SPIGOT-2629: Better thread safety for plugin class loading

By: md_5 <git@md-5.net>

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

@@ -7,6 +7,7 @@ import java.io.InputStream;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.LinkedHashMap;
@@ -48,7 +49,7 @@ public final class JavaPluginLoader implements PluginLoader {
     final Server server;
     private final Pattern[] fileFilters = new Pattern[] { Pattern.compile("\\.jar$"), };
     private final Map<String, Class<?>> classes = new HashMap<String, Class<?>>();
-    private final Map<String, PluginClassLoader> loaders = new LinkedHashMap<String, PluginClassLoader>();
+    private final Map<String, PluginClassLoader> loaders = Collections.synchronizedMap(new LinkedHashMap<String, PluginClassLoader>());
 
     /**
      * This class was not meant to be constructed explicitly
@@ -114,9 +115,6 @@ public final class JavaPluginLoader implements PluginLoader {
         }
 
         for (final String pluginName : description.getDepend()) {
-            if (loaders == null) {
-                throw new UnknownDependencyException(pluginName);
-            }
             PluginClassLoader current = loaders.get(pluginName);
 
             if (current == null) {
@@ -186,14 +184,14 @@ public final class JavaPluginLoader implements PluginLoader {
         if (cachedClass != null) {
             return cachedClass;
         } else {
-            for (String current : loaders.keySet()) {
-                PluginClassLoader loader = loaders.get(current);
-
-                try {
-                    cachedClass = loader.findClass(name, false);
-                } catch (ClassNotFoundException cnfe) {}
-                if (cachedClass != null) {
-                    return cachedClass;
+            synchronized (loaders) {
+                for (PluginClassLoader loader : loaders.values()) {
+                    try {
+                        cachedClass = loader.findClass(name, false);
+                    } catch (ClassNotFoundException cnfe) {}
+                    if (cachedClass != null) {
+                        return cachedClass;
+                    }
                 }
             }
         }