java集合中的SetFromMap

java集合中的SetFromMap

在閱讀Spring源碼的時候看見了這句代碼：

private final Set<String> alreadyCreated = 
		Collections.newSetFromMap(new ConcurrentHashMap<>(256));

於是點擊查看代碼的實現：

public class Collections {
    // Suppresses default constructor, ensuring non-instantiability.
    private Collections() {}
    
	
	
	//包裝一個內部類;以此來實現Set的安全操作
	public static <E> Set<E> newSetFromMap(Map<E, Boolean> map) {
        return new SetFromMap<>(map);
    }
	
	//所有關於set的操作都映射到map上
   private static class SetFromMap<E> extends AbstractSet<E>
        implements Set<E>, Serializable
    {
        private final Map<E, Boolean> m;  // The backing map
        private transient Set<E> s;       // Its keySet

        SetFromMap(Map<E, Boolean> map) {
            if (!map.isEmpty())
                throw new IllegalArgumentException("Map is non-empty");
            m = map;
            s = map.keySet();
        }

        public void clear()               {        m.clear(); }
        public int size()                 { return m.size(); }
        public boolean isEmpty()          { return m.isEmpty(); }
        public boolean contains(Object o) { return m.containsKey(o); }
        public boolean remove(Object o)   { return m.remove(o) != null; }
        public boolean add(E e) { return m.put(e, Boolean.TRUE) == null; }
        public Iterator<E> iterator()     { return s.iterator(); }
        public Object[] toArray()         { return s.toArray(); }
        public <T> T[] toArray(T[] a)     { return s.toArray(a); }
        public String toString()          { return s.toString(); }
        public int hashCode()             { return s.hashCode(); }
        public boolean equals(Object o)   { return o == this || s.equals(o); }
        public boolean containsAll(Collection<?> c) {return s.containsAll(c);}
        public boolean removeAll(Collection<?> c)   {return s.removeAll(c);}
        public boolean retainAll(Collection<?> c)   {return s.retainAll(c);}
        // addAll is the only inherited implementation

        // Override default methods in Collection
        @Override
        public void forEach(Consumer<? super E> action) {
            s.forEach(action);
        }
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            return s.removeIf(filter);
        }

        @Override
        public Spliterator<E> spliterator() {return s.spliterator();}
        @Override
        public Stream<E> stream()           {return s.stream();}
        @Override
        public Stream<E> parallelStream()   {return s.parallelStream();}

        private static final long serialVersionUID = 2454657854757543876L;

        private void readObject(java.io.ObjectInputStream stream)
            throws IOException, ClassNotFoundException
        {
            stream.defaultReadObject();
            s = m.keySet();
        }
    }
}

源碼內部封裝了一個Map；所有關於Set的操作都映射到了Map上。因此Set就可以使用ConcurrentHashMap 來完成線程安全的操作。