Add initial AccessWidener

2026-06-11 12:22:44 +02:00
parent e176b3bca8
commit 786257ad0e
15 changed files with 979 additions and 6 deletions
@@ -0,0 +1,117 @@
+package de.steamwar.core;
+
+import java.lang.reflect.Method;
+import java.nio.file.Path;
+import java.util.logging.Logger;
+
+/**
+ * Public API for Paper plugins that want to attach the access widener agent.
+ *
+ * <h2>Usage</h2>
+ * Call {@link #ensureAttached(Path)} from your plugin's {@code onLoad()} method
+ * (not {@code onEnable()} — onLoad fires before classes start loading):
+ *
+ * <pre>{@code
+ * public class MyPlugin extends JavaPlugin {
+ *     @Override
+ *     public void onLoad() {
+ *         try {
+ *             AccessWidenerAttacher.ensureAttached(getDataFolder());
+ *         } catch (Exception e) {
+ *             getSLF4JLogger().error("Failed to attach access widener agent", e);
+ *         }
+ *     }
+ * }
+ * }</pre>
+ *
+ * <h2>Resource file</h2>
+ * Drop a {@code plugin.accesswidener} file in your plugin's resources:
+ *
+ * <pre>
+ *   accessWidener v2 named
+ *
+ *   accessible method net/minecraft/server/level/ServerPlayer getStats ()V
+ *   mutable    field  net/minecraft/world/entity/Entity id I
+ * </pre>
+ *
+ * The agent discovers it automatically via your plugin's ClassLoader.
+ */
+public final class AccessWidenerAttacher {
+
+    private static final Logger LOG = Logger.getLogger("AccessWidenerAttacher");
+    private static final Object LOCK = new Object();
+
+    private static volatile boolean attached = false;
+
+    private AccessWidenerAttacher() {}
+
+    /**
+     * Attaches the access widener agent to the running JVM if it has not already
+     * been attached. Safe to call from multiple plugins — the agent is only
+     * attached once.
+     *
+     * @param agentJar the agentJar
+     * @throws AccessWidenerException if the agent could not be attached
+     */
+    public static void ensureAttached(Path agentJar) throws AccessWidenerException {
+        if (attached) return; // fast path — no locking needed after first attach
+
+        synchronized (LOCK) {
+            if (attached) return; // double-checked
+
+            try {
+                attachAgent(agentJar);
+                attached = true;
+                LOG.info("[AccessWidener] Agent attached successfully.");
+            } catch (Exception e) {
+                throw new AccessWidenerException("Failed to attach access widener agent", e);
+            }
+        }
+    }
+
+    /** Returns true if the agent has been successfully attached. */
+    public static boolean isAttached() {
+        return attached;
+    }
+
+    private static void attachAgent(Path agentJar) throws Exception {
+        // Verify the Attach API is available before trying
+        try {
+            Class.forName("com.sun.tools.attach.VirtualMachine");
+        } catch (ClassNotFoundException e) {
+            throw new AccessWidenerException(
+                "The JDK Attach API is not available. "
+                + "Make sure you are running on a JDK (not a JRE) "
+                + "and add '--add-opens jdk.attach/sun.tools.attach=ALL-UNNAMED' "
+                + "to your JVM flags if on Java 9+.", e);
+        }
+
+        String pid = String.valueOf(ProcessHandle.current().pid());
+
+        // Use reflection so the Attach API is not a hard compile-time dependency
+        Class<?> vmClass = Class.forName("com.sun.tools.attach.VirtualMachine");
+        Method attachMethod = vmClass.getMethod("attach", String.class);
+        Method loadAgentMethod = vmClass.getMethod("loadAgent", String.class);
+        Method detachMethod = vmClass.getMethod("detach");
+
+        Object vm = attachMethod.invoke(null, pid);
+        try {
+            loadAgentMethod.invoke(vm, agentJar.toAbsolutePath().toString());
+        } finally {
+            detachMethod.invoke(vm);
+        }
+    }
+
+    // -------------------------------------------------------------------------
+    // Exception type
+    // -------------------------------------------------------------------------
+
+    public static class AccessWidenerException extends Exception {
+        public AccessWidenerException(String message, Throwable cause) {
+            super(message, cause);
+        }
+        public AccessWidenerException(String message) {
+            super(message);
+        }
+    }
+}