Additional teardown to aid GC and release resources

Most of this would happen as part of GC clearing the runtime, but
these changes help reduce that load and get the large collections
and data structures releasable earlier.

Fixes jruby#8343

core/src/main/java/org/jruby/Ruby.java

@@ -3402,6 +3402,18 @@ private void systemTeardown(final ThreadContext context) {
         threadService.teardown();
         threadService = new ThreadService(this);
 
+        // Release classloader resources
+        releaseClassLoader();
+
+        // Tear down LoadService
+        loadService.tearDown();
+
+        // Clear runtime tables to aid GC
+        boundMethods.clear();
+        allModules.clear();
+        constantNameInvalidators.clear();
+        symbolTable.clear();
+        javaSupport = loadJavaSupport();
     }
 
     private int userTeardown(ThreadContext context) {
@@ -3454,7 +3466,6 @@ private int userTeardown(ThreadContext context) {
     public void releaseClassLoader() {
         if (jrubyClassLoader != null) {
             jrubyClassLoader.close();
-            //jrubyClassLoader = null;
         }
     }
 
@@ -5625,7 +5636,7 @@ public RubyClass getData() {
     private PrintStream err;
 
     // Java support
-    private final JavaSupport javaSupport;
+    private JavaSupport javaSupport;
     private final JRubyClassLoader jrubyClassLoader;
 
     // Object Specializer

core/src/main/java/org/jruby/RubySymbol.java

@@ -996,8 +996,7 @@ public static final class SymbolTable {
         public SymbolTable(Ruby runtime) {
             this.runtime = runtime;
             this.loadFactor = DEFAULT_LOAD_FACTOR;
-            this.threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
-            this.symbolTable = new SymbolEntry[DEFAULT_INITIAL_CAPACITY];
+            reset();
         }
 
         // note all fields are final -- rehash creates new entries when necessary.
@@ -1337,6 +1336,15 @@ public int size() {
             return size;
         }
 
+        public void clear() {
+            reset();
+        }
+
+        private void reset() {
+            this.threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
+            this.symbolTable = new SymbolEntry[DEFAULT_INITIAL_CAPACITY];
+        }
+
         private SymbolEntry[] rehash() {
             SymbolEntry[] oldTable = symbolTable;
             int oldCapacity = oldTable.length;

core/src/main/java/org/jruby/embed/ScriptingContainer.java

@@ -1844,7 +1844,6 @@ public void terminate() {
         LocalContextProvider provider = getProvider();
         if (provider.isRuntimeInitialized()) {
             provider.getRuntime().tearDown(false);
-            provider.getRuntime().releaseClassLoader();
         }
         provider.terminate();
     }

core/src/main/java/org/jruby/runtime/load/LibrarySearcher.java

@@ -808,6 +808,12 @@ private static boolean isURI(String path) {
         return false;
     }
 
+    // Clear caches and release resources
+    public void tearDown() {
+        loadedFeaturesIndex.clear();
+        loadedFeaturesSnapshot.clear();
+    }
+
     enum Suffix {
         RUBY(".rb", ResourceLibrary::new),
         CLASS(".class", ClassResourceLibrary::new),

core/src/main/java/org/jruby/runtime/load/LoadService.java

@@ -1009,4 +1009,9 @@ public String getPathForLocation(String filename) {
 
         return filename;
     }
+
+    public void tearDown() {
+        loadedFeatures.clear();
+        librarySearcher.tearDown();
+    }
 }