Name resolution rules
If you intend on creating a lot of custom exceptions, you may find this code useful. I've created an interface and an abstract exception class that ensures that all parts of the built-in Exception class are preserved in child classes. It also properly pushes all information back to the parent constructor ensuring that nothing is lost. This allows you to quickly create new exceptions on the fly. It also overrides the default __toString method with a more thorough one.
<?php
interface IException
{
/* Protected methods inherited from Exception class */
public function getMessage(); // Exception message
public function getCode(); // User-defined Exception code
public function getFile(); // Source filename
public function getLine(); // Source line
public function getTrace(); // An array of the backtrace()
public function getTraceAsString(); // Formated string of trace
/* Overrideable methods inherited from Exception class */
public function __toString(); // formated string for display
public function __construct($message = null, $code = 0);
}
abstract class CustomException extends Exception implements IException
{
protected $message = 'Unknown exception'; // Exception message
private $string; // Unknown
protected $code = 0; // User-defined exception code
protected $file; // Source filename of exception
protected $line; // Source line of exception
private $trace; // Unknown
public function __construct($message = null, $code = 0)
{
if (!$message) {
throw new $this('Unknown '. get_class($this));
}
parent::__construct($message, $code);
}
public function __toString()
{
return get_class($this) . " '{$this->message}' in {$this->file}({$this->line})\n"
. "{$this->getTraceAsString()}";
}
}
?>
Now you can create new exceptions in one line:
<?php
class TestException extends CustomException {}
?>
Here's a test that shows that all information is properly preserved throughout the backtrace.
<?php
function exceptionTest()
{
try {
throw new TestException();
}
catch (TestException $e) {
echo "Caught TestException ('{$e->getMessage()}')\n{$e}\n";
}
catch (Exception $e) {
echo "Caught Exception ('{$e->getMessage()}')\n{$e}\n";
}
}
echo '<pre>' . exceptionTest() . '</pre>';
?>
Here's a sample output:
Caught TestException ('Unknown TestException')
TestException 'Unknown TestException' in C:\xampp\htdocs\CustomException\CustomException.php(31)
#0 C:\xampp\htdocs\CustomException\ExceptionTest.php(19): CustomException->__construct()
#1 C:\xampp\htdocs\CustomException\ExceptionTest.php(43): exceptionTest()
#2 {main}
异常处理
PHP 5 添加了类似于其它语言的异常处理模块。在 PHP 代码中所产生的异常可被 throw 语句抛出并被 catch 语句捕获。需要进行异常处理的代码都必须放入 try 代码块内，以便捕获可能存在的异常。每一个 try 至少要有一个与之对应的 catch。使用多个 catch 可以捕获不同的类所产生的异常。当 try 代码块不再抛出异常或者找不到 catch 能匹配所抛出的异常时，PHP 代码就会在跳转到最后一个 catch 的后面继续执行。当然，PHP 允许在 catch 代码块内再次抛出（throw）异常。
当一个异常被抛出时，其后（译者注：指抛出异常时所在的代码块）的代码将不会继续执行，而 PHP 就会尝试查找第一个能与之匹配的 catch。如果一个异常没有被捕获，而且又没用使用 set_exception_handler() 作相应的处理的话，那么 PHP 将会产生一个严重的错误，并且输出 Uncaught Exception ... （未捕获异常）的提示信息。
Example#1 抛出一个异常
<?php
try {
$error = 'Always throw this error';
throw new Exception($error);
// 从这里开始，tra 代码块内的代码将不会被执行
echo 'Never executed';
} catch (Exception $e) {
echo 'Caught exception: ', $e->getMessage(), "\n";
}
// 继续执行
echo 'Hello World';
?>
扩展 PHP 内置的异常处理类
用户可以用自定义的异常处理类来扩展 PHP 内置的异常处理类。以下的代码说明了在内置的异常处理类中，哪些属性和方法在子类中是可访问和可继承的。译者注：以下这段代码只为说明内置异常处理类的结构，它并不是一段有实际意义的可用代码。
Example#2 内置的异常处理类
<?php
class Exception
{
protected $message = 'Unknown exception'; // 异常信息
protected $code = 0; // 用户自定义异常代码
protected $file; // 发生异常的文件名
protected $line; // 发生异常的代码行号
function __construct($message = null, $code = 0);
final function getMessage(); // 返回异常信息
final function getCode(); // 返回异常代码
final function getFile(); // 返回发生异常的文件名
final function getLine(); // 返回发生异常的代码行号
final function getTrace(); // backtrace() 数组
final function getTraceAsString(); // 已格成化成字符串的 getTrace() 信息
/* 可重载的方法 */
function __toString(); // 可输出的字符串
}
?>
如果使用自定义的类来扩展内置异常处理类，并且要重新定义构造函数的话，建议同时调用 parent::__construct() 来检查所有的变量是否已被赋值。当对象要输出字符串的时候，可以重载 __toString() 并自定义输出的样式。
Example#3 扩展 PHP 内置的异常处理类
<?php
/**
* 自定义一个异常处理类
*/
class MyException extends Exception
{
// 重定义构造器使 message 变为必须被指定的属性
public function __construct($message, $code = 0) {
// 自定义的代码
// 确保所有变量都被正确赋值
parent::__construct($message, $code);
}
// 自定义字符串输出的样式
public function __toString() {
return __CLASS__ . ": [{$this->code}]: {$this->message}\n";
}
public function customFunction() {
echo "A Custom function for this type of exception\n";
}
}
/**
* 创建一个用于测试异常处理机制的类
*/
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:
// 抛出自定义异常
throw new MyException('1 is an invalid parameter', 5);
break;
case self::THROW_DEFAULT:
// 抛出默认的异常
throw new Exception('2 isnt allowed as a parameter', 6);
break;
default:
// 没有异常的情况下，创建一个对象
$this->var = $avalue;
break;
}
}
}
// 例子 1
try {
$o = new TestException(TestException::THROW_CUSTOM);
} catch (MyException $e) { // 捕获异常
echo "Caught my exception\n", $e;
$e->customFunction();
} catch (Exception $e) { // 被忽略
echo "Caught Default Exception\n", $e;
}
// 执行后续代码
var_dump($o);
echo "\n\n";
// 例子 2
try {
$o = new TestException(TestException::THROW_DEFAULT);
} catch (MyException $e) { // 不能匹配异常的种类，被忽略
echo "Caught my exception\n", $e;
$e->customFunction();
} catch (Exception $e) { // 捕获异常
echo "Caught Default Exception\n", $e;
}
// 执行后续代码
var_dump($o);
echo "\n\n";
// 例子 3
try {
$o = new TestException(TestException::THROW_CUSTOM);
} catch (Exception $e) { // 捕获异常
echo "Default Exception caught\n", $e;
}
// 执行后续代码
var_dump($o);
echo "\n\n";
// 例子 4
try {
$o = new TestException();
} catch (Exception $e) { // 没有异常，被忽略
echo "Default Exception caught\n", $e;
}
// 执行后续代码
var_dump($o);
echo "\n\n";
?>
add a note
User Contributed Notes异常处理
ask at nilpo dot com
27-May-2009 07:19
27-May-2009 07:19
rctay89 at gmail dot com
17-Dec-2008 09:33
17-Dec-2008 09:33
The internal implementation of Exception.__toString is something like this:
<?php
class MyException extends Exception {
public function __toString() {
return "exception '".__CLASS__ ."' with message '".$this->getMessage()."' in ".$this->getFile().":".$this->getLine()."\nStack trace:\n".$this->getTraceAsString();
}
}
?>
Useful if you want to customize your exception toString format, but not to deviate too much from the built-in one.
Shot (Piotr Szotkowski)
21-Oct-2008 07:13
21-Oct-2008 07:13
‘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.’
‘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().’
These two sentences seem a bit contradicting about what happens ‘when a catch matching the thrown exception’s class is not present’ (and the second sentence is actually correct).
i dot crash17 at gmail dot com
05-Apr-2008 04:50
05-Apr-2008 04:50
As a response to Anonymous on 28-Dec-2007 07:25, please, remember that creating lots of MyXExtension classes is wrong if and only if your object oriented model is not well designed.
One should (I even think MUST) create as many custom Exception extentions as the design needs, because every different type of exception requires (or will require, even if we haven't think about it) a different response, from showing different messages to the user, to log error messages or even kill the script.
Forwarding can become confusing if you are using a big number of classes and packages, and is useless if you are using few (unless you are too lasy to write 3, 5 or 10 includes and their respective Exception classes).
Remember that object orientation was created to give more semantics to the programming code so the programmer understands better what is written without having to excecute the code, so don't do things that go against that, which is the cool thing about object orientation.
michael dot ochs at gmx dot net
21-Mar-2008 11:44
21-Mar-2008 11:44
Actually it isn't possible to do:
<?php
someFunction() OR throw new Exception();
?>
This leads to a T_THROW Syntax Error. If you want to use this kind of exceptions, you can do the following:
<?php
function throwException($message = null,$code = null) {
throw new Exception($message,$code);
}
someFunction() OR throwException();
?>
chugadie dot geo at yahoo dot com
05-Mar-2008 06:40
05-Mar-2008 06:40
@webmaster at asylum-et dot com
What Mo is describing is bug 44053 (http://bugs.php.net/bug.php?id=44053) in which exceptions cannot be caught if you are using a custom error handler to catch warnings, notices, etc.
omnibus at omnibus dot edu dot pl
04-Dec-2007 11:11
04-Dec-2007 11:11
Just to be more precise in what Frank found:
Catch the exceptions always in order from the bottom to the top of the Exception and subclasses class hierarchy. If you have class MyException extending Exception and class My2Exception extending MyException always catch My2Exception before MyException.
Hope this helps
frank at netventures dot com dot au
07-Nov-2007 11:43
07-Nov-2007 11:43
If you are going to use multiple catches within a try-catch then do not forget the stacking order of those catches!
This is important as any classes that extend the Exception class, like MyException in example 20.3, will be caught in the Exception case. This is because your newly extended class also has a class type of Exception. This baffled me for awhile as the examples here worked but mine didn't because my first catch was trying to catch Exception.
Example:
<?php
/**
* My1Exception extends Exception
* My2Exception extends Exception
*/
/**
* This will always fall in the first exception
*/
try {
throw new My1Exception("My fail english? That's unpossible", 69);
} catch (Exception $e) {
print "Incorrect Exception";
} catch (My1Exception $e) {
print "Correct Exception but I won't be here";
} catch (My2Exception $e) {
print "Again, incorrect";
}
/**
* Whereas here, the catch stacking order was changed so our throw will cascade into the correct catch
*/
try {
throw new My1Exception("My cat's breath smells like cat food", 69);
} catch (My2Exception $e) {
print "Incorrect Exception";
} catch (My1Exception $e) {
print "Correct Exception and I WILL be printed";
} catch (Exception $e) {
print "Again, incorrect";
}
?>
So, ALWAYS keep the Exception catch block at the bottom, then any of the other extended exceptions that extend from Exception, then any of your other extended exceptions that extend from those extended exceptions, etc
hartym dot dont dot like dot spam at gmail dot com
18-Oct-2007 11:41
18-Oct-2007 11:41
@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');
}
/* ... do whatever you need with database, that may mail and throw exceptions too ... */
mysql_close($connection);
}
catch (Exception $e)
{
/* ... add logging stuff there if you need ... */
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.
peter dot goodman at gmail dot com
15-Jun-2007 02:52
15-Jun-2007 02:52
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
24-Jan-2007 05:52
24-Jan-2007 05:52
Further to dexen at google dot me dot up with "use destructors to perform a cleanup in case of exception". The fact that PHP5 has destructors, exception handling, and predictable garbage collection (if there's a single reference in scope and the scope is left then the destructor is called immediately) allows for the use of the RAII idiom.
http://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initialization and my own http://www.hackcraft.net/RAII/ describe this.
dexen at google dot me dot up
18-Sep-2006 01:45
18-Sep-2006 01:45
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');
/* some SQL Operations */
someFunction();
/* more SQL Operations */
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');
/* some SQL Operations */
try {
someFunction(); }
catch ( Exception $e ) {
DB::query('UNLOCK TABLES');
throw $e;
}
/* more SQL Operations */
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( /*read lock list*/array('a', 'b'), /*write lock list*/array('b') );
/* some SQL Operations */
someFunction();
/* $SQLLocker gets destructed there if someFunction() throws an exception */
DB::query('UNLOCK TABLES');
/* other SQL Operations */
/* $SQLLocker gets destructed there if someFunction() does not throw an exception */
}
class DB {
function locker ( $read, $write ) {
DB::query( /*locks*/);
$ret = new DB;
$ret->onDestruct = 'UNLOCK TABLES';
return $ret;
}
function _destructor() {
if ( $this->onDestruct )
DB::query($this->onDestruct);
}
}
?>
jazfresh at hotmail.com
08-Aug-2006 05:18
08-Aug-2006 05:18
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 $var)", because all exceptions are sub-classes of the built-in Exception.
fjoggen at gmail dot com
26-Apr-2006 08:58
26-Apr-2006 08:58
This code will turn php errors into exceptions:
<?php
function exceptions_error_handler($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.
jd at wuputah dot com
07-May-2005 02:15
07-May-2005 02:15
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.