今天在线上出现一个UnsupportedOperationException导致接口报错：
java.lang.UnsupportedOperationException: null
at java.util.AbstractList.add(AbstractList.java:148) ~[na:1.8.0_77]
at java.util.AbstractList.add(AbstractList.java:108) ~[na:1.8.0_77]
at java.util.AbstractCollection.addAll(AbstractCollection.java:344) ~[na:1.8.0_77]
at com.souche.dawa.dao.slave.SlaveInfoDaoBySPI.getDateFlowByShop(SlaveInfoDaoBySPI.java:330) ~[classes/:na]
at com.souche.dawa.service.biz.FlowServiceImpl.getNearly30(FlowServiceImpl.java:35) ~[classes/:na]
at com.souche.dawa.json.api.MarketAPI.getNearly30Flow(MarketAPI.java:31) ~[classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_77]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_77]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_77]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_77]
at com.souche.optimus.core.engine.ParamResolverFactory.invoke(ParamResolverFactory.java:108) ~[optimus-core-1.4.0-SNAPSHOT.jar:na]
at com.souche.optimus.core.controller.JsonController.doRender(JsonController.java:95) ~[optimus-core-1.4.0-SNAPSHOT.jar:na]
at sun.reflect.GeneratedMethodAccessor139.invoke(Unknown Source) ~[na:na]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_77]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_77]
at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:221) ~[spring-web-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:136) ~[spring-web-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:114) ~[spring-webmvc-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827) ~[spring-webmvc-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738) ~[spring-webmvc-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) ~[spring-webmvc-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963) ~[spring-webmvc-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897) ~[spring-webmvc-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) [spring-webmvc-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861) [spring-webmvc-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:622) [servlet-api.jar:na]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) [spring-webmvc-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729) [servlet-api.jar:na]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292) [catalina.jar:8.0.44]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) [catalina.jar:8.0.44]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) [tomcat-websocket.jar:8.0.44]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) [catalina.jar:8.0.44]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) [catalina.jar:8.0.44]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197) [spring-web-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.2.RELEASE.jar:4.3.2.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) [catalina.jar:8.0.44]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) [catalina.jar:8.0.44]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212) [catalina.jar:8.0.44]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:94) [catalina.jar:8.0.44]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:504) [catalina.jar:8.0.44]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141) [catalina.jar:8.0.44]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) [catalina.jar:8.0.44]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88) [catalina.jar:8.0.44]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:502) [catalina.jar:8.0.44]
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1132) [tomcat-coyote.jar:8.0.44]
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684) [tomcat-coyote.jar:8.0.44]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1533) [tomcat-coyote.jar:8.0.44]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1489) [tomcat-coyote.jar:8.0.44]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_77]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_77]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-util.jar:8.0.44]
at java.lang.Thread.run(Thread.java:745) [na:1.8.0_77]定位到报错代码，发现是使用Collections.emptyList()来进行addAll()操作导致的。遂查看jdk源码：
/**
* Returns an empty list (immutable). This list is serializable.
*
* <p>This example illustrates the type-safe way to obtain an empty list:
* <pre>
* List<String> s = Collections.emptyList();
* </pre>
*
* @implNote
* Implementations of this method need not create a separate <tt>List</tt>
* object for each call. Using this method is likely to have comparable
* cost to using the like-named field. (Unlike this method, the field does
* not provide type safety.)
*
* @param <T> type of elements, if there were any, in the list
* @return an empty immutable list
*
* @see #EMPTY_LIST
* @since 1.5
*/
@SuppressWarnings("unchecked")
public static final <T> List<T> emptyList() {
return (List<T>) EMPTY_LIST;
}注释上明确表示这是一个不可变的list：Returns an empty list (immutable)。
/**
* The empty list (immutable). This list is serializable.
*
* @see #emptyList()
*/
@SuppressWarnings("rawtypes")
public static final List EMPTY_LIST = new EmptyList<>();而EmptyList是Collections类的一个静态内部类：
/**
* @serial include
*/
private static class EmptyList<E>
extends AbstractList<E>
implements RandomAccess, Serializable {
private static final long serialVersionUID = 8842843931221139166L;
public Iterator<E> iterator() {
return emptyIterator();
}
public ListIterator<E> listIterator() {
return emptyListIterator();
}
public int size() {return 0;}
public boolean isEmpty() {return true;}
public boolean contains(Object obj) {return false;}
public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
public Object[] toArray() { return new Object[0]; }
public <T> T[] toArray(T[] a) {
if (a.length > 0)
a[0] = null;
return a;
}
public E get(int index) {
throw new IndexOutOfBoundsException("Index: "+index);
}
public boolean equals(Object o) {
return (o instanceof List) && ((List<?>)o).isEmpty();
}
public int hashCode() { return 1; }
@Override
public boolean removeIf(Predicate<? super E> filter) {
Objects.requireNonNull(filter);
return false;
}
@Override
public void replaceAll(UnaryOperator<E> operator) {
Objects.requireNonNull(operator);
}
@Override
public void sort(Comparator<? super E> c) {
}
// Override default methods in Collection
@Override
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
}
@Override
public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }
// Preserves singleton property
private Object readResolve() {
return EMPTY_LIST;
}
}可以发现它继承了AbstractList，查看AbstractList类的源码，发现：public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E>
，且AbstractList并没有覆盖AbstractCollection的addAll(Collection<? extends E> c)方法，而AbstractCollection的方法如下：
public boolean addAll(Collection<? extends E> c) {
boolean modified = false;
for (E e : c)
if (add(e))
modified = true;
return modified;
}里面调用了add(e)，AbstractList是覆盖了这个方法的：
public boolean add(E e) {
add(size(), e);
return true;
}public void add(int index, E element) {
throw new UnsupportedOperationException();
}可以发现AbstractList默认的add行为会报出这个错误，但是EmptyList并没有覆盖addAll方法和add方法，所以会报错，这也是List的设计思路，父类提供默认的实现，子类根据需要去覆盖相应的方法提供自己的实现。