<condition> placed inside <if> is silently ignored, produces misleading "Janino not found" error

[freesoft]

Summary

When <condition> (the Janino-free alternative introduced in logback 1.5.x) is placed as a child of <if> instead of as a preceding sibling, both <then> and <else> branches are silently skipped. Instead of a helpful error about incorrect <condition> placement, logback falls through to the Janino code path and emits a misleading error:

ERROR in IfModelHandler - Could not find Janino library on the class path. Skipping conditional processing.

This is confusing because the user explicitly opted out of Janino by using the <condition> element.

Logback version

1.5.32 (also confirmed on 1.5.22)

Reproduction

Broken config (<condition> inside <if> — produces misleading Janino error, both branches skipped):

<configuration debug="true">
    <property name="testKey" value="testValue"/>
    <if>
        <condition class="ch.qos.logback.core.boolex.ExpressionPropertyCondition">
            <expression>propertyEquals("testKey", "testValue")</expression>
        </condition>
        <then>
            <appender name="THEN_APPENDER" class="ch.qos.logback.core.ConsoleAppender">
                <encoder><pattern>THEN: %msg%n</pattern></encoder>
            </appender>
            <root level="DEBUG"><appender-ref ref="THEN_APPENDER"/></root>
        </then>
        <else>
            <appender name="ELSE_APPENDER" class="ch.qos.logback.core.ConsoleAppender">
                <encoder><pattern>ELSE: %msg%n</pattern></encoder>
            </appender>
            <root level="DEBUG"><appender-ref ref="ELSE_APPENDER"/></root>
        </else>
    </if>
</configuration>

Working config (<condition> as preceding sibling of <if>):

<configuration debug="true">
    <property name="testKey" value="testValue"/>
    <condition class="ch.qos.logback.core.boolex.ExpressionPropertyCondition">
        <expression>propertyEquals("testKey", "testValue")</expression>
    </condition>
    <if>
        <then>
            <appender name="THEN_APPENDER" class="ch.qos.logback.core.ConsoleAppender">
                <encoder><pattern>THEN: %msg%n</pattern></encoder>
            </appender>
            <root level="DEBUG"><appender-ref ref="THEN_APPENDER"/></root>
        </then>
        <else>
            <appender name="ELSE_APPENDER" class="ch.qos.logback.core.ConsoleAppender">
                <encoder><pattern>ELSE: %msg%n</pattern></encoder>
            </appender>
            <root level="DEBUG"><appender-ref ref="ELSE_APPENDER"/></root>
        </else>
    </if>
</configuration>

Unit test (both tests pass, demonstrating the bug):

package logback.bug;

import ch.qos.logback.classic.Logger;
import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.classic.joran.JoranConfigurator;
import ch.qos.logback.core.joran.spi.JoranException;
import ch.qos.logback.core.status.Status;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

import java.net.URL;
import java.util.List;

import static org.assertj.core.api.Assertions.assertThat;

class ConditionInsideIfTest {

    private LoggerContext context;

    @BeforeEach
    void setUp() {
        context = new LoggerContext();
    }

    @Test
    void conditionBeforeIf_thenBranchExecuted() throws JoranException {
        configure("condition-before-if.xml");
        Logger root = context.getLogger(Logger.ROOT_LOGGER_NAME);

        // Works: <condition> as sibling before <if>
        assertThat(root.getAppender("THEN_APPENDER")).isNotNull();
        assertThat(root.getAppender("ELSE_APPENDER")).isNull();
    }

    @Test
    void conditionInsideIf_bothBranchesSkipped() throws JoranException {
        configure("condition-inside-if.xml");
        Logger root = context.getLogger(Logger.ROOT_LOGGER_NAME);

        // BUG: Both branches skipped
        assertThat(root.getAppender("THEN_APPENDER")).isNull();
        assertThat(root.getAppender("ELSE_APPENDER")).isNull();

        // BUG: Misleading "Janino not found" error instead of helpful message
        List<Status> errors = context.getStatusManager().getCopyOfStatusList().stream()
                .filter(s -> s.getLevel() >= Status.ERROR)
                .toList();
        assertThat(errors).isNotEmpty()
                .anyMatch(s -> s.getMessage().contains("Janino"));
    }

    private void configure(String resourceName) throws JoranException {
        URL url = getClass().getClassLoader().getResource(resourceName);
        assertThat(url).isNotNull();
        JoranConfigurator configurator = new JoranConfigurator();
        configurator.setContext(context);
        configurator.doConfigure(url);
    }
}

pom.xml dependencies: logback-classic:1.5.32, junit-jupiter, assertj-core — no Janino (testing the <condition> element path).

Expected behavior

When <condition> is placed inside <if>, logback should emit a clear warning or error, e.g.:

<condition> element found inside <if> — it must be placed as a preceding sibling of <if>, not as a child

Actual behavior

1. The <condition> element inside <if> is processed (the ExpressionPropertyCondition is instantiated and evaluates successfully) but its result is not linked to the <if>.
2. IfModelHandler doesn't find a condition result, falls through to janinoFallback().
3. Janino is not on the classpath, so it emits: "Could not find Janino library on the class path. Skipping conditional processing."
4. Both <then> and <else> branches are silently skipped.

Root cause analysis

In IfModelHandler.handle(), the condition result is looked up from BranchState which is set by ConditionModelHandler only when <condition> is a preceding sibling of <if>. When <condition> is a child of <if>, the ConditionModelHandler runs inside the <if> scope, so the BranchState is not found by IfModelHandler. The handler then falls through to janinoFallback() which:

1. Looks for a condition attribute on <if> (old Janino syntax) — not found
2. Checks for Janino on the classpath — not found
3. Emits the misleading Janino error
4. Both branches are skipped

Suggested fix

In IfModelHandler, when no BranchState is found and no condition attribute exists, check if <condition> appears as a child element of <if> and emit a helpful warning:

WARN: <condition> element found as child of <if> but it must be a preceding sibling. See https://logback.qos.ch/...

This would save users significant debugging time, as the current "Janino not found" message sends them on a wild goose chase.