mapPartitions(func) Similar to map, but runs separately on each partition (block) of the RDD, so func must be of type Iterator<T> => Iterator<U> when running on an RDD of type T.
mapPartitions
mapPartitions是map的一个变种。map的输入函数是应用于RDD中每个元素，而mapPartitions的输入函数是应用于每个分区，也就是把每个分区中的内容作为整体来处理的。
它的函数定义为：
def mapPartitions[U: ClassTag](f: Iterator[T] => Iterator[U], preservesPartitioning: Boolean = false): RDD[U]
f即为输入函数，它处理每个分区里面的内容。每个分区中的内容将以Iterator[T]传递给输入函数f，f的输出结果是Iterator[U]。最终的RDD由所有分区经过输入函数处理后的结果合并起来的。
举例：
1scala> val a = sc.parallelize(1 to 9, 3) 2scala> def myfunc[T](iter: Iterator[T]) : Iterator[(T, T)] = { 3 var res = List[(T, T)]() 4 var pre = iter.next while (iter.hasNext) { 5 val cur = iter.next; 6 res .::= (pre, cur) pre = cur; 7 } 8 res.iterator 9} 10scala> a.mapPartitions(myfunc).collect 11res0: Array[(Int, Int)] = Array((2,3), (1,2), (5,6), (4,5), (8,9), (7,8))
上述例子中的函数myfunc是把分区中一个元素和它的下一个元素组成一个Tuple。因为分区中最后一个元素没有下一个元素了，所以(3,4)和(6,7)不在结果中。
mapPartitions还有些变种，比如mapPartitionsWithContext，它能把处理过程中的一些状态信息传递给用户指定的输入函数。还有mapPartitionsWithIndex，它能把分区的index传递给用户指定的输入函数。
在scala eclipse中运行
1package yanan.spark.core.transformations.example 2 3import org.apache.spark.SparkConf 4import org.apache.spark.SparkContext 5 6object TransformationsTest { 7 def mapPartitionsFunc[T](iter: Iterator[T]): Iterator[(T, T)] = { 8 var res = List[(T, T)]() 9 var pre = iter.next 10 while (iter.hasNext) { 11 val cur = iter.next 12 res.::=(pre, cur) 13 pre = cur; 14 } 15 res.iterator 16 } 17 def mapPartitionsTest(sc: SparkContext) = { 18 val a = sc.parallelize(1 to 9, 3) 19 a.mapPartitions(mapPartitionsFunc).collect.foreach(println) 20 } 21 22 def main(args: Array[String]) { 23 val conf = new SparkConf().setAppName(s"Book example: Scala").setMaster("local[2]") 24 val sc = new SparkContext(conf) 25 mapPartitionsTest(sc) 26 sc.stop() 27 } 28}
pom.xml文件：
1<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 2 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> 3 <modelVersion>4.0.0</modelVersion> 4 5 <groupId>com.yanan.spark_maven</groupId> 6 <artifactId>spark1.3.1</artifactId> 7 <version>0.0.1-SNAPSHOT</version> 8 <packaging>jar</packaging> 9 10 <name>spark_maven</name> 11 <url>http://maven.apache.org</url> 12 13 <properties> 14 <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> 15 <jackson.version>1.9.13</jackson.version> 16 <!-- <spark.version>1.3.1</spark.version> --> 17 <spark.version>1.4.0</spark.version> 18 </properties> 19 20 <dependencies> 21 <dependency> 22 <groupId>junit</groupId> 23 <artifactId>junit</artifactId> 24 <version>3.8.1</version> 25 <scope>test</scope> 26 </dependency> 27 <dependency> 28 <groupId>org.scala-lang</groupId> 29 <artifactId>scala-library</artifactId> 30 <version>2.10.4</version> 31 </dependency> 32 <dependency> 33 <groupId>org.apache.spark</groupId> 34 <artifactId>spark-core_2.10</artifactId> 35 <version>${spark.version}</version> 36 </dependency> 37 <dependency> 38 <groupId>org.apache.spark</groupId> 39 <artifactId>spark-mllib_2.10</artifactId> 40 <version>${spark.version}</version> 41 </dependency> 42 <!--<dependency> <groupId>org.apache.spark</groupId> <artifactId>spark-sql_2.10</artifactId> 43 <version>1.3.1</version> </dependency> <dependency> <groupId>org.apache.spark</groupId> 44 <artifactId>spark-hive_2.10</artifactId> <version>1.3.1</version> </dependency> 45 <dependency> <groupId>org.apache.spark</groupId> <artifactId>spark-bagel_2.10</artifactId> 46 <version>1.3.1</version> </dependency> 47 <dependency> 48 <groupId>org.apache.spark</groupId> 49 <artifactId>spark-graphx_2.10</artifactId> 50 <version>1.3.1</version> 51 </dependency> --> 52 53 <!-- specify the version for json_truple <dependency> <groupId>org.codehaus.jackson</groupId> 54 <artifactId>jackson-core-asl</artifactId> <version>${jackson.version}</version> 55 </dependency> <dependency> <groupId>org.codehaus.jackson</groupId> <artifactId>jackson-mapper-asl</artifactId> 56 <version>${jackson.version}</version> </dependency> --> 57 58 </dependencies> 59 60 61 <build> 62 <plugins> 63 <plugin> 64 <groupId>org.scala-tools</groupId> 65 <artifactId>maven-scala-plugin</artifactId> 66 <executions> 67 <execution> 68 <goals> 69 <goal>compile</goal> 70 <goal>testCompile</goal> 71 </goals> 72 </execution> 73 </executions> 74 </plugin> 75 </plugins> 76 </build> 77 <pluginRepositories> 78 <pluginRepository> 79 <id>scala-tools.org</id> 80 <name>Scala-tools Maven2 Repository</name> 81 <url>http://scala-tools.org/repo-releases</url> 82 </pluginRepository> 83 </pluginRepositories> 84 85 <repositories> 86 <repository> 87 <id>cloudera-repo-releases</id> 88 <url>https://repository.cloudera.com/artifactory/repo/</url> 89 </repository> 90 </repositories> 91</project>