Master MyBatis Logging: Configure SLF4J, Log4j, and Custom Loggers

1. LogFactory Initialization

MyBatis provides the built‑in logging factory org.apache.ibatis.logging.LogFactory which selects a concrete logger implementation at runtime. The static block tries implementations in order: SLF4J, Commons Logging, Log4j2, Log4j, JDK logging, and finally a no‑logging fallback.

public final class LogFactory {
    public static final String MARKER = "MYBATIS";
    private static Constructor<? extends Log> logConstructor;
    static {
        tryImplementation(LogFactory::useSlf4jLogging);
        tryImplementation(LogFactory::useCommonsLogging);
        tryImplementation(LogFactory::useLog4J2Logging);
        tryImplementation(LogFactory::useLog4JLogging);
        tryImplementation(LogFactory::useJdkLogging);
        tryImplementation(LogFactory::useNoLogging);
    }
    private LogFactory() { /* disable construction */ }
    public static Log getLog(Class<?> clazz) { return getLog(clazz.getName()); }
    public static Log getLog(String logger) {
        try { return logConstructor.newInstance(logger); }
        catch (Throwable t) { throw new LogException("Error creating logger for logger " + logger, t); }
    }
    public static synchronized void useCustomLogging(Class<? extends Log> clazz) { setImplementation(clazz); }
    public static synchronized void useSlf4jLogging() { setImplementation(org.apache.ibatis.logging.slf4j.Slf4jImpl.class); }
    public static synchronized void useCommonsLogging() { setImplementation(org.apache.ibatis.logging.commons.JakartaCommonsLoggingImpl.class); }
    public static synchronized void useLog4JLogging() { setImplementation(org.apache.ibatis.logging.log4j.Log4jImpl.class); }
    public static synchronized void useLog4J2Logging() { setImplementation(org.apache.ibatis.logging.log4j2.Log4j2Impl.class); }
    public static synchronized void useJdkLogging() { setImplementation(org.apache.ibatis.logging.jdk14.Jdk14LoggingImpl.class); }
    public static synchronized void useStdOutLogging() { setImplementation(org.apache.ibatis.logging.stdout.StdOutImpl.class); }
    public static synchronized void useNoLogging() { setImplementation(org.apache.ibatis.logging.nologging.NoLoggingImpl.class); }
    private static void tryImplementation(Runnable runnable) {
        if (logConstructor == null) {
            try { runnable.run(); } catch (Throwable t) { /* ignore */ }
        }
    }
    private static void setImplementation(Class<? extends Log> implClass) {
        try {
            Constructor<? extends Log> candidate = implClass.getConstructor(String.class);
            Log log = candidate.newInstance(LogFactory.class.getName());
            if (log.isDebugEnabled()) { log.debug("Logging initialized using '" + implClass + "' adapter."); }
            logConstructor = candidate;
        } catch (Throwable t) {
            throw new LogException("Error setting Log implementation. Cause: " + t, t);
        }
    }
}

2. Test Logging Dependencies

<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-api</artifactId>
    <version>1.7.32</version>
</dependency>
<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-simple</artifactId>
    <version>1.7.32</version>
    <scope>compile</scope>
</dependency>

These dependencies enable the simple SLF4J implementation.

3. SimpleLogger Configuration

Create a simplelogger.properties file on the classpath with optional settings such as:

#org.slf4j.simpleLogger.logFile=System.out
#org.slf4j.simpleLogger.cacheOutputStream=true
org.slf4j.simpleLogger.defaultLogLevel=debug
#org.slf4j.simpleLogger.showDateTime=true
#org.slf4j.simpleLogger.dateTimeFormat=yyyy-MM-dd HH:mm:ss:SSS
#org.slf4j.simpleLogger.showThreadName=true
#org.slf4j.simpleLogger.showLogName=true
#org.slf4j.simpleLogger.showShortLogName=false
#org.slf4j.simpleLogger.levelInBrackets=false
#org.slf4j.simpleLogger.warnLevelString=WARN

4. Multiple Logging Implementations

When both slf4j-simple and slf4j-log4j12 are on the classpath, SLF4J reports multiple bindings:

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/.../slf4j-simple-1.7.32.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/.../slf4j-log4j12-1.7.32.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [org.slf4j.impl.SimpleLoggerFactory]
[main] DEBUG org.apache.ibatis.logging.LogFactory - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.

5. Switching to Log4j

Specify the desired logger in mybatis-config.xml:

<configuration>
  <settings>
    <setting name="logImpl" value="LOG4J"/>
  </settings>
</configuration>

Add a log4j.properties file:

log4j.rootLogger = debug, stdout
log4j.appender.stdout = org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target = System.out
log4j.appender.stdout.layout = org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern = [%‑5p] %d{yyyy-MM-dd HH:mm:ss,SSS} method:%l%n%m%n

6. Sample Output

Running the test now produces Log4j‑styled logs, for example:

PooledDataSource forcefully closed/removed all connections.
[DEBUG] 2021-11-04 13:04:33,475 method:org.apache.ibatis.datasource.pooled.PooledDataSource.forceCloseAll(PooledDataSource.java:363)
[main] DEBUG test.UsersMapperTest - 查询列表
Created connection 386163331.
==>  Preparing: select * from t_users
==> Parameters:
<==      Total: 2
[Users [id=1, username=admin, password=123123], Users [id=2, username=guest, password=111111]]

These logs confirm that Log4j is now the active logging implementation.