下面的代码片段是从joinedstreams的javadoc复制的

val one: DataStream[(String, Int)]  = ...
val two: DataStream[(String, Int)] = ...

val result = one.join(two)
    .where {t => ... }
    .equal {t => ... }
    .window(TumblingEventTimeWindows.of(Time.of(5, TimeUnit.SECONDS)))
    .apply(new MyJoinFunction())

两个流仅基于一个键（通过 t => ... )例如， one.a = two.a ,
我会问我怎样才能基于多个键进行连接，例如，one.a=two.a和 one.b = two.b