Java 并发编程实战学习笔记——路径查找类型并行任务的终止

1.该类问题的递归串行算法（深度优先遍历）

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package net.jcip.examples;

import java.util.*;

/**
* SequentialPuzzleSolver
* <p/>
* Sequential puzzle solver
*
* @author Brian Goetz and Tim Peierls
*/

public class SequentialPuzzleSolver <P, M> {
private final Puzzle<P, M> puzzle;
private final Set<P> seen = new HashSet<P>();

public SequentialPuzzleSolver(Puzzle<P, M> puzzle) {
this.puzzle = puzzle;
}

public List<M> solve() {
P pos = puzzle.initialPosition();
return search(new PuzzleNode<P, M>(pos, null, null));
}

private List<M> search(PuzzleNode<P, M> node) {
if (!seen.contains(node.pos)) {
seen.add(node.pos);
if (puzzle.isGoal(node.pos))
return node.asMoveList();
for (M move : puzzle.legalMoves(node.pos)) {
P pos = puzzle.move(node.pos, move);
PuzzleNode<P, M> child = new PuzzleNode<P, M>(pos, move, node);
List<M> result = search(child);
if (result != null)
return result;
}
}
return null;
}
}

2.该类问题的并行算法（CountDownLatch 用作一个简单的开/关锁存器）

广度优先算法

代码
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package net.jcip.examples;

import java.util.concurrent.*;

/**
* ValueLatch
* <p/>
* Result-bearing latch used by ConcurrentPuzzleSolver
*
* @author Brian Goetz and Tim Peierls
*/
@ThreadSafe
public class ValueLatch <T> {
@GuardedBy("this") private T value = null;
private final CountDownLatch done = new CountDownLatch(1);

public boolean isSet() {
return (done.getCount() == 0);
}

public synchronized void setValue(T newValue) {
if (!isSet()) {
value = newValue;
done.countDown();
}
}

public T getValue() throws InterruptedException {
done.await();
synchronized (this) {
return value;
}
}
}

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package net.jcip.examples;

import java.util.*;
import java.util.concurrent.*;

/**
* ConcurrentPuzzleSolver
* <p/>
* Concurrent version of puzzle solver
*
* @author Brian Goetz and Tim Peierls
*/
public class ConcurrentPuzzleSolver <P, M> {
private final Puzzle<P, M> puzzle;
private final ExecutorService exec;
private final ConcurrentMap<P, Boolean> seen;
protected final ValueLatch<PuzzleNode<P, M>> solution = new ValueLatch<PuzzleNode<P, M>>();

public ConcurrentPuzzleSolver(Puzzle<P, M> puzzle) {
this.puzzle = puzzle;
this.exec = initThreadPool();
this.seen = new ConcurrentHashMap<P, Boolean>();
if (exec instanceof ThreadPoolExecutor) {
ThreadPoolExecutor tpe = (ThreadPoolExecutor) exec;
tpe.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy());
}
}

private ExecutorService initThreadPool() {
return Executors.newCachedThreadPool();
}

public List<M> solve() throws InterruptedException {
try {
P p = puzzle.initialPosition();
exec.execute(newTask(p, null, null));
// block until solution found
PuzzleNode<P, M> solnPuzzleNode = solution.getValue();
return (solnPuzzleNode == null) ? null : solnPuzzleNode.asMoveList();
} finally {
exec.shutdown();
}
}

protected Runnable newTask(P p, M m, PuzzleNode<P, M> n) {
return new SolverTask(p, m, n);
}

protected class SolverTask extends PuzzleNode<P, M> implements Runnable {
SolverTask(P pos, M move, PuzzleNode<P, M> prev) {
super(pos, move, prev);
}

public void run() {
if (solution.isSet()
|| seen.putIfAbsent(pos, true) != null)
return; // already solved or seen this position
if (puzzle.isGoal(pos))
solution.setValue(this);
else
for (M m : puzzle.legalMoves(pos))
exec.execute(newTask(puzzle.move(pos, m), m, this));
}
}
}

该段代码有个问题，就是不能够感知到该问题没有答案。

因此可以改为，可以感知到任务不存在的解决中

代码
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package net.jcip.examples;

import java.util.concurrent.atomic.*;

/**
* PuzzleSolver
* <p/>
* Solver that recognizes when no solution exists
*
* @author Brian Goetz and Tim Peierls
*/
public class PuzzleSolver <P,M> extends ConcurrentPuzzleSolver<P, M> {
PuzzleSolver(Puzzle<P, M> puzzle) {
super(puzzle);
}

private final AtomicInteger taskCount = new AtomicInteger(0);

protected Runnable newTask(P p, M m, PuzzleNode<P, M> n) {
return new CountingSolverTask(p, m, n);
}

class CountingSolverTask extends SolverTask {
CountingSolverTask(P pos, M move, PuzzleNode<P, M> prev) {
super(pos, move, prev);
taskCount.incrementAndGet();
}

public void run() {
try {
super.run();
} finally {
if (taskCount.decrementAndGet() == 0)
solution.setValue(null);
}
}
}
}