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Maven - Repositories

What is a Maven Repository?

In Maven terminology, a repository is a place i.e. directory where all the project jars, library jar, plugins or any other project specific artifacts are stored and can be used by Maven easily.

Maven repository are of three types

Local Repository

Maven local repository is a folder location on your machine. It gets created when you run any maven command for the first time.

Maven local repository keeps your project"s all dependencies (library jars, plugin jars etc). When you run a Maven build, then Maven automatically downloads all the dependency jars into the local repository.It helps to avoid references to dependencies stored on remote machine every time a project is build.

Maven local repository by default get created by Maven in %USER_HOME% directory. To override the default location, mention another path in Maven settings.xml file available at %M2_HOME%conf directory. 

<settings xmlns="http://maven.apache.org/SETTINGS/1.0.0"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.0.0 
   http://maven.apache.org/xsd/settings-1.0.0.xsd">
      <localRepository>C:/MyLocalRepository</localRepository>
</settings>

When you run Maven command, Maven will download dependencies to your custom path.

Central Repository

Maven central repository is repository provided by Maven community. It contains a large number of commonly used libraries.

When Maven does not find any dependency in local repository, it starts searching in central repository using following URL: http://repo1.maven.org/maven2/

Key concepts of Central repository

要浏览中央maven仓库的内容，maven社区提供了一个URL：http : //search.maven.org/#browse。使用此库，开发人员可以搜索中央存储库中的所有可用库。

远程存储库

有时候，Maven在中央存储库中没有找到提到的依赖关系，因此它停止了创建过程和输出错误消息到控制台。为了防止这种情况，Maven提供了远程存储库的概念，这是开发人员自己的定制存储库，其中包含所需的库或其他项目jar。

例如，使用下面提到的POM.xml，Maven将从同一个pom.xml中提到的远程存储库中下载依赖关系（在中央存储库中不可用）。

<project xmlns="http://maven.apache.org/POM/4.0.0"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
   http://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>
   <groupId>com.companyname.projectgroup</groupId>
   <artifactId>project</artifactId>
   <version>1.0</version>
   <dependencies>
      <dependency>
         <groupId>com.companyname.common-lib</groupId>
         <artifactId>common-lib</artifactId>
         <version>1.0.0</version>
      </dependency>
   <dependencies>
   <repositories>
      <repository>
         <id>companyname.lib1</id>
         <url>http://download.companyname.org/maven2/lib1</url>
      </repository>
      <repository>
         <id>companyname.lib2</id>
         <url>http://download.companyname.org/maven2/lib2</url>
      </repository>
   </repositories>
</project>

Maven依赖搜索序列

当我们执行Maven创建命令时，Maven将按照以下顺序开始查找依赖库：

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