apache/maven · error · ExpressionEvaluationException

Error evaluating plugin parameter expression: ${expression}

Error message

Error evaluating plugin parameter expression: ${expression}

What it means

While injecting configuration into a mojo, Maven evaluated a plugin parameter expression rooted at the session object (for example ${session.localRepository.path}) via ReflectionValueExtractor, and the reflective walk threw. Maven wraps the failure in ExpressionEvaluationException carrying the exact expression, which then surfaces as a mojo configuration failure.

Source

Thrown at impl/maven-core/src/main/java/org/apache/maven/plugin/PluginParameterExpressionEvaluator.java:191

            try {
                int pathSeparator = expression.indexOf('/');

                if (pathSeparator > 0) {
                    String pathExpression = expression.substring(0, pathSeparator);
                    value = ReflectionValueExtractor.evaluate(pathExpression, session);
                    if (pathSeparator < expression.length() - 1) {
                        if (value instanceof Path path) {
                            value = path.resolve(expression.substring(pathSeparator + 1));
                        } else {
                            value = value + expression.substring(pathSeparator);
                        }
                    }
                } else {
                    value = ReflectionValueExtractor.evaluate(expression, session);
                }
            } catch (Exception e) {
                // TODO don't catch exception
                throw new ExpressionEvaluationException(
                        "Error evaluating plugin parameter expression: " + expression, e);
            }
        } else if ("reactorProjects".equals(expression)) {
            value = session.getProjects();
        } else if ("project".equals(expression)) {
            value = project;
        } else if ("executedProject".equals(expression)) {
            value = project.getExecutionProject();
        } else if (expression.startsWith("project") || expression.startsWith("pom")) {
            try {
                int pathSeparator = expression.indexOf('/');

                if (pathSeparator > 0) {
                    String pathExpression = expression.substring(0, pathSeparator);
                    value = ReflectionValueExtractor.evaluate(pathExpression, project);
                    value = value + expression.substring(pathSeparator);
                } else {
                    value = ReflectionValueExtractor.evaluate(expression, project);

View on GitHub (pinned to e4093d4e12)

Solutions

1. Test the expression standalone: mvn help:evaluate -Dexpression=session.localRepository.path -q -DforceStdout.
2. Compare the path against the MavenSession getters (getLocalRepository, getSettings, getTopLevelProject, ...) and fix the segment names.
3. If an intermediate value can be null, split the expression or provide a default through a property in <properties>.
4. Prefer plain project properties over deep session reflection paths.

Example fix

<!-- before -->
<repoDir>${session.localRepo.path}</repoDir>
<!-- after: match the real MavenSession getter chain -->
<repoDir>${session.localRepository.path}</repoDir>

mvn -q org.apache.maven.plugins:maven-help-plugin:3.4.1:evaluate \
  -Dexpression=session.localRepository.path -DforceStdout || echo 'BAD EXPRESSION'

try {
    configValue = evaluator.evaluate(expr, Object.class);
} catch (ExpressionEvaluationException e) {
    throw new MojoExecutionException("unresolvable expression: " + e.getMessage(), e);
}

When it happens

Trigger: An expression like ${session.someField} where someField is not a readable property of MavenSession; a nested path that traverses a null intermediate (a getter returns null and the next segment is dereferenced); malformed path segments with stray characters.

Common situations: Typos in POM configuration values; expressions that worked under an older Maven where session getters differed; templates copied from other projects that reference session state which is null in the current phase.

Related errors

• Cannot find default setter in {}
• Cannot set default
• Error evaluating plugin parameter expression: ${expression}
• Failed to interpolate field: " + field + " on class: " + cls
• The token '%s' at position '%d' refers to a java.util.Map, b

AI-assisted analysis of apache/maven@e4093d4e12 (2026-08-21). Data as JSON: /api/errors/1b53b878c56ce6b5. Report an issue: GitHub.