Zend Framework教程之Autoloading用法详解
本文实例讲述了Zend Framework教程之Autoloading用法。分享给大家供大家参考，具体如下： 一、概述 自动加载是一种机制，无需依赖手动编写PHP代码。参考PHP手册自动加载，一旦自动加载器被定义，你试图使用一个没有定义的类或接口的情况下，它会自动被调用。
本文实例讲述了Zend Framework教程之Autoloading用法。分享给大家供大家参考，具体如下：
一、概述
自动加载是一种机制，无需依赖手动编写PHP代码。参考»PHP手册自动加载，一旦自动加载器被定义，你试图使用一个没有定义的类或接口的情况下，它会自动被调用。
使用自动加载，在项目中你不必担心类的存放位置。定义一个良好定义的自动加载器，您不需要考虑一个类文件相对于当前类文件的位置，您只需使用类，自动加载器将自动查找文件。
此外，自动加载，确保只加载一次，提升了性能 -所以可以用它替代require_once()。
Zend Framework 鼓励使用自动加载，并提供了许多工具实现自动加载代码库以及应用程序代码。下面将介绍这些工具，以及如何有效地使用它们。
自动加载的实现约定
类命名约定
Zend Framework借鉴了 PEAR的想法，即类名与文件系统的1:1的关系。简单地说，下划线字符("_")替换目录分隔，以代表该文件的路径，然后添加后缀“.php”。例如，类“Foo_Bar_Baz”将对应文件系统上的"Foo/Bar/Baz.php"。假设已通过PHP的include_path设置类的位置，这使得可以通过 include() 和 require()找到相对include_path中设置的路径查找文件名。
此外，推荐使用供应商名称或项目名称作为前缀。这意味着，你写的所有的类都有一个共同的类前缀，例如，Zend Framework的所有代码前缀为“Zend_”。这种命名约定有助于防止命名冲突。在ZendFramework中，我们经常提到“namespace”前缀，要注意不要把它与PHP的本地命名空间混淆。
自动加载器设计约定
Zend Framework通过Zend_Loader_Autoloader实现支持自动加载的，主要提供有以下目标和设计元素：
提供命名空间匹配。如果类的命名空间前缀是没有注册的命名空间，会返回FALSE。
允许定义自动加载器作为一个备用的自动加载器。一个团队可能分布广泛，或使用一个为定义的命名空间前缀情况下，它会尝试匹配任何命名空间前缀。但是，这种做法是不推荐，因为它可能会导致不必要的查找。
允许开启禁止错误提示。 因此，默认情况下，它应该处于关闭状态。开发阶段，可以启用它。
可以自定义自动加载。一些开发商不希望使用Zend_Loader::loadClass()自动加载，但仍想使用Zend Framework的自动加载机制。 Zend_Loader_Autoloader允许使用自定义的自动加载。
允许使用SPL自动加载回调链。这样做的目的是允许指定额外的自动加载器 。
二、用法：
通常，只需将需要引入包含类，然后实例化它即可。由于Zend_Loader_Autoloader采用的单例模式，可以使用getInstance()方法来获取一个实例。
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默认情况下，可以加载命名空间前缀为"Zend_"或 "ZendX_"的任何类，只要确保已经指定include_path。
如果想使用其他的命名空间前缀？最好的，最简单的方法是调用registerNamespace() 方法。您可以通过传递一个单一的命名空间前缀，或一个数组：
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或者，你可以把Zend_Loader_Autoloader作为一个“备用”自动加载器。这意味着如果命名空间无论是否定义，都会尝试自动加载。
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（注意：这种方式是不推荐的，尽量不要使用）。
Zend_Loader_Autoloader的内部实现是使用 Zend_Loader::loadClass() 加载类的。该方法的使用 include() 来尝试加载给定的类文件。 include()将返回一个布尔值，如果没有成功返回FALSE - 而且还发出PHP警告。 可能会导致以下问题：
如果启用了display_errors，警告将包含在输出中。
根据你所配置的error_reporting级别，它也可以输出到日志中。
可以禁止这些错误消息，具体如下：（但注意，display_errors启用时，错误日志将始终显示。）
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选择一个Zend Framework的版本
ZendFramework/
|-- 1.9.2/
| |-- library/
|-- ZendFramework-1.9.1-minimal/
| |-- library/
|-- 1.8.4PL1/
| |-- library/
|-- 1.8.4/
| |-- library/
|-- ZendFramework-1.8.3/
| |-- library/
|-- 1.7.8/
| |-- library/
|-- 1.7.7/
| |-- library/
|-- 1.7.6/
| |-- library/
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|
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|
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也可以使用配置文件
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Autoloader接口
注：命名空间前缀和PHP命名空间
PHP5.3已经发布。该版本中，PHP现在已经正式支持命名空间。
然而，Zend Framework的命名空间和PHP 5.3的命名空间完全不同的。 Zend Framework中，提到的“命名空间”，是指一个类前缀。例如，所有的Zend Framework的类名称的前缀“Zend_”。 这是我们指定的“命名空间”。
在Zend Framework 2.0.0使用了原生的PHP命名空间。
自动加载器除了能够指定任意回调以外，Zend Framework还定义了一个需要自动加载类实现的接口Zend_Loader_Autoloader_Interface：
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如果您希望在Zend Framework中使用自定义的自动加载器，可以使用 Zend_Loader_Autoloader的 pushAutoloader()和unshiftAutoloader()方法。
通过这些方法将在Zend Framework的内部自动装载器之后追加或之前使用自定义的加载器。
每个方法接受一个可选的第二个参数，类的命名空间前缀。自动加载器只查找给定的类前缀。如果不是指定的类前缀，将跳过自动加载器 , 这可能是一种性能改进方式。
当使用这个接口时，你需要传递类实例到Zend_Loader_Autoloader类的pushAutoloader()和unshiftAutoloader()方法，具体如下：
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Zend_Loader_Autoloader的相关方法
| Method | Return Value | Parameters | Description |
|---|---|---|---|
| getInstance() | Zend_Loader_Autoloader | N/A |
获取实例 |
| resetInstance() | <font color="#000000" face="NSimsun">void</font> |
N/A |
重置Zend_Loader_Autoloadersingleton实例的状态，恢复它的原始状态，注销所有的自动加载器回调和所有注册的命名空间。 |
| autoload($class) | <font color="#000000"><font face="NSimsun">string|<strong><tt>FALSE</tt></strong></font></font> |
|
试图加载一个类。 |
| setDefaultAutoloader($callback) | Zend_Loader_Autoloader |
|
指定默认的加载器回调 |
| getDefaultAutoloader() | <font color="#000000" face="NSimsun">callback</font> |
N/A |
获取默认的加载器接口；默认是Zend_Loader::loadClass(). |
| setAutoloaders(array $autoloaders) | Zend_Loader_Autoloader |
|
设置在自动加载器栈使用具体的自动加载器列表。自动加载器列表中的每个项目必须是PHPcallback。 |
| getAutoloaders() | Array | N/A |
|
| getNamespaceAutoloaders($namespace) | Array |
|
获取所有已注册的自动加载器来加载一个特定的的命名空间。 |
| registerNamespace($namespace) | Zend_Loader_Autoloader |
|
注册命名空间. If$namespace is a string, it registers that namespace; if it's an array of strings, registers each as a namespace. |
| unregisterNamespace($namespace) | Zend_Loader_Autoloader |
|
|
| getRegisteredNamespaces() | Array | N/A |
|
| suppressNotFoundWarnings($flag = null) | <font color="#000000" face="NSimsun">boolean|Zend_Loader_Autoloader</font> |
|
错误提示 |
| setFallbackAutoloader($flag) | Zend_Loader_Autoloader |
|
|
| isFallbackAutoloader() | Boolean | N/A |
|
| getClassAutoloaders($class) | Array |
|
|
| unshiftAutoloader($callback, $namespace = '') | Zend_Loader_Autoloader |
|
|
| pushAutoloader($callback, $namespace = '') | Zend_Loader_Autoloader |
|
|
| removeAutoloader($callback, $namespace = '') | Zend_Loader_Autoloader |
|
更多关于zend相关内容感兴趣的读者可查看本站专题：《Zend FrameWork框架入门教程》、《php优秀开发框架总结》、《Yii框架入门及常用技巧总结》、《ThinkPHP入门教程》、《php面向对象程序设计入门教程》、《php+mysql数据库操作入门教程》及《php常见数据库操作技巧汇总》
希望本文所述对大家PHP程序设计有所帮助。
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