In this post, we'll take a look at how to implement custom logging in a Spring Boot application. We'll cover the following topics:
Log4j2 is a popular logging library for Java applications. Spring Boot provides support for Log4j2 out of the box. In order to configure Log4j2 for a Spring Boot application, we need to add a Log4j2 configuration file to the classpath. The Log4j2 configuration file must be named log4j2.xml or log4j2.yaml.
Here's a simple Log4j2 configuration file that will log all messages to the console:
<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="warn">
<Appenders>
<Console name="Console" target="SYSTEM_OUT">
<PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
</Console>
</Appenders>
<Loggers>
<Root level="info">
<AppenderRef ref="Console"/>
</Root>
</Loggers>
</Configuration>
In the Log4j2 configuration file, we need to specify the appenders that we want to use. In this example, we're using a Console appender that will log all messages to the console. We also need to specify the loggers that we want to use. In this example, we're using the root logger and we're referencing the Console appender that we defined earlier.
In order to log events to a database, we need to create a custom Log4j2 appender. A Log4j2 appender is a plugin that allows us to specify where we want our logs to be written to. In our case, we want to write our logs to a database.
Our custom appender will need to implement the following interface:
public interface Appender {
void start();
void stop();
boolean isStarted();
void append(LogEvent event);
Layout getLayout();
void setLayout(Layout layout);
ErrorHandler getErrorHandler();
void setErrorHandler(ErrorHandler errorHandler);
Name getName();
boolean ignoresThrowable();
}
We can start by creating a skeleton implementation of our custom appender:
public class DatabaseAppender implements Appender {
@Override
public void start() {
}
@Override
public void stop() {
}
@Override
public boolean isStarted() {
return false;
}
@Override
public void append(LogEvent event) {
}
@Override
public Layout getLayout() {
return null;
}
@Override
public void setLayout(Layout layout) {
}
@Override
public ErrorHandler getErrorHandler() {
return null;
}
@Override
public void setErrorHandler(ErrorHandler errorHandler) {
}
@Override
public Name getName() {
return null;
}
@Override
public boolean ignoresThrowable() {
return false;
}
}
In the start() method, we need to initialize our database connection. In the stop() method, we need to close our database connection. In the append() method, we need to insert our log event into the database.
Now that we have our custom appender, we need to configure Log4j2 to use it. We can do this by adding the following to our Log4j2 configuration file:
<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="warn">
<Appenders>
<Console name="Console" target="SYSTEM_OUT">
<PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
</Console>
<DatabaseAppender name="DatabaseAppender">
<ConnectionString>jdbc:mysql://localhost:3306/test</ConnectionString>
<DriverClassName>com.mysql.jdbc.Driver</DriverClassName>
<UserName>root</UserName>
<Password>root</Password>
</DatabaseAppender>
</Appenders>
<Loggers>
<Root level="info">
<AppenderRef ref="Console"/>
<AppenderRef ref="DatabaseAppender"/>
</Root>
</Loggers>
</Configuration>
In the Log4j2 configuration file, we've added a element. This element has a few attributes that we need to set:
We've also added a element to the root logger. This element references our DatabaseAppender.
Now that we have our custom appender configured, we can use it to log events to our database.