/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.base.internal;

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.logging.Level;
import java.util.logging.Logger;
import org.checkerframework.checker.nullness.qual.Nullable;

Thread that finalizes referents. All references should implement 
com.google.common.base.FinalizableReference. 
While this class is public, we consider it to be *internal* and not part of our published API.
It is public so we can access it reflectively across class loaders in secure environments.
This class can't depend on other Guava code. If we were to load this class in the same class
loader as the rest of Guava, this thread would keep an indirect strong reference to the class
loader and prevent it from being garbage collected. This poses a problem for environments where
you want to throw away the class loader. For example, dynamically reloading a web application or
unloading an OSGi bundle.
com.google.common.base.FinalizableReferenceQueue loads this class in its own class loader. That way, this class doesn't prevent the main class loader from getting garbage collected, and this class can detect when the main class loader has been garbage collected and stop itself. /**
 * Thread that finalizes referents. All references should implement {@code
 * com.google.common.base.FinalizableReference}.
 *
 * <p>While this class is public, we consider it to be *internal* and not part of our published API.
 * It is public so we can access it reflectively across class loaders in secure environments.
 *
 * <p>This class can't depend on other Guava code. If we were to load this class in the same class
 * loader as the rest of Guava, this thread would keep an indirect strong reference to the class
 * loader and prevent it from being garbage collected. This poses a problem for environments where
 * you want to throw away the class loader. For example, dynamically reloading a web application or
 * unloading an OSGi bundle.
 *
 * <p>{@code com.google.common.base.FinalizableReferenceQueue} loads this class in its own class
 * loader. That way, this class doesn't prevent the main class loader from getting garbage
 * collected, and this class can detect when the main class loader has been garbage collected and
 * stop itself.
 */
public class Finalizer implements Runnable {

  private static final Logger logger = Logger.getLogger(Finalizer.class.getName());

  
Name of FinalizableReference.class. /** Name of FinalizableReference.class. */
  private static final String FINALIZABLE_REFERENCE = "com.google.common.base.FinalizableReference";

  
Starts the Finalizer thread. FinalizableReferenceQueue calls this method reflectively.
Params:
finalizableReferenceClass – FinalizableReference.class.
queue – a reference queue that the thread will poll.
frqReference – a phantom reference to the FinalizableReferenceQueue, which will be queued
    either when the FinalizableReferenceQueue is no longer referenced anywhere, or when its
    close() method is called./**
   * Starts the Finalizer thread. FinalizableReferenceQueue calls this method reflectively.
   *
   * @param finalizableReferenceClass FinalizableReference.class.
   * @param queue a reference queue that the thread will poll.
   * @param frqReference a phantom reference to the FinalizableReferenceQueue, which will be queued
   *     either when the FinalizableReferenceQueue is no longer referenced anywhere, or when its
   *     close() method is called.
   */
  public static void startFinalizer(
      Class<?> finalizableReferenceClass,
      ReferenceQueue<Object> queue,
      PhantomReference<Object> frqReference) {
    /*
     * We use FinalizableReference.class for two things:
     *
     * 1) To invoke FinalizableReference.finalizeReferent()
     *
     * 2) To detect when FinalizableReference's class loader has to be garbage collected, at which
     * point, Finalizer can stop running
     */
    if (!finalizableReferenceClass.getName().equals(FINALIZABLE_REFERENCE)) {
      throw new IllegalArgumentException("Expected " + FINALIZABLE_REFERENCE + ".");
    }

    Finalizer finalizer = new Finalizer(finalizableReferenceClass, queue, frqReference);
    String threadName = Finalizer.class.getName();
    Thread thread = null;
    if (bigThreadConstructor != null) {
      try {
        boolean inheritThreadLocals = false;
        long defaultStackSize = 0;
        thread =
            bigThreadConstructor.newInstance(
                (ThreadGroup) null, finalizer, threadName, defaultStackSize, inheritThreadLocals);
      } catch (Throwable t) {
        logger.log(
            Level.INFO, "Failed to create a thread without inherited thread-local values", t);
      }
    }
    if (thread == null) {
      thread = new Thread((ThreadGroup) null, finalizer, threadName);
    }
    thread.setDaemon(true);

    try {
      if (inheritableThreadLocals != null) {
        inheritableThreadLocals.set(thread, null);
      }
    } catch (Throwable t) {
      logger.log(
          Level.INFO,
          "Failed to clear thread local values inherited by reference finalizer thread.",
          t);
    }

    thread.start();
  }

  private final WeakReference<Class<?>> finalizableReferenceClassReference;
  private final PhantomReference<Object> frqReference;
  private final ReferenceQueue<Object> queue;

  // By preference, we will use the Thread constructor that has an `inheritThreadLocals` parameter.
  // But before Java 9, our only way not to inherit ThreadLocals is to zap them after the thread
  // is created, by accessing a private field.
  private static final @Nullable Constructor<Thread> bigThreadConstructor =
      getBigThreadConstructor();

  private static final @Nullable Field inheritableThreadLocals =
      (bigThreadConstructor == null) ? getInheritableThreadLocalsField() : null;

  
Constructs a new finalizer thread. /** Constructs a new finalizer thread. */
  private Finalizer(
      Class<?> finalizableReferenceClass,
      ReferenceQueue<Object> queue,
      PhantomReference<Object> frqReference) {
    this.queue = queue;

    this.finalizableReferenceClassReference =
        new WeakReference<Class<?>>(finalizableReferenceClass);

    // Keep track of the FRQ that started us so we know when to stop.
    this.frqReference = frqReference;
  }

  
Loops continuously, pulling references off the queue and cleaning them up. /** Loops continuously, pulling references off the queue and cleaning them up. */
  @SuppressWarnings("InfiniteLoopStatement")
  @Override
  public void run() {
    while (true) {
      try {
        if (!cleanUp(queue.remove())) {
          break;
        }
      } catch (InterruptedException e) {
        // ignore
      }
    }
  }

  
Cleans up a single reference. Catches and logs all throwables.
Returns:
true if the caller should continue, false if the associated FinalizableReferenceQueue
    is no longer referenced./**
   * Cleans up a single reference. Catches and logs all throwables.
   *
   * @return true if the caller should continue, false if the associated FinalizableReferenceQueue
   *     is no longer referenced.
   */
  private boolean cleanUp(Reference<?> reference) {
    Method finalizeReferentMethod = getFinalizeReferentMethod();
    if (finalizeReferentMethod == null) {
      return false;
    }
    do {
      /*
       * This is for the benefit of phantom references. Weak and soft references will have already
       * been cleared by this point.
       */
      reference.clear();

      if (reference == frqReference) {
        /*
         * The client no longer has a reference to the FinalizableReferenceQueue. We can stop.
         */
        return false;
      }

      try {
        finalizeReferentMethod.invoke(reference);
      } catch (Throwable t) {
        logger.log(Level.SEVERE, "Error cleaning up after reference.", t);
      }

      /*
       * Loop as long as we have references available so as not to waste CPU looking up the Method
       * over and over again.
       */
    } while ((reference = queue.poll()) != null);
    return true;
  }

  
Looks up FinalizableReference.finalizeReferent() method. /** Looks up FinalizableReference.finalizeReferent() method. */
  private @Nullable Method getFinalizeReferentMethod() {
    Class<?> finalizableReferenceClass = finalizableReferenceClassReference.get();
    if (finalizableReferenceClass == null) {
      /*
       * FinalizableReference's class loader was reclaimed. While there's a chance that other
       * finalizable references could be enqueued subsequently (at which point the class loader
       * would be resurrected by virtue of us having a strong reference to it), we should pretty
       * much just shut down and make sure we don't keep it alive any longer than necessary.
       */
      return null;
    }
    try {
      return finalizableReferenceClass.getMethod("finalizeReferent");
    } catch (NoSuchMethodException e) {
      throw new AssertionError(e);
    }
  }

  private static @Nullable Field getInheritableThreadLocalsField() {
    try {
      Field inheritableThreadLocals = Thread.class.getDeclaredField("inheritableThreadLocals");
      inheritableThreadLocals.setAccessible(true);
      return inheritableThreadLocals;
    } catch (Throwable t) {
      logger.log(
          Level.INFO,
          "Couldn't access Thread.inheritableThreadLocals. Reference finalizer threads will "
              + "inherit thread local values.");
      return null;
    }
  }

  private static @Nullable Constructor<Thread> getBigThreadConstructor() {
    try {
      return Thread.class.getConstructor(
          ThreadGroup.class, Runnable.class, String.class, long.class, boolean.class);
    } catch (Throwable t) {
      // Probably pre Java 9. We'll fall back to Thread.inheritableThreadLocals.
      return null;
    }
  }
}