原文地址：http://blog.csdn.net/oopsoom/article/details/41318599
一、问题定义
最近在用sbt打assembly包时出现问题，在package的时候，发生jar包冲突/文件冲突问题，两个相同的class来自不同的jar包在classpath内引起冲突。
具体是：我有一个self4j的jar， 还有一个hadoop-common-hdfs的jar包，其中hadoop-common-hdfs.jar内包含了self4j这个jar包，导致冲突。
此类异常一般是由于打包不规范和打包疏忽引起的。
（个人认为正确的打包策略是：只打包自己核心功能，不将依赖打包在一起，但是有时为了方便或者有时不得不打在一起，要注意可能会出现上述问题）
异常log如下：
[trace] Stack trace suppressed: run last *:assembly for the full output.
[error] (*:assembly) deduplicate: different file contents found in the following:
[error] C:\Users\shengli.victor\.ivy2\cache\org.slf4j\slf4j-api\jars\slf4j-api-1.7.7.jar:org/slf4j/IMarkerFactory.class
[error] C:\Users\shengli.victor\.ivy2\cache\com.xxx.xx.hdfsfile\hdfscommon\jars\hdfscommon-1.1.jar:org/slf4j/IMarkerFactory.class
[error] Total time: 4 s, completed 2014-11-20 19:07:33
异常很明显，来自2个不同的jar包self4j, hdfscommon-1.1.jar里，在org/slf4j/IMarkerFactory.class这个类冲突了。
如下图：
二、解决方案
解决jar包冲突有两种方案：
1、删除其中的某个jar,或者说，在打包的时候，不将2个相同的class打在同一个jar包内的classpath内，即exclude jar。
2、合并冲突
1. Excluding JARs and files
% "provided"
将相同的jar中排除一个，因为重复，可以使用"provided"关键字。
例如spark是一个容器类，编写spark应用程序我们需要spark core jar. 但是真正打包提交到集群上执行，则不需要将它打入jar包内。
这是我们使用 % "provided" 关键字来exclude它。
libraryDependencies ++= Seq(
"org.apache.spark" %% "spark-core" % "0.8.0-incubating" % "provided",
"org.apache.hadoop" % "hadoop-client" % "2.0.0-cdh4.4.0" % "provided"
)
Maven defines "provided" as:
This is much like compile, but indicates you expect the JDK or a container to provide the dependency at runtime. For example, when building a web application for the Java Enterprise Edition, you would set the dependency on the Servlet API and related Java EE APIs to scopeprovided because the web container provides those classes. This scope is only available on the compilation and test classpath, and is not transitive.
2、Merge Strategy
如果在相对路径下，有多个相同的文件或者jar，这时我们可以使用Merge策略。
在build.sbt中对assemblyMergeStrategy 进行定义。
如下：
mergeStrategy in assembly <<= (mergeStrategy in assembly) { (old) =>
{
case PathList("org", "slf4j", xs @ _*) => MergeStrategy.first
case PathList(ps @ _*) if ps.last endsWith "axiom.xml" => MergeStrategy.filterDistinctLines
case PathList(ps @ _*) if ps.last endsWith "Log$Logger.class" => MergeStrategy.first
case PathList(ps @ _*) if ps.last endsWith "ILoggerFactory.class" => MergeStrategy.first
case x => old(x)
}
}
解决方法：将org, slf4j 这个下的所有类和文件，都做合并， 采用的策略是：在classpath里，2选1，选的是classpath顺序里第一个self4j。
这里支持多种格式，例如ps.lat endsWith "axiom.xml" ，是以axiom.xml为结尾的文件，都采用filterDistinctLines策略，即合并两个文件，舍去重复的部分。
通过以上修改，终于解决了slf4j冲突的问题，即deduplicate: different file contents found in the following问题。
再次sbt assembly:
[warn] Merging 'META-INF\INDEX.LIST' with strategy 'discard'
[warn] Merging 'META-INF\MANIFEST.MF' with strategy 'discard'
[warn] Merging 'META-INF\maven\log4j\log4j\pom.properties' with strategy 'first'
[warn] Merging 'META-INF\maven\log4j\log4j\pom.xml' with strategy 'first'
[warn] Merging 'META-INF\maven\org.slf4j\slf4j-api\pom.properties' with strategy 'first'
[warn] Merging 'META-INF\maven\org.slf4j\slf4j-api\pom.xml' with strategy 'first'
[warn] Merging 'META-INF\maven\org.slf4j\slf4j-log4j12\pom.properties' with strategy 'first'
[warn] Merging 'META-INF\maven\org.slf4j\slf4j-log4j12\pom.xml' with strategy 'first'
[warn] Merging 'com\esotericsoftware\minlog\Log$Logger.class' with strategy 'first'
[warn] Merging 'com\esotericsoftware\minlog\Log.class' with strategy 'first'
[warn] Merging 'org\apache\log4j\helpers\LogLog.class' with strategy 'first'
[warn] Merging 'org\slf4j\ILoggerFactory.class' with strategy 'first'
[warn] Merging 'org\slf4j\IMarkerFactory.class' with strategy 'first'
[warn] Merging 'org\slf4j\Logger.class' with strategy 'first'
[warn] Merging 'org\slf4j\LoggerFactory.class' with strategy 'first'
[warn] Merging 'org\slf4j\MDC.class' with strategy 'first'
[warn] Merging 'org\slf4j\Marker.class' with strategy 'first'
[warn] Merging 'org\slf4j\MarkerFactory.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\BasicMDCAdapter.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\BasicMarker.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\BasicMarkerFactory.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\FormattingTuple.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\MarkerIgnoringBase.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\MessageFormatter.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\NOPLogger.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\NOPLoggerFactory.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\NOPMDCAdapter.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\NamedLoggerBase.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\SubstituteLoggerFactory.class' with strategy 'first'
[warn] Merging 'org\slf4j\helpers\Util.class' with strategy 'first'
[warn] Merging 'org\slf4j\impl\Log4jLoggerAdapter.class' with strategy 'first'
[warn] Merging 'org\slf4j\impl\Log4jLoggerFactory.class' with strategy 'first'
[warn] Merging 'org\slf4j\impl\Log4jMDCAdapter.class' with strategy 'first'
[warn] Merging 'org\slf4j\impl\StaticLoggerBinder.class' with strategy 'first'
[warn] Merging 'org\slf4j\impl\StaticMDCBinder.class' with strategy 'first'
[warn] Merging 'org\slf4j\impl\StaticMarkerBinder.class' with strategy 'first'
[warn] Merging 'org\slf4j\spi\LocationAwareLogger.class' with strategy 'first'
[warn] Merging 'org\slf4j\spi\LoggerFactoryBinder.class' with strategy 'first'
[warn] Merging 'org\slf4j\spi\MDCAdapter.class' with strategy 'first'
[warn] Merging 'org\slf4j\spi\MarkerFactoryBinder.class' with strategy 'first'
[warn] Merging 'rootdoc.txt' with strategy 'concat'
[warn] Strategy 'concat' was applied to a file
[info] Strategy 'deduplicate' was applied to 373 files (Run the task at debug level to see details)
[warn] Strategy 'discard' was applied to 2 files
[warn] Strategy 'first' was applied to 38 files
[java] view plain copy 在CODE上查看代码片派生到我的代码片
[info] Done packaging.
[success] Total time: 84 s, completed 2014-11-20 19:04:52
合并策略有很多种：
可以参考官方sbt assembly文档：https://github.com/sbt/sbt-assembly
http://stackoverflow.com/questions/19606243/resolving-dependencies-in-creating-jar-through-sbt-assembly
MergeStrategy.deduplicate is the default described above
MergeStrategy.first picks the first of the matching files in classpath order
MergeStrategy.last picks the last one
MergeStrategy.singleOrError bails out with an error message on conflict
MergeStrategy.concat simply concatenates all matching files and includes the result
MergeStrategy.filterDistinctLines also concatenates, but leaves out duplicates along the way
MergeStrategy.rename renames the files originating from jar files
MergeStrategy.discard simply discards matching files
更多的写法，example：
assemblyMergeStrategy in assembly := {
case PathList("javax", "servlet", xs @ _*) => MergeStrategy.first
case PathList(ps @ _*) if ps.last endsWith ".html" => MergeStrategy.first
case "application.conf" => MergeStrategy.concat
case "unwanted.txt" => MergeStrategy.discard
case x =>
val oldStrategy = (assemblyMergeStrategy in assembly).value
oldStrategy(x)
}
Final Qucik Hack：
如果以上写法都不奏效，还有最好一种，强制默认全部合并，不到万不得已，不要用。。
mergeStrategy in assembly <<= (mergeStrategy in assembly) { mergeStrategy => {
case entry => {
val strategy = mergeStrategy(entry)
if (strategy == MergeStrategy.deduplicate) MergeStrategy.first
else strategy
}
}}
三、总结
碰到类似的问题不要慌张，仔细看log描述的是什么意思。
异常报出内容冗余的冲突，在看路径，发现在classpath内有完全相同的2个类，这是导致问题的根本原因。
找出原因，解决方发，消除冲突两种方法，一直是去除法，另一种是合并法。
相对于maven和gradle，sbt的冲突解决方法还是比较接近底层。如果没记错的话，maven和gradle都能自动解决冲突。
本文仅针对该问题提出解决方案和思路，具体的各个配置还需要继续研究。
