SLF4J SubstituteLoggerFactory Concurrency Quirk

In SLF4J 2.x, concurrent initialization can cause LoggerFactory.getILoggerFactory() to temporarily return a SubstituteLoggerFactory instead of the expected logging backend factory (such as Logback's LoggerContext), resulting in unexpected ClassCastException in multi-threaded environments.

Problem Description

When bootstrapping a Java application or test suite that runs tests concurrently (such as JUnit 5 parallel execution), multiple threads may interact with logging simultaneously.

If code attempts to cast the underlying ILoggerFactory to a concrete implementation:

var context = (LoggerContext) LoggerFactory.getILoggerFactory();

A ClassCastException can be thrown intermittently:

java.lang.ClassCastException: class org.slf4j.helpers.SubstituteLoggerFactory cannot be cast to class ch.qos.logback.classic.LoggerContext

Root Cause Analysis

In SLF4J 2.x, org.slf4j.LoggerFactory tracks its lifecycle via internal states: UNINITIALIZED, ONGOING_INITIALIZATION, SUCCESSFUL_INITIALIZATION, and FAILED_INITIALIZATION.

When the first thread triggers SLF4J initialization:

1. INITIALIZATION_STATE transitions from UNINITIALIZED to ONGOING_INITIALIZATION.
2. The initializing thread enters performInitialization(), which uses Java SPI (ServiceLoader<SLF4JServiceProvider>) to discover, instantiate, and initialize the provider (e.g. LogbackServiceProvider).

During this window, if another thread calls LoggerFactory.getILoggerFactory():

1. It checks if (INITIALIZATION_STATE == UNINITIALIZED). Because the state is ONGOING_INITIALIZATION, it bypasses synchronization on LoggerFactory.class.
2. In the following switch statement, case ONGOING_INITIALIZATION: returns SUBST_PROVIDER (intended to support re-entrant logging on the initializing thread without deadlocks).
3. As a result, the non-initializing thread immediately receives SubstituteLoggerFactory rather than waiting for initialization to finish.

Resolution

When programmatically configuring or accessing the backend context, do not assume LoggerFactory.getILoggerFactory() immediately returns the concrete factory during early startup. Instead, poll with a timeout when SubstituteLoggerFactory is encountered:

private static LoggerContext getLoggerContext() {
    var factory = LoggerFactory.getILoggerFactory();
    if (factory instanceof SubstituteLoggerFactory) {
        var sleeper = Sleeper.of(Duration.ofMillis(10), Duration.ofSeconds(5));
        while (factory instanceof SubstituteLoggerFactory) {
            sleeper.sleep("Timed out waiting for SLF4J initialization");
            factory = LoggerFactory.getILoggerFactory();
        }
    }
    if (factory instanceof LoggerContext context) {
        return context;
    }
    throw new IllegalStateException("Expected LoggerContext but got " + factory.getClass().getName());
}