Make it possible for plugins to request a shared class loader

In #2280 we changed the plugin loader so that all plugins share one
class loader and they can see each other. (needed for plugin
inder-dependencies)

This is problematic when different plugins have conflicting
dependencies. (see #2436)

With this change, every plugin gets its own class loader by default so
we can avoid dependency clashes. Plugins which depend on other plugins
can request a shared class loader via a graylog-plugin.properties file.

Fixes #2436

graylog2-server/src/main/java/org/graylog2/shared/plugins/PluginLoader.java

@@ -31,6 +31,7 @@
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.net.URLClassLoader;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -99,9 +100,24 @@ private Iterable<Plugin> loadJarPlugins() {
                 .filter(Objects::nonNull)
                 .collect(Collectors.toList());
 
-        // Create one URL class loader for all plugins so they can see each other.
-        // This makes plugin inter-dependencies work.
-        classLoader.addClassLoader(URLClassLoader.newInstance(urls.toArray(new URL[urls.size()])));
+        final List<URL> sharedClassLoaderUrls = new ArrayList<>();
+        urls.forEach(url -> {
+            final PluginProperties properties = PluginProperties.fromJarFile(url.getFile());
+
+            // Decide whether to create a separate, isolated class loader for the plugin. When the plugin is isolated
+            // (the default), it gets its own class loader and cannot see other plugins. Plugins which are not
+            // isolated share one class loader so they can see each other. (makes plugin inter-dependencies work)
+            if (properties.isIsolated()) {
+                LOG.debug("Create isolated class loader for {}", url);
+                classLoader.addClassLoader(URLClassLoader.newInstance(new URL[]{url}));
+            } else {
+                LOG.debug("Use shared class loader for {}", url);
+                sharedClassLoaderUrls.add(url);
+            }
+        });
+
+        LOG.debug("Create shared class loader for {} plugins: {}", sharedClassLoaderUrls.size(), sharedClassLoaderUrls);
+        classLoader.addClassLoader(URLClassLoader.newInstance(sharedClassLoaderUrls.toArray(new URL[sharedClassLoaderUrls.size()])));
 
         final ServiceLoader<Plugin> pluginServiceLoader = ServiceLoader.load(Plugin.class, classLoader);
 

graylog2-server/src/main/java/org/graylog2/shared/plugins/PluginProperties.java

@@ -0,0 +1,61 @@
+/**
+ * This file is part of Graylog.
+ *
+ * Graylog is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Graylog is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Graylog.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.graylog2.shared.plugins;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.InputStream;
+import java.util.Properties;
+import java.util.jar.JarFile;
+
+import static java.util.Objects.requireNonNull;
+
+public class PluginProperties {
+    private static final Logger LOG = LoggerFactory.getLogger(PluginProperties.class);
+
+    private static final String GRAYLOG_PLUGIN_PROPERTIES = "graylog-plugin.properties";
+
+    private static final String PROPERTY_ISOLATED = "isolated";
+    private static final String PROPERTY_ISOLATED_DEFAULT = "true";
+
+    private final boolean isolated;
+
+    public PluginProperties(Properties properties) {
+        this.isolated = Boolean.parseBoolean((String) properties.getOrDefault(PROPERTY_ISOLATED, PROPERTY_ISOLATED_DEFAULT));
+    }
+
+    public static PluginProperties fromJarFile(final String filename) {
+        // Try to load the plugin properties from the JAR file. This fails for plugins which do not have the
+        // properties file.
+        final Properties properties = new Properties();
+        try {
+            LOG.debug("Loading {} from {}", GRAYLOG_PLUGIN_PROPERTIES, filename);
+            final JarFile jarFile = new JarFile(requireNonNull(filename));
+            final InputStream inputStream = jarFile.getInputStream(jarFile.getEntry(GRAYLOG_PLUGIN_PROPERTIES));
+            properties.load(inputStream);
+        } catch (Exception e) {
+            LOG.debug("Unable to load {} from plugin {}", GRAYLOG_PLUGIN_PROPERTIES, filename, e);
+        }
+
+        return new PluginProperties(properties);
+    }
+
+    public boolean isIsolated() {
+        return this.isolated;
+    }
+}