1_基础知识_chapter04_对象的组合_3_线程安全性的委托

• (1) 从头开始构建一个类, 或将多个非线程安全的类组合为一个类时, Java监视器模式很有效(见"2_实例封闭.md");

  多个线程安全的类组合为一个类时, 可以考虑委托方式

  (2) 示例

  1° 基于监视器模式的车辆追踪

  MonitorVehicleTracker.java

    @ThreadSafe
    public class MonitorVehicleTracker {
  
        @GuardedBy("this")
        private final Map locations;
  
        public MonitorVehicleTracker(Map locations) {
  
            this.locations = deepCopy(locations);
        }
  
        public synchronized Map 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 deepCopy(Map m) {
  
            Map result = new HashMap();
  
            for (String id : m.keySet()) {
                result.put(id, new MutablePoint(m.get(id)));
            }
  
            return Collections.unmodifiableMap(result);
        }
    }
  

  MutablePoint.java

    @NotThreadSafe
    public class MutablePoint {
        public int x, y;
  
        public MutablePoint() {
            x = 0;
            y = 0;
        }
  
        public MutablePoint(MutablePoint p) {
            this.x = p.x;
            this.y = p.y;
        }
    }
  

  这个示例中, 虽然MutablePoint线程不安全, 但是MonitorVehicleTracker是安全的: 它采用了监视器模式, 用this作为锁, 并且内含的Map对象和Mutable对象都未曾发布

__2° 基于委托的车辆追踪__

DelegatingVehicleTracker.java

    @ThreadSafe
    public class DelegatingVehicleTracker {

        private final ConcurrentMap locations;
        private final Map unmodifiableMap;

        public DelegatingVehicleTracker(Map points) {

            locations = new ConcurrentHashMap(points);
            unmodifiableMap = Collections.unmodifiableMap(locations);
        }

        public Point getLocation(String id) {

            return locations.get(id);
        }

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

            if (locations.replace(id, new Point(x, y)) == null) {
                throw new IllegalArgumentException("invalid vehicle name: " + id);
            }
        }

        public Map getLocations() {

            return unmodifiableMap;
        }

        // Alternate version of getLocations 
        public Map getLocationsAsStatic() {
    
            return Collections.unmodifiableMap(new HashMap(locations));
        }
    }

Point.java

    @Immutable
    public class Point {

        public final int x, y;

        public Point(int x, int y) {
            this.x = x;
            this.y = y;
        }
    }

这种方式将线程安全委托给了ConcurrentMap类型的locations, 由于Point类时不可变的，所以如果想要实时更新点的位置, 可以调用getLocations()函数, 这样更新会立刻显示; 如果不需要发生变化的车辆位置, 可以调用getLocationsAsStatic()函数, 在这里采用了__浅拷贝__的方式(因为Point不可变, 复制Map的结构即可, 不需要复制它的内容)

(3) __线程安全性可以委托给多个状态变量, 只要它们是彼此独立的__

示例

    public class VisualComponent {

        private final List keyListeners = new CopyOnWriteArrayList();

        private final List mouseListeners = new CopyOnWriteArrayList();

        public void addKeyListener(KeyListener listener) {
    
            keyListeners.add(listener);
        }

        public void addMouseListener(MouseListener listener) {
    
            mouseListeners.add(listener);
        }

        public void removeKeyListener(KeyListener listener) {
    
            keyListeners.remove(listener);
        }

        public void removeMouseListener(MouseListener listener) {
    
            mouseListeners.remove(listener);
        }
    }

这个示例中, keyListeners和mouseListeners的类型是CopyOnWriteArrayList, 这是一个线程安全的容器类型, 并且由于它们互相独立, 所以VisualComponent的线程安全委托给了keyListeners和mouseListeners

(4) __但是, 如果某个类含有复合操作, 那么委托方式就会失效, 此时必须依靠类自己的加锁机制__(这个类似于如果只有一个状态变量时用volatile就行，但是有多个变量仅用volatile就会失效)