Java

Debugger: JDWP (Java Debug Wire Protocol, built into the JVM) Status: Stable JDK version: 11+

Prerequisites

JDWP is built into the JVM — no separate debugger installation needed.

Verify: java --version (must be 11+)

Quick Start

# Debug a plain Java program
krometrail launch "java -cp target/classes com.example.Main" \
	--break src/main/java/com/example/OrderService.java:147

# Debug with Maven
krometrail launch "mvn exec:java -Dexec.mainClass=com.example.Main" \
	--break src/main/java/com/example/OrderService.java:147

# Debug JUnit tests
krometrail launch "mvn test -Dtest=OrderServiceTest" \
	--break src/main/java/com/example/OrderService.java:147

How It Works

The adapter starts the JVM with JDWP agent flags (-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=PORT) injected automatically. No changes to your code or build config are needed.

Conditional Breakpoints

Java expressions:

krometrail break "OrderService.java:147 when discount < 0"
krometrail break "OrderService.java:147 when user.getTier().equals(\"GOLD\")"

Thread Debugging

Java programs typically have many threads (GC, finalizer, etc.). Use debug_threads to navigate:

krometrail threads
# Focus on application threads — filter out GC/finalizer by name

Inspecting Java Objects

The viewport renders Java objects using their field values:

Locals:
  order    = Order@482 {id=482, total=149.97, tier=GOLD}
  discount = -149.97
  items    = ArrayList(3): [Item@12, Item@13, Item@14]

Use debug_evaluate for deeper inspection:

krometrail eval "order.getDiscount().getClass().getName()"
krometrail eval "items.stream().mapToDouble(Item::getPrice).sum()"

Tips

• Compile with -g flag for full debug information (javac -g or set in build tool)
• For Spring Boot: launch with ./mvnw spring-boot:run or the equivalent Gradle command
• Inner classes appear as OuterClass$InnerClass in breakpoint paths
• Lambda breakpoints work — set the breakpoint on the line containing the lambda expression