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//
// ========================================================================
// Copyright (c) 1995-2021 Mort Bay Consulting Pty Ltd and others.
//
// This program and the accompanying materials are made available under the
// terms of the Eclipse Public License v. 2.0 which is available at
// https://www.eclipse.org/legal/epl-2.0, or the Apache License, Version 2.0
// which is available at https://www.apache.org/licenses/LICENSE-2.0.
//
// SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
// ========================================================================
//
package org.eclipse.jetty.util;
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import org.eclipse.jetty.util.component.AbstractLifeCycle;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* A facility to detect improper usage of resource pools.
*
* Resource pools usually have a method to acquire a pooled resource and a method to released it back to the pool.
*
* To detect if client code acquires a resource but never releases it, the resource pool can be modified to use a
* {@link LeakDetector}. The modified resource pool should call {@link #acquired(Object)} every time the method to
* acquire a resource is called, and {@link #released(Object)} every time the method to release the resource is called.
* {@link LeakDetector} keeps track of these resources and invokes method
* {@link #leaked(org.eclipse.jetty.util.LeakDetector.LeakInfo)} when it detects that a resource has been leaked (that
* is, acquired but never released).
*
* To detect whether client code releases a resource without having acquired it, the resource pool can be modified to
* check the return value of {@link #released(Object)}: if false, it means that the resource was not acquired.
*
* IMPLEMENTATION NOTES
*
* This class relies on {@link System#identityHashCode(Object)} to create a unique id for each resource passed to
* {@link #acquired(Object)} and {@link #released(Object)}. {@link System#identityHashCode(Object)} does not guarantee
* that it will not generate the same number for different objects, but in practice the chance of collision is rare.
*
* {@link LeakDetector} uses {@link PhantomReference}s to detect leaks. {@link PhantomReference}s are enqueued in their
* {@link ReferenceQueue} after they have been garbage collected (differently from {@link WeakReference}s that
* are enqueued before). Since the resource is now garbage collected, {@link LeakDetector} checks whether it
* has been released and if not, it reports a leak. Using {@link PhantomReference}s is better than overriding
* {@link #finalize()} and works also in those cases where {@link #finalize()} is not overridable.
*
* @param the resource type.
*/
public class LeakDetector extends AbstractLifeCycle implements Runnable
{
private static final Logger LOG = LoggerFactory.getLogger(LeakDetector.class);
private final ReferenceQueue queue = new ReferenceQueue<>();
private final ConcurrentMap resources = new ConcurrentHashMap<>();
private Thread thread;
/**
* Tracks the resource as been acquired.
*
* @param resource the resource that has been acquired
* @return true whether the resource has been acquired normally, false if the resource has detected a leak (meaning
* that another acquire occurred before a release of the same resource)
* @see #released(Object)
*/
public boolean acquired(T resource)
{
String id = id(resource);
LeakInfo info = resources.putIfAbsent(id, new LeakInfo(resource, id));
if (info != null)
{
// Leak detected, prior acquire exists (not released) or id clash.
return false;
}
// Normal behavior.
return true;
}
/**
* Tracks the resource as been released.
*
* @param resource the resource that has been released
* @return true whether the resource has been released normally (based on a previous acquire). false if the resource
* has been released without a prior acquire (such as a double release scenario)
* @see #acquired(Object)
*/
public boolean released(T resource)
{
String id = id(resource);
LeakInfo info = resources.remove(id);
if (info != null)
{
// Normal behavior.
return true;
}
// Leak detected (released without acquire).
return false;
}
/**
* Generates a unique ID for the given resource.
*
* @param resource the resource to generate the unique ID for
* @return the unique ID of the given resource
*/
public String id(T resource)
{
return String.valueOf(System.identityHashCode(resource));
}
@Override
protected void doStart() throws Exception
{
super.doStart();
thread = new Thread(this, getClass().getSimpleName());
thread.setDaemon(true);
thread.start();
}
@Override
protected void doStop() throws Exception
{
super.doStop();
thread.interrupt();
}
@Override
public void run()
{
try
{
while (isRunning())
{
@SuppressWarnings("unchecked")
LeakInfo leakInfo = (LeakInfo)queue.remove();
if (LOG.isDebugEnabled())
LOG.debug("Resource GC'ed: {}", leakInfo);
if (resources.remove(leakInfo.id) != null)
leaked(leakInfo);
}
}
catch (InterruptedException x)
{
// Exit
}
}
/**
* Callback method invoked by {@link LeakDetector} when it detects that a resource has been leaked.
*
* @param leakInfo the information about the leak
*/
protected void leaked(LeakInfo leakInfo)
{
LOG.warn("Resource leaked: {}", leakInfo.description, leakInfo.stackFrames);
}
/**
* Information about the leak of a resource.
*/
public class LeakInfo extends PhantomReference
{
private final String id;
private final String description;
private final Throwable stackFrames;
private LeakInfo(T referent, String id)
{
super(referent, queue);
this.id = id;
this.description = referent.toString();
this.stackFrames = new Throwable();
}
/**
* @return the resource description as provided by the resource's {@link Object#toString()} method.
*/
public String getResourceDescription()
{
return description;
}
/**
* @return a Throwable instance that contains the stack frames at the time of resource acquisition.
*/
public Throwable getStackFrames()
{
return stackFrames;
}
@Override
public String toString()
{
return description;
}
}
}