# Child Injectors

## Overview

![](/files/CbV17J1ReI7PzUIx2nt5)

Welcome to the world of hierarchical dependency injection.  We had the ability before to add a parent injector to WireBox, but you can not only add a parent, but also many children to the hierarchy.

Every injector has the capability to store an ordered collection (`ordered struct`) of child injectors via the `childInjectors` property. Child injectors are used internally in many instances to provide a hierarchical approach to DI where instances can be **searched** for locally, in the parent and in the children. 

### Child Injector Methods

Here are some of the new methods to assist with child injectors:

* `hasChildInjector( name )` - Verify if a child injector has been registered
* `registerChildInjector( name, child )` - Register a child injector by name
* `removeChildInjector( name )` - Remove a child injector by name
* `getChildInjector( name )` - Get a child injector by name
* `getChildInjectors()` - Get all the child injectors registered
* `getChildInjectorNames()` - Get an array of all the registered child injectors

### Child Enhanced Methods

* `getInstance()`
  * The `getInstance()`method has an `injector` argument so you can **EXPLICITLY** request an instance from a child injector by name `getInstance( name : "service", injector : "childInjector" )`
  * Apart from the explicit lookup it can also do implicit hierarchical lookups using the following order:
    * Locally
    * Parent
    * All Children (in order of registration)
* `containsInstance( name )` - This method now also searches in the child collection for the specific `name` instance. The lookup searches in the following order:
  1. Locally
  2. Parent
  3. Children (in order of registration)
* `shutdown()` - The shutdown method has been enhanced to issue shutdown method calls to all child injectors registered.

### Getting Instances From Specific Child Injectors

The `getInstance()` has been modified to have an `injector` argument that you can use to specifically ask for an instance from that child injector. If the child injector has not been registered you will get a `InvalidChildInjector` Exception.

```javascript
getInstance( name: "CategoryService", injector : "ChildInjector" )
```

### Child Injector Explicit DSL

The following is the DSL you can use to **explicitly** target a child injector for a dependency. You will prefix it with `wirebox:child:{name}` and the name of the injector:

```javascript
// Use the property name as the instance name
property name="categoryService" inject="wirebox:child:childInjector"
// Use a specific instance name
property name="categoryService" inject="wirebox:child:childInjector:CategoryService"
// Use any DSL
property name="categoryService" inject="wirebox:child:childInjector:{DSL}"
```

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