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Java 13 Dynamic CDS Archives Guide

Overview

Dynamic Class-Data Sharing (CDS) Archives extend the existing CDS feature by allowing classes to be archived dynamically at application exit, not just during JDK build time.

What is CDS?

Class-Data Sharing (CDS) improves startup performance by sharing common class metadata between different Java processes. This reduces memory footprint and speeds up application startup.

Dynamic CDS vs Static CDS

Static CDS (Java 5+)

  • Created during JDK build time
  • Contains only JDK classes
  • Fixed at JDK installation

Dynamic CDS (Java 13+)

  • Created at application runtime
  • Contains application-specific classes
  • Generated when application exits
  • Can be updated without rebuilding JDK

Creating Dynamic CDS Archive

Step 1: Run Application with Archive Option

java -XX:ArchiveClassesAtExit=app-cds.jsa \
     -cp myApp.jar \
     com.example.MyApp

This command:

  • Runs your application normally
  • Archives all loaded classes when the application exits
  • Creates app-cds.jsa file

Step 2: Use Dynamic Archive in Subsequent Runs

java -XX:SharedArchiveFile=app-cds.jsa \
     -cp myApp.jar \
     com.example.MyApp

This command:

  • Loads classes from the archive
  • Faster startup time
  • Reduced memory footprint

Complete Example

1. First Run (Create Archive)

# Run application and create archive
java -XX:ArchiveClassesAtExit=myapp-cds.jsa \
     -cp target/myapp.jar \
     com.example.Main

# Archive is created at: myapp-cds.jsa

2. Subsequent Runs (Use Archive)

# Use the archive for faster startup
java -XX:SharedArchiveFile=myapp-cds.jsa \
     -cp target/myapp.jar \
     com.example.Main

JVM Options

Creating Archive

  • -XX:ArchiveClassesAtExit=<filename>: Create archive at exit

Using Archive

  • -XX:SharedArchiveFile=<filename>: Use existing archive
  • -Xshare:on: Force CDS (fail if archive not found)
  • -Xshare:auto: Use CDS if available (default)
  • -Xshare:off: Disable CDS

Benefits

  1. Faster Startup: Classes are pre-loaded from archive
  2. Reduced Memory: Shared class metadata across processes
  3. Application-Specific: Includes your application classes
  4. Dynamic: No JDK rebuild required
  5. Easy Updates: Regenerate archive when classes change

Use Cases

Microservices

  • Frequent restarts benefit from faster startup
  • Multiple instances can share memory

Development

  • Faster iteration cycles
  • Quicker test execution

Production

  • Reduced startup time
  • Lower memory footprint
  • Better resource utilization

Best Practices

  1. Create Archive in Production Environment

    • Archive should match production classpath
    • Use same JDK version

  2. Update Archive When Needed

    • Regenerate when classes change
    • Update when dependencies change

  3. Test Archive Compatibility

    • Verify archive works with new code
    • Test startup performance

  4. Monitor Performance

    • Measure startup time improvement
    • Check memory usage reduction

Troubleshooting

Archive Not Found

Error: Could not open shared archive file

Solution: Ensure archive path is correct and file exists

Archive Incompatible

Error: Shared archive file is incompatible

Solution: Regenerate archive with current classes

Archive Creation Failed

Error: Failed to create archive

Solution: Check file permissions and disk space

Example Workflow

# 1. Build application
mvn clean package

# 2. First run - create archive
java -XX:ArchiveClassesAtExit=app-cds.jsa \
     -cp target/app.jar \
     com.example.Main

# 3. Verify archive created
ls -lh app-cds.jsa

# 4. Use archive in subsequent runs
java -XX:SharedArchiveFile=app-cds.jsa \
     -cp target/app.jar \
     com.example.Main

# 5. Measure performance improvement
time java -XX:SharedArchiveFile=app-cds.jsa \
          -cp target/app.jar \
          com.example.Main

Performance Impact

Typical improvements:

  • Startup Time: 10-30% faster
  • Memory: 5-10% reduction
  • First Request: Slightly faster (classes pre-loaded)

Limitations

  1. Archive must match classpath
  2. Archive is JDK version specific
  3. Some classes cannot be archived
  4. Archive size grows with application size

Additional Resources

Summary

Dynamic CDS Archives provide an easy way to improve application startup performance by archiving application classes at runtime. This feature is particularly beneficial for applications that restart frequently or have long startup times.