一、前置知識詳解
Spark SQL重要是操作DataFrame，DataFrame本身提供了save和load的操作，
Load：可以創建DataFrame，
Save：把DataFrame中的數據保存到文件或者說與具體的格式來指明我們要讀取的文件的類型以及與具體的格式來指出我們要輸出的文件是什么類型。
二、Spark SQL讀寫數據代碼實戰
import org.apache.spark.SparkConf;import org.apache.spark.api.java.JavaRDD;import org.apache.spark.api.java.JavaSparkContext;import org.apache.spark.api.java.function.Function;import org.apache.spark.sql.*;import org.apache.spark.sql.types.DataTypes;import org.apache.spark.sql.types.StructField;import org.apache.spark.sql.types.StructType;import java.util.ArrayList;import java.util.List;public class SparkSQLLoadSaveOps { public static void main(String[] args) { SparkConf conf = new SparkConf().setMaster("local").setAppName("SparkSQLLoadSaveOps"); JavaSparkContext sc = new JavaSparkContext(conf); SQLContext = new SQLContext(sc); /** * read()是DataFrameReader類型，load可以將數據讀取出來 */ DataFrame peopleDF = sqlContext.read().format("json").load("E://Spark//Sparkinstanll_package//Big_Data_Software//spark-1.6.0-bin-hadoop2.6//examples//src//main//resources//people.json"); /** * 直接對DataFrame進行操作 * Json: 是一種自解釋的格式，讀取Json的時候怎么判斷其是什么格式？ * 通過掃描整個Json。掃描之后才會知道元數據 */ //通過mode來指定輸出文件的是append。創建新文件來追加文件 peopleDF.select("name").write().mode(SaveMode.Append).save("E://personNames"); }} 讀取過程源碼分析如下：
1. read方法返回DataFrameReader，用于讀取數據。
/** * :: Experimental :: * Returns a [[DataFrameReader]] that can be used to read data in as a [[DataFrame]]. * {{{ * sqlContext.read.parquet("/path/to/file.parquet") * sqlContext.read.schema(schema).json("/path/to/file.json") * }}} * * @group genericdata * @since 1.4.0 */@Experimental//創建DataFrameReader實例，獲得了DataFrameReader引用def read: DataFrameReader = new DataFrameReader(this)2. 然后再調用DataFrameReader類中的format，指出讀取文件的格式。
/** * Specifies the input data source format. * * @since 1.4.0 */def format(source: String): DataFrameReader = { this.source = source this}3. 通過DtaFrameReader中load方法通過路徑把傳入過來的輸入變成DataFrame。
/** * Loads input in as a [[DataFrame]], for data sources that require a path (e.g. data backed by * a local or distributed file system). * * @since 1.4.0 */// TODO: Remove this one in Spark 2.0.def load(path: String): DataFrame = { option("path", path).load()} 至此，數據的讀取工作就完成了，下面就對DataFrame進行操作。
下面就是寫操作?。?！
1. 調用DataFrame中select函數進行對列篩選
/** * Selects a set of columns. This is a variant of `select` that can only select * existing columns using column names (i.e. cannot construct expressions). * * {{{ * // The following two are equivalent: * df.select("colA", "colB") * df.select($"colA", $"colB") * }}} * @group dfops * @since 1.3.0 */@scala.annotation.varargsdef select(col: String, cols: String*): DataFrame = select((col +: cols).map(Column(_)) : _*)2. 然后通過write將結果寫入到外部存儲系統中。
/** * :: Experimental :: * Interface for saving the content of the [[DataFrame]] out into external storage. * * @group output * @since 1.4.0 */@Experimentaldef write: DataFrameWriter = new DataFrameWriter(this)
3. 在保持文件的時候mode指定追加文件的方式
/** * Specifies the behavior when data or table already exists. Options include:// Overwrite是覆蓋 * - `SaveMode.Overwrite`: overwrite the existing data.//創建新的文件，然后追加 * - `SaveMode.Append`: append the data. * - `SaveMode.Ignore`: ignore the operation (i.e. no-op). * - `SaveMode.ErrorIfExists`: default option, throw an exception at runtime. * * @since 1.4.0 */def mode(saveMode: SaveMode): DataFrameWriter = { this.mode = saveMode this}4. 最后，save()方法觸發action，將文件輸出到指定文件中。
/** * Saves the content of the [[DataFrame]] at the specified path. * * @since 1.4.0 */def save(path: String): Unit = { this.extraOptions += ("path" -> path) save()}三、Spark SQL讀寫整個流程圖如下
四、對于流程中部分函數源碼詳解
DataFrameReader.Load（）
1. Load（）返回DataFrame類型的數據集合，使用的數據是從默認的路徑讀取。
/** * Returns the dataset stored at path as a DataFrame, * using the default data source configured by spark.sql.sources.default. * * @group genericdata * @deprecated As of 1.4.0, replaced by `read().load(path)`. This will be removed in Spark 2.0. */@deprecated("Use read.load(path). This will be removed in Spark 2.0.", "1.4.0")def load(path: String): DataFrame = {//此時的read就是DataFrameReader read.load(path)} 2. 追蹤load源碼進去，源碼如下：
在DataFrameReader中的方法。Load()通過路徑把輸入傳進來變成一個DataFrame。
/** * Loads input in as a [[DataFrame]], for data sources that require a path (e.g. data backed by * a local or distributed file system). * * @since 1.4.0 */// TODO: Remove this one in Spark 2.0.def load(path: String): DataFrame = { option("path", path).load()}3. 追蹤load源碼如下：
/** * Loads input in as a [[DataFrame]], for data sources that don't require a path (e.g. external * key-value stores). * * @since 1.4.0 */def load(): DataFrame = {//對傳入的Source進行解析 val resolved = ResolvedDataSource( sqlContext, userSpecifiedSchema = userSpecifiedSchema, partitionColumns = Array.empty[String], provider = source, options = extraOptions.toMap) DataFrame(sqlContext, LogicalRelation(resolved.relation))}DataFrameReader.format()
1. Format：具體指定文件格式，這就獲得一個巨大的啟示是：如果是Json文件格式可以保持為Parquet等此類操作。
Spark SQL在讀取文件的時候可以指定讀取文件的類型。例如，Json,Parquet.
/** * Specifies the input data source format.Built-in options include “parquet”,”json”,etc. * * @since 1.4.0 */def format(source: String): DataFrameReader = { this.source = source //FileType this}DataFrame.write()
1. 創建DataFrameWriter實例
/** * :: Experimental :: * Interface for saving the content of the [[DataFrame]] out into external storage. * * @group output * @since 1.4.0 */@Experimentaldef write: DataFrameWriter = new DataFrameWriter(this)1
2. 追蹤DataFrameWriter源碼如下：
以DataFrame的方式向外部存儲系統中寫入數據。
/** * :: Experimental :: * Interface used to write a [[DataFrame]] to external storage systems (e.g. file systems, * key-value stores, etc). Use [[DataFrame.write]] to access this. * * @since 1.4.0 */@Experimentalfinal class DataFrameWriter private[sql](df: DataFrame) {DataFrameWriter.mode()
1. Overwrite是覆蓋，之前寫的數據全都被覆蓋了。
Append:是追加，對于普通文件是在一個文件中進行追加，但是對于parquet格式的文件則創建新的文件進行追加。
/** * Specifies the behavior when data or table already exists. Options include: * - `SaveMode.Overwrite`: overwrite the existing data. * - `SaveMode.Append`: append the data. * - `SaveMode.Ignore`: ignore the operation (i.e. no-op).//默認操作 * - `SaveMode.ErrorIfExists`: default option, throw an exception at runtime. * * @since 1.4.0 */def mode(saveMode: SaveMode): DataFrameWriter = { this.mode = saveMode this}2. 通過模式匹配接收外部參數
/** * Specifies the behavior when data or table already exists. Options include: * - `overwrite`: overwrite the existing data. * - `append`: append the data. * - `ignore`: ignore the operation (i.e. no-op). * - `error`: default option, throw an exception at runtime. * * @since 1.4.0 */def mode(saveMode: String): DataFrameWriter = { this.mode = saveMode.toLowerCase match { case "overwrite" => SaveMode.Overwrite case "append" => SaveMode.Append case "ignore" => SaveMode.Ignore case "error" | "default" => SaveMode.ErrorIfExists case _ => throw new IllegalArgumentException(s"Unknown save mode: $saveMode. " + "Accepted modes are 'overwrite', 'append', 'ignore', 'error'.") } this}DataFrameWriter.save()
1. save將結果保存傳入的路徑。
/** * Saves the content of the [[DataFrame]] at the specified path. * * @since 1.4.0 */def save(path: String): Unit = { this.extraOptions += ("path" -> path) save()}2. 追蹤save方法。
/** * Saves the content of the [[DataFrame]] as the specified table. * * @since 1.4.0 */def save(): Unit = { ResolvedDataSource( df.sqlContext, source, partitioningColumns.map(_.toArray).getOrElse(Array.empty[String]), mode, extraOptions.toMap, df)} 3. 其中source是SQLConf的defaultDataSourceName
private var source: String = df.sqlContext.conf.defaultDataSourceName
其中DEFAULT_DATA_SOURCE_NAME默認參數是parquet。
// This is used to set the default data sourceval DEFAULT_DATA_SOURCE_NAME = stringConf("spark.sql.sources.default", defaultValue = Some("org.apache.spark.sql.parquet"), doc = "The default data source to use in input/output.")DataFrame.scala中部分函數詳解：
1. toDF函數是將RDD轉換成DataFrame
/** * Returns the object itself. * @group basic * @since 1.3.0 */// This is declared with parentheses to prevent the Scala compiler from treating// `rdd.toDF("1")` as invoking this toDF and then apply on the returned DataFrame.def toDF(): DataFrame = this2. show()方法：將結果顯示出來
/** * Displays the [[DataFrame]] in a tabular form. For example: * {{{ * year month AVG('Adj Close) MAX('Adj Close) * 1980 12 0.503218 0.595103 * 1981 01 0.523289 0.570307 * 1982 02 0.436504 0.475256 * 1983 03 0.410516 0.442194 * 1984 04 0.450090 0.483521 * }}} * @param numRows Number of rows to show * @param truncate Whether truncate long strings. If true, strings more than 20 characters will * be truncated and all cells will be aligned right * * @group action * @since 1.5.0 */// scalastyle:off printlndef show(numRows: Int, truncate: Boolean): Unit = println(showString(numRows, truncate))// scalastyle:on println追蹤showString源碼如下：showString中觸發action收集數據。
/** * Compose the string representing rows for output * @param _numRows Number of rows to show * @param truncate Whether truncate long strings and align cells right */private[sql] def showString(_numRows: Int, truncate: Boolean = true): String = { val numRows = _numRows.max(0) val sb = new StringBuilder val takeResult = take(numRows + 1) val hasMoreData = takeResult.length > numRows val data = takeResult.take(numRows) val numCols = schema.fieldNames.length以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持VeVb武林網。
新聞熱點
疑難解答
