Avoid Java 8‑to‑17 Upgrade Pitfalls: Compilation, Unified Logging, and GC Migration Guide

Compilation Issues

Java 11 introduced JEP 320, which removes the Java EE and CORBA modules. Projects that still use classes such as javax.annotation.* must add the javax.annotation-api dependency manually:

<dependency>
    <groupId>javax.annotation</groupId>
    <artifactId>javax.annotation-api</artifactId>
    <version>1.3.2</version>
</dependency>

Packages under sun.misc (e.g., BASE64Encoder) are no longer available; replace them with standard alternatives. Older Netty versions that depend on sun.misc.* will fail to compile.

Lombok versions prior to the latest use com.sun.tools.javac internals, causing compilation errors on Java 17. Upgrading Lombok (e.g., to 1.18.24) resolves the issue.

Kotlin projects must use Kotlin 1.6 or newer because earlier versions do not support the Java 17 bytecode target.

Run a dependency analysis with

jdeps --jdk-internals --multi-release 17 --class-path . your‑app.jar

to identify modules that need updating.

Parameter Migration – Unified Logging

Java 9 introduced Unified Logging, replacing the fragmented logging flags of earlier releases. The general syntax is:

-Xlog:[selectors]:[output]:[decorators][:output‑options]

Selectors (what) combine tags and levels, e.g., -Xlog:gc=debug prints debug‑level GC logs. Wildcards ( *) allow broader matching, such as -Xlog:gc*=debug for all GC‑related tags.

Output (where) defaults to stdout. To log to a file with rotation:

-Xlog:all=debug:file=/path/to/app.log:filesize=104857600,filecount=5

Decorators (additional context) add timestamps, thread IDs, etc. Example:

-Xlog:all=debug:stdout:level,tags,time,uptime,pid

Run java -Xlog:help to list all available tags.

GC Parameter Migration

Many -XX flags are deprecated in Java 17. Use Unified Logging equivalents instead: -XX:+PrintGCDetails →

-Xlog:gc*

-XX:+PrintGCApplicationStoppedTime

and -XX:+PrintGCApplicationConcurrentTime →

-Xlog:safepoint

-XX:+PrintTenuringDistribution

→

-Xlog:gc+age=trace

-XX:+PrintAdaptiveSizePolicy

→ -Xlog:gc+ergo*=trace Typical replacement configuration:

-Xlog:
  gc*,
  safepoint,
  gc+heap=debug,
  gc+ergo*=trace,
  gc+age*=trace,
  :file=/var/log/%t-gc.log,
  :time,tags,level,
  :filesize=104857600,filecount=5

Runtime Issues – Reflection and Private APIs

Java 9’s module system blocks reflective access to internal packages. To open packages, add JVM options such as:

--add-opens java.base/java.lang=ALL-UNNAMED
--add-opens java.base/java.io=ALL-UNNAMED
--add-opens java.base/jdk.internal.misc=ALL-UNNAMED

GC Algorithm Choice

CMS is removed; G1 is the default and generally the safest choice. ZGC offers low‑pause times but can appear to consume three times the resident memory due to its colored‑pointer implementation.

G1 Tuning Recommendations

Avoid configuring explicit young‑generation sizes (e.g., -Xmn, -XX:NewSize); let G1 adjust dynamically.

Adjust -XX:InitiatingHeapOccupancyPercent (default 45) to control when concurrent marking starts, based on your application’s average heap usage.

Reflection and Private API Access

When using libraries that rely on internal APIs (e.g., older Netty or Lombok), add the appropriate --add-opens flags to the JVM command line to avoid IllegalAccessError at runtime.

Conclusion

By updating dependencies, switching to Unified Logging, replacing deprecated GC flags, and applying the recommended G1 tuning parameters, teams can smoothly migrate Java 8 applications to Java 17 while maintaining performance and stability.