摘要：說明本文主要學(xué)習(xí)模塊的源碼。這里，就已經(jīng)得到了鏈接器實(shí)例了，該中還裝著一個(gè)，下文在其使用時(shí)再聊下其具體連接邏輯。

說明：本文主要學(xué)習(xí)Laravel Database模塊的Query Builder源碼。實(shí)際上，Laravel通過Schema Builder來設(shè)計(jì)數(shù)據(jù)庫(kù)，通過Query Builder來CURD數(shù)據(jù)庫(kù)。Query Builder并不復(fù)雜或神秘，只是在PDO擴(kuò)展的基礎(chǔ)上又開放封閉的包裝了一層，提供了fluent api，使得書寫的代碼也很簡(jiǎn)潔流暢。在看下Query Builder源碼之前，先大概探索下illuminate/database package的目錄結(jié)構(gòu)。

開發(fā)環(huán)境： Laravel5.3 + PHP7

Query Builder主要在Query文件夾下，以一行簡(jiǎn)單又經(jīng)常使用的代碼為例來學(xué)習(xí)下內(nèi)部實(shí)現(xiàn)的原理吧：

Route::get("/query_builder", function() {
    // Query Builder
    return DB::table("users")->where("id", "=", 1)->get();
});

// Illuminate/Support/Facades/DB
class DB extends Facade
{
    /**
     * Get the registered name of the component.
     *
     * @return string
     */
    protected static function getFacadeAccessor()
    {
        return "db";
    }
}

在DatabaseServiceProvider已經(jīng)注冊(cè)了名為"db"的服務(wù)即DatabaseManager對(duì)象，則實(shí)際上魔術(shù)調(diào)用DatabaseManager中的table()方法，看下__call()魔術(shù)方法源碼：

    // $method = "table", $parameters = "users"
    public function __call($method, $parameters)
    {
        return $this->connection()->$method(...$parameters);
    }

所以重點(diǎn)是connection()方法，該方法返回的是Connection對(duì)象，看下connection()方法源碼：

public function connection($name = null)
    {
        // $name = "mysql", $type = null
        list($name, $type) = $this->parseConnectionName($name);

        // 首次在$connections[]中沒有"mysql" => $mysql_connection，所以需要根據(jù)配置創(chuàng)建對(duì)應(yīng)DB連接
        if (! isset($this->connections[$name])) {
            // 重點(diǎn)是makeConnection()創(chuàng)建了mysql連接實(shí)例
            $connection = $this->makeConnection($name);
            
            // 由于$type是null，不是"write"或"read"，所以實(shí)際上啥也沒做
            $this->setPdoForType($connection, $type);

            // 得到連接實(shí)例$connection后，還需要對(duì)該實(shí)例做準(zhǔn)備工作，如綁定事件，設(shè)置connector
            $this->connections[$name] = $this->prepare($connection);
        }

        return $this->connections[$name];
    }
    
    protected function parseConnectionName($name)
    {
        $name = $name ?: $this->getDefaultConnection();
        // 檢查是否以::read, ::write結(jié)尾
        return Str::endsWith($name, ["::read", "::write"])
                            ? explode("::", $name, 2) : [$name, null];
    }
    
    public function getDefaultConnection()
    {
        // laravel默認(rèn)是mysql，這里假定是常用的mysql連接
        return $this->app["config"]["database.default"];
    }

通過上面源碼知道重點(diǎn)是makeConnection($name)方法，該方法根據(jù)傳入的mysql名稱，來實(shí)例化出一個(gè)Connection對(duì)象，重點(diǎn)看下makeConnection()源碼：

    protected function makeConnection($name)
    {
        // 從config/database.php中獲取"connections.mysql"的配置
        $config = $this->getConfig($name);

        // 如果已經(jīng)自定義了連接，如在AppServiceProvider的boot()中又使用DatabaseManager::extend()方法自定義了一個(gè)"mysql"連接實(shí)例，
        // 那就用該實(shí)例，這里假設(shè)沒有自定義
        if (isset($this->extensions[$name])) {
            return call_user_func($this->extensions[$name], $config, $name);
        }

        // $driver = "mysql"
        $driver = $config["driver"];

        if (isset($this->extensions[$driver])) {
            return call_user_func($this->extensions[$driver], $config, $name);
        }
        
        // 通過ConnectionFactory類工廠模式獲取Mysql的連接類    
        return $this->factory->make($config, $name);
    }

實(shí)際上最后還是通過IlluminateDatabaseConnectorsConnectionFactory來解析出對(duì)應(yīng)的connection，這里使用了工廠模式，看下該工廠類的make()方法源碼：

    public function make(array $config, $name = null)
    {
        $config = $this->parseConfig($config, $name);

        if (isset($config["read"])) {
            return $this->createReadWriteConnection($config);
        }

        return $this->createSingleConnection($config);
    }
    
    protected function createSingleConnection(array $config)
    {
        // $pdo是個(gè)閉包
        $pdo = $this->createPdoResolver($config);

        return $this->createConnection(
            // $config["driver"] = "mysql", $config["database"] = "homestead"(數(shù)據(jù)庫(kù)名稱)
            $config["driver"], $pdo, $config["database"], $config["prefix"], $config
        );
    }
    
    protected function createPdoResolver(array $config)
    {
        return function () use ($config) {
            return $this->createConnector($config)->connect($config);
        };
    }

深入代碼發(fā)現(xiàn)，最后是通過該工廠類的createConnection()方法來造出的一個(gè)Connection對(duì)象，createConnection()源碼就是常見的傻瓜式的工廠構(gòu)造函數(shù)：

    protected function createConnection($driver, $connection, $database, $prefix = "", array $config = [])
    {
        // 容器中已經(jīng)綁定了"db.connection.mysql"服務(wù)就解析出該服務(wù)，這里是沒有注冊(cè)的
        if ($this->container->bound($key = "db.connection.{$driver}")) {
            return $this->container->make($key, [$connection, $database, $prefix, $config]);
        }

        // $driver = "mysql"
        switch ($driver) {
            case "mysql":
                return new MySqlConnection($connection, $database, $prefix, $config);
            case "pgsql":
                return new PostgresConnection($connection, $database, $prefix, $config);
            case "sqlite":
                return new SQLiteConnection($connection, $database, $prefix, $config);
            case "sqlsrv":
                return new SqlServerConnection($connection, $database, $prefix, $config);
        }

        throw new InvalidArgumentException("Unsupported driver [$driver]");
    }

總之，通過以上一步步分析就拿到了Connection這個(gè)對(duì)象了，DatabaseManager中的__call()方法中最后執(zhí)行的是(new MysqlConnection(*))->table("users")->where("id", 1)->get()。

OK, 這里注意下MySqlConnection的構(gòu)造參數(shù)$connection是個(gè)閉包，該閉包的值是ConnectionFactory::createPdoResolver()的返回值，看下閉包里的操作：

    protected function createPdoResolver(array $config)
    {
        return function () use ($config) {
            return $this->createConnector($config)->connect($config);
        };
    }
    
    public function createConnector(array $config)
    {
        if (! isset($config["driver"])) {
            throw new InvalidArgumentException("A driver must be specified.");
        }

        if ($this->container->bound($key = "db.connector.{$config["driver"]}")) {
            return $this->container->make($key);
        }

        switch ($config["driver"]) {
            case "mysql":
                return new MySqlConnector;
            case "pgsql":
                return new PostgresConnector;
            case "sqlite":
                return new SQLiteConnector;
            case "sqlsrv":
                return new SqlServerConnector;
        }

        throw new InvalidArgumentException("Unsupported driver [{$config["driver"]}]");
    }

很簡(jiǎn)單就能知道該閉包一旦執(zhí)行時(shí)，實(shí)際上執(zhí)行的行為類似于(new MySqlConnector)->connect($config)。

這里，就已經(jīng)得到了鏈接器實(shí)例MySqlConnection了，該connection中還裝著一個(gè)(new MySqlConnector)->connect($config)，下文在其使用時(shí)再聊下其具體連接邏輯。

總結(jié)：第一步數(shù)據(jù)庫(kù)連接實(shí)例化已經(jīng)走完了，已經(jīng)拿到了連接實(shí)例MySqlConnection，下一步將學(xué)習(xí)下connect()連接器是如何連接數(shù)據(jù)庫(kù)的，和如何編譯執(zhí)行SQL語句得到user_id為1的結(jié)果值。到時(shí)見。

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