昨晚测试环境报了一个错误，

exceptionHandler, error:java.lang.UnsupportedOperationException
    at java.util.AbstractList.add(AbstractList.java:148)
    at java.util.AbstractList.add(AbstractList.java:108)
    at java.util.AbstractCollection.addAll(AbstractCollection.java:344)
    at com.xxxxxx.services.facade.impl.HomXXXXFacadeImpl.projecDistributionState(HomeXXXXFacadeImpl.java:190)
    at com.xxxxx.api.controller.HomePageController.projecXXXXXXXXState(HomeXXXController.java:59)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

看到这个错误第一时间就想到了Arrays.asList生成的list执行添加删除操作导致的报错，但转念一想，这块代码运行几个月了都没报错，不会啊。点进方法一看，好家伙，还真改了。

我们进源码看看,可以看到此处的ArrayList可不是我们理解的Arraylist,这个ArrayList是一个静态内部类，和java.util.ArrayList不同这个是一个低配版的，只有初始化和一些获取属性方法的ArrayList,也是基于数组实现，但是不能添加和删除。

package java.util;
/**
 * @author Josh Bloch
 * @author Neal Gafter
 * @author John Rose
 * @since  1.2
 */
public class Arrays {
    public static <T> List<T> asList(T... a) {
        return new ArrayList<>(a);
    }
     /**
     * @serial include
     */
    private static class ArrayList<E> extends AbstractList<E>
        implements RandomAccess, java.io.Serializable
    {
        private static final long serialVersionUID = -2764017481108945198L;
        private final E[] a;

        ArrayList(E[] array) {
            a = Objects.requireNonNull(array);
        }

        @Override
        public int size() {
            return a.length;
        }

        @Override
        public Object[] toArray() {
            return a.clone();
        }

        @Override
        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            // xxxx
        }

        @Override
        public E get(int index) {
            // xxxx
        }

        @Override
        public E set(int index, E element) {
            // xxxx
        }

        @Override
        public int indexOf(Object o) {
            E[] a = this.a;
           // xxxx
        }

        @Override
        public boolean contains(Object o) {
            // xxxx
        }

        @Override
        public Spliterator<E> spliterator() {
            // xxxx
        }

        @Override
        public void forEach(Consumer<? super E> action) {
           // xxxx action.accept(e);
        }

        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            // xxxx
        }

        @Override
        public void sort(Comparator<? super E> c) {
        // xxxx
    }

}

与Arrays.asList穿一条裤子的还有Collections.singletonList(arr),这个也是final修饰，也不支持添加删除操作。

package java.util;
public class Collections {
    // Suppresses default constructor, ensuring non-instantiability.
    private Collections() {
    }
    public static <T> List<T> singletonList(T o) {
        return new SingletonList<>(o);
    }

    /**
     * @serial include
     */
    private static class SingletonList<E>
        extends AbstractList<E>
        implements RandomAccess, Serializable {

        private static final long serialVersionUID = 3093736618740652951L;

        private final E element;

        SingletonList(E obj)                {element = obj;}

        public Iterator<E> iterator() {
            return singletonIterator(element);
        }

        public int size()                   {return 1;}

        public boolean contains(Object obj) {return eq(obj, element);}

        public E get(int index) {
            if (index != 0)
              throw new IndexOutOfBoundsException("Index: "+index+", Size: 1");
            return element;
        }

        // Override default methods for Collection
        @Override
        public void forEach(Consumer<? super E> action) {
            action.accept(element);
        }
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void sort(Comparator<? super E> c) {
        }
        @Override
        public Spliterator<E> spliterator() {
            return singletonSpliterator(element);
        }
    }
}