Resolving Maven Resources Plugin 3.2.0 Failure in Spring Boot Projects

Introduction to the Maven Resources Plugin Failure

The Maven build process relies heavily on the resources plugin to handle file copying and filtering during project compilation. When using Spring Boot 2.4.x with Maven resources plugin version 3.2.0, developers frequently encounter the error message: "Failed to execute goal org.apache.maven.plugins:maven-resources-plugin:3.2.0:resources (default-resources) on project [project-name]: Input length = 1". This error typically occurs during the mvn clean install command execution and can halt the entire build process.

Root Cause Analysis

The primary cause of this failure stems from character encoding mismatches between resource files and the Maven plugin's expectations. The maven-resources-plugin 3.2.0 assumes UTF-8 encoding by default, but when property files or other resources contain characters encoded in different formats (such as ISO-8859-1), the plugin encounters malformed input sequences. This results in the java.nio.charset.MalformedInputException: Input length = 1 that propagates up through the Maven execution stack.

Consider the following scenario: A properties file contains accented characters saved in ISO-8859-1 encoding. When Maven attempts to process this file as UTF-8, the byte sequences don't form valid UTF-8 characters, causing the decoding failure. The stack trace clearly shows this progression from the charset level up to the Maven plugin execution:

java.nio.charset.MalformedInputException: Input length = 1
    at java.nio.charset.CoderResult.throwException(CoderResult.java:274)
    at org.apache.maven.shared.filtering.DefaultMavenFileFilter.copyFile(DefaultMavenFileFilter.java:113)
    at org.apache.maven.plugins.resources.ResourcesMojo.execute(ResourcesMojo.java:362)

Temporary Workaround Solution

For immediate resolution, downgrading the maven-resources-plugin to version 3.1.0 provides a reliable workaround. This version handles character encoding more leniently and avoids the strict UTF-8 validation that causes the failure. Add the following plugin configuration to your pom.xml:

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-resources-plugin</artifactId>
            <version>3.1.0</version>
        </plugin>
    </plugins>
</build>

This configuration explicitly specifies the plugin version, overriding the default version provided by Spring Boot's parent POM. The downgrade to 3.1.0 maintains functionality while avoiding the encoding strictness introduced in version 3.2.0.

Permanent Encoding Solution

The proper long-term solution involves ensuring all resource files use consistent UTF-8 encoding. Start by explicitly defining the project encoding in your pom.xml properties section:

<properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <java.version>15</java.version>
</properties>

Next, convert all existing property files to UTF-8 encoding. Most modern IDEs provide encoding conversion tools. For example, in IntelliJ IDEA, you can select multiple files, right-click, and choose "File Encoding" → "Convert to UTF-8". This ensures that special characters, including accented letters, are properly represented and processed during the Maven build.

Additional Configuration Considerations

Beyond encoding issues, the original problem description also revealed dependency duplication warnings in the POM file. These warnings, while not directly causing the resources plugin failure, indicate structural problems that could lead to other build issues. The duplicate declarations of spring-boot-starter-data-jpa and spring-boot-starter-test should be resolved by removing redundant dependency entries.

For projects containing binary resources that should not undergo filtering, configure the resources plugin to exclude specific file extensions:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-resources-plugin</artifactId>
    <configuration>
        <nonFilteredFileExtensions>
            <nonFilteredFileExtension>pdf</nonFilteredFileExtension>
            <nonFilteredFileExtension>png</nonFilteredFileExtension>
            <nonFilteredFileExtension>jpg</nonFilteredFileExtension>
        </nonFilteredFileExtensions>
    </configuration>
</plugin>

Best Practices for Resource Management

To prevent similar issues in future projects, adopt these Maven resource management practices:

1. Always specify project.build.sourceEncoding in your POM properties
2. Use consistent UTF-8 encoding across all text resources
3. Regularly validate POM structure to eliminate duplicate dependencies
4. Configure resource filtering exclusions for binary files
5. Test builds with both clean and incremental scenarios

When creating new Spring Boot projects using Spring Initializr, verify that the generated POM includes proper encoding configuration. If necessary, add the encoding property manually to ensure build consistency across different development environments.

Conclusion

The Maven resources plugin failure in Spring Boot 2.4.x projects primarily results from encoding mismatches between resource files and plugin expectations. While downgrading to version 3.1.0 provides immediate relief, the permanent solution involves standardizing on UTF-8 encoding across all project resources. By implementing proper encoding configuration and following Maven best practices, developers can maintain stable, reproducible builds while avoiding the character encoding pitfalls that frequently disrupt the development workflow.