Kafka：ZK+Kafka+Spark Streaming集群环境搭建（二十四）Structured Streaming：Encoder

一般情况下我们在使用Dataset<Row>进行groupByKey时，你会发现这个方法最后一个参数需要一个encoder，那么这些encoder如何定义呢？

一般数据类型

static Encoder<byte[]>    BINARY()                           An encoder for arrays of bytes.
static Encoder<Boolean>    BOOLEAN()                         An encoder for nullable boolean type.
static Encoder<Byte>    BYTE()                               An encoder for nullable byte type.
static Encoder<java.sql.Date>    DATE()                      An encoder for nullable date type.
static Encoder<java.math.BigDecimal>    DECIMAL()            An encoder for nullable decimal type.
static Encoder<Double>    DOUBLE()                           An encoder for nullable double type.
static Encoder<Float>    FLOAT()                             An encoder for nullable float type.
static Encoder<Integer>    INT()                             An encoder for nullable int type.
static Encoder<Long>    LONG()                               An encoder for nullable long type.
static Encoder<Short>    SHORT()                             An encoder for nullable short type.
static Encoder<String>    STRING()                           An encoder for nullable string type.
static Encoder<java.sql.Timestamp>    TIMESTAMP()            An encoder for nullable timestamp type.

示例：

== Scala == Encoders are generally created automatically through implicits from a SparkSession, or can be explicitly created by calling static methods on Encoders.
   import spark.implicits._
   val ds = Seq(1, 2, 3).toDS() // implicitly provided (spark.implicits.newIntEncoder) 
== Java == Encoders are specified by calling static methods on Encoders.
   List<String> data = Arrays.asList("abc", "abc", "xyz");
   Dataset<String> ds = context.createDataset(data, Encoders.STRING()); 

Class类型：

Or constructed from Java Beans:
   Encoders.bean(MyClass.class); 

Tuple类型：

一般类型的Tuple

   Encoder<Tuple2<Integer, String>> encoder2 = Encoders.tuple(Encoders.INT(), Encoders.STRING());
   List<Tuple2<Integer, String>> data2 = Arrays.asList(new scala.Tuple2(1, "a");
   Dataset<Tuple2<Integer, String>> ds2 = context.createDataset(data2, encoder2);

Tuple包含类的：

Encoder<Tuple2<String, MyClass>> encoder = Encoders.tuple(Encoders.STRING(), Encoders.bean(MyClass.class));

关于Encoder请参考《http://spark.apache.org/docs/latest/api/java/org/apache/spark/sql/Encoder.html》

关于Encoders请参考《http://spark.apache.org/docs/latest/api/java/org/apache/spark/sql/Encoders.html》