java - 同步方法中的Java线程安全错误

我有这个课

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。有时会得到以下输出：BUG! MORE: 12 max: 12这是因为我在getBitmap()方法中检查了值AGAIN。为什么？

我不明白，如果动画是synchronized，moveCount怎么会大于11？

如果模拟器滞后，则这种情况会更频繁地发生。

最佳答案

您正在递增和重置moveCount块中的synchronized，但是当您访问moveCount方法中的getBitmap()变量时，您并未在同一锁上进行同步。

这意味着线程A可能位于animate()方法的中间，并且已将moveCount递增为等于moveCountMax。然后，线程B输入getBitmap()并读取moveCount的值，然后线程A将moveCount重置为0。

通常，不仅需要在写入变量的值时同步，还需要在读取变量时（在同一锁上）进行同步，尤其是在对该变量进行的一项操作（例如您的animate()方法）时，尤其要进行同步。涉及复合操作（递增，然后可能重置为0）

顺便说一句，如果moveCountMax是恒定值（= moves.length），则将其标记为final。