SUMMARY 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 Throwing an exception Catching exceptions Chaining Assertions Using assertions Logging Using loggers Handlers Appendix: Checked or Unchecked? 2 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 Throwing exceptions A method can signal a problem by throwing an exception Decoupling of the process of detecting and handling errors private static Random generator = new Random(); public static int readint(int low, int high) { return low + (int) (generator.nextdouble() * (high low + 1)); Trying to fix in loco is not a good idea......let s rise an exception, instead! if (low > high) throw new IllegalArgumentException( String.format("%d is greater than %d!", low, high); What if low > high? 3 4
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 The Exception hierarchy methoda(arg1, arg2) methodb(arg1) methodc(arg1, arg2, arg3) methodd(arg1, arg2) Exception 5 6 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 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) public void write(string str) throws IOException 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 /** * @throws NullPointerException if filename is null */ public void write(string str) throws IOException 7 8
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 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); Supply every method you need to give information on the ex. Catching exceptions The handling of an exception is accomplished with a try block // 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 Sharing one handler among multiple exception classes The catch clauses are matched top to bottom and they respect type hierarchies Java 7 and // Statments that could throw an exception above catch (ExceptionClass1 ExceptionClass2 ex) { // Handling of the exception of type ExceptionClass1 and 2 9 10 Resource management try (ResourceType1 res1 = init1; ResourceType2 res2 = init2) { // Statments that use res1 and res2 and // that could throw and exception catch (Exception ex) { /*... */ 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» The finally clause It is executed when the try block comes to an end, either normally or due to an exeption // Statments catch (Exception ex) { // Handle exception finally { // Do some cleanup (release locks, close db connection,...) Avoid throwing exception in the finally block Shadowing of the original exception finally block should not contain a return statement 11 12
Rethrowing and Chaining Exception It is possible in a catch block to rethrow and exception // Statments catch (Exception ex) { // Do something throw new Exception("Something is going on here", ex); 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) 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 public void process(string direction) { this.direction = Objects.requireNonNull(direction); Put a marker in the stack trace, simplifying debugging ops Java 7 and above 13 14 ASSERTIONS LOGGING A common idiom of defensive programming assert condition; assert condition : expression; 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 java enableassertions MainClass // or -ea java disableassertions MainClass // or -da The logging API overcomes the problems to deal with System.out.println during debugging The logging system manages a default logger Logger.getGlobal().info("Opening file " + filename); // Prints: Aug 04, 2014 09:53:34 AM com.company.myclass read // INFO: Opening file data.txt You can define your own logger First time you request a logger with a name, it is created Logger names are hierarchical Logger logger = Logger.getLogger("com.company.app"); Seven logging levels Logger.setLevel(Level.FINE) OFF SEVERE WARNING INFO CONFIG FINE FINER FINEST ALL 15 16
LOGGING 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 # In jre/lib/logging.properties java.util.logging.consolehandler.level=info You can use a custom log handler Handler handler = new ConsoleHandler(); handler.setlevel(level.fine); logger.setuseparenthandlers(false); // Inhibit parent handling logger.addhandler(handler); 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 logger.setuseparenthandlers(false); 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 17 18 CHECKED OR UNCHECKED? 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 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. Joshua Bloch Item 58: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors "The use of checked is a path to the Dark Side" Robert C. Martin 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. Violation of the Open / Close Principle Martin Fowler...on the whole I think that exceptions are good, but Java checked exceptions are more trouble than they are worth. Proposes the Notification pattern 19 20
CHECKED OR UNCHECKED? EXAMPLES https://github.com/rcardin/pcd snippets 21 22 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 23