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HomeBackend DevelopmentPHP Tutorial{php 5异常(Exception)}

异常(Exception):异常处理用于在指定的错误(异常)情况发生时改变脚本的正常流程。

当异常被触发时,通常会发生:

当前代码状态被保存 代码执行被切换到预定义的异常处理器函数 根据情况,处理器也许会从保存的代码状态重新开始执行代码,终止脚本执行,或从代码中另外的位置继续执行脚本

我们将展示不同的错误处理方法:

异常的基本使用 创建自定义的异常处理器 多个异常 重新抛出异常 设置顶层异常处理器

              <p class="sycode">              基本使用 :              </p>              <p class="sycode">              Try,     throw     和     catch    要避免上面例子出现的错误,我们需要创建适当的代码来处理异常。处理处理程序应当包括:Try     -     使用异常的函数应该位于     "    try    "     代码块内。如果没有触发异常,则代码将照常继续执行。但异常被触发,会抛出一个异常    。              </p>              <p class="sycode">              Throw     -     这里规定如何触发异常。每一个     "    throw    "     必须对应至少一个     "    catch    "    Catch     -         "    catch    "     代码块会捕获异常,并创建一个包含异常信息的对象    <?    phpfunction inverse($x) {     if     (    !    $x) {     throw         new     Exception(    '    Division by zero.    '    ); }     else         return         1    /    $x;}    try     { echo inverse(    5    ) .     "    \n    "    ; echo inverse(    0    ) .     "    \n    "    ;}     catch     (Exception $e) { echo     '    Caught exception:     '    , $e    ->    getMessage(),     "    \n    "    ;}    //     Continue execution        echo     '    Hello World    '    ;    ?>              </p>


~创建一个自定义的 Exception 类

创建自定义的异常处理程序非常简单。我们简单地创建了一个专门的类,当 PHP 中发生异常时,可调用其函数。该类必须是 exception 类的一个扩展。

这个自定义的 exception 类继承了 PHP 的 exception 类的所有属性,您可向其添加自定义的函数。

我们开始创建 exception 类:

              <p class="sycode">              <?    php    class     customException extends Exception {     public     function errorMessage() {     //    error message         $errorMsg     =         '    Error on line     '    .$    this    ->    getLine().    '     in     '    .$    this    ->    getFile() .    '    : <b>    '    .$    this    ->    getMessage().    '    </b> is not a valid E-Mail address    '    ;     return     $errorMsg; } }$email     =         "    someone@example...com    "    ;    try     {     //    check if              if    (filter_var($email, FILTER_VALIDATE_EMAIL)     ===     FALSE) {     //    throw exception if email is not valid             throw         new     customException($email); } }    catch     (customException $e) {     //    display custom message         echo $e    ->    errorMessage(); }    ?>              </p>

~多个异常

可以为一段脚本使用多个异常,来检测多种情况。

可以使用多个 if..else 代码块,或一个 switch 代码块,或者嵌套多个异常。这些异常能够使用不同的 exception 类,并返回不同的错误消息:

              <p class="sycode">              <?    php    class     customException extends Exception{    public     function errorMessage(){    //    error message        $errorMsg     =         '    Error on line     '    .$    this    ->    getLine().    '     in     '    .$    this    ->    getFile().    '    : <b>    '    .$    this    ->    getMessage().    '    </b> is not a valid E-Mail address    '    ;    return     $errorMsg;}}$email     =         "    someone@example.com    "    ;    try     {     //    check if              if    (filter_var($email, FILTER_VALIDATE_EMAIL)     ===     FALSE) {     //    throw exception if email is not valid             throw         new     customException($email); }     //    check for "example" in mail address             if    (strpos($email,     "    example    "    )     !==     FALSE) {     throw         new     Exception(    "    $email is an example e-mail    "    ); } }    catch     (customException $e) { echo $e    ->    errorMessage(); }    catch    (Exception $e) { echo $e    ->    getMessage(); }    ?>              </p>


~重新抛出异常

有时,当异常被抛出时,您也许希望以不同于标准的方式对它进行处理。可以在一个 "catch" 代码块中再次抛出异常。

脚本应该对用户隐藏系统错误。对程序员来说,系统错误也许很重要,但是用户对它们并不感兴趣。为了让用户更容易使用,您可以再次抛出带有对用户比较友好的消息的异常:

              <p class="sycode">              <?    php    class     customException extends Exception {     public     function errorMessage() {     //    error message         $errorMsg     =     $    this    ->    getMessage().    '     is not a valid E-Mail address.    '    ;     return     $errorMsg; } }$email     =         "    someone@example.com    "    ;    try     {     try     {     //    check for "example" in mail address             if    (strpos($email,     "    example    "    )     !==     FALSE) {     //    throw exception if email is not valid             throw         new     Exception($email); } }     catch(Exception $e) { //re-throw exception throw new     customException($email); } }    catch     (customException $e) {     //    display custom message         echo $e    ->    errorMessage(); }    ?>              </p>


~设置顶层异常处理器 (Top Level Exception Handler)

set_exception_handler() 函数可设置处理所有未捕获异常的用户定义函数。

<?phpfunction myException($exception){echo "<b>Exception:</b> " , $exception->getMessage();}set_exception_handler('myException');throw new Exception('Uncaught Exception occurred');?>

以上代码的输出应该类似这样:

Exception: Uncaught Exception occurred

在上面的代码中,不存在 "catch" 代码块,而是触发顶层的异常处理程序。应该使用此函数来捕获所有未被捕获的异常。

异常的规则 需要进行异常处理的代码应该放入 try 代码块内,以便捕获潜在的异常。 每个 try 或 throw 代码块必须至少拥有一个对应的 catch 代码块。 使用多个 catch 代码块可以捕获不同种类的异常。 可以在 try 代码块内的 catch 代码块中再次抛出(re-thrown)异常。

简而言之:如果抛出了异常,就必须捕获它。

~

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