先把问题解决掉：更换较低版本maven(<=3.6.1)
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IntelliJ IDEA从 2019.3 更新到 2020.1+ 后，旧项目全线飘红，无法加载依赖。
明明External Libraries下都加载了相关依赖，但很多类却还是报“Cannot resolve symbol”？？？查看日志发现以下异常：
yyyy-MM-dd HH:mm:ss [ xxxxxx] INFO - #org.jetbrains.idea.maven -
java.lang.NullPointerException
at org.apache.maven.model.interpolation.StringSearchModelInterpolator$InterpolateObjectAction$StringField.doInterpolate(StringSearchModelInterpolator.java:372)
at org.apache.maven.model.interpolation.StringSearchModelInterpolator$InterpolateObjectAction$CacheField.interpolate(StringSearchModelInterpolator.java:329)
at org.apache.maven.model.interpolation.StringSearchModelInterpolator$InterpolateObjectAction$CacheItem.interpolate(StringSearchModelInterpolator.java:305)
at org.apache.maven.model.interpolation.StringSearchModelInterpolator$InterpolateObjectAction.traverseObjectWithParents(StringSearchModelInterpolator.java:178)
at org.apache.maven.model.interpolation.StringSearchModelInterpolator$InterpolateObjectAction.run(StringSearchModelInterpolator.java:154)
at java.security.AccessController.doPrivileged(Native Method)
at org.apache.maven.model.interpolation.StringSearchModelInterpolator.interpolateObject(StringSearchModelInterpolator.java:83)
at org.apache.maven.model.interpolation.StringSearchModelInterpolator.interpolateModel(StringSearchModelInterpolator.java:70)
at org.apache.maven.model.building.DefaultModelBuilder.interpolateModel(DefaultModelBuilder.java:789)
at org.apache.maven.model.building.DefaultModelBuilder.build(DefaultModelBuilder.java:393)
at org.apache.maven.repository.internal.DefaultArtifactDescriptorReader.loadPom(DefaultArtifactDescriptorReader.java:292)
at org.apache.maven.repository.internal.DefaultArtifactDescriptorReader.readArtifactDescriptor(DefaultArtifactDescriptorReader.java:171)
at org.eclipse.aether.internal.impl.collect.DefaultDependencyCollector.resolveCachedArtifactDescriptor(DefaultDependencyCollector.java:541)
at org.eclipse.aether.internal.impl.collect.DefaultDependencyCollector.getArtifactDescriptorResult(DefaultDependencyCollector.java:524)
at org.eclipse.aether.internal.impl.collect.DefaultDependencyCollector.processDependency(DefaultDependencyCollector.java:412)
at org.eclipse.aether.internal.impl.collect.DefaultDependencyCollector.processDependency(DefaultDependencyCollector.java:365)
at org.eclipse.aether.internal.impl.collect.DefaultDependencyCollector.process(DefaultDependencyCollector.java:352)
at org.eclipse.aether.internal.impl.collect.DefaultDependencyCollector.collectDependencies(DefaultDependencyCollector.java:254)
at org.eclipse.aether.internal.impl.DefaultRepositorySystem.collectDependencies(DefaultRepositorySystem.java:284)
at org.apache.maven.project.DefaultProjectDependenciesResolver.resolve(DefaultProjectDependenciesResolver.java:169)
at org.jetbrains.idea.maven.server.Maven3XServerEmbedder.resolveDependencies(Maven3XServerEmbedder.java:788)
at org.jetbrains.idea.maven.server.Maven3XServerEmbedder.access$700(Maven3XServerEmbedder.java:115)
at org.jetbrains.idea.maven.server.Maven3XServerEmbedder$3.run(Maven3XServerEmbedder.java:722)
at org.jetbrains.idea.maven.server.Maven3ServerEmbedder$2.run(Maven3ServerEmbedder.java:347)
at org.jetbrains.idea.maven.server.Maven3ServerEmbedder.executeWithSessionScope(Maven3ServerEmbedder.java:422)
at org.jetbrains.idea.maven.server.Maven3ServerEmbedder.executeWithMavenSession(Maven3ServerEmbedder.java:355)
at org.jetbrains.idea.maven.server.Maven3ServerEmbedder.executeWithMavenSession(Maven3ServerEmbedder.java:344)
at org.jetbrains.idea.maven.server.Maven3XServerEmbedder.doResolveProject(Maven3XServerEmbedder.java:664)
at org.jetbrains.idea.maven.server.Maven3XServerEmbedder.resolveProject(Maven3XServerEmbedder.java:626)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:361)
at sun.rmi.transport.Transport$1.run(Transport.java:200)
at sun.rmi.transport.Transport$1.run(Transport.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at sun.rmi.transport.Transport.serviceCall(Transport.java:196)
at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:568)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:826)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:683)
at java.security.AccessController.doPrivileged(Native Method)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:682)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
这种情况，首先怀疑maven配置问题，一番对比后并没有发现什么问题，唯一不同就是maven默认版本号不同。
2019.3 --> 3.6.1
2020.1 --> 3.6.3
所以猜测是较新版本maven带入了新的bug。更换较低版本后恢复正常~
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以上是表现和解决方案，具体原因欢迎指出^_^
