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Глава 20. Exceptions
PHP 5 has an exception model similar to that of other programming languages.
An exception can be thrown, and caught
("catched") within PHP. Code may be surrounded in a
try block, to facilitate the catching of potential
exceptions. Each try must have at least one
corresponding catch block. Multiple
catch blocks can be used to catch different classes of
exeptions. Normal execution (when no exception is thrown within the
try block, or when a catch matching
the thrown exception's class is not present) will continue after that last catch
block defined in sequence. Exceptions can be thrown (or
re-thrown) within a catch block.
When an exception is thrown, code following the statement will not be
executed, and PHP will attempt to find the first matching
catch block. If an
exception is not caught, a PHP Fatal Error will be issued with an
"Uncaught Exception ..." message, unless a handler has
been defined with set_exception_handler().
Пример 20-1. Throwing an Exception |
<?php
try {
$error = 'Always throw this error';
throw new Exception($error);
echo 'Never executed';
} catch (Exception $e) {
echo 'Caught exception: ', $e->getMessage(), "\n";
}
echo 'Hello World';
?>
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A User defined Exception class can be defined by extending the built-in
Exception class. The members and properties below, show what is accessible
within the child class that derives from the built-in Exception class.
Пример 20-2. The Built in Exception class |
<?php
class Exception
{
protected $message = 'Unknown exception'; protected $code = 0; protected $file; protected $line; function __construct($message = null, $code = 0);
final function getMessage(); final function getCode(); final function getFile(); final function getLine(); final function getTrace(); final function getTraceAsString(); function __toString(); }
?>
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If a class extends the built-in Exception class and re-defines the constructor, it is highly recomended
that it also call parent::__construct()
to ensure all available data has been properly assigned. The __toString() method can be overriden
to provide a custom output when the object is presented as a string.
Пример 20-3. Extending the Exception class |
<?php
class MyException extends Exception
{
public function __construct($message, $code = 0) {
parent::__construct($message, $code);
}
public function __toString() {
return __CLASS__ . ": [{$this->code}]: {$this->message}\n";
}
public function customFunction() {
echo "A Custom function for this type of exception\n";
}
}
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 new MyException('1 is an invalid parameter', 5);
break;
case self::THROW_DEFAULT:
throw new Exception('2 isnt allowed as a parameter', 6);
break;
default:
$this->var = $avalue;
break;
}
}
}
try {
$o = new TestException(TestException::THROW_CUSTOM);
} catch (MyException $e) { echo "Caught my exception\n", $e;
$e->customFunction();
} catch (Exception $e) { echo "Caught Default Exception\n", $e;
}
var_dump($o);
echo "\n\n";
try {
$o = new TestException(TestException::THROW_DEFAULT);
} catch (MyException $e) { echo "Caught my exception\n", $e;
$e->customFunction();
} catch (Exception $e) { echo "Caught Default Exception\n", $e;
}
var_dump($o);
echo "\n\n";
try {
$o = new TestException(TestException::THROW_CUSTOM);
} catch (Exception $e) { echo "Default Exception caught\n", $e;
}
var_dump($o);
echo "\n\n";
try {
$o = new TestException();
} catch (Exception $e) { echo "Default Exception caught\n", $e;
}
var_dump($o);
echo "\n\n";
?>
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Georgiana Beju
25-Oct-2007 11:33
The first example should read
<?php
echo "Caught Custom Exception\n", $e;
?>
instead of
<?php
echo "Caught Default Exception\n", $e;
?>
hartym dot dont dot like dot spam at gmail dot com
18-Oct-2007 04:41
@serenity: of course you need to throw exception within the try block, catch will not watch fatal errors, nor less important errors but only exceptions that are instanceof the exception type you're giving. Of course by within the try block, i mean within every functions call happening in try block.
For example, to nicely handle old mysql errors, you can do something like this:
<?php
try
{
$connection = mysql_connect(...);
if ($connection === false)
{
throw new Exception('Cannot connect do mysql');
}
mysql_close($connection);
}
catch (Exception $e)
{
echo "This page cannot be displayed";
}
?>
By doing so, you're aiming at the don't repeat yourself (D.R.Y) concept, by managing error handling at only one place for the whole.
serenity at exscape dot org
23-Aug-2007 04:20
WOWA: You certainly do *not* need to throw an exception within the try block, that was just an example.
The only difference from JavaScript is that you need to add a class to catch (i.e. catch (Exception $e) instead of catch (e)).
Example:
<?php
try
{
}
catch (Exception $e)
{
echo "Caught exception: " . $e->getMessage();
}
?>
Jouni
20-Jun-2007 01:59
If you are having troubles with exceptions and you are using eAccelerator, it is because of an eAccelerator v. 0.9.5 bug. See http://www.eaccelerator.net/ticket/242 for more information. I posted this here so that other eAccelerator users can save their time.
peter dot goodman at gmail dot com
14-Jun-2007 07:52
I've found that exception destructors are not called unless the exception is caught.
I've created a simple solution to this problem (calling __destruct() from __toString() ) and have written up a lengthy article detailing one good use case for this method at http://ioreader.com/2007/06/14/taking-advantage-of-exceptions-in-php5/
Also, one of the useful things about using a destructor as a clean up method is that it is called at the end of a catch statement.
jon at hackcraft dot net
08-Jun-2007 05:00
Hi corrupted_wise.
Your method works, but has a disadvantage in mixing two different error-handling methods.
There is clarity to be had in just using set_error_handler or just using try...catch (I recommend the latter).
If the code you want to avoid using is further inside the same try block than the code that could throw the exception (or called by code in the same block) then it won't be called in the case of the try block being exited due to an error. Therefore there is no need to use set_error_handler.
corrupted_wise at yahoo dot com
30-Apr-2007 04:37
There was a certain section of my code that I needed to completely ignore if it failed. So I trapped it all in a try/catch and utilized the set_error_message, which in essence is like a pseudo-goto clutch. The reason for it all was if it failed during the code, I didn't want it to execute the rest of the respective section but continue outside of it. In the hopes it might help someone else out, here's what I did.
I'm using PHP 5.2.1
<?php
....
try {
set_error_handler(create_function('', "throw new Exception(); return true;"));
...code...
} catch(Exception $e) {
}
restore_error_handler();
?>
romain dot boisnard at gmail dot com
16-Jan-2007 09:43
If you need a Java-like exception with a sort of "Throwable" cause to report the whole stackTrace, you can use this class I wrote and I release under the LGPL GNU Lesser General Public Liscence.
It looks more or less like the original PHP "Exception" but you might want to change the formatting of the returned string in the two last functions. Enjoy :)
<?php
class LinkedException extends Exception {
private $cause;
function __construct($_message = null, $_code = 0, Exception $_cause = null) {
parent::__construct($_message, $_code);
$this->cause = $_cause;
}
public function getCause() {
return $this->cause;
}
public function getStackTrace() {
if ($this->cause !== null) {
$arr = array();
$trace = $this->getTrace();
array_push($arr, $trace[0]);
unset($trace);
if (get_class($this->cause) == "LinkedException") {
foreach ($this->cause->getStackTrace() as $key => $trace) {
array_push($arr, $trace);
}
}
else {
foreach ($this->cause->getTrace() as $key => $trace) {
array_push($arr, $trace);
}
}
return $arr;
}
else {
return $this->getTrace();
}
}
public function showStackTrace() {
$htmldoc = "<p style=\"font-family: monospace; border: solid 1px #000000\"><span style=\"font-weight: bold; color: #000000;\">An exception was thrown :<br/></span>";
$htmldoc.= "Exception code : $this->code<br/>";
$htmldoc.= "Exception message : $this->message<br/>";
$htmldoc.= "<span style=\"color: #0000FF;\">";
$i = 0;
foreach ($this->getStackTrace() as $key => $trace) {
$htmldoc.= $this->showTrace($trace, $i);
$i++;
}
$htmldoc.= "#$i {main}<br/>";
unset($i);
$htmldoc.= "</span></p>";
return $htmldoc;
}
private function showTrace($_trace, $_i) {
$htmldoc = "#$_i ";
if (array_key_exists("file",$_trace)) {
$htmldoc.= $_trace["file"];
}
if (array_key_exists("line",$_trace)) {
$htmldoc.= "(".$_trace["line"]."): ";
}
if (array_key_exists("class",$_trace) && array_key_exists("type",$_trace)) {
$htmldoc.= $_trace["class"].$_trace["type"];
}
if (array_key_exists("function",$_trace)) {
$htmldoc.= $_trace["function"]."(";
if (array_key_exists("args",$_trace)) {
if (count($_trace["args"]) > 0) {
$args = $_trace["args"];
$type = gettype($args[0]);
$value = $args[0];
unset($args);
if ($type == "boolean") {
if ($value) {
$htmldoc.= "true";
}
else {
$htmldoc.= "false";
}
}
elseif ($type == "integer" || $type == "double") {
if (settype($value, "string")) {
if (strlen($value) <= 20) {
$htmldoc.= $value;
}
else {
$htmldoc.= substr($value,0,17)."...";
}
}
else {
if ($type == "integer" ) {
$htmldoc.= "? integer ?";
}
else {
$htmldoc.= "? double or float ?";
}
}
}
elseif ($type == "string") {
if (strlen($value) <= 18) {
$htmldoc.= "'$value'";
}
else {
$htmldoc.= "'".substr($value,0,15)."...'";
}
}
elseif ($type == "array") {
$htmldoc.= "Array";
}
elseif ($type == "object") {
$htmldoc.= "Object";
}
elseif ($type == "resource") {
$htmldoc.= "Resource";
}
elseif ($type == "NULL") {
$htmldoc.= "null";
}
elseif ($type == "unknown type") {
$htmldoc.= "? unknown type ?";
}
unset($type);
unset($value);
}
if (count($_trace["args"]) > 1) {
$htmldoc.= ",...";
}
}
$htmldoc.= ")<br/>";
}
return $htmldoc;
}
}
?>
dexen at google dot me dot up
18-Sep-2006 06:45
Summary:
* use destructors to perform a cleanup in case of exception.
PHP calls method __destruct() on instance of class when variable storing the instance goes out-of-scope (or gets unset). This works for function leave by Exception, aside of plain return. (same as for C++, AFAIK)
aFunction() {
$i = new LockerClass();
throw new MinorErrorEx('Warn user & perform some other activity');
// $i->__destruct() gets called before stack unwind begins, unlocking whatever get locked by new LockerClass();
return $bar;
}
(A lengthy) example:
Let's say you need to perform a series of operaions on SQL database that should not get disrupted. You lock the tables:
<?php
function updateStuff() {
DB::query('LOCK TABLES `a`, `b`, `c` WRITE');
someFunction();
DB::query('UNLOCK TABLES');
} ?>
Now, let's supouse that someFunction() may throw an exception. This would leave us with the tables locked, as the second DB::query() will not get called. This pretty much will cause the next query to fail. You can do it like:
<?php
function updateStuff() {
DB::query('LOCK TABLES `a`, `b` WRITE');
try {
someFunction(); }
catch ( Exception $e ) {
DB::query('UNLOCK TABLES');
throw $e;
}
DB::query('UNLOCK TABLES')
} ?>
However, this is rather ugly as we get code duplication. And what if somebody later modifies updateStuff() function in a way it needs another step of cleanup, but forget to add it to catch () {}? Or when we have multiple things to be cleaned up, of which not all will be valid all the time?
My solution using destructor: i create an instance of class DB holding a query unlocking tables which will be executed on destruction.
<?php
function updateStuff() {
$SQLLocker = DB::locker( array('a', 'b'), array('b') );
someFunction();
DB::query('UNLOCK TABLES');
}
class DB {
function locker ( $read, $write ) {
DB::query( );
$ret = new DB;
$ret->onDestruct = 'UNLOCK TABLES';
return $ret;
}
function _destructor() {
if ( $this->onDestruct )
DB::query($this->onDestruct);
}
}
?>
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> $var) will match the given <classname> *or any of it's sub-classes*.
For example:
<?php
class DisplayException extends Exception {};
class FileException extends Exception {};
class AccessControl extends FileException {}; class IOError extends FileException {}; try {
if(!is_readable($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 $e) {
echo "File error: ".$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.
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
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) { throw; throw $ie; }
}
catch (Exception $oe) { 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.
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