1.yii中與數(shù)據(jù)庫連接的時候，是通過ActiveRecord進行連接的，一般需要與數(shù)據(jù)庫表進行對應(yīng)的類需要繼承ActiveRecord，而對于該表中數(shù)據(jù)庫的查詢，同樣也是在該 User類中定義的查詢方法，不同的是，該查詢的方法定義是static的。
2. 對于yii與數(shù)據(jù)庫的鏈接是在配置文件中已經(jīng)描寫過的

run();

3.而對應(yīng)的common/config/main-local.php文件內(nèi)容是這樣的：

?[
????????'db'?=>?[
????????????'class'?=>?'yiidbConnection',
????????????'dsn'?=>?'mysql:host=10.1.20.36;dbname=db_XXX',
????????????'username'?=>?'weihu_dev',
????????????'password'?=>?'xxxxxx',
????????????'charset'?=>?'utf8',
????????],
????????'mailer'?=>?[
????????????'class'?=>?'yiiswiftmailerMailer',
????????????'viewPath'?=>?'@common/mail',
????????????//?send?all?mails?to?a?file?by?default.?You?have?to?set
????????????//?'useFileTransport'?to?false?and?configure?a?transport
????????????//?for?the?mailer?to?send?real?emails.
????????????'useFileTransport'?=>?true,
????????],
????],
];

4.整個初始化Yii::$app->db的初始化過程是這樣的，整個配置字符是在main-local.php當(dāng)中的，而其內(nèi)容是在$config中的，這個過程是在base/Application中的

Component::__construct($config);最終實現(xiàn)的是Yii::configure其內(nèi)容是：

public?static?function?configure($object,?$properties)
{??
????foreach?($properties?as?$name?=>?$value)?{

????????$object->$name?=?$value;
????}
????return?$object;
}

$object的實參是$app而其中有一個參數(shù)是component，而對component的參數(shù)的設(shè)置，還有一個真正的函數(shù)覆蓋，setComponent，該函數(shù)的定義是在ServiceLocator，因為Application的父類是Module而Module的父類則是ServiceLocator

該類中定義了

public?function?setComponents($components)
{
????foreach?($components?as?$id?=>?$component)?{
????????$this->set($id,?$component);
????}
}

其set的定義是：

public?function?set($id,?$definition)
{
????if?($definition?===?null)?{
????????unset($this->_components[$id],?$this->_definitions[$id]);
????????return;
????}

????unset($this->_components[$id]);

????if?(is_object($definition)?||?is_callable($definition,?true))?{
????????//?an?object,?a?class?name,?or?a?PHP?callable
????????$this->_definitions[$id]?=?$definition;
????}?elseif?(is_array($definition))?{
????????//?a?configuration?array
????????if?(isset($definition['class']))?{
????????????$this->_definitions[$id]?=?$definition;
????????}?else?{
????????????throw?new?InvalidConfigException("The?configuration?for?the?"$id"?component?must?contain?a?"class"?element.");
????????}
????}?else?{
????????throw?new?InvalidConfigException("Unexpected?configuration?type?for?the?"$id"?component:?"?.?gettype($definition));
????}
}

這樣我們就知道會有一個$id是"db"??而$definition則是，具體db的數(shù)組

'db'?=>?[
????'class'?=>?'yiidbConnection',
????'dsn'?=>?'mysql:host=db1-dev.bj1.haodf.net;dbname=db_Hdf',
????'username'?=>?'weihu_dev',
????'password'?=>?'hdf@haodf.com',
????'charset'?=>?'utf8',
]

這樣db的配置就放在了$_definitions?當(dāng)中，當(dāng)我們不使用的時候，存放的就是數(shù)組，當(dāng)我們使用的時候，則會調(diào)用ServiceLocator的get函數(shù)。

當(dāng)我們調(diào)用Yii::$app->db的時候出發(fā)?base/application中的

public?function?getDb()
{
????return?$this->get('db');
}

函數(shù)，而get函數(shù)則會調(diào)用父類的get函數(shù)

public?function?get($id,?$throwException?=?true)
{

????if?(isset($this->_components[$id]))?{
????????return?$this->_components[$id];
????}

????if?(isset($this->_definitions[$id]))?{
????????$definition?=?$this->_definitions[$id];
????????if?(is_object($definition)?&&?!$definition?instanceof?Closure)?{
????????????return?$this->_components[$id]?=?$definition;
????????}?else?{
????????????return?$this->_components[$id]?=?Yii::createObject($definition);
????????}
????}?elseif?($throwException)?{
????????throw?new?InvalidConfigException("Unknown?component?ID:?$id");
????}?else?{
????????return?null;
????}
}

之前將數(shù)據(jù)庫的配置放在了$_definition當(dāng)中，那么此時會判斷如果??$definition?是數(shù)組，那么就會創(chuàng)建對應(yīng)的對象Yii::createObject($definition);

至此就創(chuàng)建了該對象?。?！可以與數(shù)據(jù)庫進行交互了