https://segmentfault.com/a/1190000014948542
深入解析 composer 的自动加载原理
FIG制定的PHP规范，简称PSR，是PHP开发的事实标准。
PSR原本有四个规范，分别是：
PSR-0 自动加载
PSR-1 基本代码规范
PSR-2 代码样式
PSR-3 日志接口
2013年底，新出了第5个规范——PSR-4。
PSR-4规范了如何指定文件路径从而自动加载类定义，同时规范了自动加载文件的位置。这个乍一看和PSR-0重复了，实际上，在功能上确实有所重复。区别在于PSR-4的规范比较干净，去除了兼容PHP 5.3以前版本的内容，有一点PSR-0升级版的感觉。当然，PSR-4也不是要完全替代PSR-0，而是在必要的时候补充PSR-0——当然，如果你愿意，PSR-4也可以替代PSR-0。PSR-4可以和包括PSR-0在内的其他自动加载机制共同使用。
PSR-4和PSR-0最大的区别是对下划线（underscore)的定义不同。PSR-4中，在类名中使用下划线没有任何特殊含义。而PSR-0则规定类名中的下划线_会被转化成目录分隔符。
代码样例
以下代码展示了遵循PSR-4的类定义，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.
*
* Given a foo-bar package of classes in the file system at the following
* paths ...
*
* /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:
*
* <?php
* // instantiate the loader
* $loader = new \Example\Psr4AutoloaderClass;
*
* // register the autoloader
* $loader->register();
*
* // register the base directories for the namespace prefix
* $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:
*
* <?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:
*
* <?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.
*
* @var array
*/
protected $prefixes = array(); /**
* Register loader with SPL autoloader stack.
*
* @return void
*/
public function register()
{
spl_autoload_register(array($this, 'loadClass'));
} /**
* Adds a base directory for a namespace prefix.
*
* @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
$prefix = trim($prefix, '\\') . '\\'; // normalize the base directory with a trailing separator
$base_dir = rtrim($base_dir, '/') . DIRECTORY_SEPARATOR; $base_dir = rtrim($base_dir, DIRECTORY_SEPARATOR) . '/'; // initialize the namespace prefix array
if (isset($this->prefixes[$prefix]) === false) { $this->prefixes[$prefix] = array();
} // retain the base directory for the namespace prefix
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.
*
* @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
while (false !== $pos = strrpos($prefix, '\\')) { // retain the trailing namespace separator in the prefix
$prefix = substr($class, 0, $pos + 1); // the rest is the relative class name
$relative_class = substr($class, $pos + 1); // try to load a mapped file for the prefix and relative class
$mapped_file = $this->loadMappedFile($prefix, $relative_class); if ($mapped_file) { return $mapped_file;
} // remove the trailing namespace separator for the next iteration
// of strrpos()
$prefix = rtrim($prefix, '\\');
} // never found a mapped file
return false;
} /**
* Load the mapped file for a namespace prefix and relative class.
*
* @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, $relative_class)
{ // are there any base directories for this namespace prefix?
if (isset($this->prefixes[$prefix]) === false) { return false;
} // look through base directories for this namespace prefix
foreach ($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
$file = $base_dir
. str_replace('\\', DIRECTORY_SEPARATOR, $relative_class)
. '.php'; $file = $base_dir
. str_replace('\\', '/', $relative_class)
. '.php'; // if the mapped file exists, require it
if ($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;
}
}相应的单元测试代码
<?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);
}
}Composer
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。