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Spring Dependency Injection and Types


Any application is composed of many objects that collaborate with each other to perform some useful stuff. Traditionally each object is responsible for obtaining its own references to the dependent objects (dependencies) it collaborate with. This leads to highly coupled classes and hard-to-test code.

For example, consider a Car object. A Car depends on Wheels, Engine, Fuel, Battery, etc to run. Traditionally we define the brand of such dependent objects along with the definition of the Car object.

class Car{
  private Wheel wh= new NepaliRubberWheel();
  private Battery bt= new ExcideBattery();
  //rest
}

Here, the Car object is responsible for creating the dependent objects.

What if we want to change the type of its dependent object - say Wheel - after the initial NepaliRubberWheel() punctures? We need to recreate the Car object with its new dependency say ChineseRubberWheel(), but only the Car manufacturer can do that.

Then what the Dependency Injection do us for ...

When using Dependency Injection, objects are given their dependencies at run time rather than compile time (car manufacturing time). So that we can now change the Wheel whenever we want. Here, the Dependency (Wheel) can be injected into Car at run time.

Inversion of Control (IoC) is a general concept, and it can be expressed in many different ways and Dependency Injection is merely one concrete example of Inversion of Control.

This concept says that you do not create your objects but describe how they should be created. You don't directly connect your components and services together in code but describe which services are needed by which components in a configuration file. A container is then responsible for hooking it all up.

Spring supports 2 types of dependency injection, they are:

1) Constructor-based dependency injection: It is accomplished when the container invokes a class constructor with a number of arguments, each representing a dependency on other class.

2) Setter-based dependency injection: It is accomplished by the container calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean.

Examples will be covered in the next sections.

Reference: Understanding Dependency Injection

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Spring framework examples

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Knowledge Centre
Different types of Access Modifiers
public: Any thing declared as public can be accessed from anywhere.

private: Any thing declared as private can't be seen outside of its class.

protected: Any thing declared as protected can be accessed by classes in the same package and subclasses in the other packages.

default modifier: Can be accessed only to classes in the same package.
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Reference: Java™ Platform Standard Ed. 7 - API Specification | Java is registered trademark of Oracle.
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