TemplateInputException with Thymeleaf 3.x on Spring Boot 1.5

Problem Context

When a Spring Boot 1.5 application declares a direct dependency on Thymeleaf 3.x, startup fails with a TemplateInputException that wraps a NoSuchMethodError for org.thymeleaf.spring5.SpringTemplateEngine.process(...). The error indicates that the Spring Boot 1.5 starter expects the older org.thymeleaf.spring4 integration classes.

Constraints and Uncertainty

• Spring Boot 1.5 is locked by legacy project requirements.
• Thymeleaf 3.x is desired for its new features and bug fixes.
• The Spring Boot 1.5 starter pulls in Thymeleaf 2.x transitively, leading to a classpath conflict.

Unresolved Decision

Developers must decide whether to upgrade Spring Boot to a version that bundles Thymeleaf 3.x, or to downgrade the Thymeleaf dependency to 2.x to maintain compatibility with Spring Boot 1.5.

Questions

• Is Thymeleaf 3.x officially supported by Spring Boot 1.5, or is a minimum Spring Boot 2.x required for the spring5 integration?
• What is the recommended approach for resolving the NoSuchMethodError without breaking auto‑configuration?
• Can the transitive Thymeleaf dependency be excluded from the Spring Boot starter and replaced with a compatible version, and what are the risks?