java - 构造期间的线程安全

Question

在 “Java Concurrency in Practice” 书中有一个 MonitorVehicleTracker 代码示例，它声称是线程安全的。

@ThreadSafe
public class MonitorVehicleTracker {
    @GuardedBy("this") private final Map<String, MutablePoint> locations;

    public MonitorVehicleTracker(Map<String, MutablePoint> locations) {
        this.locations = deepCopy(locations);
    }

    public synchronized Map<String, MutablePoint> getLocations() {
        return deepCopy(locations);
    }

    public synchronized MutablePoint getLocation(String id) {
        MutablePoint loc = locations.get(id);
        return loc == null ? null : new MutablePoint(loc);
    }

    public synchronized void setLocation(String id, int x, int y) {
        MutablePoint loc = locations.get(id);
        if (loc == null)
            throw new IllegalArgumentException("No such ID: " + id);
        loc.x = x;
        loc.y = y;
    }

    private static Map<String, MutablePoint> deepCopy(Map<String, MutablePoint> m) {
        Map<String, MutablePoint> result = new HashMap<String, MutablePoint>();

        for (String id : m.keySet())
            result.put(id, new MutablePoint(m.get(id)));

        return Collections.unmodifiableMap(result);
    }
}

但让我们考虑一下构造函数 locations 参数在 deepCopy() 调用期间被另一个线程修改 的情况。这可能会导致在遍历 keySet() 时抛出 ConcurrentModificationException。

那么这是否意味着 MonitorVehicleTracker 不是完全线程安全的？还是thread-safety只有在对象构造完成后才会出现，调用代码负责确保MonitorVehicleTracker实例化时locations不会被修改？

Answer 1

不，该类仍然是线程安全的。

如果对象初始化失败，并不意味着它的类不再是线程安全的。它是一个线程安全的类，但未能初始化，因为它的参数被另一个线程错误地修改，破坏了它的 thread-saftey 契约。