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漫谈Java高并发方案-线程池

2018.07.03 14:20 5421浏览

ThreadPoolExecutor UML图:

image

image

8.1 在任务和执行策略之间隐形耦合

避免Thread starvation deadlock

8.2 设置线程池大小

8.3 配置ThreadPoolExecutor

image

构造函数如下:

public ThreadPoolExecutor(int corePoolSize,
 int maximumPoolSize,
 long keepAliveTime,
 TimeUnit unit,
 BlockingQueue<Runnable> workQueue,
 ThreadFactory threadFactory,
 RejectedExecutionHandler handler) { ... } 

  • 核心和最大池大小:如果运行的线程少于 corePoolSize,则创建新线程来处理请求(即一个Runnable实例),即使其它线程是空闲的。如果运行的线程多于 corePoolSize 而少于 maximumPoolSize,则仅当队列满时才创建新线程。
  • 保持活动时间:如果池中当前有多于 corePoolSize 的线程,则这些多出的线程在空闲时间超过 keepAliveTime 时将会终止。
  • 排队:如果运行的线程等于或多于 corePoolSize,则 Executor 始终首选将请求加入队列BlockingQueue,而不添加新的线程。
  • 被拒绝的任务:当 Executor 已经关闭,或者队列已满且线程数量达到maximumPoolSize时(即线程池饱和了),请求将被拒绝。这些拒绝的策略叫做Saturation Policy,即饱和策略。包括AbortPolicy, CallerRunsPolicy, DiscardPolicy, and DiscardOldestPolicy.

另外注意:

  • 如果运行的线程少于 corePoolSize,ThreadPoolExecutor 会始终首选创建新的线程来处理请求;注意,这时即使有空闲线程也不会重复使用(这和数据库连接池有很大差别)。
  • 如果运行的线程等于或多于 corePoolSize,则 ThreadPoolExecutor 会将请求加入队列BlockingQueue,而不添加新的线程(这和数据库连接池也不一样)。
  • 如果无法将请求加入队列(比如队列已满),则创建新的线程来处理请求;但是如果创建的线程数超出 maximumPoolSize,在这种情况下,请求将被拒绝。

newCachedThreadPool使用了SynchronousQueue,并且是无界的。

线程工厂ThreadFactory

8.4 扩展ThreadPoolExecutor

重写beforeExecute和afterExecute方法。

8.5 递归算法的并行化

实际就是类似Number of Islands或者N-Queens等DFS问题的一种并行处理。

串行版本如下:

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;
    }
}

并行版本如下:

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));
        }
    }
}
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