This article explores the compatibility issues in Java projects when dealing with dependencies on different JDK versions. The core principle is that if a project depends on a library compiled with a higher version of the JDK, the project itself must be compiled with at least the same or higher version of the JDK. The article explains the reason for this limitation and provides a potential solution if feasible, while emphasizing the importance of prioritizing Java LTS (Long Term Support) versions in library development to ensure broader compatibility and ecosystem stability.

Understand JDK version compatibility limitations

In the Java ecosystem, a common misunderstanding is that as long as the dependent library does not use the new version of the language features, it can run in a lower version of the JDK environment. However, this is not the case. When a Java project (for example, a library compiled with Java 11) depends on another library compiled with a later version of the JDK (for example, Java 14), the main project must be compiled with at least Java 14, even if the dependent library does not use any Java 14-specific language features.

Reason analysis:

1. Bytecode version (Major.Minor Version): Each JDK version generates a specific version of bytecode when compiling Java source code. For example, Java 11 compiles to bytecode version 55, while Java 14 compiles to bytecode version 58. The Java Virtual Machine (JVM) is generally able to run bytecode compiled by an earlier version of the JDK (forward-compatible), but cannot run bytecode compiled by a later version of itself (backward-incompatible). This means that if a Java 11 JVM tries to load a Java 14 compiled class file, it will fail because it does not recognize the bytecode version.
2. Compiler and runtime environment: The compilation process itself requires a JDK environment that understands the bytecode of all dependencies. If your project depends on a library compiled with Java 14, your compiler (and ultimately the JVM) must be at least Java 14 to properly handle these dependencies.

Therefore, if your Java 11 project depends on a Java 14 compiled library, your only option is to upgrade your project to Java 14 or higher.

Potential solution: Recompile dependencies

In some specific cases, if you have sufficient control over the third-party library that is a dependency (for example, have access to its source code), and you are sure that the library does not use any language features specific to the target JDK version (such as Java 14), then you can try a "curve-saving" solution:

1. Get the source code of dependent libraries.
2. Recompile the dependent library with the JDK version required by your project (e.g. Java 11).
3. Make the recompiled library a local dependency of your project.

Things to note:

• This method only works if you have access to and recompiled the third-party library, and if the library does not depend on specific features of the higher version of the JDK.
• This increases maintenance costs because you need to manage specific versions of that dependent library yourself rather than directly using the official released version.
• If the dependent library introduces features of a higher version of the JDK in subsequent versions, you will need to repeat this process or eventually upgrade your project JDK version.

Best Practice: Embrace LTS Releases

For library developers, to ensure that your code can be used by a wider audience and to reduce the compatibility burden on downstream consumers, it is highly recommended to lock your library on a long-term support (LTS) version of Java.

Java LTS releases include:

• Java 8
• Java 11
• Java 17
• Future LTS releases (usually every two years)

Benefits of choosing LTS version:

• Longer support cycle: LTS versions offer longer maintenance and security update cycles, which means greater stability and reliability.
• Wider compatibility: Most enterprise-level applications and third-party libraries tend to use LTS versions. If your library is also based on LTS versions, it will be easier to be integrated into other projects.
• Less upgrade pressure: Using LTS versions can reduce the need to frequently upgrade JDK versions, thereby reducing project maintenance costs.
• Mature ecosystem: LTS versions have more mature tools, frameworks and community support.

In comparison, non-LTS versions (such as Java 9, 10, 12, 13, 14, 15, 16) have very short life cycles and usually end support shortly after the next version is released. This means that if your library is based on a non-LTS version, your users may need to frequently upgrade their JDK version to maintain compatibility, which will cause unnecessary trouble.

Summarize

In Java project dependency management, JDK version compatibility is a key factor that cannot be ignored. The core principle is "downward compatible, upward incompatible" - your project must be compiled with the same or higher version of the JDK as the dependent library. While it is possible to try to recompile dependent libraries under certain conditions, this is usually not a sustainable solution. For library developers, the wisest strategy is to stick to LTS versions of Java to maximize the compatibility, stability, and maintainability of your libraries, thereby promoting the development of a healthier Java ecosystem.

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