报错背景
Java代码偶然报错，但是正常情况下运行不会报错。
报错现象
exception: java.lang.reflect.InvocationTargetException: null
2020-05-11 at 15:17:39 CST traceId:[] ERROR io.netty.util.internal.logging.AbstractInternalLogger 91 error - Unexpected exception: java.lang.reflect.InvocationTargetException: null at sun.reflect.GeneratedMethodAccessor214.invoke(Unknown Source) ~[?:?] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_192] at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_192] at org.yeauty.pojo.PojoEndpointServer.doOnClose(PojoEndpointServer.java:121) [netty-websocket-spring-boot-starter-0.8.0.jar!/:?] at org.yeauty.standard.WebSocketServerHandler.channelInactive(WebSocketServerHandler.java:29) [netty-websocket-spring-boot-starter-0.8.0.jar!/:?] at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:257) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:243) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:236) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.handler.codec.ByteToMessageDecoder.channelInputClosed(ByteToMessageDecoder.java:393) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.handler.codec.ByteToMessageDecoder.channelInactive(ByteToMessageDecoder.java:358) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:257) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:243) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:236) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.channel.DefaultChannelPipeline$HeadContext.channelInactive(DefaultChannelPipeline.java:1416) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:257) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:243) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.channel.DefaultChannelPipeline.fireChannelInactive(DefaultChannelPipeline.java:912) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.channel.AbstractChannel$AbstractUnsafe$8.run(AbstractChannel.java:816) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:163) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:416) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:515) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:918) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) [netty-all-4.1.38.Final.jar!/:4.1.38.Final] at java.lang.Thread.run(Thread.java:748) [?:1.8.0_192] Caused by: java.util.ConcurrentModificationException at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445) ~[?:1.8.0_192] at java.util.HashMap$EntryIterator.next(HashMap.java:1479) ~[?:1.8.0_192] at java.util.HashMap$EntryIterator.next(HashMap.java:1477) ~[?:1.8.0_192] at com.jcdz.hbdservice.websocket.ServerWebSocket1.onClose(ServerWebSocket1.java:38) ~[coalminehbdservice-1.0.jar!/:1.0] ... 25 more
报错原因
报错原因有很多，我只说我接触到的这个原因：
（1）if(==) 判断语句出错
if(devMap.getOrDefault("locationcode","") == null)
devMap.get("locationcode")：有可能传过来是一个null，此时就会出现空指针的现象
查看 getOrDefault() 源码
1 /** 2 * Returns the value to which the specified key is mapped, or 3 * {@code defaultValue} if this map contains no mapping for the key. 4 * 5 * @implSpec 6 * The default implementation makes no guarantees about synchronization 7 * or atomicity properties of this method. Any implementation providing 8 * atomicity guarantees must override this method and document its 9 * concurrency properties. 10 * 11 * @param key the key whose associated value is to be returned 12 * @param defaultValue the default mapping of the key 13 * @return the value to which the specified key is mapped, or 14 * {@code defaultValue} if this map contains no mapping for the key 15 * @throws ClassCastException if the key is of an inappropriate type for 16 * this map 17 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 18 * @throws NullPointerException if the specified key is null and this map 19 * does not permit null keys 20 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 21 * @since 1.8 22 */ 23 default V getOrDefault(Object key, V defaultValue) { 24 V v; 25 return (((v = get(key)) != null) || containsKey(key)) 26 ? v 27 : defaultValue; 28 }
@throws NullPointerException如果指定的键为null并且此映射不允许使用空键
我们可以看到 getOrDefault() 方法里面有 get(key) 方法，此时如果key的值是一个null就会报错，例如 key:null。
map 传入值的时候要避免传入map.put(key：null)现象的发生，可以以map.put(key："")来代替。
（2）if(equals) 判断语句出错
if(!time.equals("1900-01-01 00:00:00"))
time：有可能传过来是一个null，此时就会出现空指针的现象
报错解决
修改判断语句的前后顺序，如下：
if(null == devMap.getOrDefault("locationcode",""))
if(!"1900-01-01 00:00:00".equals(time))