Understanding MyBatis Logging Implementation and LogFactory Mechanism

Logging is a crucial aspect of software development, and MyBatis, a popular Java ORM framework, provides a unified logging interface while internally adapting to various logging frameworks such as SLF4J, Log4j, and java.util.logging through an adapter pattern.

MyBatis allows the logImpl setting in its global configuration to specify one of six logging implementations (SLF4J, LOG4J, LOG4J2, JDK_LOGGING, COMMONS_LOGGING, STDOUT_LOGGING, or NO_LOGGING). If no implementation is configured, MyBatis defaults to NO_LOGGING.

SLF4J is a simple logging façade that does not perform actual logging itself; it delegates to a concrete logging framework like Logback. When only SLF4J is configured without an underlying implementation, MyBatis emits a warning and produces no log output.

The log implementation is resolved by MyBatis during configuration parsing: the specified name is looked up in TypeAliasRegistry, and if not found, the class is loaded via Class.forName. Custom log classes can be used by implementing the Log interface.

After locating the log class, MyBatis calls setLogImpl, which ultimately invokes LogFactory. The static initializer of LogFactory attempts to instantiate each built‑in logger in order, and the tryImplementation method selects the first successful one, falling back to the next if an exception occurs.

MyBatis also defines a jdbc logging package that contains four proxy classes— ConnectionLogger, PreparedStatementLogger, ResultSetLogger, and StatementLogger —all extending BaseJdbcLogger and implementing InvocationHandler. These proxies use JDK dynamic proxies to intercept JDBC calls and log SQL statements, parameters, and execution details.

Each proxy’s invoke method ultimately calls the debug method of BaseJdbcLogger, which formats the log message with a prefix (e.g., "==>" for SQL execution) and outputs it through the selected logging framework.

The article concludes that MyBatis’s logging mechanism relies on the LogFactory adapter to choose an appropriate logger, and uses dynamic proxies to separate logging concerns for different JDBC objects, enabling flexible and detailed SQL logging.