java - 如何避免并发访问资源？

我有四个线程使用CompletableFuture异步运行，如下代码所示。并且所有人都应该访问“ grownSeedXYList”。有时，当我运行代码时，我没有收到任何错误，但有时，我收到了“ java.util.concurrent.completionexception”
我认为这是因为“ grownSeedXYList”未同步。

请让我知道如何同步“ grownSeedXYList”吗？

更新：

this.grownSeedXYList is a list that will be populated with some Point objects based on the runnable class used (GrowSeedSERun, GrowSeedNWRun, GrowSeedNERun, GrowSeedSWRun)

四个线程使用Compltable future运行

this.grownSeedXYList = new ArrayList<Point>();
this.growSeedFutureList = CompletableFuture.runAsync(new GrowSeedSERun(this.saliencyMat, this.seedXY, this.seedVal), this.growSeedExecutor);
                this.growSeedFutureList = CompletableFuture.runAsync(new GrowSeedNWRun(this.saliencyMat, this.seedXY, this.seedVal), this.growSeedExecutor);
                this.growSeedFutureList = CompletableFuture.runAsync(new GrowSeedNERun(this.saliencyMat, this.seedXY, this.seedVal), this.growSeedExecutor);
                this.growSeedFutureList = CompletableFuture.runAsync(new GrowSeedSWRun(this.saliencyMat, this.seedXY, this.seedVal), this.growSeedExecutor);
                CompletableFuture.allOf(this.growSeedFutureList).join();

GrowSeedSERun类：

private class GrowSeedSERun implements Runnable {

    private Mat saliencyMat = null;
    private double seedVal;
    private Point seedXY = null;

    public GrowSeedSERun(Mat saliencyMat, Point seedXY, double seedVal) {
        // TODO Auto-generated constructor stub
        this.saliencyMat = saliencyMat;
        this.seedXY = seedXY;
        this.seedVal = seedVal;
    }
    public void run() {
        // TODO Auto-generated method stub
        growSeedsSE(this.saliencyMat, this.seedXY, this.seedVal);
    }
}

growSeedsSE：

private void growSeedsSE(Mat saliencyMat, Point seedXY, Double seedVal) {
    // TODO Auto-generated method stub
    int origX = (int) seedXY.x;
    int origY = (int) seedXY.y;

    if ( this.withinRange(saliencyMat.get(origY, ++origX)[0]) ) {

        if ( (this.grownSeedXYList != null) && (!this.grownSeedXYList.contains(new Point(origX, origY))) ) {

            //Log.D(TAG, "growSeedsSE", "newX: origX: "+origX);
            //Log.D(TAG, "growSeedsSE", "newX: origY: "+origY);
            //Log.D(TAG, "growSeedsSE", "newX: value: "+saliencyMat.get(origY, origX)[0]);

            this.grownSeedXYList.add(new Point(origX, origY));

        } else {
            Log.D(TAG, "growSeedsSE", "point: "+ new Point(origX, origY)+" contained in the list");
        }
        this.growSeedsSE(this.saliencyMat, new Point(origX, origY), this.saliencyMat.get(origY, origX)[0]);

    } else if ( this.withinRange(saliencyMat.get(++origY, (int) this.seedXY.x)[0]) ) {
        origX = (int) this.seedXY.x;

        if ( (this.grownSeedXYList != null) && (!this.grownSeedXYList.contains(new Point(origX, origY))) ) {

            //Log.D(TAG, "growSeedsSE", "newY: origX: "+origX);
            //Log.D(TAG, "growSeedsSE", "newY: origY: "+origY);
            //Log.D(TAG, "growSeedsSE", "newY: value: "+saliencyMat.get(origY, origX)[0]);

            this.grownSeedXYList.add(new Point(origX, origY));

        }  else {
            Log.D(TAG, "growSeedsSE", "point: "+ new Point(origX, origY)+" contained in the list");
        }
        this.growSeedsSE(this.saliencyMat, new Point(origX, origY), this.saliencyMat.get(origY, origX)[0]);
    }
}

最佳答案

您的grownSeedXYList在线程之间共享。您最简单的选择是使用声明如下：

this.grownSeedXYList = Collections.synchronizedList(new ArrayList<Point>());

这将导致grownSeedXYList集合是线程安全的。仅供参考，我相信，由于没有CompletionException的完整遍历，您可能在调用join时收到了它，因为其中有一个线程捕获了ConcurrentModificationException

编辑：正如@ user270349在下面的注释中所指出的那样，并且如javadoc所指出的那样，如果对它进行迭代，您仍然需要在grownSeedXYList上进行同步。在这种情况下，您将执行以下操作：

synchronized(grownSeedXYList) {
    Iterator i = grownSeedXYList.iterator();
    while (i.hasNext())
        foo(i.next());
    }
}

但是，如果使用for / each块，则无需同步，如下所示：

for (Point point : grownSeedXYList) {
    foo(point);
}