Use ASM for event executors.

Uses method handles for private or static methods.

paper-api/src/main/java/org/bukkit/plugin/EventExecutor.java

@@ -5,9 +5,75 @@ import org.bukkit.event.EventException;
 import org.bukkit.event.Listener;
 import org.jetbrains.annotations.NotNull;
 
+// Paper start
+import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.function.Function;
+
+import com.destroystokyo.paper.event.executor.MethodHandleEventExecutor;
+import com.destroystokyo.paper.event.executor.StaticMethodHandleEventExecutor;
+import com.destroystokyo.paper.event.executor.asm.ASMEventExecutorGenerator;
+import com.destroystokyo.paper.event.executor.asm.ClassDefiner;
+import com.google.common.base.Preconditions;
+// Paper end
+
 /**
  * Interface which defines the class for event call backs to plugins
  */
 public interface EventExecutor {
     public void execute(@NotNull Listener listener, @NotNull Event event) throws EventException;
+
+    // Paper start
+    ConcurrentMap<Method, Class<? extends EventExecutor>> eventExecutorMap = new ConcurrentHashMap<Method, Class<? extends EventExecutor>>() {
+        @NotNull
+        @Override
+        public Class<? extends EventExecutor> computeIfAbsent(@NotNull Method key, @NotNull Function<? super Method, ? extends Class<? extends EventExecutor>> mappingFunction) {
+            Class<? extends EventExecutor> executorClass = get(key);
+            if (executorClass != null)
+                return executorClass;
+
+            //noinspection SynchronizationOnLocalVariableOrMethodParameter
+            synchronized (key) {
+                executorClass = get(key);
+                if (executorClass != null)
+                    return executorClass;
+
+                return super.computeIfAbsent(key, mappingFunction);
+            }
+        }
+    };
+
+    @NotNull
+    public static EventExecutor create(@NotNull Method m, @NotNull Class<? extends Event> eventClass) {
+        Preconditions.checkNotNull(m, "Null method");
+        Preconditions.checkArgument(m.getParameterCount() != 0, "Incorrect number of arguments %s", m.getParameterCount());
+        Preconditions.checkArgument(m.getParameterTypes()[0] == eventClass, "First parameter %s doesn't match event class %s", m.getParameterTypes()[0], eventClass);
+        ClassDefiner definer = ClassDefiner.getInstance();
+        if (Modifier.isStatic(m.getModifiers())) {
+            return new StaticMethodHandleEventExecutor(eventClass, m);
+        } else if (definer.isBypassAccessChecks() || Modifier.isPublic(m.getDeclaringClass().getModifiers()) && Modifier.isPublic(m.getModifiers())) {
+            // get the existing generated EventExecutor class for the Method or generate one
+            Class<? extends EventExecutor> executorClass = eventExecutorMap.computeIfAbsent(m, (__) -> {
+                String name = ASMEventExecutorGenerator.generateName();
+                byte[] classData = ASMEventExecutorGenerator.generateEventExecutor(m, name);
+                return definer.defineClass(m.getDeclaringClass().getClassLoader(), name, classData).asSubclass(EventExecutor.class);
+            });
+
+            try {
+                EventExecutor asmExecutor = executorClass.newInstance();
+                // Define a wrapper to conform to bukkit stupidity (passing in events that don't match and wrapper exception)
+                return (listener, event) -> {
+                    if (!eventClass.isInstance(event)) return;
+                    asmExecutor.execute(listener, event);
+                };
+            } catch (InstantiationException | IllegalAccessException e) {
+                throw new AssertionError("Unable to initialize generated event executor", e);
+            }
+        } else {
+            return new MethodHandleEventExecutor(eventClass, m);
+        }
+    }
+    // Paper end
 }