CapÃtulo 20. Excepciones

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Extendiendo excepciones

PHP 5 tiene un modelo de excepciones similar al de otros lenguajes de programaciÃ³n. Una excepciÃ³n puede ser lanzada, intentada o capturada en PHP. Un bloque de intento (try) debe incluir por lo menos un bloque de captura (catch). Los bloques de captura mÃºltiples pueden ser usados para capturar diferentes tipos de clases; la ejecuciÃ³n continuarÃ¡ despuÃ©s del Ãºltimo bloque de captura definido. Las excepciones pueden ser lanzadas dentro de bloques de captura.

Cuando es lanzada una excepciÃ³n, la siguiente lÃnea de cÃ³digo no serÃ¡ ejecutada y PHP intentarÃ¡ encontrar el primer bloque de captura de excepciones. Si una excepciÃ³n no es capturada se despliega un error fatal de PHP con un mensaje de que la excepciÃ³n no fue capturada, a menos que exista un manejador de errores definido como set_exception_handler().

Ejemplo 20-1. Lanzando una ExcepciÃ³n


<?php

try {

    $error = 'Always throw this error';

    throw new Exception($error);



    // Code following an exception is not executed.

    echo 'Never executed';



} catch (Exception $e) {

    echo 'Caught exception: ',  $e->getMessage(), "\n";

}



// Continue execution

echo 'Hello World';

?>

Extendiendo excepciones

Una clase de definiciÃ³n de excepciones del usuario puede ser definida extendiendo la clase de excepciones incorporado. Los miembros y propiedades mencionados en seguida, muestran lo que estÃ¡ accesible dentro de la clase "hijo" que se deriba de la clase de excepciones incorporados.

Ejemplo 20-2. La clase de excepciones incorporada


<?php

class Exception

{

    protected $message = 'Unknown exception';   // exception message

    protected $code = 0;                        // user defined exception code

    protected $file;                            // source filename of exception

    protected $line;                            // source line of exception



    function __construct($message = null, $code = 0);



    final function getMessage();                // message of exception 

    final function getCode();                   // code of exception

    final function getFile();                   // source filename

    final function getLine();                   // source line

    final function getTrace();                  // an array of the backtrace()

    final function getTraceAsString();          // formated string of trace



    /* Overrideable */

    function __toString();                       // formated string for display

}

?>

Si una clase se extiende de la clase Exception incorporada y redefine el constructor, es altamente recomendado que tambiÃ©n llame parent::__construct() para asegurarse que todos los datos disponibles han sido asignados apropiadamente. El mÃ©todo __toString() puede ser evitado para proveer una salida personalizada cuando el objeto es presentado como una cadena.

Ejemplo 20-3. Extendiendo la clase Exception


<?php

/**

 * Define a custom exception class

 */

class MyException extends Exception

{

    // Redefine the exception so message isn't optional

    public function __construct($message, $code = 0) {

        // some code

    

        // make sure everything is assigned properly

        parent::__construct($message, $code);

    }



    // custom string representation of object */

    public function __toString() {

        return __CLASS__ . ": [{$this->code}]: {$this->message}\n";

    }



    public function customFunction() {

        echo "A Custom function for this type of exception\n";

    }

}





/**

 * Create a class to test the exception

 */

class TestException

{

    public $var;



    const THROW_NONE    = 0;

    const THROW_CUSTOM  = 1;

    const THROW_DEFAULT = 2;



    function __construct($avalue = self::THROW_NONE) {



        switch ($avalue) {

            case self::THROW_CUSTOM:

                // throw custom exception

                throw new MyException('1 is an invalid parameter', 5);

                break;



            case self::THROW_DEFAULT:

                // throw default one.

                throw new Exception('2 isnt allowed as a parameter', 6);

                break;



            default: 

                // No exception, object will be created.

                $this->var = $avalue;

                break;

        }

    }

}





// Example 1

try {

    $o = new TestException(TestException::THROW_CUSTOM);

} catch (MyException $e) {      // Will be caught

    echo "Caught my exception\n", $e;

    $e->customFunction();

} catch (Exception $e) {        // Skipped

    echo "Caught Default Exception\n", $e;

}



// Continue execution

var_dump($o);

echo "\n\n";





// Example 2

try {

    $o = new TestException(TestException::THROW_DEFAULT);

} catch (MyException $e) {      // Doesn't match this type

    echo "Caught my exception\n", $e;

    $e->customFunction();

} catch (Exception $e) {        // Will be caught

    echo "Caught Default Exception\n", $e;

}



// Continue execution

var_dump($o);

echo "\n\n";





// Example 3

try {

    $o = new TestException(TestException::THROW_CUSTOM);

} catch (Exception $e) {        // Will be caught

    echo "Default Exception caught\n", $e;

}



// Continue execution

var_dump($o);

echo "\n\n";





// Example 4

try {

    $o = new TestException();

} catch (Exception $e) {        // Skipped, no exception

    echo "Default Exception caught\n", $e;

}



// Continue execution

var_dump($o);

echo "\n\n";

?>

add a note User Contributed Notes
Excepciones

jazfresh at hotmail.com
07-Aug-2006 10:18


Sometimes you want a single catch() to catch multiple types of Exception. In a language like Python, you can specify multiple types in a catch(), but in PHP you can only specify one. This can be annoying when you want handle many different Exceptions with the same catch() block.



However, you can replicate the functionality somewhat, because catch(<classname> $var) will match the given <classname> *or any of it's sub-classes*.



For example:



<?php

class DisplayException extends Exception {};

class FileException extends Exception {};

class AccessControl extends FileException {}; // Sub-class of FileException

class IOError extends FileException {}; // Sub-class of FileException



try {

  if(!is_readable($somefile))

     throw new IOError("File is not readable!");

  if(!user_has_access_to_file($someuser, $somefile))

     throw new AccessControl("Permission denied!");

  if(!display_file($somefile))

     throw new DisplayException("Couldn't display file!");



} catch (FileException $e) {

  // This block will catch FileException, AccessControl or IOError exceptions, but not Exceptions or DisplayExceptions.

  echo "File error: ".$e->getMessage();

  exit(1);

}

?>



Corollary: If you want to catch *any* exception, no matter what the type, just use "catch(Exception $var)", because all exceptions are sub-classes of the built-in Exception.

linus at flowingcreativity dot net
19-Jul-2006 02:06


Here is a basic example of a way to mimick the convenience of exception handling in PHP4:



<?php



do {

    if (!$test_condition) {

        $error = 'test condition failed';

        break;

    }

    

    if (!test_function()) {

        $error = 'test function failed';

        break;

    }

    

    echo 'success!';

    

} while (false);



if (isset($error)) {

    echo $error;

}



?>



Obviously this falls far short of PHP5 real exception handling in terms of normalisation.  Also, $error won't propogate up the call stack like a real exception (i.e. test_function() can't itself throw an exception, nor call 'break').  But for me, the most important thing about exception handling is to be able to run through code and deal with errors separately, and not have to have a million nested IFs.  Compare that code to this:



<?php



if (!$test_condition) {

    $error = 'test condition failed';

} elseif (!test_function()) {

    $error = 'test function failed';

} else {

    echo 'success!';

}



if (isset($error)) {

    echo $error;

}



?>



At first this seems no more cumbersome, but what if test_function took arguments that required complicated preparation?  You would need a mess like this:



<?php



if (!$test_condition) {

    $error = 'test condition failed';

} else {

    $fooRes = mysql_query('SELECT foo FROM bar LIMIT 1');

    $fooRow = mysql_fetch_assoc($fooRes);

    

    if (!test_function($fooRow)) {

        $error = 'test function failed';

    } else {

        echo 'success!';

    }

}



?>



Obviously this could get out of hand quickly.  In the first example, you can just prepare the argument before the 'if (!test_function($fooRow))', and on the same nesting level.



This method is also somewhat more flexible in that you can generate 'exceptions', and then deal with them much later (which may be desirable if they aren't really threatening).  In PHP5, a catch block must always follow a try block directly.

tatarynowicz at gmail dot com
09-Jul-2006 06:26


Carlos Konstanski: You can't directly extract previous lexical environment in non-functional languages.



What you probably _can_ do is to create a function that dumps the current lexical environment (to an external file or to stdout) and then use it to recreate past states.



function log_lex ($vars, $functions) {

  $f = fopen('lex_env.log', 'a');

  fwrite($f, serialize(array($vars, $functions, get_defined_constants(), debug_backtrace()));

  fclose($f);

}



log_lex (get_defined_vars(), get_defined_functions());



Writing the parser/browser for dumps is the difficult part. Then just sprinkle the code with log_lex() and see how the program runs.



Sure, it works better in Lisp, but Lisp isn't supported on the webservers I work with, and making PHP more like Lisp would probably make it as popular as Lisp is.

13-May-2006 04:49


@armenio at inmidiaweb dot com dot br:

When echoing into JavaScript code must use htmlspecialchars(addslashes($this->msg)) - think what would happen if msg was "')</script>";

armenio at inmidiaweb dot com dot br
10-May-2006 01:23


<?php

class alert{

    

    public $return;

    public $msg;

    

    public function __construct($value){

        $this->msg = $value;

    }

    public function OutPut(){

        $this->return  = '<script language="JavaScript">';

        $this->return .= '    alert("'.$this->msg.'")';

        $this->return .= '</script>';

        return $this->return;

    }

}

$msg = new alert('classe criada com sucesso');

$msg->OutPut();

?>

fjoggen at gmail dot com
26-Apr-2006 01:58


This code will turn php errors into exceptions:



<?php

function exceptions_error_handler($severity, $message, $filename, $lineno) { 

    throw new ErrorException($message, 0, $severity, $filename, $lineno); 

}



set_error_handler('exceptions_error_handler');

?>



However since <?php set_error_handler()?> doesn't work with fatal errors, you will not be able to throw them as Exceptions.

gerry03 at 4warding dot com
13-Nov-2005 09:39


Good PHP5 article about exceptions. I have read quite a few now and this is the only one I've liked:



http://www.andreashalter.ch/phpug/20040115/

Carlos Konstanski
29-Sep-2005 10:17


Being able to catch the exception up the call stack from where it is thrown is a good idea, in that it lets you handle the exception closer to where it ought to be handled - in the calling code.  Not as good as common lisp though, where you can call a condition handler that resides up the stack without actually unwinding the stack.  With this additional feature, you have the lexical environment at the point where the exception occured, combined with the relocation of the handler to a place where the lower-level function's failure can be addressed from the standpoint of the calling code.



Even so, this fluke of PHP that lets you place the try...catch block up the stack from the actual point of error could be used to implement a neat shift in error-handling architecture, where the caller gets to decide how to proceed after an error.

jd at wuputah dot com
06-May-2005 07:15


PHP5 supports exception throwing inside a function, and catching it outside that function call. There is no mention of this in documentation but it works just fine, as tested by this sample code:



<?php



function exceptionFunction() { 

        throw new Exception("Throwing an exception!"); 

}



try {

        exceptionFunction();

} catch (Exception $e) {

        echo "Exception caught!\n";

}



?>



The result in PHP 5.0.3 is "Exception caught!"



Further tests show that nested functions with exceptions, methods throwing exceptions, etc all work the same way. This is like declaring all classes (or methods) in Java as "class ClassName throws Exception". While I consider this a good thing, you should be aware that any thrown exception will propagate up your stack until it is either caught or runs out of stack.

ravindra at gatewaytechnolabs dot com
29-Oct-2004 01:34


<?php

/*I add here example for nested Exception Handling.*/

 try

 {

   try

   {

     throw new exception();

   }

   catch(exception $m)

   {

    print $m;

    print "<br>";

    print "inner exception"."<br>";

   }

   throw new exception();

 }

 catch(exception $e)

 {

  print $e;

  print "outer exception";

 }

 ?>

gomikochar at yahoo dot com
10-Oct-2004 07:00


To re-throw an exception, you must specify the name of the variable after throw in the catch block, i.e., 



<?php 



try {

  try {

    throw new Exception("Unknown error");

  }

  catch (Exception $ie) {  // Inner Catch Block

    throw;  // this will NOT work; parse error

    throw $ie;  // this will re-throw the exception to the outer catch block

  }

}

catch (Exception $oe) {  // Outer Catch Block

  echo $oe;

}



?>

php5 at grapio dot nl
06-Oct-2004 06:16


The base exception class, which is build in PHP5 has also a function getLine(). This is as expected if you look at the class define. But it is not noticed there. 

I had just this code:

<?php

try

{

     throw new Exception("test")

}

catch (Exception $e)

{

  echo $e->getLine()

}

?>

And this worked.

ravindra at gatewaytechnolabs dot com
01-Oct-2004 04:23


Like Java php5 also supports nesting of try catch.

moirae at centrum dot cz
21-Aug-2004 06:16


Really good article about exceptions in php5: http://www.zend.com/php5/articles/php5-exceptions.php

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MANUAL DE PHP

windsurf mercedes camper

CapÃtulo 20. Excepciones

Extendiendo excepciones

MANUAL DE PHP

windsurf mercedes camper

CapÃ­tulo 20. Excepciones

Extendiendo excepciones

CapÃtulo 20. Excepciones