[Spring] Dependency injection

Definition

DI - Dependency Injection

• A technique in which objects are passively given their dependencies instead of creating or looking for dependent objects for themselves. 
• [Nattily] DI means how we are going to make sure that all your components are related to one another.
• [Nattily] same concept IoC.
• [Nattily] It's a form of push configuration.
• Based on Hollywood Principle: "Don't call me, I'll call you."

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Push configuration

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How Non-IoC code looks like

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How the code changed with dependency injection

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Benefits of Dependency Injection

• Your object is handed what it needs to work
• Free your object from the burden of resolving its dependencies.
• Simplifies your code, improves code reuseability
• Promotes programming to interfaces
• Conceals (~hide) the implementation details of each dependency 
• [Nattily] so that can be performed "Swap out"
• Remove the responsibility of finding or creating dependent objects and moves it into configuration.
• Reduces coupling between implementation objects and encourages interface based design.
• Allows an application to be reconfigured outside of code.
• Improves testability
• Dependencies can be easily stubbed out for unit testing 
• Can encourage writing testable components
• Allows for centralized control over object lifecycle 
• Opens the door for new possiblities

Notes:
「保留抽象介面，讓組件依賴於抽象介面，當組件要與其它實際的物件發生依賴關係時，藉過抽象介面來注入依 賴的實際物件。  [link]

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Type of Dependency injection

(1) Setter Injection - The injection of dependencies via JavaBean setter methods.
public void setName (String name){
    this.name = name;
}

(2) Constructor Injection - The injection of dependencies via constructor arguments.
public Test (String name){
    this.name = name;
}

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