java.lang.UnsupportedOperationException分析及解决办法
作者：仙草不加料
今天在写业务的时候，需要对从数据库返回的List集合根据对象属性进行排序，那么常规的做法就是使用Collections的sort方法，实现Compartor接口重写compare方法，或者直接使用该list的sort方法，但是无论使用那种方法都遇到了这样的报错原因：
java.lang.UnsupportedOperationException: null
at java.util.Collections$UnmodifiableList.sort(Collections.java:1333) ~[na:1.8.0_331]
at java.util.Collections.sort(Collections.java:177) ~[na:1.8.0_331]
无法支持的操作，再看报错第二行UnmodifiableList，这是一个不可变的集合，它继承 UnmodifiableCollection类，UnmodifiableCollection 中涉及到元素改动（新增、删除、清空…）的方法都直接抛出 UnsupportedOperationException 异常，并不改动元素；Iterator 中涉及到元素修改的方法也一样不进行元素的改动。
那么是不是把这个集合变成可变的集合问题是不就解决了呢？
list = new ArrayList<>(list);
在排序前给这个list再次封装，问题得以解决。
static class UnmodifiableList<E> extends UnmodifiableCollection<E>
implements List<E> {
private static final long serialVersionUID = -283967356065247728L;
final List<? extends E> list;
UnmodifiableList(List<? extends E> list) {
super(list);
this.list = list;
}
public boolean equals(Object o) {return o == this || list.equals(o);}
public int hashCode() {return list.hashCode();}
public E get(int index) {return list.get(index);}
public E set(int index, E element) {
throw new UnsupportedOperationException();
}
public void add(int index, E element) {
throw new UnsupportedOperationException();
}
public E remove(int index) {
throw new UnsupportedOperationException();
}
public int indexOf(Object o) {return list.indexOf(o);}
public int lastIndexOf(Object o) {return list.lastIndexOf(o);}
public boolean addAll(int index, Collection<? extends E> c) {
throw new UnsupportedOperationException();
}
@Override
public void replaceAll(UnaryOperator<E> operator) {
throw new UnsupportedOperationException();
}
@Override
public void sort(Comparator<? super E> c) {
throw new UnsupportedOperationException();
}
public ListIterator<E> listIterator() {return listIterator(0);}
public ListIterator<E> listIterator(final int index) {
return new ListIterator<E>() {
private final ListIterator<? extends E> i
= list.listIterator(index);
public boolean hasNext() {return i.hasNext();}
public E next() {return i.next();}
public boolean hasPrevious() {return i.hasPrevious();}
public E previous() {return i.previous();}
public int nextIndex() {return i.nextIndex();}
public int previousIndex() {return i.previousIndex();}
public void remove() {
throw new UnsupportedOperationException();
}
public void set(E e) {
throw new UnsupportedOperationException();
}
public void add(E e) {
throw new UnsupportedOperationException();
}
@Override
public void forEachRemaining(Consumer<? super E> action) {
i.forEachRemaining(action);
}
};
}
public List<E> subList(int fromIndex, int toIndex) {
return new UnmodifiableList<>(list.subList(fromIndex, toIndex));
}
/**
* UnmodifiableRandomAccessList instances are serialized as
* UnmodifiableList instances to allow them to be deserialized
* in pre-1.4 JREs (which do not have UnmodifiableRandomAccessList).
* This method inverts the transformation. As a beneficial
* side-effect, it also grafts the RandomAccess marker onto
* UnmodifiableList instances that were serialized in pre-1.4 JREs.
*
* Note: Unfortunately, UnmodifiableRandomAccessList instances
* serialized in 1.4.1 and deserialized in 1.4 will become
* UnmodifiableList instances, as this method was missing in 1.4.
*/
private Object readResolve() {
return (list instanceof RandomAccess
? new UnmodifiableRandomAccessList<>(list)
: this);
}
}这里从UnmodifiableList的源码来分析：除了equals、hashCode、get、indexOf、lastIndexOf等方法外，其他方法都会抛出UnsupportedOperationException()异常，而这些方法的共同点都是返回该list的属性，说明UnmodifiableList是一个只读的集合，因此对这个List不能进行添加或删除元素等操作。
如果需要将list转化为不可变的集合，Collections也提供了这样的方法，最终转化为UnmodifiableList。
public static <T> List<T> unmodifiableList(List<? extends T> list) {
return (list instanceof RandomAccess ?
new UnmodifiableRandomAccessList<>(list) :
new UnmodifiableList<>(list));
}同样的 Collections 集合类中的 unmodifiableMap、unmodifiableSet也是只读内部类。它们可以在多线程环境下使用，或者在需要保护列表免受修改的场景下使用。
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