don’t know how to manage it, but want to log the failure
 The compiler tracks the correct flow of exception types
 Change the class of the thrown exception
 Use the proper constructor or the initCause method (old
school)
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try {
// Statments
} catch (Exception ex) {
// Do something
throw new Exception("Something is going on here", ex);
}

14. Programmazione concorrente e distribuita
EXCEPTION HANDLING
 The Stack trace
 If an exception is not caught anywhere, a stack trace
is displayed. By default it is sent to System.err
 Thread.setDefaultUncaughtExceptionHandler
changes the default exception handling policy
 ex.printStackTrace() prints on System.out the
stack trace of an exception
 It’s possible to pass a stream to the above method
 Checking nullability
 Put a marker in the stack trace, simplifying debugging ops
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public void process(String direction) {
this.direction = Objects.requireNonNull(direction);
}
Java 7 and
above

15. Programmazione concorrente e distribuita
ASSERTIONS
 A common idiom of defensive programming
 Assertions allow to put in checks during test and to
have them automatically removed in production code
 Throws and AssertionError if it is false
 Expression value is passed into the error
 Intended as a debugging aid for validating internal
assumptions
 Enable / disable assertion at runtime
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assert condition;
assert condition : expression;
java –enableassertions MainClass // or -ea
java –disableassertions MainClass // or -da

16. Programmazione concorrente e distribuita
LOGGING
 The logging API overcomes the problems to deal
with System.out.println during debugging
 The logging system manages a default logger
 You can define your own logger
 First time you request a logger with a name, it is created
 Logger names are hierarchical
 Seven logging levels
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Logger.getGlobal().info("Opening file " + filename);
// Prints: Aug 04, 2014 09:53:34 AM com.company.MyClass read
// INFO: Opening file data.txt
Logger logger = Logger.getLogger("com.company.app");
OFF SEVERE WARNING INFO CONFIG FINE FINER FINEST ALL
Logger.setLevel(Level.FINE)

17. Programmazione concorrente e distribuita
LOGGING
 Log Handlers
 Log handler are hierarchical, too
 Default handler (ancestor of all handler) has name " " and it
has type ConsoleHandler
 For a log, its logging level must be above the
threshold of both the logger and the handler
 You can use a custom log handler
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# In jre/lib/logging.properties
java.util.logging.ConsoleHandler.level=INFO
Handler handler = new ConsoleHandler();
handler.setLevel(Level.FINE);
logger.setUseParentHandlers(false); // Inhibit parent handling
logger.addHandler(handler);

18. Programmazione concorrente e distribuita
LOGGING
 Log Handlers
 By default, a logger sends records both to its own
handlers and the handlers of the parent.
 To prevent double logging, use setUseParentHandlers
 There exist two other handlers in the logging API
 SocketHandler
 Sends records to a specified host and port
 FileHandler
 Collects records in a file (javan.log in user’s home dir.)
 Written in XML
 Highly configurable using the logging configuration file
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logger.setUseParentHandlers(false);

19. Programmazione concorrente e distribuita
CHECKED OR UNCHECKED?
 Checked or un unchecked, which is better?
 There is a active and never ending debate on this
question in Java, but no «right absolute answer».
 "Use the checked ones, Luke!"
 A checked exception is part of a method API
 Cay Horstmann
 Joshua Bloch
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Unchecked exceptions indicate logic errors caused by programmers, not
by unavoidable external risks [..] Checked exceptions are used in a
situation where failure should be anticipated.
Item 58: Use checked exceptions for recoverable conditions and runtime
exceptions for programming errors

20. Programmazione concorrente e distribuita
CHECKED OR UNCHECKED?
 "The use of checked is a path to the Dark Side"
 Robert C. Martin
 Violation of the Open / Close Principle
 Martin Fowler
 Proposes the Notification pattern
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If you throw a checked exception from a method in your code and the
catch is three levels above, you must declare that exception in the
signature of each method between you and the catch. This means that a
change at a low level of the software can force signature changes on many
higher levels. The changed modules must be rebuilt and redeployed, even
though nothing they care about changed.
...on the whole I think that exceptions are good, but Java checked
exceptions are more trouble than they are worth.

21. Programmazione concorrente e distribuita
CHECKED OR UNCHECKED?
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22. Programmazione concorrente e distribuita
EXAMPLES
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https://github.com/rcardin/pcd-snippets

23. Programmazione concorrente e distribuita
REFERENCES
 Chap. 5 «Exceptions, Assertions, and Logging», Core Java for the
Impatient, Cay Horstmann, 2015, Addison-Wesley
 Replacing Throwing Exceptions with Notification in Validations
http://martinfowler.com/articles/replaceThrowWithNotification.ht
ml
 Chap. 7 «Error handling», Clean Code – A Handbook of Agile
Software Craftmanship, Robert C. Martin, 2008, Prentice Hall
 «Item 58: Use checked exceptions for recoverable conditions and
runtime exceptions for programming errors», Effective Java, Joshua
Bloch, 2008, Addison-Wesley
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