Extension loading from classpath #71

to ensure that kernel is accessible in the notebook unconditonally
(whether extensions loading is on or off), provide BaseKernel.notebookKernel()
method and remove Extension lifecycle from BaseNotebookStatics and JavaNotebookStatics

Author: andrus

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

@@ -62,6 +62,8 @@
 
 public abstract class BaseKernel {
 
+    protected static BaseKernel notebookKernel;
+
     public static final String IS_COMPLETE_YES = "complete";
     public static final String IS_COMPLETE_BAD = "invalid";
     public static final String IS_COMPLETE_MAYBE = "unknown";
@@ -79,10 +81,21 @@ public abstract class BaseKernel {
     protected final MagicParser magicParser;
     protected final MagicsRegistry magicsRegistry;
     protected final Map<String, Extension> extensions;
+    // this flag is for custom extensions only. It does not affect the default extensions that are loaded unconditionally
     protected final boolean extensionsEnabled;
     protected final StringStyler errorStyler;
     protected final AtomicInteger executionCount;
 
+    /**
+     * Returns a non-null instance of the kernel associated with the current notebook. Throws an exception if called
+     * outside the notebook lifecycle.
+     */
+    public static BaseKernel notebookKernel() {
+        return Objects.requireNonNull(
+                BaseKernel.notebookKernel,
+                "No kernel running. Likely called outside of the notebook lifecycle");
+    }
+
     protected BaseKernel(
             String name,
             String version,
@@ -283,9 +296,7 @@ public DisplayData eval(String expr) {
      * initializes kernel extensions.
      */
     public void onStartup() {
-        if (extensionsEnabled) {
-            installDefaultExtensions();
-        }
+        installNotebookKernel();
     }
 
     /**
@@ -297,7 +308,11 @@ public void onStartup() {
      *                     is likely to be started up again.
      */
     public void onShutdown(boolean isRestarting) {
+        uninstallExtension();
+        uninstallNotebookKernel();
+    }
 
+    protected void uninstallExtension() {
         Set<Extension> localExts = new HashSet<>(extensions.values());
         extensions.clear();
 
@@ -311,12 +326,28 @@ public void onShutdown(boolean isRestarting) {
         }
     }
 
+    protected void installNotebookKernel() {
+        if (BaseKernel.notebookKernel != null) {
+            throw new IllegalStateException("A different notebook kernel was already started: " + BaseKernel.notebookKernel.getBanner());
+        }
+
+        BaseKernel.notebookKernel = this;
+    }
+
+    protected void uninstallNotebookKernel() {
+        if (BaseKernel.notebookKernel != null && BaseKernel.notebookKernel != this) {
+            throw new IllegalStateException("A different notebook kernel is running: " + BaseKernel.notebookKernel.getBanner());
+        }
+
+        BaseKernel.notebookKernel = null;
+    }
+
     /**
      * Invoked when the kernel.json specifies an {@code interrupt_mode} of {@code message}
      * and the frontend requests an interrupt of the currently running cell.
      */
     public void interrupt() {
-        //no-op
+        // no-op
     }
 
     /**
@@ -546,14 +577,6 @@ protected ClassLoader getClassLoader() {
         return ClassLoader.getSystemClassLoader();
     }
 
-    /**
-     * Locates, loads and initializes {@code Extension}s. Extension classes are discovered via {@link ServiceLoader},
-     * using the kernel's default ClassLoader.
-     */
-    protected void installDefaultExtensions() {
-        installExtensionsFromClassLoader(getClassLoader());
-    }
-
     /**
      * Locates, loads and initializes {@code Extension}s. Extension classes are discovered via {@link ServiceLoader}.
      * It is passed a custom ClassLoader created internally based on a combination of the kernel ClassLoader
@@ -579,8 +602,13 @@ protected void installExtensionsFromClasspath(String classpath) {
     protected void installExtensionsFromClassLoader(ClassLoader classLoader) {
         ServiceLoader.load(Extension.class, classLoader).stream()
                 .map(ServiceLoader.Provider::get)
-                .filter(e -> extensions.putIfAbsent(e.getClass().getName(), e) == null)
-                .forEach(e -> e.install(this));
+                .forEach(this::installExtension);
+    }
+
+    protected void installExtension(Extension ext) {
+        if (extensions.putIfAbsent(ext.getClass().getName(), ext) == null) {
+            ext.install(this);
+        }
     }
 
     private static URL pathToURL(String path) {

jjava-jupyter/src/main/java/org/dflib/jjava/jupyter/kernel/BaseNotebookStatics.java

@@ -1,57 +1,27 @@
 package org.dflib.jjava.jupyter.kernel;
 
-import org.dflib.jjava.jupyter.Extension;
 import org.dflib.jjava.jupyter.kernel.display.DisplayData;
 import org.dflib.jjava.jupyter.kernel.magic.UndefinedMagicException;
 
 import java.util.List;
-import java.util.Objects;
 import java.util.UUID;
 
 /**
  * An automatically-loaded extension that exposes a collection of static methods for notebook code to interact with the
  * kernel.
  */
-public class BaseNotebookStatics implements Extension {
-
-    private static BaseKernel kernel;
-
-    @Override
-    public void install(BaseKernel kernel) {
-
-        if (BaseNotebookStatics.kernel != null) {
-            throw new IllegalStateException("Already initialized with a different kernel: " + BaseNotebookStatics.kernel.getBanner());
-        }
-
-        BaseNotebookStatics.kernel = kernel;
-    }
-
-    @Override
-    public void uninstall(BaseKernel kernel) {
-
-        if (BaseNotebookStatics.kernel != null && BaseNotebookStatics.kernel != kernel) {
-            throw new IllegalStateException("Was initialized with a different kernel: " + BaseNotebookStatics.kernel.getBanner());
-        }
-
-        BaseNotebookStatics.kernel = null;
-    }
-
-    private static BaseKernel nonNullKernel() {
-        return Objects.requireNonNull(
-                BaseNotebookStatics.kernel,
-                "No kernel running. Likely called outside of the notebook lifecycle");
-    }
+public class BaseNotebookStatics {
 
     public static void printf(String format, Object... args) {
         System.out.printf(format, args);
     }
 
     public static Object eval(String expr) {
-        return nonNullKernel().evalRaw(expr);
+        return BaseKernel.notebookKernel().evalRaw(expr);
     }
 
     public static <T> T lineMagic(String name, List<String> args) {
-        BaseKernel kernel = nonNullKernel();
+        BaseKernel kernel = BaseKernel.notebookKernel();
 
         try {
             return kernel.getMagicsRegistry().evalLineMagic(kernel, name, args);
@@ -63,7 +33,7 @@ public static <T> T lineMagic(String name, List<String> args) {
     }
 
     public static <T> T cellMagic(String name, List<String> args, String body) {
-        BaseKernel kernel = nonNullKernel();
+        BaseKernel kernel = BaseKernel.notebookKernel();
 
         try {
             return kernel.getMagicsRegistry().evalCellMagic(kernel, name, args, body);
@@ -75,11 +45,11 @@ public static <T> T cellMagic(String name, List<String> args, String body) {
     }
 
     public static DisplayData render(Object o) {
-        return nonNullKernel().getRenderer().render(o);
+        return BaseKernel.notebookKernel().getRenderer().render(o);
     }
 
     public static DisplayData render(Object o, String... as) {
-        return nonNullKernel().getRenderer().renderAs(o, as);
+        return BaseKernel.notebookKernel().getRenderer().renderAs(o, as);
     }
 
     public static String display(Object o) {
@@ -92,7 +62,7 @@ public static String display(Object o) {
             data.setDisplayId(id);
         }
 
-        nonNullKernel().display(data);
+        BaseKernel.notebookKernel().display(data);
         return id;
     }
 
@@ -105,17 +75,17 @@ public static String display(Object o, String... as) {
             data.setDisplayId(id);
         }
 
-        nonNullKernel().display(data);
+        BaseKernel.notebookKernel().display(data);
         return id;
     }
 
     public static void updateDisplay(String id, Object o) {
         DisplayData data = render(o);
-        nonNullKernel().getIO().display.updateDisplay(id, data);
+        BaseKernel.notebookKernel().getIO().display.updateDisplay(id, data);
     }
 
     public static void updateDisplay(String id, Object o, String... as) {
         DisplayData data = render(o, as);
-        nonNullKernel().getIO().display.updateDisplay(id, data);
+        BaseKernel.notebookKernel().getIO().display.updateDisplay(id, data);
     }
 }

jjava-jupyter/src/main/resources/META-INF/services/org.dflib.jjava.jupyter.Extension

@@ -1,42 +1,12 @@
 package org.dflib.jjava.kernel;
 
-import org.dflib.jjava.jupyter.Extension;
 import org.dflib.jjava.jupyter.kernel.BaseKernel;
 
-import java.util.Objects;
-
 /**
  * An automatically-loaded extension that exposes a collection of static methods for notebook code to interact with the
  * kernel.
  */
-public class JavaNotebookStatics implements Extension {
-
-    static JavaKernel kernel;
-
-    @Override
-    public void install(BaseKernel kernel) {
-
-        if (JavaNotebookStatics.kernel != null) {
-            throw new IllegalStateException("Already initialized with a different kernel: " + JavaNotebookStatics.kernel.getBanner());
-        }
-
-        if (kernel instanceof JavaKernel) {
-            JavaNotebookStatics.kernel = (JavaKernel) kernel;
-        } else {
-            // TODO: should we have some kind of abstract logger?
-            System.err.println("Ignoring unexpected kernel '" + kernel.getClass().getName() + "'. Only '" + JavaKernel.class.getName() + "' is supported.");
-        }
-    }
-
-    @Override
-    public void uninstall(BaseKernel kernel) {
-
-        if (JavaNotebookStatics.kernel != null && JavaNotebookStatics.kernel != kernel) {
-            throw new IllegalStateException("Was initialized with a different kernel: " + JavaNotebookStatics.kernel.getBanner());
-        }
-
-        JavaNotebookStatics.kernel = null;
-    }
+public class JavaNotebookStatics {
 
     /**
      * @deprecated in favor of {@link #kernel()}
@@ -49,8 +19,6 @@ public static JavaKernel getKernelInstance() {
     }
 
     public static JavaKernel kernel() {
-        return Objects.requireNonNull(
-                JavaNotebookStatics.kernel,
-                "No Java kernel running. Likely called outside of the notebook lifecycle");
+        return (JavaKernel) BaseKernel.notebookKernel();
     }
 }

jjava-kernel/src/main/java/org/dflib/jjava/kernel/JavaNotebookStatics.java

@@ -10,23 +10,6 @@
 
 public class JavaKernelExtensionsLifecycleTest {
 
-    @Test
-    public void defaultExtension() {
-        assertNull(JavaNotebookStatics.kernel);
-
-        JavaKernel kernel = JavaKernel.builder().name("TestKernel").build();
-        assertNull(JavaNotebookStatics.kernel, "Kernel should not have loaded any extensions as of creation");
-
-        try {
-            kernel.onStartup();
-            assertNotNull(JavaNotebookStatics.kernel, "Built-in extension was not installed");
-        } finally {
-            kernel.onShutdown(false);
-        }
-
-        assertNull(JavaNotebookStatics.kernel);
-    }
-
     @Test
     public void extraClasspathExtension() throws Exception {
         Path extensionJar = TestJarFactory.buildJar(
@@ -46,7 +29,6 @@ public void extraClasspathExtension() throws Exception {
                 .build();
         try {
             kernel.onStartup();
-            assertNotNull(JavaNotebookStatics.kernel, "Built-in extension was not installed");
             assertNull(System.getProperty(extInstalledProp), "Custom extension should not be installed yet");
 
             kernel.addToClasspath(extraClasspath);
@@ -58,7 +40,6 @@ public void extraClasspathExtension() throws Exception {
             kernel.onShutdown(false);
         }
 
-        assertNull(JavaNotebookStatics.kernel);
         assertNull(System.getProperty(extInstalledProp));
     }