manifold.util.concurrent.LockingLazyVar Maven / Gradle / Ivy
/*
* Copyright (c) 2018 - Manifold Systems LLC
*
* 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 manifold.util.concurrent;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public abstract class LockingLazyVar
{
private final static Object NULL = new Object();
private volatile T _val = null;
private final ReadWriteLock _rwLock;
protected LockingLazyVar()
{
this( new ReentrantReadWriteLock() );
}
protected LockingLazyVar( ReadWriteLock lock )
{
_rwLock = lock;
}
/**
* @return the value of this lazy var, created if necessary
*/
public final T get()
{
T result;
Lock rLock = _rwLock.readLock();
rLock.lock();
try
{
result = _val;
}
finally
{
rLock.unlock();
}
if( result == NULL )
{
return null;
}
if( result != null )
{
return result;
}
Lock wLock = _rwLock.writeLock();
wLock.lock();
try
{
result = _val;
if( result == NULL )
{
return null;
}
if( result == null )
{
result = init();
if( result == null )
{
_val = (T)NULL;
}
else
{
_val = result;
}
}
}
finally
{
wLock.unlock();
}
return result;
}
public T set( T value )
{
if( value == null )
{
value = (T)NULL;
}
T prev = get();
Lock wLock = _rwLock.writeLock();
wLock.lock();
try
{
_val = value;
}
finally
{
wLock.unlock();
}
return prev;
}
protected abstract T init();
/**
* Clears the variable, forcing the next call to {@link #get()} to re-calculate
* the value.
*/
public final T clear()
{
T prev;
Lock wLock = _rwLock.writeLock();
wLock.lock();
try
{
prev = _val;
_val = null;
}
finally
{
wLock.unlock();
}
return prev;
}
public final void clearNoLock()
{
_val = null;
}
public boolean isLoaded()
{
Lock rLock = _rwLock.readLock();
rLock.lock();
try
{
return _val != null;
}
finally
{
rLock.unlock();
}
}
/**
* A simple init interface to make LockingLazyVar's easier to construct.
*/
public interface LazyVarInit
{
Q init();
}
/**
* Creates a new LockingLazyVar based on the type of the LazyVarInit passed in.
*/
public static LockingLazyVar make( final LazyVarInit init )
{
return new LockingLazyVar()
{
protected Q init()
{
return init.init();
}
};
}
public static LockingLazyVar make( ReadWriteLock lock, final LazyVarInit init )
{
return new LockingLazyVar( lock )
{
protected Q init()
{
return init.init();
}
};
}
}