project/build.properties 指定sbt的版本

When a val is declared as lazy, its initialization is deferred until it is accessed for the first time.

三种类型的key

SettingKey[T]
  a key for a value computed once (the value is computed when loading the subproject, and kept around) 

TaskKey[T]
  a key for a value, called a task, that has to be recomputed each time, potentially with side effects.
  
InputKey[T]
  a key for a task that has command line arguments as input


内置  key
自定义 key


定义 Task 和 Settings

TaskKey[T] 用来定义task ，task是一些操作，比如 compile  package 等

settints可以直接在build.sbt中定义，不用写在.settings()中，这种定义称为bare style

bare  光秃秃的 裸露的 裸体的

ThisBuild / useCoursier := false





lazy val root = (project in file("."))
  .enablePlugins(plugin1, plugin2, ...)
  .disablePlugins(plugin1 , plugin1 ,...)
  .settings(
    slick := slick.value,
    Compile / run := {
      (xxxProject / Test / test).value
      (Compile / run).evaluated
    },
    excludeDependencies ++= Seq()
  ) 
  .aggregate(project1, project2, ...)
  .dependsOn(project1, project2, ...)
  
  

lazy val subProject1 = project in file("subProjectRoot/subProject1")
lazy val subProject2 = project in file("subProjectRoot/subProject2")


subproject 之间 可以独立 ，可以建立依赖

aggregate and classpath

Aggregation means that running a task on the aggregate project will also run it on the aggregated projects

lazy val root = (project in file("."))
    .aggregate(core, util)
    
lazy val util = project in file("util")
lazy val core = (project in file("core") ).dependsOn(util)

Library dependencies
非托管库 unmanaged dependencies 放在lib/路径下的jar包
托管库   managed dependencies  在构建定义中（build definition）配置，由sbt自动下载


unmanagedBase := baseDirectory.value / "custom_lib"

libraryDependencies ++= Seq(  
  "org.apache.spark" % "spark-core_2.10" % "1.6.0" % "provided",
  "org.apache.hadoop" % "hadoop-client" % "2.6.4" % "provided"
)

// 仓库
resolvers ++= Seq( 
  "Admonitor Repository" at "http://maven.mzsvn.com/repository/xxx",
  "Local Maven Repository" at "file:///home/xxx/.m2/repository"
)