Isolate plugins from the classpath so that the framework doesn't override plugin dependencies

Author: hlxid

src/main/scala/org/codeoverflow/chatoverflow/framework/helper/PluginClassLoader.scala

@@ -2,12 +2,52 @@ package org.codeoverflow.chatoverflow.framework.helper
 
 import java.net.{URL, URLClassLoader}
 
+import org.codeoverflow.chatoverflow.WithLogger
+
 /**
- * This plugin class loader does only exist for plugin security policy checks and
- * to expose the addURL method to the package inorder to add all required dependencies after dependency resolution.
+ * This plugin class loader is used for plugin security policy checks,
+ * to expose the addURL method to the package inorder to add all required dependencies after dependency resolution
+ * and most importantly to isolate the plugin from the normal classpath and only access the classpath if it needs to load the ChatOverflow api.
+ * Also if this PluginClassLoader had access to the classpath the same classes of the classpath would have
+ * higher priority over the classes in this classloader which could be a problem  if a plugin uses a newer version
+ * of a dependency that the framework.
  *
  * @param urls Takes an array of urls an creates a simple URLClassLoader with it
  */
-class PluginClassLoader(urls: Array[URL]) extends URLClassLoader(urls) {
+class PluginClassLoader(urls: Array[URL]) extends URLClassLoader(urls, PluginClassLoader.platformClassloader) {
+  // Note the platform classloader in the constructor of the URLClassLoader as the parent.
+  // That way the classloader skips the app classloader with the classpath when it is asks it's parents for classes.
+
   protected[helper] override def addURL(url: URL): Unit = super.addURL(url) // just exposes this method to be package-private instead of class internal protected
+
+  override def loadClass(name: String, resolve: Boolean): Class[_] = {
+    if (name.startsWith("org.codeoverflow.chatoverflow.api")) {
+      PluginClassLoader.appClassloader.loadClass(name) // Api needs to be loaded from the classpath
+    } else {
+      super.loadClass(name, resolve) // non api class. load it as normal
+    }
+  }
+}
+
+/**
+ * This companion object holds references to the app classloader (normal classloader, includes java and classpath)
+ * and to the extension/platform classloader depending on the java version that excludes the classpath,
+ * but still includes everything from java.
+ */
+private object PluginClassLoader extends WithLogger {
+  val appClassloader: ClassLoader = this.getClass.getClassLoader
+  val platformClassloader: ClassLoader = {
+    var current = appClassloader
+    while (current != null && !current.getClass.getName.contains("ExtClassLoader") && // ExtClassLoader is java < 9
+      !current.getClass.getName.contains("PlatformClassLoader")) { // PlatformClassLoader is java >= 9
+      current = current.getParent
+    }
+
+    if (current != null) {
+      current
+    } else {
+      logger error "Platform classloader couldn't be found. Falling back to normal app classloader."
+      appClassloader
+    }
+  }
 }