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joplin notion extension/Web Development/Java/Spring/Getting started.md

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+Getting started
+
+
+**Dependency injection**
+https://www.youtube.com/watch?v=J1f5b4vcxCQ  : Dependeny injection, the best pattern
+
+Dependency
+- there r functions or objects that use other objects or functions and they r called dependency. There are 2 types of coupling.
+  1. Hard coupling: Here dependency cant be changed
+  2. Loose coupling: To change dependency in future
+ 
+
+Dependency injection
+- Instead of instantiating new code in constructor or function in same braces/scope we pass it instead through parameter to be used.
+- Main goal of dependency injection is to achieve loose coupling.
+eg
+without DI(Dependency Injection)
+```java
+public class Laptop{
+	HardDrive obj1 = new HitachiHD();
+}
+```
+with DI
+```java
+public class Laptop{
+	HardDrive obj1;
+	Laptop(HardDrive obj){
+		this.obj1= obj;
+	}
+}
+```
+- It is possible using external service. That service is responsible for creating objects.
+- Example in spring boot
+```java
+class Laptop{
+	@Autowired
+	HardDrive obj1;
+}
+
+@Component
+class HitachiHD implements HardDrive{
+	// ...
+}
+```
+With this `@Autorired` annotation it will find the component which is tagged by `@Component` to connect to it.
+- Not only this is useful to prevent tight coupling, but also for testing.
+  When we wanna test laptop
+then we may not need to check harddrive bcz it is already tested so with these we can separate it too. We can create *mock object* of the class if it is loosely coupled.