SpringBoot project/dependencies and versions problems

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Summary

The project fails to compile with a Toolchain Mismatch error (java.lang.ExceptionInInitializerError com.sun.tools.javac.code.TypeTag :: UNKNOWN). This error occurs because the build environment is using an incompatible JDK version relative to the Spring Boot and Maven Compiler configurations. Specifically, while the project targets Java 21, the active JDK running the Maven build is likely an older version (JDK 11, 17, or early 21) that cannot handle the specific javac internals required by the newer Spring Boot 3.2.1 plugin ecosystem. The error manifests during the annotation processing phase, where the compiler attempts to initialize internal type tags that do not exist in the running JDK.

Root Cause

The root cause is a JDK Version Incompatibility between the developer’s environment and the project requirements.

  • Spring Boot 3.x Requirement: The project uses Spring Boot 3.2.1, which strictly requires JDK 17 or higher. It does not support JDK 11.
  • Java 21 Target: The <java.version>21</java.version> property forces the compiler to target Java 21 features.
  • Annotation Processing Conflict: The stack trace points to com.sun.tools.javac.code.TypeTag. This is an internal JDK class. The error UNKNOWN indicates that the running JVM’s compiler does not recognize the bytecode or language features being processed. This typically happens when:
    1. The Maven Compiler Plugin inherited from Spring Boot Parent is configured for Java 21, but the JAVA_HOME environment variable points to an older JDK (e.g., Java 11 or Java 8).
    2. Lombok (which is present in the POM) generates bytecode that interacts with javac internals. Lombok versions bundled with older Spring Boot releases are incompatible with Java 21, causing this specific crash during compilation.

Why This Happens in Real Systems

In real-world engineering, this issue is a classic Environment Drift scenario:

  • IDE vs. CLI Mismatch: Developers often upgrade their IDE’s (IntelliJ/Eclipse) bundled JDK to 21, but the system JAVA_HOME or Maven toolchains.xml remains pointed at a legacy JDK required for other projects.
  • Container Inconsistency: A developer builds locally with JDK 21 but the CI/CD pipeline (Jenkins/GitHub Actions) uses a Docker image with JDK 17, or vice versa, leading to “works on my machine” errors.
  • Legacy Dependency Inheritance: Transitive dependencies (like specific spring-boot-dependencies or javafx plugins) may pull in configurations optimized for Java 17, which conflict with explicit Java 21 flags, causing the javac internal state to become corrupted.

Real-World Impact

  • Blocked Builds: The pipeline fails immediately; no artifacts can be produced.
  • Developer Productivity Loss: Engineers waste hours reinstalling JDKs, cleaning Maven caches (.m2), and guessing environment variables.
  • False Positives: The error message (TypeTag :: UNKNOWN) is cryptic and misleading. It looks like a Spring or Lombok bug, leading to incorrect issue reports or dependency downgrades, rather than a simple environment fix.
  • Unpredictable Behavior: If a bad build is forced or bypassed, runtime NoSuchMethodError or ClassFormatError exceptions may occur later, which are much harder to debug.

Example or Code

To reproduce or verify this issue, you can check the running Maven Java version. If this command returns anything lower than 17, the root cause is confirmed.

mvn -v

Example INVALID output (causes the error):

Apache Maven 3.9.5
Maven home: /usr/local/Cellar/maven/3.9.5
Java version: 11.0.19, vendor: Amazon.com Inc.

Example VALID output (required for Spring Boot 3.2.1 + Java 21):

Apache Maven 3.9.5
Maven home: /usr/local/Cellar/maven/3.9.5
Java version: 21.0.1, vendor: Oracle

How Senior Engineers Fix It

Senior engineers address this by enforcing Strict Environment Parity and Dependency Alignment:

  1. Verify JAVA_HOME:
    Run echo $JAVA_HOME (or echo %JAVA_HOME% on Windows) and ensure it points to a JDK 21 installation (e.g., jdk-21.jdk).
  2. Configure Maven Toolchains (Best Practice):
    Create/update ~/.m2/toolchains.xml to explicitly define the JDK 21 path, then configure the Maven Compiler Plugin to use it.

    
    org.apache.maven.plugins
    maven-compiler-plugin
    
        21
        21
  3. Validate Dependency Alignment:
    Explicitly set the maven-compiler-plugin version to a modern one compatible with Java 21 (e.g., 3.11.0 or 3.13.0) to override any stale defaults inherited from older parent POMs.
  4. IDE Recalculation:
    Invalidate caches and restart the IDE (File -> Invalidate Caches in IntelliJ), ensuring the project JDK is set to 21 in Project Structure.

Why Juniors Miss It

  • Reliance on IDE Magic: Juniors assume the IDE “just works” without verifying the underlying JDK used by the build tool (Maven/Gradle). They often change the “Project SDK” but forget to change the “Build and Run” SDK or the Maven runner JDK.
  • Misinterpreting the Stack Trace: They focus on com.sun.tools.javac and assume it is a bug in the Java Compiler itself or a Spring Boot issue, leading them to downgrade Spring Boot versions unnecessarily.
  • Skipping mvn -v: They rarely check the global Maven version or Java configuration, assuming that pom.xml dictates everything, not realizing the build tool must physically run on a compatible JDK to interpret that POM.