fix(agent): Support running agent on bootstrap classpath

- Update `Agent.java` to use `Agent.class.getResource()` instead of
  `Agent.class.getClassLoader().getResource()` when locating the agent JAR.
  This prevents a `NullPointerException` when the agent is loaded by the
  bootstrap class loader (where `getClassLoader()` returns null).
- Modify `Properties.java` to automatically default `appmap.debug.disableGit`
  to `true` if the agent is running on the bootstrap classpath. This avoids
  crashes in JGit initialization, which relies on `ResourceBundle` loading
  that is problematic in the bootstrap context.
- Add a warning log in `Agent.premain` when running on the bootstrap
  classpath, advising that this configuration is for troubleshooting only.

Author: dividedmind

agent/src/main/java/com/appland/appmap/Agent.java

@@ -74,6 +74,10 @@ public static void premain(String agentArgs, Instrumentation inst) {
     logger.info("config: {}", AppMapConfig.get());
     logger.debug("System properties: {}", System.getProperties());
 
+    if (Agent.class.getClassLoader() == null) {
+      logger.warn("AppMap agent is running on the bootstrap classpath. This is not a recommended configuration and should only be used for troubleshooting. Git integration will be disabled.");
+    }
+
     addAgentJars(agentArgs, inst);
 
 
@@ -162,12 +166,18 @@ private static void addAgentJars(String agentArgs, Instrumentation inst) {
     Path agentJarPath = null;
     try {
       Class<Agent> agentClass = Agent.class;
-      URL resourceURL = agentClass.getClassLoader()
-          .getResource(agentClass.getName().replace('.', '/') + ".class");
+      // When the agent is loaded by the bootstrap class loader (e.g., via -Xbootclasspath/a:),
+      // agentClass.getClassLoader() returns null, leading to a NullPointerException. To handle
+      // this, we use Class.getResource() which correctly resolves resources even when the
+      // class is loaded by the bootstrap class loader. The leading '/' in the resource name
+      // is crucial for absolute path resolution when using Class.getResource().
+      URL resourceURL = agentClass.getResource("/" + agentClass.getName().replace('.', '/') + ".class");
+
       // During testing of the agent itself, classes get loaded from a directory, and will have the
       // protocol "file". The rest of the time (i.e. when it's actually deployed), they'll always
-      // come from a jar file.
-      if (resourceURL.getProtocol().equals("jar")) {
+      // come from a jar file. We must also check that resourceURL is not null before using it,
+      // as getResource() can return null if the resource is not found.
+      if (resourceURL != null && resourceURL.getProtocol().equals("jar")) {
         String resourcePath = resourceURL.getPath();
         URL jarURL = new URL(resourcePath.substring(0, resourcePath.indexOf('!')));
         logger.debug("jarURL: {}", jarURL);

agent/src/main/java/com/appland/appmap/config/Properties.java

@@ -21,7 +21,12 @@ public class Properties {
   public static final String DebugClassPrefix = resolveProperty("appmap.debug.classPrefix", (String) null);
   public static final Boolean SaveInstrumented =
       resolveProperty("appmap.debug.saveInstrumented", false);
-  public static final Boolean DisableGit = resolveProperty("appmap.debug.disableGit", false);
+  public static final Boolean DisableGit =
+      // Git integration (JGit) uses resource bundles, which are not reliably available
+      // when the agent is loaded by the bootstrap class loader (i.e., when
+      // getClassLoader() returns null). In such cases, automatically disable Git
+      // to prevent NullPointerExceptions during initialization.
+      resolveProperty("appmap.debug.disableGit", Properties.class.getClassLoader() == null);
 
   public static final Boolean RecordingAuto = resolveProperty("appmap.recording.auto", false);
   public static final String RecordingName = resolveProperty("appmap.recording.name", (String) null);