java中如何创建不可变并且static的静态集合

最新推荐文章于 2024-09-01 13:57:44 发布

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最新推荐文章于 2024-09-01 13:57:44 发布

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文章标签： java 数据库

原文链接：https://my.oschina.net/MjShao/blog/2872388

为什么80%的码农都做不了架构师？>>>

以map为例

findbug错误提示：

MS_MUTABLE_COLLECTION_PKGPROTECT, Priority: Normal

XX.XX.XXX.XX is a mutable collection which should be package protected

A mutable collection instance is assigned to a final static field, thus can be changed by malicious code or by accident from another package. The field could be made package protected to avoid this vulnerability. Alternatively you may wrap this field into Collections.unmodifiableSet/List/Map/etc. to avoid this vulnerability.

错误案例：

public class TestMap {
  private static final Map<Integer,String> map = new LinkedHashMap<Integer, String>();
  static {
    map = new HashMap();
    map.put(1, "one");
    map.put(2, "two");
  }
}

正确的做法 (通过Collections.unmodifiableMap)

public class TestMap {
  private static final Map<Integer,String> map ;
  static {
    Map<Integer,String> tempMap  = new HashMap();
    tempMap.put(1, "one");
    tempMap.put(2, "two");
    map = Collections.unmodifiableMap(tempMap);
  }
}

如果现在往map里添加元素，则抛出UnsupportedOperationException异常

一下附上一段源码

/**
         * We need this class in addition to UnmodifiableSet as
         * Map.Entries themselves permit modification of the backing Map
         * via their setValue operation.  This class is subtle: there are
         * many possible attacks that must be thwarted.
         *
         * @serial include
         */
        static class UnmodifiableEntrySet<K,V>
            extends UnmodifiableSet<Map.Entry<K,V>> {
            private static final long serialVersionUID = 7854390611657943733L;

            @SuppressWarnings({"unchecked", "rawtypes"})
            UnmodifiableEntrySet(Set<? extends Map.Entry<? extends K, ? extends V>> s) {
                // Need to cast to raw in order to work around a limitation in the type system
                super((Set)s);
            }

            static <K, V> Consumer<Map.Entry<K, V>> entryConsumer(Consumer<? super Entry<K, V>> action) {
                return e -> action.accept(new UnmodifiableEntry<>(e));
            }

            public void forEach(Consumer<? super Entry<K, V>> action) {
                Objects.requireNonNull(action);
                c.forEach(entryConsumer(action));
            }

            static final class UnmodifiableEntrySetSpliterator<K, V>
                    implements Spliterator<Entry<K,V>> {
                final Spliterator<Map.Entry<K, V>> s;

                UnmodifiableEntrySetSpliterator(Spliterator<Entry<K, V>> s) {
                    this.s = s;
                }

                @Override
                public boolean tryAdvance(Consumer<? super Entry<K, V>> action) {
                    Objects.requireNonNull(action);
                    return s.tryAdvance(entryConsumer(action));
                }

                @Override
                public void forEachRemaining(Consumer<? super Entry<K, V>> action) {
                    Objects.requireNonNull(action);
                    s.forEachRemaining(entryConsumer(action));
                }

                @Override
                public Spliterator<Entry<K, V>> trySplit() {
                    Spliterator<Entry<K, V>> split = s.trySplit();
                    return split == null
                           ? null
                           : new UnmodifiableEntrySetSpliterator<>(split);
                }
				..................

转载于:https://my.oschina.net/MjShao/blog/2872388