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

Concept Flow - Application.properties basics

Start Spring Boot App

↓

Load application.properties

↓

Read key=value pairs

↓

Apply settings to app context

↓

Run app with configured settings

↓

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.

Practice

1. What is the primary purpose of the application.properties file in a Spring Boot project?

easy

A. To store configuration settings as key=value pairs

B. To write Java code for the application

C. To define HTML templates for the UI

D. To manage database schema migrations

Solution

1. Step 1: Understand the role of application.properties

   This file is used to store configuration settings in a simple key=value format.

2. Step 2: Compare with other options

   Writing Java code, defining HTML templates, or managing database migrations are done elsewhere, not in this file.

3. Final Answer:

   To store configuration settings as key=value pairs -> Option A

4. Quick Check:

   Config file = key=value pairs [OK]

Hint: Remember: properties file holds settings, not code or templates [OK]

Common Mistakes:

• Thinking it contains Java code
• Confusing it with HTML or template files
• Assuming it manages database migrations

2. Which of the following is the correct syntax to set the server port to 8081 in application.properties?

easy

A. server.port=8081

B. server-port: 8081

C. server_port=8081

D. port.server=8081

Solution

1. Step 1: Recall Spring Boot property syntax

   Spring Boot uses dot notation with key=value pairs in application.properties.

2. Step 2: Identify correct key for server port

   The correct key is server.port, and the value is set with an equals sign.

3. Final Answer:

   server.port=8081 -> Option A

4. Quick Check:

   Dot notation + equals sign = correct syntax [OK]

Hint: Use dot notation and equals sign for properties [OK]

Common Mistakes:

• Using underscores instead of dots
• Using colon instead of equals sign
• Swapping key parts order

3. Given this application.properties snippet:

spring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=pass123

What will Spring Boot do with these properties when the app starts?

medium

A. Create a new database named 'mydb'

B. Ignore these properties because they are invalid

C. Start a web server on port 3306

D. Connect to MySQL database at localhost with given credentials

Solution

1. Step 1: Identify datasource properties

   The keys starting with spring.datasource configure database connection details.

2. Step 2: Understand Spring Boot behavior

   Spring Boot uses these to connect to the specified MySQL database with username and password.

3. Final Answer:

   Connect to MySQL database at localhost with given credentials -> Option D

4. Quick Check:

   Datasource properties = DB connection [OK]

Hint: Datasource keys configure DB connection automatically [OK]

Common Mistakes:

• Thinking it starts a web server on DB port
• Assuming properties are ignored
• Believing it creates the database automatically

4. What is wrong with this application.properties line?

server.port-8080

medium

A. The port number 8080 is invalid

B. The key and value are separated by a dash instead of equals sign

C. The key should be port.server instead

D. There should be no spaces around the dash

Solution

1. Step 1: Check property syntax

   Properties require key and value separated by an equals sign (=), not a dash (-).

2. Step 2: Validate the key and value

   The key server.port is correct, and 8080 is a valid port number.

3. Final Answer:

   The key and value are separated by a dash instead of equals sign -> Option B

4. Quick Check:

   Use '=' to separate key and value [OK]

Hint: Always use '=' between key and value in properties [OK]

Common Mistakes:

• Using dash instead of equals sign
• Swapping key parts order
• Thinking spaces fix the syntax

5. You want to add a custom property app.title in application.properties and access it in your Spring Boot code. Which of these steps is correct?

hard

A. Add app.title=MyApp in properties and access it with System.getenv("app.title")

B. Add app.title: MyApp in properties and read it with System.getProperty("app.title")

C. Add app.title=MyApp in properties and use @Value("${app.title}") in code

D. Add app.title=MyApp in properties and use @Autowired String app.title in code

Solution

1. Step 1: Add custom property correctly

   Use key=value syntax: app.title=MyApp in application.properties.

2. Step 2: Access property in Spring Boot code

   Use @Value("${app.title}") annotation to inject the property value.

3. Final Answer:

   Add app.title=MyApp in properties and use @Value("${app.title}") in code -> Option C

4. Quick Check:

   Custom property + @Value injection = correct [OK]

Hint: Use @Value("${key}") to inject properties in code [OK]

Common Mistakes:

• Using colon instead of equals in properties
• Trying to get property from environment variables
• Incorrect use of @Autowired for simple values