| /usr/includes/Zend/ | /usr/includes/Zend/Acl.php | | 备注：以第一行为例来说明，完全合规的类名是“\Acme\Log\Writer\File_Writer”， 去掉前面的命名空间分隔符'\', 则命名空间前缀为"Acme\Log\Writer", 类名为"File_Writer"。这个命名空间前缀对应的base目录为"./acme-log-writer/lib/", 因此最终加载的文件名为：base目录+类名+".php"， 即"./acme-log-writer/lib/File_Writer.php"
遵循本规范的自动加载器的实现举例， 可参见下面的代码样例。这些实现样例一定不能（MUST NOT）被视为本规范的内容，它们可能（MAY）随时发生改变。
2. 代码样例 以下代码展示了遵循PSR-4的类定义，
闭包(Closure)举例：
<?php/** * An example of a project-specific implementation. * * After registering this autoload function with SPL, the following line * would cause the function to attempt to load the \Foo\Bar\Baz\Qux class * from /path/to/project/src/Baz/Qux.php: * * new \Foo\Bar\Baz\Qux; * * @param string $class The fully-qualified class name. * @return void */spl_autoload_register(function ($class) { // project-specific namespace prefix // 项目的命名空间前缀 $prefix = 'Foo\\Bar\\'; // base directory for the namespace prefix // 命名空间前缀对应的base目录 $base_dir = __DIR__ . '/src/'; // does the class use the namespace prefix? // 检查$class中是否包含命名空间前缀 $len = strlen($prefix); if (strncmp($prefix, $class, $len) !== 0) { // no, move to the next registered autoloader // 未包含，立即返回 return; } // get the relative class name // 获取相对类名 $relative_class = substr($class, $len); // replace the namespace prefix with the base directory, replace namespace // separators with directory separators in the relative class name, append // with .php // 用base目录替代命名空间前缀, // 在相对类名中用目录分隔符'/'来替换命名空间分隔符'\', // 并在后面追加.php组成$file的绝对路径 $file = $base_dir . str_replace('\\', '/', $relative_class) . '.php'; // if the file exists, require it // 如果文件存在，则通过require关键字包含文件 if (file_exists($file)) { require $file; }}); 下面这个类处理多个命名空间：
<?phpnamespace Example;/** * An example of a general-purpose implementation that includes the optional * functionality of allowing multiple base directories for a single namespace * prefix. * 下面例子中在一个命名空间前缀下有多个base目录。 * * Given a foo-bar package of classes in the file system at the following * paths ... * 在下面路径中foo-bar包中存在以下类： * * /path/to/packages/foo-bar/ * src/ * Baz.php # Foo\Bar\Baz * Qux/ * Quux.php # Foo\Bar\Qux\Quux * tests/ * BazTest.php # Foo\Bar\BazTest * Qux/ * QuuxTest.php # Foo\Bar\Qux\QuuxTest * * ... add the path to the class files for the \Foo\Bar\ namespace prefix * as follows: * ...对\Foo\Bar\命名空间前缀，添加类文件的路径 * * <?php * // instantiate the loader * // 初始化loader * $loader = new \Example\Psr4AutoloaderClass; * * // register the autoloader * // 注册autoloader * $loader->register(); * * // register the base directories for the namespace prefix * // 注册命名空间前缀的多个base目录 * $loader->addNamespace('Foo\Bar', '/path/to/packages/foo-bar/src'); * $loader->addNamespace('Foo\Bar', '/path/to/packages/foo-bar/tests'); * * The following line would cause the autoloader to attempt to load the * \Foo\Bar\Qux\Quux class from /path/to/packages/foo-bar/src/Qux/Quux.php: * 下面代码将用/path/to/packages/foo-bar/src/Qux/Quux.php文件来加载\Foo\Bar\Qux\Quux类。 * * <?php * new \Foo\Bar\Qux\Quux; * * The following line would cause the autoloader to attempt to load the * \Foo\Bar\Qux\QuuxTest class from /path/to/packages/foo-bar/tests/Qux/QuuxTest.php: * 下面代码将用/path/to/packages/foo-bar/tests/Qux/QuuxTest.php文件来加载 * \Foo\Bar\Qux\QuuxTest类。 * * <?php * new \Foo\Bar\Qux\QuuxTest; */class Psr4AutoloaderClass{ /** * An associative array where the key is a namespace prefix and the value * is an array of base directories for classes in that namespace. * 定义一个数组：key为命名空间前缀，value为一个数组，每一项表示命名空间中类对应的base目录. * * @var array */ protected $prefixes = array(); /** * Register loader with SPL autoloader stack. * 利用SPL自动加载器来注册loader * * @return void */ public function register() { spl_autoload_register(array($this, 'loadClass')); } /** * Adds a base directory for a namespace prefix. * 为一个命名空间前缀添加对应的base目录 * * @param string $prefix The namespace prefix. * @param string $base_dir A base directory for class files in the * namespace. * @param bool $prepend If true, prepend the base directory to the stack * instead of appending it; this causes it to be searched first rather * than last. * @return void */ public function addNamespace($prefix, $base_dir, $prepend = false) { // normalize namespace prefix // 规范命名空间前缀 $prefix = trim($prefix, '\\') . '\\'; // normalize the base directory with a trailing separator // 用'/'字符来规范base目录 $base_dir = rtrim($base_dir, DIRECTORY_SEPARATOR) . '/'; // initialize the namespace prefix array // 初始化命名空间前缀数组 if (isset($this->prefixes[$prefix]) === false) { $this->prefixes[$prefix] = array(); } // retain the base directory for the namespace prefix // 绑定命名空间前缀对应的base目录 if ($prepend) { array_unshift($this->prefixes[$prefix], $base_dir); } else { array_push($this->prefixes[$prefix], $base_dir); } } /** * Loads the class file for a given class name. * 根据类名来加载类文件。 * * @param string $class The fully-qualified class name. * @return mixed The mapped file name on success, or boolean false on * failure. */ public function loadClass($class) { // the current namespace prefix $prefix = $class; // work backwards through the namespace names of the fully-qualified // class name to find a mapped file name // 从后面开始遍历完全合格类名中的命名空间名称, 来查找映射的文件名 while (false !== $pos = strrpos($prefix, '\\')) { // retain the trailing namespace separator in the prefix // 保留命名空间前缀中尾部的分隔符 $prefix = substr($class, 0, $pos + 1); // the rest is the relative class name // 剩余的就是相对类名称 $relative_class = substr($class, $pos + 1); // try to load a mapped file for the prefix and relative class // 利用命名空间前缀和相对类名来加载映射文件 $mapped_file = $this->loadMappedFile($prefix, $relative_class); if ($mapped_file) { return $mapped_file; } // remove the trailing namespace separator for the next iteration // of strrpos() // 删除命名空间前缀尾部的分隔符，以便用于下一次strrpos()迭代 $prefix = rtrim($prefix, '\\'); } // never found a mapped file // 未找到映射文件 return false; } /** * Load the mapped file for a namespace prefix and relative class. * 根据命名空间前缀和相对类来加载映射文件 * * @param string $prefix The namespace prefix. * @param string $relative_class The relative class name. * @return mixed Boolean false if no mapped file can be loaded, or the * name of the mapped file that was loaded. */ protected function loadMappedFile($prefix, $relative_class) { // are there any base directories for this namespace prefix? // 命名空间前缀中有base目录吗？ if (isset($this->prefixes[$prefix]) === false) { return false; } // look through base directories for this namespace prefix // 遍历命名空间前缀的base目录 foreach ($this->prefixes[$prefix] as $base_dir) { // replace the namespace prefix with the base directory, // replace namespace separators with directory separators // in the relative class name, append with .php // 用base目录替代命名空间前缀, // 在相对类名中用目录分隔符'/'来替换命名空间分隔符'\', // 并在后面追加.php组成$file的绝对路径 $file = $base_dir . str_replace('\\', '/', $relative_class) . '.php'; // if the mapped file exists, require it // 若映射文件存在，则require该文件 if ($this->requireFile($file)) { // yes, we're done return $file; } } // never found it return false; } /** * If a file exists, require it from the file system. * * @param string $file The file to require. * @return bool True if the file exists, false if not. */ protected function requireFile($file) { if (file_exists($file)) { require $file; return true; } return false; }} 3. 单元测试 下面是对应的单元测试代码：
<?phpnamespace Example\Tests;class MockPsr4AutoloaderClass extends Psr4AutoloaderClass{ protected $files = array(); public function setFiles(array $files) { $this->files = $files; } protected function requireFile($file) { return in_array($file, $this->files); }}class Psr4AutoloaderClassTest extends \PHPUnit_Framework_TestCase{ protected $loader; protected function setUp() { $this->loader = new MockPsr4AutoloaderClass; $this->loader->setFiles(array( '/vendor/foo.bar/src/ClassName.php', '/vendor/foo.bar/src/DoomClassName.php', '/vendor/foo.bar/tests/ClassNameTest.php', '/vendor/foo.bardoom/src/ClassName.php', '/vendor/foo.bar.baz.dib/src/ClassName.php', '/vendor/foo.bar.baz.dib.zim.gir/src/ClassName.php', )); $this->loader->addNamespace( 'Foo\Bar', '/vendor/foo.bar/src' ); $this->loader->addNamespace( 'Foo\Bar', '/vendor/foo.bar/tests' ); $this->loader->addNamespace( 'Foo\BarDoom', '/vendor/foo.bardoom/src' ); $this->loader->addNamespace( 'Foo\Bar\Baz\Dib', '/vendor/foo.bar.baz.dib/src' ); $this->loader->addNamespace( 'Foo\Bar\Baz\Dib\Zim\Gir', '/vendor/foo.bar.baz.dib.zim.gir/src' ); } public function testExistingFile() { $actual = $this->loader->loadClass('Foo\Bar\ClassName'); $expect = '/vendor/foo.bar/src/ClassName.php'; $this->assertSame($expect, $actual); $actual = $this->loader->loadClass('Foo\Bar\ClassNameTest'); $expect = '/vendor/foo.bar/tests/ClassNameTest.php'; $this->assertSame($expect, $actual); } public function testMissingFile() { $actual = $this->loader->loadClass('No_Vendor\No_Package\NoClass'); $this->assertFalse($actual); } public function testDeepFile() { $actual = $this->loader->loadClass('Foo\Bar\Baz\Dib\Zim\Gir\ClassName'); $expect = '/vendor/foo.bar.baz.dib.zim.gir/src/ClassName.php'; $this->assertSame($expect, $actual); } public function testConfusion() { $actual = $this->loader->loadClass('Foo\Bar\DoomClassName'); $expect = '/vendor/foo.bar/src/DoomClassName.php'; $this->assertSame($expect, $actual); $actual = $this->loader->loadClass('Foo\BarDoom\ClassName'); $expect = '/vendor/foo.bardoom/src/ClassName.php'; $this->assertSame($expect, $actual); }} 4. PSR-4应用
PHP的包管理系统Composer已经支持PSR-4，同时也允许在composer.json中定义不同的prefix使用不同的自动加载机制。
Composer使用PSR-0风格
vendor/ vendor_name/ package_name/ src/ Vendor_Name/ Package_Name/ ClassName.php # Vendor_Name\Package_Name\ClassName tests/ Vendor_Name/ Package_Name/ ClassNameTest.php # Vendor_Name\Package_Name\ClassName
Composer使用PSR-4风格
vendor/ vendor_name/ package_name/ src/ ClassName.php # Vendor_Name\Package_Name\ClassName tests/ ClassNameTest.php # Vendor_Name\Package_Name\ClassNameTest
对比以上两种结构，明显可以看出PSR-4带来更简洁的文件结构。
5. 参考资料
[PHP-FIG] php-fig, http://www.php-fig.org/
[PSR-0] Autoloading Standard, http://www.php-fig.org/psr/psr-0/
[PSR-4] Autoloader, http://www.php-fig.org/psr/psr-4/
[PSR-4-Meta] PSR-4 Meta Document, http://www.php-fig.org/psr/psr-4/meta/
[PSR-4-Example] Example Implementations of PSR-4, http://www.php-fig.org/psr/psr-4/examples/
