Demo code結束后，有一句話沒太理解：“PlublishingVehicleTracker是線程安全的，但是如果它在車輛位置的有效值上施加了任何約束，那么就不再是線程安全的”，SafePoint本身已經通過內置鎖保證了對象內數據的線程安全，那么即使在set方法內加入了一些判斷location位置的有效性代碼也不會導致線程不安全吧。求指點！

代碼如下：

@ThreadSafe

public class PublishingVehicleTracker {

private final Map locations;

private final Map unmodifiableMap;

public PublishingVehicleTracker(

Map locations) {

this.locations

= new ConcurrentHashMap(locations);

this.unmodifiableMap

= Collections.unmodifiableMap(this.locations);

}

public Map getLocations() {

return unmodifiableMap;

}

public SafePoint getLocation(String id) {

return locations.get(id);

}

public void setLocation(String id, int x, int y) {

if (!locations.containsKey(id))

throw new IllegalArgumentException(

"invalid vehicle name: " + id);

locations.get(id).set(x, y);

}

}

@ThreadSafe

public class SafePoint {

@GuardedBy("this") private int x, y;

private SafePoint(int[] a) { this(a[0], a[1]); }

public SafePoint(SafePoint p) { this(p.get()); }

public SafePoint(int x, int y) {

this.x = x;

this.y = y;

}

public synchronized int[] get() {

return new int[] { x, y };

}

public synchronized void set(int x, int y) {

this.x = x;

this.y = y;

}

}

總結

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