Simplify the implementation of the kernel extensions

jjava/pom.xml

@@ -189,6 +189,15 @@
                     </filesets>
                 </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>
         </plugins>
     </build>
 

jjava/src/main/java/org/dflib/jjava/JJavaExtension.java

@@ -25,10 +25,6 @@ public class JJavaExtension implements Extension {
 
     @Override
     public void install(BaseKernel kernel) {
-        try {
-            kernel.eval(STARTUP_SCRIPT);
-        } catch (Exception e) {
-            throw new RuntimeException(e);
-        }
+        kernel.eval(STARTUP_SCRIPT);
     }
 }

jjava/src/main/java/org/dflib/jjava/JavaKernel.java

@@ -210,7 +210,7 @@ public boolean autoLoadExtensions() {
     }
 
     @Override
-    public List<String> formatError(Exception e) {
+    public List<String> formatError(Throwable e) {
         if (e instanceof CompilationException) {
             return formatCompilationException((CompilationException) e);
         } else if (e instanceof IncompleteSourceException) {
@@ -331,22 +331,21 @@ private List<String> formatEvaluationInterruptedException(EvaluationInterruptedE
         return fmt;
     }
 
-    public Object evalRaw(String expr) throws Exception {
+    public Object evalRaw(String expr) {
         expr = this.magicsTransformer.transformMagics(expr);
 
         return this.evaluator.eval(expr);
     }
 
     @Override
-    public DisplayData eval(String expr) throws Exception {
+    public DisplayData eval(String expr) {
         Object result = this.evalRaw(expr);
-
-        if (result != null)
-            return result instanceof DisplayData
-                    ? (DisplayData) result
-                    : this.getRenderer().render(result);
-
-        return null;
+        if (result == null) {
+            return null;
+        }
+        return result instanceof DisplayData
+                ? (DisplayData) result
+                : this.getRenderer().render(result);
     }
 
     @Override

jjava/src/main/java/org/dflib/jjava/execution/CodeEvaluator.java

@@ -79,14 +79,14 @@ private SourceCodeAnalysis.CompletionInfo analyzeCompletion(String source) {
         return this.sourceAnalyzer.analyzeCompletion(source);
     }
 
-    private void init() throws Exception {
+    private void init() {
         for (String script : this.startupScripts)
             eval(script);
 
         this.startupScripts.clear();
     }
 
-    protected Object evalSingle(String code) throws Exception {
+    protected Object evalSingle(String code) {
         JJavaExecutionControl executionControl =
                 this.executionControlProvider.getRegisteredControlByID(this.executionControlID);
 
@@ -139,11 +139,11 @@ protected Object evalSingle(String code) throws Exception {
                             case JJavaExecutionControl.EXECUTION_INTERRUPTED_NAME:
                                 throw new EvaluationInterruptedException(code.trim());
                             default:
-                                throw e;
+                                throw new RuntimeException(e);
                         }
                     }
 
-                    throw e;
+                    throw new RuntimeException(e);
                 }
 
                 if (!event.status().isDefined())
@@ -154,7 +154,7 @@ protected Object evalSingle(String code) throws Exception {
         return result;
     }
 
-    public Object eval(String code) throws Exception {
+    public Object eval(String code) {
         // The init() method runs some code in the shell to initialize the environment. As such
         // it is deferred until the first user requested evaluation to cleanly return errors when
         // they happen.
@@ -178,7 +178,7 @@ public Object eval(String code) throws Exception {
     /**
      * Try to clean up information linked to a code snippet and the snippet itself
      */
-    private void dropSnippet(Snippet snippet) throws Exception {
+    private void dropSnippet(Snippet snippet) {
         JJavaExecutionControl executionControl =
                 this.executionControlProvider.getRegisteredControlByID(this.executionControlID);
         this.shell.drop(snippet);

jjava/src/main/java/org/dflib/jjava/execution/CompilationException.java

@@ -25,7 +25,7 @@
 
 import jdk.jshell.SnippetEvent;
 
-public class CompilationException extends Exception {
+public class CompilationException extends RuntimeException {
     private final SnippetEvent badSnippetCompilation;
 
     public CompilationException(SnippetEvent badSnippetCompilation) {

jjava/src/main/java/org/dflib/jjava/execution/EvaluationInterruptedException.java

@@ -23,7 +23,7 @@
  */
 package org.dflib.jjava.execution;
 
-public class EvaluationInterruptedException extends Exception {
+public class EvaluationInterruptedException extends RuntimeException {
     private final String source;
 
     public EvaluationInterruptedException(String source) {

jjava/src/main/java/org/dflib/jjava/execution/EvaluationTimeoutException.java

@@ -25,7 +25,7 @@
 
 import java.util.concurrent.TimeUnit;
 
-public class EvaluationTimeoutException extends Exception {
+public class EvaluationTimeoutException extends RuntimeException {
     private final long duration;
     private final TimeUnit unit;
     private final String source;

jjava/src/main/java/org/dflib/jjava/execution/IncompleteSourceException.java

@@ -23,7 +23,7 @@
  */
 package org.dflib.jjava.execution;
 
-public class IncompleteSourceException extends Exception {
+public class IncompleteSourceException extends RuntimeException {
     private final String source;
 
     public IncompleteSourceException(String source) {

jjava/src/main/java/org/dflib/jjava/execution/JJavaExecutionControl.java

@@ -103,7 +103,7 @@ public Object takeResult(String key) {
         return result == NULL ? null : result;
     }
 
-    private Object execute(String key, Method doitMethod) throws TimeoutException, Exception {
+    private Object execute(String key, Method doitMethod) throws Exception {
         Future<Object> runningTask = this.executor.submit(() -> doitMethod.invoke(null));
 
         this.running.put(key, runningTask);
@@ -137,7 +137,7 @@ private Object execute(String key, Method doitMethod) throws TimeoutException, E
             else if (cause instanceof SPIResolutionException)
                 throw new ResolutionException(((SPIResolutionException) cause).id(), cause.getStackTrace());
             else
-                throw new UserException(String.valueOf(cause.getMessage()), String.valueOf(cause.getClass().getName()), cause.getStackTrace());
+                throw new UserException(String.valueOf(cause.getMessage()), cause.getClass().getName(), cause.getStackTrace());
         } catch (TimeoutException e) {
             throw new UserException(
                     String.format("Execution timed out after configured timeout of %d %s.", this.timeoutTime, this.timeoutUnit.toString().toLowerCase()),
@@ -199,7 +199,7 @@ protected Class<?> findClass(String name) throws ClassNotFoundException {
         return loaderDelegate.findClass(name);
     }
 
-    void unloadClass(String className) throws ClassNotFoundException {
+    void unloadClass(String className) {
         this.loaderDelegate.unloadClass(className);
     }
 

jupyter-jvm-basekernel/src/main/java/org/dflib/jjava/jupyter/kernel/BaseKernel.java

@@ -59,7 +59,9 @@
 
 public abstract class BaseKernel {
     protected final AtomicInteger executionCount = new AtomicInteger(1);
-    protected static final Map<String, String> KERNEL_META = ((Supplier<Map<String, String>>) () -> {
+    protected static final Map<String, String> KERNEL_META;
+
+    static {
         Map<String, String> meta = null;
 
         InputStream metaStream = BaseKernel.class.getClassLoader().getResourceAsStream("kernel-metadata.json");
@@ -83,8 +85,8 @@ public abstract class BaseKernel {
             meta.put("project", "unknown");
         }
 
-        return meta;
-    }).get();
+        KERNEL_META = meta;
+    }
 
     private final JupyterIO io;
     private boolean shouldReplaceStdStreams;
@@ -161,7 +163,28 @@ public HistoryManager getHistoryManager() {
         return null;
     }
 
-    public abstract DisplayData eval(String expr) throws Exception;
+    /**
+     * Evaluates a code expression in the kernel's language environment and returns the result
+     * as display data. This is the core evaluation method called when executing code cells
+     * in a Jupyter notebook.
+     *
+     * <p>The implementation should:
+     * <ul>
+     *   <li>Parse and evaluate the provided expression string</li>
+     *   <li>Convert the evaluation result into appropriate display data formats</li>
+     *   <li>Handle any language-specific evaluation context/scope</li>
+     * </ul>
+     *
+     * <p>The returned {@link DisplayData} can contain multiple representations of the result
+     * (e.g. text/plain, text/html, image/png) to allow rich display in the notebook.
+     * Return null if the expression produces no displayable result.
+     *
+     * @param expr The code expression to evaluate as received from the Jupyter frontend
+     *
+     * @return A {@link DisplayData} object containing the evaluation result in one or more
+     *         MIME formats, or null if there is no displayable result
+     */
+    public abstract DisplayData eval(String expr);
 
     /**
      * Inspect the code to get things such as documentation for a function. This is
@@ -178,10 +201,9 @@ public HistoryManager getHistoryManager() {
      *
      * @return an output bundle for displaying the documentation or null if nothing is found
      *
-     * @throws Exception if the code cannot be inspected for some reason (such as it not
-     *                   compiling)
+     * @throws RuntimeException if the code cannot be inspected for some reason (such as it not compiling)
      */
-    public DisplayData inspect(String code, int at, boolean extraDetail) throws Exception {
+    public DisplayData inspect(String code, int at, boolean extraDetail) {
         return null;
     }
 
@@ -204,11 +226,11 @@ public DisplayData inspect(String code, int at, boolean extraDetail) throws Exce
      * @return the replacement options containing a list of replacement texts and a
      *         source range to overwrite with a user selected replacement from the list
      *
-     * @throws Exception if code cannot be completed due to code compilation issues, or
-     *                   similar. This should not be thrown if not replacements are available but rather just
-     *                   an empty replacements returned.
+     * @throws RuntimeException if code cannot be completed due to code compilation issues, or similar.
+     *                          This should not be thrown if not replacements are available but rather just
+     *                          an empty replacements returned.
      */
-    public ReplacementOptions complete(String code, int at) throws Exception {
+    public ReplacementOptions complete(String code, int at) {
         return null;
     }
 
@@ -283,7 +305,7 @@ public void interrupt() {
      *         not include strings with newlines but rather separate strings each to go on a
      *         new line.
      */
-    public List<String> formatError(Exception e) {
+    public List<String> formatError(Throwable e) {
         List<String> lines = new LinkedList<>();
         lines.add(this.errorStyler.secondary("---------------------------------------------------------------------------"));