BUG-7052: Move RecursiveObjectLeader to util

[yangtools.git] / common / util / src / main / java / org / opendaylight / yangtools / util / RecursiveObjectLeaker.java

diff --git a/common/util/src/main/java/org/opendaylight/yangtools/util/RecursiveObjectLeaker.java b/common/util/src/main/java/org/opendaylight/yangtools/util/RecursiveObjectLeaker.java
new file mode 100644 (file)
index 0000000..f69134f
--- /dev/null
+++ b/common/util/src/main/java/org/opendaylight/yangtools/util/RecursiveObjectLeaker.java
@@ -0,0 +1,130 @@
+/*
+ * Copyright (c) 2015 Cisco Systems, Inc. and others.  All rights reserved.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v1.0 which accompanies this distribution,
+ * and is available at http://www.eclipse.org/legal/epl-v10.html
+ */
+package org.opendaylight.yangtools.util;
+
+import com.google.common.annotations.Beta;
+import com.google.common.base.Preconditions;
+import java.util.AbstractMap.SimpleEntry;
+import java.util.ArrayDeque;
+import java.util.Deque;
+import java.util.Map.Entry;
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Thread-local hack to make recursive extensions work without too much hassle. The idea is that prior to instantiating
+ * an extension, the definition object checks whether it is already present on the stack, recorded object is returned.
+ *
+ * <p>
+ * If it is not, it will push itself to the stack as unresolved and invoke the constructor. The constructor's lowermost
+ * class calls to this class and if the topmost entry is not resolved, it will leak itself.
+ *
+ * <p>
+ * Upon return from the constructor, the topmost entry is removed and if the queue is empty, the thread-local variable
+ * will be cleaned up.
+ *
+ * <p>
+ * WARNING: BE CAREFUL WHEN USING THIS CLASS. IT LEAKS OBJECTS WHICH ARE NOT COMPLETELY INITIALIZED.
+ *
+ * <p>
+ * WARNING: THIS CLASS EAVES THREAD-LOCAL RESIDUE. MAKE SURE IT IS OKAY OR CALL {@link #cleanup()} IN APPROPRIATE
+ *          PLACES.
+ *
+ * <p>
+ * THIS CLASS IS EXTREMELY DANGEROUS (okay, not as much as sun.misc.unsafe). YOU HAVE BEEN WARNED. IF SOMETHING BREAKS
+ * IT IS PROBABLY YOUR FAULT AND YOU ARE ON YOUR OWN.
+ *
+ * @author Robert Varga
+ */
+@Beta
+@ThreadSafe
+public final class RecursiveObjectLeaker {
+    // Logging note. Only keys passed can be logged, as objects beng resolved may not be properly constructed.
+    private static final Logger LOG = LoggerFactory.getLogger(RecursiveObjectLeaker.class);
+
+    // Initial value is set to null on purpose, so we do not allocate anything (aside the map)
+    private static final ThreadLocal<Deque<Entry<?, Object>>> STACK = new ThreadLocal<>();
+
+    private RecursiveObjectLeaker() {
+        throw new UnsupportedOperationException();
+    }
+
+    // Key is checked for identity
+    public static void beforeConstructor(final Object key) {
+        Deque<Entry<?, Object>> stack = STACK.get();
+        if (stack == null) {
+            // Biased: this class is expected to be rarely and shallowly used
+            stack = new ArrayDeque<>(1);
+            STACK.set(stack);
+        }
+
+        LOG.debug("Resolving key {}", key);
+        stack.push(new SimpleEntry<>(key, null));
+    }
+
+    // Can potentially store a 'null' mapping. Make sure cleanup() is called
+    public static void inConstructor(final Object obj) {
+        final Deque<Entry<?, Object>> stack = STACK.get();
+        if (stack != null) {
+            final Entry<?, Object> top = stack.peek();
+            if (top != null) {
+                if (top.getValue() == null) {
+                    LOG.debug("Resolved key {}", top.getKey());
+                    top.setValue(obj);
+                }
+            } else {
+                LOG.info("Cleaned stale empty stack", new Exception());
+                STACK.set(null);
+            }
+        } else {
+            LOG.trace("No thread stack");
+        }
+    }
+
+    // Make sure to call this from a finally block
+    public static void afterConstructor(final Object key) {
+        final Deque<Entry<?, Object>> stack = STACK.get();
+        Preconditions.checkState(stack != null, "No stack allocated when completing %s", key);
+
+        final Entry<?, Object> top = stack.pop();
+        if (stack.isEmpty()) {
+            LOG.trace("Removed empty thread stack");
+            STACK.set(null);
+        }
+
+        Preconditions.checkState(key == top.getKey(), "Expected key %s, have %s", top.getKey(), key);
+        Preconditions.checkState(top.getValue() != null, "");
+    }
+
+    // BEWARE: this method returns incpmpletely-initialized objects (that is the purpose of this class).
+    //
+    //         BE VERY CAREFUL WHAT OBJECT STATE YOU TOUCH
+    public static @Nullable <T> T lookup(final Object key, final Class<T> requiredClass) {
+        final Deque<Entry<?, Object>> stack = STACK.get();
+        if (stack != null) {
+            for (Entry<?, Object> e : stack) {
+                // Keys are treated as identities
+                if (key == e.getKey()) {
+                    Preconditions.checkState(e.getValue() != null, "Object for %s is not resolved", key);
+                    LOG.debug("Looked up key {}", e.getKey());
+                    return requiredClass.cast(e.getValue());
+                }
+            }
+        }
+
+        return null;
+    }
+
+    // Be sure to call this in from a finally block when bulk processing is done, so that this class can be unloaded
+    public static void cleanup() {
+        STACK.remove();
+        LOG.debug("Removed thread state");
+    }
+}