Application.properties basics in Spring Boot - Step-by-Step Execution

Concept Flow - Application.properties basics

Start Spring Boot App

↓

Load application.properties

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Read key=value pairs

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Apply settings to app context

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Run app with configured settings

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App ready with custom config

Spring Boot reads the application.properties file at startup, loads key-value pairs, applies them to configure the app, then runs with those settings.

Execution Sample

Spring Boot

server.port=8081
spring.application.name=MyApp
logging.level.root=INFO

This file sets the server port to 8081, names the app 'MyApp', and sets logging level to INFO.

Execution Table

Step	Action	Key	Value	Effect
1	Read line	server.port	8081	Server will listen on port 8081
2	Read line	spring.application.name	MyApp	App name set to 'MyApp'
3	Read line	logging.level.root	INFO	Logging level set to INFO
4	Apply all settings	-	-	App configured with above settings
5	Start app	-	-	App runs with custom port, name, and logging

💡 All properties read and applied; app starts with these settings.

Variable Tracker

Property	Initial	After Load	Final
server.port	8080 (default)	8081	8081
spring.application.name	SpringBootApp (default)	MyApp	MyApp
logging.level.root	INFO (default)	INFO	INFO

Key Moments - 3 Insights

Why does changing server.port in application.properties change the port the app listens on?

Because Spring Boot reads server.port from application.properties at startup (see execution_table step 1) and applies it to configure the embedded server port.

What happens if a property is missing in application.properties?

Spring Boot uses default values for missing properties, so the app still runs with defaults (see variable_tracker initial values).

Can I use application.properties to set any configuration?

Only properties recognized by Spring Boot or your app are applied; unknown keys are ignored (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'server.port' after step 1?

A 8081

B 8080

C 80

D Not set yet

💡 Hint

Check the 'Value' column in execution_table row with Step 1.

At which step does the app apply all settings from application.properties?

A Step 5

B Step 2

C Step 4

D Step 3

💡 Hint

Look for the row where 'Action' is 'Apply all settings'.

If you remove 'logging.level.root' from application.properties, what happens to its value in variable_tracker?

A It reverts to DEBUG

B It stays INFO

C It becomes undefined

D App fails to start

💡 Hint

Check the 'Initial' column for logging.level.root in variable_tracker.

Concept Snapshot

application.properties is a simple text file with key=value pairs.
Spring Boot reads it at startup to configure app settings.
Common keys: server.port, spring.application.name, logging.level.root.
Missing keys use default values.
Changes here affect app behavior without code changes.

Full Transcript

Spring Boot applications use a file named application.properties to set configuration options. This file contains lines with keys and values separated by an equals sign. When the app starts, Spring Boot reads this file line by line, loading each key and its value. It then applies these settings to configure the app, such as setting the server port or the app name. If a property is missing, Spring Boot uses a default value. This allows easy customization of app behavior without changing code. For example, setting server.port=8081 makes the app listen on port 8081 instead of the default 8080. The process ends with the app running using these configured settings.