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/*
* %CopyrightBegin%
*
* Copyright Ericsson AB 2000-2016. All Rights Reserved.
*
* 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.
*
* %CopyrightEnd%
*/
package com.ericsson.otp.erlang;
/**
* This class implements a generic FIFO queue. There is no upper bound on the
* length of the queue, items are linked.
*/
public class GenericQueue {
private static final int open = 0;
private static final int closing = 1;
private static final int closed = 2;
private int status;
private Bucket head;
private Bucket tail;
private int count;
private void init() {
head = null;
tail = null;
count = 0;
}
/** Create an empty queue */
public GenericQueue() {
init();
status = open;
}
/** Clear a queue */
public void flush() {
init();
}
public void close() {
status = closing;
}
/**
* Add an object to the tail of the queue.
*
* @param o
* Object to insert in the queue
*/
public synchronized void put(final Object o) {
final Bucket b = new Bucket(o);
if (tail != null) {
tail.setNext(b);
tail = b;
} else {
// queue was empty but has one element now
head = tail = b;
}
count++;
// notify any waiting tasks
notify();
}
/**
* Retrieve an object from the head of the queue, or block until one
* arrives.
*
* @return The object at the head of the queue.
*/
public synchronized Object get() {
Object o = null;
while ((o = tryGet()) == null) {
try {
this.wait();
} catch (final InterruptedException e) {
}
}
return o;
}
/**
* Retrieve an object from the head of the queue, blocking until one arrives
* or until timeout occurs.
*
* @param timeout
* Maximum time to block on queue, in ms. Use 0 to poll the
* queue.
*
* @exception InterruptedException
* if the operation times out.
*
* @return The object at the head of the queue, or null if none arrived in
* time.
*/
public synchronized Object get(final long timeout)
throws InterruptedException {
if (status == closed) {
return null;
}
long currentTime = System.currentTimeMillis();
final long stopTime = currentTime + timeout;
Object o = null;
while (true) {
if ((o = tryGet()) != null) {
return o;
}
currentTime = System.currentTimeMillis();
if (stopTime <= currentTime) {
throw new InterruptedException("Get operation timed out");
}
try {
this.wait(stopTime - currentTime);
} catch (final InterruptedException e) {
// ignore, but really should retry operation instead
}
}
}
// attempt to retrieve message from queue head
public Object tryGet() {
Object o = null;
if (head != null) {
o = head.getContents();
head = head.getNext();
count--;
if (head == null) {
tail = null;
count = 0;
}
}
return o;
}
public synchronized int getCount() {
return count;
}
/*
* The Bucket class. The queue is implemented as a linked list of Buckets.
* The container holds the queued object and a reference to the next Bucket.
*/
class Bucket {
private Bucket next;
private final Object contents;
public Bucket(final Object o) {
next = null;
contents = o;
}
public void setNext(final Bucket newNext) {
next = newNext;
}
public Bucket getNext() {
return next;
}
public Object getContents() {
return contents;
}
}
}
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