File-based logging in Spring Boot - Mini Project: Build & Apply

File-based logging in Spring Boot

📖 Scenario: You are building a Spring Boot application that needs to keep track of important events and errors by writing logs to a file. This helps developers and system administrators understand what happened during the app's runtime.

🎯 Goal: Set up file-based logging in a Spring Boot application by creating a log file configuration, adding a log message in the code, and verifying the log output is written to the file.

📋 What You'll Learn

Create a Spring Boot application main class

Configure logging to write to a file named app.log

Add a log message using Spring Boot's logger

Run the application and verify the log message appears in app.log

💡 Why This Matters

🌍 Real World

File-based logging helps track application behavior and errors over time, which is essential for troubleshooting and monitoring in real-world software.

💼 Career

Understanding how to configure and use logging in Spring Boot is a key skill for backend developers and DevOps engineers to maintain reliable applications.

1

Create Spring Boot main application class

Create a Java class called LoggingApplication in package com.example.logging with a main method that starts the Spring Boot application using SpringApplication.run(LoggingApplication.class, args).

Spring Boot

package com.example.logging;

// Your code here

Hint

This is the standard way to start a Spring Boot app. Use the @SpringBootApplication annotation and a main method.

2

Configure logging to write to a file

Create a file named src/main/resources/application.properties and add a property to set the logging file name to app.log using logging.file.name=app.log.

Spring Boot

package com.example.logging;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class LoggingApplication {
    public static void main(String[] args) {
        SpringApplication.run(LoggingApplication.class, args);
    }
}

# Your code here: Create application.properties with logging.file.name=app.log

Hint

The application.properties file controls Spring Boot settings. Add logging.file.name=app.log to write logs to app.log.

3

Add a log message in the application

In the LoggingApplication class, add a private static final logger using LoggerFactory.getLogger(LoggingApplication.class). Then add a System.out.println line inside main to print "Application started" and a log info message with the same text using logger.info("Application started").

Spring Boot

package com.example.logging;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@SpringBootApplication
public class LoggingApplication {
    private static final Logger logger = LoggerFactory.getLogger(LoggingApplication.class);

    public static void main(String[] args) {
        SpringApplication.run(LoggingApplication.class, args);
        // Your code here: print and log "Application started"
    }
}

# File: src/main/resources/application.properties
logging.file.name=app.log

Hint

Use SLF4J Logger to add a log message. Also print to console to see immediate output.

4

Run and verify log output

Run the Spring Boot application. Then open the app.log file in the project root folder and verify it contains the line with "Application started" logged by the logger.

Spring Boot

package com.example.logging;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@SpringBootApplication
public class LoggingApplication {
    private static final Logger logger = LoggerFactory.getLogger(LoggingApplication.class);

    public static void main(String[] args) {
        SpringApplication.run(LoggingApplication.class, args);
        System.out.println("Application started");
        logger.info("Application started");
    }
}

# File: src/main/resources/application.properties
logging.file.name=app.log

# Run the application and check app.log for the log message
# Your code here

Hint

Run the app using your IDE or command line. Then open app.log to see the logged message.

Practice

1. What is the main purpose of file-based logging in a Spring Boot application?

easy

A. To save application messages to a file for later review

B. To send application messages only to the console

C. To delete old log files automatically

D. To encrypt application data before saving

Solution

1. Step 1: Understand logging basics in Spring Boot

   Logging captures messages about app behavior. File-based logging stores these messages in a file.

2. Step 2: Identify the purpose of file-based logging

   It allows reviewing logs later for debugging or monitoring, unlike console-only logging.

3. Final Answer:

   To save application messages to a file for later review -> Option A

4. Quick Check:

   File-based logging = save logs to file [OK]

Hint: File-based logging means saving logs to a file [OK]

Common Mistakes:

• Confusing file logging with console logging
• Thinking it deletes old logs automatically
• Assuming it encrypts logs by default

2. Which of the following is the correct way to enable file-based logging in application.properties?

easy

A. logging.filepath=logs/app.log

B. logging.file.name=logs/app.log

C. log.file=logs/app.log

D. logging.pathfile=logs/app.log

Solution

1. Step 1: Recall Spring Boot logging property names

   The correct property to set the log file name is logging.file.name.

2. Step 2: Check each option for correctness

   Only logging.file.name=logs/app.log uses the exact property logging.file.name with a valid file path.

3. Final Answer:

   logging.file.name=logs/app.log -> Option B

4. Quick Check:

   Correct property for log file name = logging.file.name [OK]

Hint: Use logging.file.name to set log file path [OK]

Common Mistakes:

• Using incorrect property names like logging.filepath
• Misspelling property keys
• Confusing logging.file.name with logging.pathfile

3. Given this application.yml snippet:

logging:
  file:
    name: logs/myapp.log
  level:
    root: INFO

What will happen when the Spring Boot app runs?

medium

A. Logs will only print to console with DEBUG level

B. Application will fail to start due to syntax error

C. Logs will be saved to logs/myapp.log but only ERROR level

D. Logs will be saved to logs/myapp.log with INFO level and above

Solution

1. Step 1: Analyze logging.file.name setting

   The logs will be saved to the file path logs/myapp.log as configured.

2. Step 2: Analyze logging.level.root setting

   The root log level is INFO, so logs INFO and above will be recorded.

3. Final Answer:

   Logs will be saved to logs/myapp.log with INFO level and above -> Option D

4. Quick Check:

   File path + INFO level = logs/myapp.log with INFO+ logs [OK]

Hint: logging.file.name sets file, logging.level.root sets minimum log level [OK]

Common Mistakes:

• Assuming logs only go to console
• Confusing log level INFO with DEBUG or ERROR
• Thinking YAML syntax is invalid here

4. You configured file logging in application.properties as:
logging.file.name=logs/app.log
But no log file is created after running the app. What is the most likely cause?

medium

A. The property should be logging.file.path, not logging.file.name

B. The log level is set to OFF by default

C. The logs directory does not exist and is not created automatically

D. Spring Boot does not support file logging

Solution

1. Step 1: Check directory existence for log file

   If the logs directory does not exist, Spring Boot may not create it automatically, so no file is created.

2. Step 2: Verify other options

   Log level defaults to INFO, property name is correct, and Spring Boot supports file logging.

3. Final Answer:

   The logs directory does not exist and is not created automatically -> Option C

4. Quick Check:

   Missing log directory = no log file created [OK]

Hint: Ensure log directory exists before running app [OK]

Common Mistakes:

• Using wrong property name
• Assuming Spring Boot disables file logging
• Ignoring missing directory errors

5. You want to keep log files organized by date in Spring Boot. Which configuration snippet in application.yml correctly enables daily rolling log files using Logback?

hard

A. logging:
file:
name: logs/app.log
logback:
rollingpolicy:
file-name-pattern: logs/app-%d{yyyy-MM-dd}.log

B. logging:
file:
name: logs/app.log
logback:
rollingPolicy:
filePattern: logs/app-%d{yyyy-MM-dd}.log

C. logging:
file:
name: logs/app.log
logback:
rollingpolicy:
fileNamePattern: logs/app-%d{yyyy-MM-dd}.log

D. logging:
file:
path: logs
logback:
rollingPolicy:
fileNamePattern: logs/app-%d{yyyy-MM-dd}.log

Solution

1. Step 1: Identify correct property names for Logback rolling policy

   The correct key is rollingpolicy (lowercase 'p'), and file-name-pattern (kebab-case) is the correct pattern key.

2. Step 2: Check each option for exact spelling and structure

   logging: file: name: logs/app.log logback: rollingpolicy: file-name-pattern: logs/app-%d{yyyy-MM-dd}.log uses correct keys: rollingpolicy and file-name-pattern.

3. Final Answer:

   logging: file: name: logs/app.log logback: rollingpolicy: file-name-pattern: logs/app-%d{yyyy-MM-dd}.log -> Option A

4. Quick Check:

   Correct keys for rollingpolicy = logging: file: name: logs/app.log logback: rollingpolicy: file-name-pattern: logs/app-%d{yyyy-MM-dd}.log [OK]

Hint: Use exact keys: rollingpolicy and file-name-pattern [OK]

Common Mistakes:

• Using uppercase 'rollingPolicy' instead of 'rollingpolicy'
• Using 'filePattern' or 'fileNamePattern' instead of 'file-name-pattern'
• Confusing logging.file.name with logging.file.path