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.
Short Story: Unveiling the Reasoning Abilities of Large Language Models by Ke...
Java Exception Handling, Assertions and Logging
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
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.
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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