java.lang.RuntimeException: Cannot reconnect.
    at org.jetbrains.idea.maven.server.RemoteObjectWrapper.perform(RemoteObjectWrapper.java:82)
    at org.jetbrains.idea.maven.server.MavenEmbedderWrapper.customizeForResolve(MavenEmbedderWrapper.java:42)
    at org.jetbrains.idea.maven.project.MavenProjectResolver.downloadSourcesAndJavadocs(MavenProjectResolver.java:256)
    at org.jetbrains.idea.maven.project.MavenProjectsProcessorArtifactsDownloadingTask.perform(MavenProjectsProcessorArtifactsDownloadingTask.java:40)
    at org.jetbrains.idea.maven.project.MavenProjectsProcessor.doProcessPendingTasks(MavenProjectsProcessor.java:146)
    at org.jetbrains.idea.maven.project.MavenProjectsProcessor$1.run(MavenProjectsProcessor.java:115)
    at org.jetbrains.idea.maven.utils.MavenUtil.lambda$runInBackground$6(MavenUtil.java:536)
    at com.intellij.util.RunnableCallable.call(RunnableCallable.java:20)
    at com.intellij.util.RunnableCallable.call(RunnableCallable.java:11)
    at com.intellij.openapi.application.impl.ApplicationImpl$1.call(ApplicationImpl.java:265)
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1$1.run(Executors.java:668)
    at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1$1.run(Executors.java:665)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1.run(Executors.java:665)
    at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.rmi.ConnectException: Connection refused to host: 127.0.0.1; nested exception is: 
    java.net.ConnectException: Connection refused (Connection refused)
    at java.rmi/sun.rmi.transport.tcp.TCPEndpoint.newSocket(TCPEndpoint.java:623)
    at java.rmi/sun.rmi.transport.tcp.TCPChannel.createConnection(TCPChannel.java:209)
    at java.rmi/sun.rmi.transport.tcp.TCPChannel.newConnection(TCPChannel.java:196)
    at java.rmi/sun.rmi.server.UnicastRef.invoke(UnicastRef.java:132)
    at java.rmi/java.rmi.server.RemoteObjectInvocationHandler.invokeRemoteMethod(RemoteObjectInvocationHandler.java:217)
    at java.rmi/java.rmi.server.RemoteObjectInvocationHandler.invoke(RemoteObjectInvocationHandler.java:162)
    at com.sun.proxy.$Proxy144.createEmbedder(Unknown Source)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at com.intellij.execution.rmi.RemoteUtil.invokeRemote(RemoteUtil.java:150)
    at com.intellij.execution.rmi.RemoteUtil.access$400(RemoteUtil.java:21)
    at com.intellij.execution.rmi.RemoteUtil$1.lambda$invoke$0(RemoteUtil.java:134)
    at com.intellij.openapi.util.ClassLoaderUtil.computeWithClassLoader(ClassLoaderUtil.java:31)
    at com.intellij.execution.rmi.RemoteUtil.executeWithClassLoader(RemoteUtil.java:202)
    at com.intellij.execution.rmi.RemoteUtil$1.invoke(RemoteUtil.java:134)
    at com.sun.proxy.$Proxy144.createEmbedder(Unknown Source)
    at org.jetbrains.idea.maven.server.MavenServerConnector.createEmbedder(MavenServerConnector.java:57)
    at org.jetbrains.idea.maven.server.MavenServerManager$4.create(MavenServerManager.java:448)
    at org.jetbrains.idea.maven.server.MavenServerManager$4.create(MavenServerManager.java:436)
    at org.jetbrains.idea.maven.server.RemoteObjectWrapper.getOrCreateWrappee(RemoteObjectWrapper.java:41)
    at org.jetbrains.idea.maven.server.MavenEmbedderWrapper.doCustomize(MavenEmbedderWrapper.java:83)
    at org.jetbrains.idea.maven.server.MavenEmbedderWrapper.lambda$customizeForResolve$0(MavenEmbedderWrapper.java:43)
    at org.jetbrains.idea.maven.server.RemoteObjectWrapper.perform(RemoteObjectWrapper.java:76)
    ... 17 more
Caused by: java.net.ConnectException: Connection refused (Connection refused)
    at java.base/java.net.PlainSocketImpl.socketConnect(Native Method)
    at java.base/java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:399)
    at java.base/java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:242)
    at java.base/java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:224)
    at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
    at java.base/java.net.Socket.connect(Socket.java:609)
    at java.base/java.net.Socket.connect(Socket.java:558)
    at java.base/java.net.Socket.<init>(Socket.java:454)
    at java.base/java.net.Socket.<init>(Socket.java:231)
    at com.intellij.execution.rmi.RemoteServer$1.createSocket(RemoteServer.java:174)
    at java.rmi/sun.rmi.transport.tcp.TCPEndpoint.newSocket(TCPEndpoint.java:617)
    ... 41 more

二、解决方案

settings—》Build，Execution，Deployment—》Build Tools—》Maven—》Importing—》JDK for importer选择你安装的JDK版本