{"id":1411,"date":"2016-09-14T21:42:07","date_gmt":"2016-09-14T14:42:07","guid":{"rendered":"http:\/\/huongdanjava.com\/?p=1411"},"modified":"2025-11-23T08:16:45","modified_gmt":"2025-11-23T01:16:45","slug":"understanding-about-dependency-injection","status":"publish","type":"post","link":"https:\/\/huongdanjava.com\/understanding-about-dependency-injection.html","title":{"rendered":"Understanding about Dependency Injection"},"content":{"rendered":"

Searching on Google, you can see that: there are a lot of tutorials about Dependency Injection, each one with a different style. Perhaps, some of you can perceive it quickly, someone is not. So, I would like to summarize again and try to write about it in an easy way to understand. (I will try my best \ud83d\ude00 ).<\/p>\n

The main idea of Dependency Injection is: you don’t depend on anyone<\/strong> and no one depends on you. I will call you when I need<\/strong> and you too.<\/p>\n

Let me talk more specifically about it!<\/p>\n

Someday in the past, I usually wrote my code like the below:<\/p>\n

Assume, you have an object Circle, this object has a method named draw() as below:<\/p>\n

package com.huongdanjava.softwaredesign;\r\n\r\npublic class Circle {\r\n\tpublic void draw() {\r\n\t\tSystem.out.println(\"Drawing circle ...\");\r\n\t}\r\n}<\/pre>\n
Now, I want to use this object to draw a circle, I will initialize the object Circle\u00a0within the constructor and I will write the code as below:<\/p>\n
package com.huongdanjava.softwaredesign;\r\n\r\npublic class Drawing {\r\n\tpublic Circle circle;\r\n\t\r\n\tpublic Drawing() {\r\n\t\tcircle = new Circle();\r\n\t}\r\n      \r\n\tpublic void preparing() {\r\n\t\tSystem.out.println(\"Preparing ...\");\r\n\t}\r\n\t\r\n\tpublic void draw() {\r\n\t\tcircle.draw();\r\n\t}\r\n\t\r\n\tpublic static void main(String[] args) {\r\n\t\tDrawing drawing = new Drawing();\r\n\t\tdrawing.draw();\r\n\t}\r\n}<\/pre>\n
Obviously, you see my object,\u00a0Drawing,<\/strong>\u00a0is depending on your object Circle\u00a0(that means I am depending on you), because every\u00a0time I\u00a0run the application, the object Drawing\u00a0must keep the information of the object Circle. <\/strong>This is the first drawback of this code.<\/p>\n
The second drawback is that if in the future, I want to draw a triangle, I must declare again another object to satisfy my needs. This is cause the object Drawing\u00a0will constantly change according to the needs.<\/p>\n
So, how can we resolve this drawback?<\/p>\n
The second drawback can resolve by using the interface. Your\u00a0object Circle will implement an interface named\u00a0Shape, details as below:<\/p>\n
package com.huongdanjava.softwaredesign;\r\n\r\npublic interface Shape {\r\n\tpublic void draw();\r\n}<\/pre>\n
package com.huongdanjava.softwaredesign;\r\n\r\npublic class Circle implement Shape {\r\n\tpublic void draw() {\r\n\t\tSystem.out.println(\"Drawing circle ...\");\r\n\t}\r\n}<\/pre>\n
In the future, if I want to draw a triangle, I only need to create a new class and implement the interface Shape.<\/p>\n
For the first drawback, Dependency Injection will resolve it.<\/p>\n
Let’s see how Dependency Injection can resolve this problem, I will modify the object Drawing\u00a0as below:<\/p>\n
package com.huongdanjava.softwaredesign;\r\n\r\npublic class Drawing {\r\n\tprivate Shape shape;\r\n\t\r\n\tpublic Drawing(Shape shape) {\r\n\t\tthis.shape = shape;\r\n\t}\r\n\t\r\n\tpublic void setShape(Shape shape) {\r\n\t\tthis.shape = shape;\r\n\t}\r\n      \r\n\tpublic void preparing() {\r\n\t\tSystem.out.println(\"Preparing ...\");\r\n\t}\r\n\t\r\n\tpublic void draw() {\r\n\t\tshape.draw();\r\n\t}\r\n}<\/pre>\n
As you can see, if I only need my object Drawing\u00a0to prepare the tool to draw, I don’t need to call your object\u00a0Circle, code will be:<\/p>\n
Drawing drawing = new Drawing(null);\r\ndrawing.preparing();<\/pre>\n
After preparing complete, I will need you to draw a circle, I will call you:<\/p>\n
Drawing drawing = new Drawing(null);\r\ndrawing.preparing();\r\n\r\nShape shape = new Circle();\r\ndrawing.setShape(shape);\r\ndrawing.draw();<\/pre>\n
Actually, maybe you already worked on the code but you didn’t know that this is Dependency Injection.<\/p>\n
You can watch the video here:<\/strong><\/p>\n