# Migrating from Developing Asynchronous Multi-Threading Programs to Reactive Programs in Java
## 6 Implementation Details  
### Updating Java and Spring Versions  
Recent versions of Java and Spring provide access to the latest functionality, optimizations, and enhancements while ensuring long-term relevance, compatibility, and essential security fixes against numerous vulnerabilities. Upgrading to the latest versions is therefore crucial for a successful migration process and lays a solid foundation for future adjustments. However, it is worth noting that most industry applications are still running on lower versions of Java. While the asynchronous and reactive projects are based on Java 21, many organizations may find it more practical to adopt an intermediate step. Java 17 serves as a bridge between the two major versions of Spring Boot, namely Spring Boot 2 and Spring Boot 3. It is compatible with both, whereas Java 11 is not supported by Spring Boot 3. Therefore, for existing applications, particularly those on older Spring Boot versions, the first step should be updating to Java 17. This transition allows for a smoother migration path, as Java 17 facilitates compatibility while mitigating potential issues arising from outdated or deprecated specifications. This approach ensures that applications remain in line with industry standards while taking advantage of the new features and improvements available in later versions. This step should also be reasonably straightforward, as most Java versions are designed to maintain very good backward compatibility, facilitating a smooth transition. One of the most significant changes is the management of Jakarta EE dependencies. Newer versions of Spring Boot no longer use \"javax\" dependencies, necessitating a migration to their \"jakarta\" equivalents. As a result, developers need to update the import statements in the source code wherever these old specifications are used. While this transition might seem straightforward in the context of the own codebase, complications often arise with external dependencies because there is no interoperability between the two specifications. The most reliable approach is to update the maintained dependencies to their latest versions, which should be compatible with Jakarta EE, and then thoroughly test the application to ensure that its functionality remains intact. However, if an immediate upgrade is not feasible, either because a compatible version of the dependency is not available or because the upgrade process is too complex, a temporary solution might involve using tools such as Eclipse Transformer [58]. This tool works by directly modifying the bytecode of the existing JAR files, converting \"javax\" references to \"jakarta\" equivalents without requiring access to the original source code. This process begins by identifying all dependencies that still rely on the \"javax\" namespace. Once identified, Eclipse Transformer or a similar utility can be used to rewrite the bytecode within these JAR files, adapting them to the new Jakarta specifications. After the transformation, the modified JAR files are then integrated back into the project, replacing the old versions. Although this approach allows you to continue using older dependencies temporarily, it is crucial to conduct thorough testing of the application to ensure that these transformations have not introduced any issues or altered the application's behavior. The transformation process must be repeated for each affected dependency, which can make the procedure quite time-consuming and complex. Moreover, because this process does not guarantee complete success, extensive testing is essential to confirm that the application's functionality is preserved. A diagram presenting the mentioned steps is depicted in Figure 4.  
Another change that may potentially impact the operability of the application is the changed default behavior for a trailing slash in HTTP request paths. In the previous version of Spring, a controller method annotated with @_GetMapping(\"/feed\") would have accepted requests coming in on both the \"/feed\" path, as in the example, and also on the \"/feed/\".