java同步方法中的线程安全错误

Question

我有这门课

package net.omnosis.mazegame.components;

import net.omnosis.mazegame.SlicedBitmap;
import android.graphics.Bitmap;

public class PlayerLayer extends DrawableLayer {

    private Player player;
    private XY tileImageSize;

    private int[] move = new int[] { 1, 2, 3, 4, 3, 2, 1, 6, 7, 8, 7, 6 };
    //private int[] move = new int[] { 8 };

    private int moveCount;
    private int moveCountMax = move.length;

    private Bitmap playerBitmap;
    public SlicedBitmap playerTiles;

    private int line;

    private static final int VERTICAL = 0;
    private static final int HORIZONTAL = 8;

    public PlayerLayer(Player player, Bitmap playerBitmap, XY tileImageSize) {

        this.playerBitmap = playerBitmap;
        this.tileImageSize = tileImageSize;
        playerTiles = new SlicedBitmap(playerBitmap, tileImageSize.x(), tileImageSize.y());

        setPlayer(player);

        update();
    }

    public final void setPlayer(Player player) {
        if (this.player != null) {
            this.player.removeListener(this);
        }

        this.player = player;
        player.addListener(this);
        update();
    }

    public void updateDirection() {
        Direction dir = player.getHeading();

        if (dir == Direction.LEFT || dir == Direction.RIGHT) {
            line = HORIZONTAL;
        } else if (dir == Direction.TOP || dir == Direction.BOTTOM) {
            line = VERTICAL;
        }
    }

    public synchronized void animate() {

        if (player.isMoving()) {

            moveCount++;

            if (moveCount >= moveCountMax) {

                player.finishMove();
                moveCount = 0;
            }
        } else {

        }

        updateDirection();
        super.update();
    }

    public void update() {
        updateDirection();
        super.update();
    }

    public XY getSpritePos() {
        XY playerPos = new XY(player.getCurrentPosition().x() * tileImageSize.x() + (tileImageSize.x() / 2), player.getCurrentPosition().y() * tileImageSize.y() + (tileImageSize.y() / 2));
        XY animationPos = getAnimationPos();
        return playerPos.add(animationPos);
    }

    public XY getAnimationPos() {
        double step = (double) tileImageSize.x() / moveCountMax * moveCount;
        return player.getHeading().multiply((int) step);
    }

    public Bitmap getBitmap() {

        if (moveCount >= moveCountMax) {
            System.out.println("BUG! MORE: " + moveCount + "  max: " + moveCountMax);
            moveCount = 0;
        }
        return playerTiles.getTile(move[moveCount] + line);
    }
}

线程每 10 毫秒调用一次 animate 方法。有时我会得到这样的输出:BUG! MORE: 12 max: 12 这是因为我在 getBitmap() 方法中再次检查了该值。为什么？

我不明白，如果动画是同步的，moveCount怎么会超过11？

如果模拟器滞后，这种情况会更频繁地发生。

Answer 1

您正在 synchronized block 中递增和重置 moveCount，但是当您访问 moveCount 变量时，您并未在同一个锁上同步在 getBitmap() 方法中。

这意味着线程 A 可能位于 animate() 方法的中间，并且已将 moveCount 增加到等于 moveCountMax 。然后线程 B 进入 getBitmap() 并在线程 A 将 moveCount 重置为 0 之前读取 moveCount 的值。

一般来说，您不仅需要在写入变量的值时进行同步，还需要在读取变量的值时进行同步(在同一个锁上)，特别是如果对该变量进行其中一项操作(例如您的 animate() 方法)涉及复合操作(递增，然后可能重置为 0)

顺便说一句，如果 moveCountMax 是一个常量值 (= move.length)，则将其标记为 final。