Demo code结束后，有一句话没太理解：“PlublishingVehicleTracker是线程安全的，但是如果它在车辆位置的有效值上施加了任何约束，那么就不再是线程安全的”，SafePoint本身已经通过内置锁保证了对象内数据的线程安全，那么即使在set方法内加入了一些判断location位置的有效性代码也不会导致线程不安全吧。求指点！
代码如下：
@ThreadSafepublic class PublishingVehicleTracker { private final Map<String, SafePoint> locations; private final Map<String, SafePoint> unmodifiableMap; public PublishingVehicleTracker( Map<String, SafePoint> locations) { this.locations = new ConcurrentHashMap<String, SafePoint>(locations); this.unmodifiableMap = Collections.unmodifiableMap(this.locations); } public Map<String, SafePoint> 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); }}@ThreadSafepublic 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; }}解决方案
正好有这本书，重新看了下，对这最后两句我的理解是在强调虽然PublishingVehicleTracker和SafePoint本身都是threadsafe的，但是PublishingVehicleTracker发布出来的SafePoint是可变的，因此如果使用场景里需要基于PublishingVehicleTracker发布出来的Location（即SafePoint）做一些逻辑判断或者其他的操作时，结果可能是有问题的。
例如，如果A线程对通过getLocations获取的Map做遍历的同时，B线程通过setLocation方法修改其中的一个SafePoint值，那么A线程的处理结果就有了问题。
写了一个A线程内处理过程的样例代码，不一定确切，仅供参考：
Map<String, SafePoint> locations = new HashMap<String, SafePoint>(); locations.put("1", new SafePoint(1, 1)); locations.put("2", new SafePoint(2, 2)); PublishingVehicleTracker tracker = new PublishingVehicleTracker(locations); Map<String, SafePoint> m1 = tracker.getLocations(); int greatThanTenCount = 0; for (String s1 : m1.keySet()) { SafePoint p1 = m1.get(s1); int [] v = p1.get(); if (v[0] + v[1] > 10) { //可能的问题场景：此时B线程修改了某一个location的SafePoint值 //在符合x+y和值大于10时，作业务处理 greatThanTenCount ++; System.out.println(s1 + "的(x+y)和值大于10了"); } } if (greatThanTenCount > 2) { //可能的问题场景：此时B线程修改了某一个location的SafePoint值 System.out.println("有超过2个的Location的x+y和值大于10，因此停车场不能再放车进来了"); }