Fixing main class ambiguity in Spring Boot

Searching for the main class. When building a Spring Boot application, the framework relies on the spring-boot-maven-plugin to package the project into an executable JAR. During the packaging phase, this plugin scans the compiled classes to locate the entry point—specifically the class annotated with @SpringBootApplication that contains the public static void main method. Under standard conditions, this discovery process is automatic and transparent. However, as projects scale or incorporate multiple modules and integration tests, the plugin may encounter multiple candidates or fail to identify the correct one, leading to a critical build error.

The Problem: Ambiguity in the Bootstrap Process

The error “Error: Unable to find a single main class from the following candidates” typically occurs during the mvn package or mvn install command. This happens when the classpath contains more than one class with a main method that looks like a Spring Boot entry point. This is common in microservices architectures where utility classes or test configurations might inadvertently satisfy the plugin’s search criteria. Without an explicit instruction, Maven cannot decide which class should be responsible for bootstrapping the application context. This results in a failed build.

The Explicit Solution via Maven Plugin

To resolve this ambiguity, you must explicitly tell the spring-boot-maven-plugin which class to use as the application’s entry point. This is handled within the <plugins> section of your pom.xml file. By defining the <mainClass> property inside the plugin configuration, you override the automatic search mechanism. Also you provide a definitive path to your bootstrap class.

Practical Maven Configuration:

In the following example, we specify that IdrobyteApplication is the designated main class for the build process.

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>
                <mainClass>com.idrobyte.IdrobyteApplication</mainClass>
            </configuration>
            <executions>
                <execution>
                    <goals>
                        <goal>repackage</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

Using the fully qualified name (including the package) is mandatory. This configuration ensures that even if other main methods exist in the project, the generated JAR manifest will correctly point to the intended bootstrap logic.

Alternative Configuration using Start-Class Property

In some scenarios, particularly with older versions of Spring Boot or specific parent POM structures, you might prefer using a property-based approach. The plugin is designed to look for a property named start-class if the explicit configuration is missing. Adding this to your <properties> section can achieve the same result while keeping the plugin declaration cleaner.

Properties-based Example

<properties>
    <java.version>17</java.version>
    <start-class>com.idrobyte.IdrobyteApplication</start-class>
</properties>

While effective, the plugin configuration method shown previously is generally considered more robust. This because it resides directly within the build tool’s logic.

Troubleshooting and Verification for the right main class

If you have defined the main class but the build still fails, the issue is often related to the class path or a naming mismatch.

Verifying the fully qualified name

Ensure that the string in your pom.xml matches the actual package and class name in your Java source. A single typo in the package path will cause the plugin to report that we cannot find the class.

Classpath conflicts

Sometimes, a main class might be present in a dependency that is incorrectly scoped. If a dependency brings in its own Spring Boot bootstrap class, Maven might see it as a candidate. Ensure that your dependencies are properly scoped and that utility libraries do not accidentally include @SpringBootApplication annotations.

Testing environment interference

If the error lists classes located in src/test/java, it means the plugin is scanning test sources during a phase where it should only be looking at production code. While the spring-boot-maven-plugin usually filters these out, explicit configuration is the only way to guarantee it ignores these candidates during the repackage goal.

Summary of Spring Boot main class configuration

Defining the entry point is a fundamental step in ensuring a reproducible build pipeline.

• Automatic Discovery Failure: Occurs when multiple main methods exist or the search fails to identify the correct candidate.
• Plugin Configuration: The most effective fix involves setting the <mainClass> inside the spring-boot-maven-plugin.
• Fully Qualified Name: Always use the full package path to avoid ambiguity.
• Property Fallback: The <start-class> property serves as a secondary method for defining the entry point.
• Build Integrity: Explicitly defining the main class prevents unexpected failures during CI/CD processes when we will add new classes to the project.