Log formatting configuration in Spring Boot - Step-by-Step Execution

Concept Flow - Log formatting configuration

Start Application

↓

Load Logging Config

↓

Apply Log Format Settings

↓

Log Event Occurs

↓

Format Log Message

↓

Output to Console/File

↓

Repeat for Next Log Event

This flow shows how Spring Boot loads logging configuration, applies formatting, and outputs logs.

Execution Sample

Spring Boot

logging.pattern.console=%d{HH:mm:ss} %-5level %logger{36} - %msg%n
logger.info("App started")

This config sets console log format; then a log message is printed using that format.

Execution Table

Step	Action	Input Log Event	Format Applied	Output Log Message
1	Load config	N/A	%d{HH:mm:ss} %-5level %logger{36} - %msg%n	N/A
2	Log event triggered	INFO, 'App started'	Apply pattern	14:23:01 INFO com.example.MyApp - App started
3	Log event triggered	ERROR, 'Error occurred'	Apply pattern	14:23:05 ERROR com.example.MyApp - Error occurred
4	Log event triggered	DEBUG, 'Debug info'	Apply pattern	14:23:10 DEBUG com.example.MyApp - Debug info
5	Application stops	N/A	N/A	N/A

💡 Application stops or logging ends

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
logging.pattern.console	undefined	%d{HH:mm:ss} %-5level %logger{36} - %msg%n	%d{HH:mm:ss} %-5level %logger{36} - %msg%n	%d{HH:mm:ss} %-5level %logger{36} - %msg%n	%d{HH:mm:ss} %-5level %logger{36} - %msg%n
log message	N/A	INFO, 'App started'	ERROR, 'Error occurred'	DEBUG, 'Debug info'	N/A
formatted output	N/A	14:23:01 INFO com.example.MyApp - App started	14:23:05 ERROR com.example.MyApp - Error occurred	14:23:10 DEBUG com.example.MyApp - Debug info	N/A

Key Moments - 2 Insights

Why does the log message show time and level before the message text?

Because the pattern '%d{HH:mm:ss} %-5level %logger{36} - %msg%n' formats logs to show time, level, logger name, then message as seen in execution_table rows 2-4.

What happens if the logging pattern is missing or incorrect?

Spring Boot uses a default format. The execution_table step 1 shows loading the pattern; if missing, default applies, so logs still print but differently.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the log level shown in the output?

A INFO

B ERROR

C DEBUG

D WARN

💡 Hint

Check the 'Output Log Message' column at step 3 in execution_table

At which step does the logging pattern get loaded and applied?

A Step 4

B Step 2

C Step 1

D Step 5

💡 Hint

Look at the 'Action' column in execution_table where config is loaded

If the pattern changed to include thread name, how would the output change?

A Thread name would appear before the message

B Log level would disappear

C Timestamp would be removed

D Logger name would be replaced by message

💡 Hint

Refer to how pattern controls output format in variable_tracker and execution_table

Concept Snapshot

Log formatting in Spring Boot uses patterns like '%d{HH:mm:ss} %-5level %logger{36} - %msg%n'.
This pattern controls how logs show time, level, logger, and message.
Configured in application.properties or YAML.
Logs use this format each time a log event happens.
Changing pattern changes log appearance immediately.

Full Transcript

In Spring Boot, log formatting configuration controls how log messages appear. The application loads a pattern from configuration files like application.properties. This pattern defines the order and style of log parts such as timestamp, log level, logger name, and message. When a log event happens, Spring Boot applies this pattern to format the message before outputting it to console or file. For example, a pattern '%d{HH:mm:ss} %-5level %logger{36} - %msg%n' shows time, level, logger, and message in that order. If the pattern changes, the log output changes accordingly. This process repeats for every log event until the application stops.