Add method to get plugin by its class. Adds BUKKIT-5240

Currently, the only way to get a plugin is by name or using a static
variable. This adds two methods to get a plugin based on its classes,
utilizing the plugin classloader.

By: Wesley Wolfe <weswolf@aol.com>

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

@@ -12,6 +12,7 @@ import java.util.List;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
+import org.apache.commons.lang.Validate;
 import org.bukkit.Server;
 import org.bukkit.Warning.WarningState;
 import org.bukkit.command.Command;
@@ -405,4 +406,59 @@ public abstract class JavaPlugin extends PluginBase {
     public String toString() {
         return description.getFullName();
     }
+
+    /**
+     * This method provides fast access to the plugin that has {@link
+     * #getProvidingPlugin(Class) provided} the given plugin class, which is
+     * usually the plugin that implemented it.
+     * <p>
+     * An exception to this would be if plugin's jar that contained the class
+     * does not extend the class, where the intended plugin would have
+     * resided in a different jar / classloader.
+     *
+     * @param clazz the class desired
+     * @return the plugin that provides and implements said class, or null
+     *     if called from an inconsistent static initialization block
+     * @throws IllegalArgumentException if clazz is null
+     * @throws IllegalArgumentException if clazz does not extend {@link
+     *     JavaPlugin}
+     * @throws IllegalArgumentException if clazz was not provided by a
+     *     plugin, for example, if called with
+     *     <code>JavaPlugin.getPlugin(JavaPlugin.class)</code>
+     * @throws ClassCastException if plugin that provided the class does not
+     *     extend the class
+     */
+    public static <T extends JavaPlugin> T getPlugin(Class<T> clazz) {
+        Validate.notNull(clazz, "Null class cannot have a plugin");
+        if (!JavaPlugin.class.isAssignableFrom(clazz)) {
+            throw new IllegalArgumentException(clazz + " does not extend " + JavaPlugin.class);
+        }
+        final ClassLoader cl = clazz.getClassLoader();
+        if (!(cl instanceof PluginClassLoader)) {
+            throw new IllegalArgumentException(clazz + " is not initialized by " + PluginClassLoader.class);
+        }
+        JavaPlugin plugin = ((PluginClassLoader) cl).plugin;
+        if (plugin == null) {
+            throw new IllegalStateException("Cannot get plugin for " + clazz + " from a static initializer");
+        }
+        return clazz.cast(plugin);
+    }
+
+    /**
+     * This method provides fast access to the plugin that has {@link
+     * #getProvidingPlugin(Class) provided} the given plugin class, which is
+     * usually the plugin that implemented it.
+     */
+    public static JavaPlugin getProvidingPlugin(Class<?> clazz) {
+        Validate.notNull(clazz, "Null class cannot have a plugin");
+        final ClassLoader cl = clazz.getClassLoader();
+        if (!(cl instanceof PluginClassLoader)) {
+            throw new IllegalArgumentException(clazz + " is not provided by " + PluginClassLoader.class);
+        }
+        JavaPlugin plugin = ((PluginClassLoader) cl).plugin;
+        if (plugin == null) {
+            throw new IllegalStateException("Cannot get plugin for " + clazz + " from a static initializer");
+        }
+        return plugin;
+    }
 }