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强制更新Maven 依赖

有时候依赖的jar包始终没法更新，可以强制更新

命令行方式

mvn clean install -U #强制刷新本地合库不存在的release版和所有的snapshots版本jar包。

-U,–update-snapshots Forces a check for missing releases and updated snapshots on remote repositories

IDEA方式

打开IDEA Settings（Mac上是IDEA Preferences），找到Maven，勾选“Always update snapshots"。再用Maven / Reimport来更新依赖。

settings.xml方式

参考Maven 3.6.0 settings.xml 修改settings.xml，在repository和pluginRepository下配置总是更新Maven snapshots依赖：

<snapshots>
  <enabled>true</enabled>
  <updatePolicy>always</updatePolicy>
</snapshots>

enabled: Whether to use this repository for downloading this type of artifact. Default value is: true.
updatePolicy: The frequency for downloading updates - can be “always”, “daily” (default), “interval:XXX” (in minutes) or “never” (only if it doesn’t exist locally). Each repository in the project has its own update policy:
- always - always check when Maven is started for newer versions of snapshots
- never - never check for newer remote versions. Once off manual updates can be performed.
- daily (default) - check on the first run of the day (local time)
- interval:XXX - check every XXX minutes

终极方法

将本地Maven缓存（~/.m2/repository）的snapshot依赖删除掉，再重新更新依赖。

parent项目使用dependencyManagement统一管理依赖的版本号

如果在parent项目中用dependencyManagement来统一管理依赖的版本号，需要检查本地Maven repository（~/.m2/repository)下的parent项目的.pom文件是否是最新的。

参考文档

https://stackoverflow.com/questions/3805329/how-does-the-updatepolicy-in-maven-really-work https://javabeat.net/maven-updatepolicy/ https://stackoverflow.com/questions/4701532/how-to-force-maven-update http://maven.apache.org/ref/3.6.0/maven-settings/settings.html#class_repository

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