Capture output of @Sl4j for a package in a SpringBoot application

I have a class which is using Flyway to apply migrations for an application

import lombok.extern.slf4j.Slf4j;
import org.flywaydb.core.Flyway;
import org.flywaydb.core.api.MigrationInfo;
import org.flywaydb.core.api.MigrationInfoService;
import org.flywaydb.core.api.MigrationVersion;
import org.flywaydb.core.api.configuration.FluentConfiguration;
import org.flywaydb.core.api.output.MigrateResult;
import org.flywaydb.core.internal.info.MigrationInfoDumper;

import java.util.Properties;

@Slf4j
public class FlywayService {

    Flyway flyway;

    public FlywayService(Properties props, String dbServer, String schema, String user, String password) {
        FluentConfiguration flywayConfig = Flyway.configure();
        flywayConfig.configuration(props);

        flywayConfig.dataSource("jdbc:oracle:thin:@" + dbServer, user, password);
        flywayConfig.schemas(schema.toUpperCase());
        flyway = flywayConfig.load();
    }

    public void runInfo() {
        MigrationInfoService info = flyway.info();
        MigrationInfo current = info.current();
        MigrationVersion currentSchemaVersion = current == null ? MigrationVersion.EMPTY : current.getVersion();

        MigrationVersion schemaVersionToOutput = currentSchemaVersion == null ? MigrationVersion.EMPTY : currentSchemaVersion;
        StringBuffer buffer = new StringBuffer();
        buffer.append("Schema version: ")
                .append(schemaVersionToOutput).append("\n")
                .append(MigrationInfoDumper.dumpToAsciiTable(info.all()));

        log.info(buffer.toString());

    }
}

Logging is made through lombok(version 1.18.22) @Sl4j. What I need is to capture the all the log lines Flyway output to a String for some further processing but also keeping the lines logged by Flyway in spring.log.

Also, I'm running Flyway multithreaded as I'm applying migrations to several hundreds schemas.
Even I've searched for several hours, I still could not find an approach to do this.

How about configuring all the flyway loggers (i.e. logger start with the name org.flywaydb) to output the logs to a file appender with a fixed file path , then your codes read all content from this log file to a variable for further processing ?

The file appender also need to be configured as additive such that it will forward the log to the original root logger which print the logs to console. It is also required to be configured as "non-append" mode which will clear the file before writing the logs to ensure that the log file will not contain the log from the previous run .

If you need to execute Flyway concurrently in multiple threads , you can use routing appender to forward the logs of different threads to their own separate log files based on some thread identifier you defined in MDC

An example configuration is :

<Appenders>
   <Routing name="Routing">
			<Routes pattern="$${ctx:threadId}">
				<Route>
					<File fileName="/logs/flyway-${ctx:threadId}.log"
							name="flyway-${ctx:threadId}"
							append="false">						
					</File>
				</Route>
			</Routes>
		</Routing>
</Appenders>


<Loggers>
   <Logger name="org.flywaydb" level="DEBUG" >
			<AppenderRef ref="Routing"/>
	</Logger>
</Loggers>

In codes before you execute flyway , you configure a thread identifier and put it in MDC , something likes :

String threadId = threadId +  UUID.randomUUID().toString();

ThreadContext.put("threadId", threadId);

//execute flyway

//read all content from log to a string to further process
Path logFilePath = Path.of("/logs/flyway-" + threadId + ".log");
String logs = Files.readString(logFilePath);

ThreadContext.clearAll();