diff --git a/docs/guide/structure-application-components.md b/docs/guide/structure-application-components.md
index eeaac00baf..3f18c2b327 100644
--- a/docs/guide/structure-application-components.md
+++ b/docs/guide/structure-application-components.md
@@ -58,12 +58,22 @@ If it is not accessed at all during a request, it will not be instantiated. Some
 to instantiate an application component for every request, even if it is not explicitly accessed.
 To do so, you may list its ID in the [[yii\base\Application::bootstrap|bootstrap]] property of the application.
 
+You can also use Closures to bootstrap customized components. Returning a instantiated component is not 
+required. A Closure can also be used simply for running code after [[yii\base\Application]] instantiation.
+
 For example, the following application configuration makes sure the `log` component is always loaded:
 
 ```php
 [
     'bootstrap' => [
         'log',
+        function($app){
+            return new ComponentX();
+        },
+        function($app){
+            // some code
+           return;
+        }
     ],
     'components' => [
         'log' => [
diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index cbe680c380..c6b7a1f9c0 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -3,7 +3,7 @@ Yii Framework 2 Change Log
 
 2.0.13 under development
 ------------------------
-
+- Bug #14449: Fix PHP 7.2 compatibility bugs and add explicit closure support in `yii\base\Application` (dynasource)
 - Bug #7890: Allow `migrate/mark` to mark history at the point of the base migration (cebe)
 - Bug #14206: `MySqlMutex`, `PgsqlMutex` and `OracleMutex` now use `useMaster()` to ensure lock is aquired on the same DB server (cebe, ryusoft)
 - Chg #14321: `yii\widgets\MaskedInput` is now registering its JavaScript `clientOptions` initialization code in head section (DaveFerger)
diff --git a/framework/base/Application.php b/framework/base/Application.php
index 2cb68cac9e..441f00cf03 100644
--- a/framework/base/Application.php
+++ b/framework/base/Application.php
@@ -168,6 +168,7 @@ abstract class Application extends Module
      * - a module ID as specified via [[modules]].
      * - a class name.
      * - a configuration array.
+     * - a Closure
      *
      * During the bootstrapping process, each component will be instantiated. If the component class
      * implements [[BootstrapInterface]], its [[BootstrapInterface::bootstrap()|bootstrap()]] method
@@ -300,19 +301,25 @@ abstract class Application extends Module
             }
         }
 
-        foreach ($this->bootstrap as $class) {
+        foreach ($this->bootstrap as $mixed) {
             $component = null;
-            if (is_string($class)) {
-                if ($this->has($class)) {
-                    $component = $this->get($class);
-                } elseif ($this->hasModule($class)) {
-                    $component = $this->getModule($class);
-                } elseif (strpos($class, '\\') === false) {
-                    throw new InvalidConfigException("Unknown bootstrapping component ID: $class");
+            if ($mixed instanceof \Closure) {
+                Yii::trace('Bootstrap with Closure', __METHOD__);
+                if (!$component = call_user_func($mixed, $this)) {
+                    continue;
+                }
+            } elseif (is_string($mixed)) {
+                if ($this->has($mixed)) {
+                    $component = $this->get($mixed);
+                } elseif ($this->hasModule($mixed)) {
+                    $component = $this->getModule($mixed);
+                } elseif (strpos($mixed, '\\') === false) {
+                    throw new InvalidConfigException("Unknown bootstrapping component ID: $mixed");
                 }
             }
+
             if (!isset($component)) {
-                $component = Yii::createObject($class);
+                $component = Yii::createObject($mixed);
             }
 
             if ($component instanceof BootstrapInterface) {
diff --git a/framework/db/QueryBuilder.php b/framework/db/QueryBuilder.php
index 20b81f4fde..5a4b1fc7ee 100644
--- a/framework/db/QueryBuilder.php
+++ b/framework/db/QueryBuilder.php
@@ -1350,7 +1350,7 @@ class QueryBuilder extends \yii\base\Object
             $values = (array) $values;
         }
 
-        if ($column instanceof \Traversable || count($column) > 1) {
+        if ($column instanceof \Traversable || ((is_array($column) || $column instanceof \Countable) && count($column) > 1)) {
             return $this->buildCompositeInCondition($operator, $column, $values, $params);
         } elseif (is_array($column)) {
             $column = reset($column);
diff --git a/tests/framework/base/ApplicationTest.php b/tests/framework/base/ApplicationTest.php
index f3ce06cabb..8834f9a499 100644
--- a/tests/framework/base/ApplicationTest.php
+++ b/tests/framework/base/ApplicationTest.php
@@ -8,6 +8,9 @@
 namespace yiiunit\framework\base;
 
 use Yii;
+use yii\base\BootstrapInterface;
+use yii\base\Component;
+use yii\base\Module;
 use yii\log\Dispatcher;
 use yiiunit\TestCase;
 
@@ -29,8 +32,50 @@ class ApplicationTest extends TestCase
 
         $this->assertInstanceOf(DispatcherMock::className(), Yii::$app->log);
     }
+
+    public function testBootstrap()
+    {
+        Yii::getLogger()->flush();
+
+
+        $this->mockApplication([
+            'components' => [
+                'withoutBootstrapInterface' => [
+                    'class' => Component::class
+                ],
+                'withBootstrapInterface' => [
+                    'class' => BootstrapComponentMock::class
+                ]
+            ],
+            'modules' => [
+                'moduleX' => [
+                    'class' => Module::class
+                ]
+            ],
+            'bootstrap' => [
+                'withoutBootstrapInterface',
+                'withBootstrapInterface',
+                'moduleX',
+                function () {
+                }
+
+            ],
+        ]);
+        $this->assertSame('Bootstrap with yii\base\Component', Yii::getLogger()->messages[0][0]);
+        $this->assertSame('Bootstrap with yiiunit\framework\base\BootstrapComponentMock::bootstrap()', Yii::getLogger()->messages[1][0]);
+        $this->assertSame('Loading module: moduleX', Yii::getLogger()->messages[2][0]);
+        $this->assertSame('Bootstrap with yii\base\Module', Yii::getLogger()->messages[3][0]);
+        $this->assertSame('Bootstrap with Closure', Yii::getLogger()->messages[4][0]);
+    }
 }
 
 class DispatcherMock extends Dispatcher
 {
 }
+
+class BootstrapComponentMock extends Component implements BootstrapInterface
+{
+    public function bootstrap($app)
+    {
+    }
+}
\ No newline at end of file