Migrate to JDI execution control

.github/workflows/maven.yml

@@ -12,11 +12,9 @@ jobs:
 
     strategy:
       matrix:
-        os: [ ubuntu-latest ]
-        java: [ 11, 17, 21, 25 ]
-        include:
-          - os: windows-latest
-            java: 25
+        os: [ ubuntu-latest, windows-latest ]
+        java: [ 21, 25 ]
+      fail-fast: false
 
     steps:
       - name: Checkout

.github/workflows/release-prepare.yml

@@ -24,7 +24,7 @@ jobs:
       - name: Set up JDK
         uses: actions/setup-java@v4
         with:
-          java-version: 17
+          java-version: 21
           distribution: 'temurin'
           cache: 'maven'
 

CLAUDE.md

@@ -5,13 +5,13 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 ## Build, Test, Run
 
 - `mvn clean package` — full build; produces the shaded kernel jar and the kernelspec zip under `jjava-distro/target/`.
-- `mvn clean verify` — what CI runs on Linux/macOS. Adds Failsafe integration tests (the `*IT.java` cases in `jjava-distro`, which spin up `eclipse-temurin` containers via Testcontainers). Testcontainers does not work on Windows; CI uses `mvn clean test` there.
+- `mvn clean verify` — runs Failsafe integration tests (`*IT.java` in `jjava-distro`) via Testcontainers if Testcontainers can reach Docker (Windows GitHub-hosted runners cannot run WSL-backed containers). If Docker is unavailable, use `mvn clean test`.
 - `mvn test -pl jjava-kernel -am` — single-module unit tests (with required upstream modules built).
 - `mvn -pl jjava-kernel test -Dtest=JavaKernelExtensionsLifecycleTest` — run a single test class.
 - Surefire in `jjava-kernel` and `jjava-distro` is configured with `--add-opens jdk.jshell/jdk.jshell=ALL-UNNAMED` — required for JShell reflection. Anything that exercises `JavaKernel`/`CodeEvaluator` directly needs the same flag.
 - Local install of the freshly built kernel into Jupyter: see `DEV-GUIDE.md` (`unzip` the kernelspec zip, then `jupyter kernelspec install ... --name=java --user`).
 
-Minimum toolchain: JDK 11 (`maven.compiler.release=11`). CI matrix builds against 11, 17, 21, 25.
+Minimum toolchain: JDK 21 (`maven.compiler.release=21`). CI matrix builds against 21 and 25.
 
 ## Module Layout and Dependency Graph
 

DEV-GUIDE.md

@@ -22,7 +22,7 @@ You will need a **JDK** >= 11 and proper credentials for the `sonatype-central`
 
 Two submodules of this project have different release strategies:
 
-- the library modules (`jjava-jupyter`, `jjava-kernel`, `jjava-maven`, `jjava-launcher`) are released
+- the library modules (`jjava-jupyter`, `jjava-kernel`, `jjava-maven`) are released
   on Maven Central via the Sonatype Central Portal
 - the `jjava-distro` kernel assembly is released through GitHub Releases, and from there picked up by
   the [Homebrew tap](https://github.com/dflib/homebrew-tap) and PyPI

README.md

@@ -8,7 +8,7 @@ _JJava is an evolution of the earlier [IJava kernel](https://github.com/SpencerP
 
 ## Requirements
 
-1.  Java 11 or newer
+1.  Java 21 or newer
 2.  Python and a Jupyter-like environment to use the kernel in.
 
 ## Installation

jjava-distro/assembly/zipped-kernel.xml

@@ -8,10 +8,6 @@
             <source>${project.build.directory}/jjava-distro-${project.version}.jar</source>
             <destName>jjava.jar</destName>
         </file>
-        <file>
-            <source>../jjava-launcher/${project.build.directory}/jjava-launcher-${project.version}.jar</source>
-            <destName>jjava-launcher.jar</destName>
-        </file>
         <file>
             <source>../kernelspec/java/kernel.json</source>
         </file>

jjava-distro/pom.xml

@@ -133,15 +133,6 @@
                     </transformers>
                 </configuration>
             </plugin>
-            <plugin>
-                <groupId>org.apache.maven.plugins</groupId>
-                <artifactId>maven-surefire-plugin</artifactId>
-                <configuration>
-                    <argLine>
-                        --add-opens jdk.jshell/jdk.jshell=ALL-UNNAMED
-                    </argLine>
-                </configuration>
-            </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-assembly-plugin</artifactId>

jjava-distro/src/main/java/org/dflib/jjava/distro/Env.java

@@ -24,7 +24,6 @@ final class Env {
     public static final String JJAVA_STARTUP_SCRIPT = "JJAVA_STARTUP_SCRIPT";
     public static final String JJAVA_LOAD_EXTENSIONS = "JJAVA_LOAD_EXTENSIONS";
 
-    // not used by JJava, but rather by the kernel launcher script
     public static final String JJAVA_JVM_OPTS = "JJAVA_JVM_OPTS";
 
     public static String timeout() {
@@ -48,6 +47,11 @@ public static List<String> compilerOpts() {
         return optsString != null ? Opts.splitOpts(optsString) : java.util.List.of();
     }
 
+    public static List<String> jvmOpts() {
+        String optsString = System.getenv(Env.JJAVA_JVM_OPTS);
+        return optsString != null ? Opts.splitOpts(optsString) : List.of();
+    }
+
     public static String extraClasspath() {
         return System.getenv(Env.JJAVA_CLASSPATH);
     }

jjava-distro/src/main/java/org/dflib/jjava/distro/JJava.java

@@ -63,6 +63,7 @@ public static void main(String[] args) throws Exception {
 
                 .extensionsEnabled(Env.extensionsEnabled())
                 .compilerOpts(Env.compilerOpts())
+                .remoteVMOptions(Env.jvmOpts())
                 .timeout(timeout.time, timeout.timeUnit)
 
                 .lineMagic("load", new LoadMagic())

jjava-distro/src/main/java/org/dflib/jjava/distro/NotebookInitializer.java

@@ -2,17 +2,17 @@
 
 import org.dflib.jjava.jupyter.Extension;
 import org.dflib.jjava.jupyter.kernel.BaseKernel;
-import org.dflib.jjava.jupyter.kernel.BaseNotebookStatics;
-import org.dflib.jjava.kernel.JavaNotebookStatics;
+import org.dflib.jjava.jupyter.kernel.ExecutorNotebookStatics;
+import org.dflib.jjava.kernel.ExecutorJavaNotebookStatics;
 
 /**
  * An automatically-loaded extension that adds common imports to the notebook. This includes static method imports
- * from {@link BaseNotebookStatics} and {@link JavaNotebookStatics}.
+ * from the executor helper classes {@link ExecutorNotebookStatics} and {@link ExecutorJavaNotebookStatics}.
  */
 public class NotebookInitializer implements Extension {
 
-    private static final String BASE_STATICS_CLASS = BaseNotebookStatics.class.getName();
-    private static final String JAVA_STATICS_CLASS = JavaNotebookStatics.class.getName();
+    private static final String BASE_STATICS_CLASS = ExecutorNotebookStatics.class.getName();
+    private static final String JAVA_STATICS_CLASS = ExecutorJavaNotebookStatics.class.getName();
 
     private static final String STARTUP_SCRIPT = "import java.util.*;\n" +
             "import java.io.*;\n" +

jjava-distro/src/test/java/org/dflib/jjava/distro/CellOutput.java

@@ -87,7 +87,7 @@ public CellOutput assertError(String... errorFragments) {
         return this;
     }
 
-    private String field(String name) {
+    String field(String name) {
         return fields.getOrDefault(name, "");
     }
 

jjava-distro/src/test/java/org/dflib/jjava/distro/KernelEnvIT.java

@@ -5,6 +5,7 @@
 import java.util.Map;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 public class KernelEnvIT extends ContainerizedKernelCase {
 
@@ -25,9 +26,34 @@ public void compilerOpts() throws Exception {
     public void timeout() throws Exception {
         Map<String, String> env = Map.of(Env.JJAVA_TIMEOUT, "3000");
         String cell = "Thread.sleep(5000);";
-        KernelRun run = executeInKernel(env, cell);
+        KernelRun run = executeInKernel(env, cell, "2 + 2");
 
         run.cell(1).assertError("|   " + cell, "Evaluation timed out after 3000 milliseconds.");
+        assertEquals("4", run.cell(2).result());
+    }
+
+    @Test
+    public void timeoutCoversTheWholeCell() throws Exception {
+        String cell = "Thread.sleep(2000); System.out.println(\"first complete\"); Thread.sleep(2000);";
+        KernelRun run = executeInKernel(Map.of(Env.JJAVA_TIMEOUT, "3000"), "1 + 1", cell);
+
+        assertEquals("2", run.cell(1).result());
+        run.cell(2).assertError("Evaluation timed out after 3000 milliseconds.");
+        assertTrue(run.cell(2).stdout().contains("first complete"), run.cell(2).toString());
+    }
+
+    @Test
+    public void zeroTimeoutDisablesDeadline() throws Exception {
+        KernelRun run = executeInKernel(Map.of(Env.JJAVA_TIMEOUT, "0"), "Thread.sleep(100); 42").assertNoErrors();
+
+        assertEquals("42", run.cell(1).result());
+    }
+
+    @Test
+    public void negativeTimeoutDisablesDeadline() throws Exception {
+        KernelRun run = executeInKernel(Map.of(Env.JJAVA_TIMEOUT, "-1"), "Thread.sleep(100); 42").assertNoErrors();
+
+        assertEquals("42", run.cell(1).result());
     }
 
     @Test
@@ -38,23 +64,23 @@ public void classpath() throws Exception {
                 "\"className = \" + Dummy.class.getName();"
         ).assertNoErrors();
 
-        assertEquals("className = org.dflib.jjava.Dummy", run.cell(2).result());
+        assertEquals("\"className = org.dflib.jjava.Dummy\"", run.cell(2).result());
     }
 
     @Test
     public void startUpScriptsPath() throws Exception {
         Map<String, String> env = Map.of(Env.JJAVA_STARTUP_SCRIPTS_PATH, CONTAINER_RESOURCES + "/test-ping.jshell");
         KernelRun run = executeInKernel(env, "ping()").assertNoErrors();
 
-        assertEquals("pong!", run.cell(1).result());
+        assertEquals("\"pong!\"", run.cell(1).result());
     }
 
     @Test
     public void startUpScript() throws Exception {
         Map<String, String> env = Map.of(Env.JJAVA_STARTUP_SCRIPT, "public String ping() { return \"pong!\"; }");
         KernelRun run = executeInKernel(env, "ping()").assertNoErrors();
 
-        assertEquals("pong!", run.cell(1).result());
+        assertEquals("\"pong!\"", run.cell(1).result());
     }
 
     @Test

jjava-distro/src/test/java/org/dflib/jjava/distro/KernelExecutionIT.java

@@ -3,6 +3,7 @@
 import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 public class KernelExecutionIT extends ContainerizedKernelCase {
 
@@ -22,7 +23,7 @@ public void variableSurvivesLaterImports() throws Exception {
                 )
         ).assertNoErrors();
 
-        assertEquals("com.fasterxml.jackson.databind.ObjectMapper", run.cell(2).result());
+        assertEquals("\"com.fasterxml.jackson.databind.ObjectMapper\"", run.cell(2).result());
     }
 
     @Test
@@ -45,7 +46,89 @@ public void variableRedeclarationInSameCell() throws Exception {
                 "var v = \"b\";\nv"
         ).assertNoErrors();
 
-        assertEquals("a", run.cell(1).result());
-        assertEquals("b", run.cell(2).result());
+        assertEquals("\"a\"", run.cell(1).result());
+        assertEquals("\"b\"", run.cell(2).result());
+    }
+
+    @Test
+    public void textPlainStringsAndCustomObjects() throws Exception {
+        KernelRun run = executeInKernel(
+                "\"quoted\"",
+                "\"line1\\nline2\"",
+                "class DisplayValue { public String toString() { return \"custom-value\"; } }",
+                "new DisplayValue()"
+        ).assertNoErrors();
+
+        assertEquals("\"quoted\"", run.cell(1).result());
+        assertEquals("\"line1\\nline2\"", run.cell(2).result());
+        assertEquals("custom-value", run.cell(4).result());
+    }
+
+    @Test
+    public void methodRedefinitionKeepsSavedValues() throws Exception {
+        KernelRun run = executeInKernel(
+                "int calculate() { return 1; }",
+                "int saved = calculate();",
+                "int calculate() { return 2; }",
+                "saved",
+                "calculate()"
+        ).assertNoErrors();
+
+        assertEquals("1", run.cell(4).result());
+        assertEquals("2", run.cell(5).result());
+    }
+
+    @Test
+    public void classRedefinitionKeepsSavedValuesAndInstances() throws Exception {
+        KernelRun run = executeInKernel(
+                "class Reloadable { int value() { return 1; } }",
+                "Reloadable instance = new Reloadable();",
+                "int saved = instance.value();",
+                "class Reloadable { int value() { return 2; } }",
+                "saved",
+                "instance.value()",
+                "new Reloadable().value()"
+        ).assertNoErrors();
+
+        assertEquals("1", run.cell(5).result());
+        assertEquals("2", run.cell(6).result());
+        assertEquals("2", run.cell(7).result());
+    }
+
+    @Test
+    public void interruptStopsBusyLoopAndKeepsExecutor() throws Exception {
+        KernelRun run = executeInKernel(
+                "@@INTERRUPT_AFTER_MS=500@@while (true) {}",
+                "1 + 1"
+        );
+
+        run.cell(1).assertError("Evaluation interrupted.");
+        assertTrue(Long.parseLong(run.cell(1).field("interruptLatencyMs")) <= 2000, "interrupt was not handled within two seconds");
+        assertEquals("2", run.cell(2).result());
+    }
+
+    @Test
+    public void executorStreamsReachNotebook() throws Exception {
+        KernelRun run = executeInKernel(
+                "System.out.print(\"stdout-1\"); System.err.print(\"stderr-1\");",
+                "System.out.print(\"stdout-2\"); System.err.print(\"stderr-2\");"
+        ).assertNoErrors();
+
+        assertEquals("stdout-1", run.cell(1).stdout());
+        assertEquals("stderr-1", run.cell(1).stderr());
+        assertEquals("stdout-2", run.cell(2).stdout());
+        assertEquals("stderr-2", run.cell(2).stderr());
+    }
+
+    @Test
+    public void userExceptionDoesNotLoseExecutorState() throws Exception {
+        KernelRun run = executeInKernel(
+                "int saved = 7;",
+                "throw new IllegalStateException(\"outer\", new IllegalArgumentException(\"inner\"));",
+                "saved + 1"
+        );
+
+        run.cell(2).assertError("IllegalStateException", "outer", "IllegalArgumentException", "inner", "at .(#");
+        assertEquals("8", run.cell(3).result());
     }
 }