目的
对象池模式是一种提前准备了一组已经初始化了的对象『池』而不是按需创建或者销毁的创建型设计模式。对象池的客户端会向对象池中请求一个对象，然后使用这个返回的对象执行相关操作。当客户端使用完毕，它将把这个特定类型的工厂对象返回给对象池，而不是销毁掉这个对象。
在初始化实例成本高，实例化率高，可用实例不足的情况下，对象池可以极大地提升性能。在创建对象（尤其是通过网络）时间花销不确定的情况下，通过对象池在可期时间内就可以获得所需的对象。
无论如何，对象池模式在需要耗时创建对象方面，例如创建数据库连接，套接字连接，线程和大型图形对象（比方字体或位图等），使用起来都是大有裨益的。在某些情况下，简单的对象池（无外部资源，只占内存）可能效率不高，甚至会有损性能。
代码
WorkerPool.php
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<?php
namespace DesignPatterns\Creational\Pool;
class WorkerPool implements \Countable
{
private $occupiedWorkers = [];
private $freeWorkers = [];
public function get(): StringReverseWorker
{
if (count($this->freeWorkers) == 0) {
$worker = new StringReverseWorker();
} else {
$worker = array_pop($this->freeWorkers);
}
$this->occupiedWorkers[spl_object_hash($worker)] = $worker;
return $worker;
}
public function dispose(StringReverseWorker $worker)
{
$key = spl_object_hash($worker);
if (isset($this->occupiedWorkers[$key])) {
unset($this->occupiedWorkers[$key]);
$this->freeWorkers[$key] = $worker;
}
}
public function count(): int
{
return count($this->occupiedWorkers) + count($this->freeWorkers);
}
}
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StringReverseWorker.php
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<?php
namespace DesignPatterns\Creational\Pool;
class StringReverseWorker
{
private $createdAt;
public function __construct()
{
$this->createdAt = new \DateTime();
}
public function run(string $text)
{
return strrev($text);
}
}
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测试
Tests/PoolTest.php
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<?php
namespace DesignPatterns\Creational\Pool\Tests;
use DesignPatterns\Creational\Pool\WorkerPool;
use PHPUnit\Framework\TestCase;
class PoolTest extends TestCase
{
public function testCanGetNewInstancesWithGet()
{
$pool = new WorkerPool();
$worker1 = $pool->get();
$worker2 = $pool->get();
$this->assertCount(2, $pool);
$this->assertNotSame($worker1, $worker2);
}
public function testCanGetSameInstanceTwiceWhenDisposingItFirst()
{
$pool = new WorkerPool();
$worker1 = $pool->get();
$pool->dispose($worker1);
$worker2 = $pool->get();
$this->assertCount(1, $pool);
$this->assertSame($worker1, $worker2);
}
}
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