package org.apache.hadoop.hive.contrib.udf.example;import org.apache.hadoop.hive.ql.exec.Description;import org.apache.hadoop.hive.ql.exec.UDF;/** * UDFExampleAdd. * *///UDF是作用于单个数据行，产生一个数据行 //用户必须要继承UDF，且必须至少实现一个evalute方法，该方法并不在UDF中 //但是Hive会检查用户的UDF是否拥有一个evalute方法@Description(name = "example_add", value = "http://www.mamicode.com/_FUNC_(expr) - Example UDAF that returns the sum")public class UDFExampleAdd extends UDF {//实现具体逻辑  public Integer evaluate(Integer... a) {    int total = 0;    for (Integer element : a) {      if (element != null) {        total += element;      }    }    return total;  }  public Double evaluate(Double... a) {    double total = 0;    for (Double element : a) {      if (element != null) {        total += element;      }    }    return total;  }}

package org.apache.hadoop.hive.contrib.udaf.example;import org.apache.hadoop.hive.ql.exec.Description;import org.apache.hadoop.hive.ql.exec.UDAF;import org.apache.hadoop.hive.ql.exec.UDAFEvaluator;/** * This is a simple UDAF that calculates average. *  * It should be very easy to follow and can be used as an example for writing * new UDAFs. *  * Note that Hive internally uses a different mechanism (called GenericUDAF) to * implement built-in aggregation functions, which are harder to program but * more efficient. *  *///UDAF是输入多个数据行，产生一个数据行 //用户自定义的UDAF必须是继承了UDAF，且内部包含多个实现了exec的静态类@Description(name = "example_avg",value = "http://www.mamicode.com/_FUNC_(col) - Example UDAF to compute average")public final class UDAFExampleAvg extends UDAF {  /**   * The internal state of an aggregation for average.   *    * Note that this is only needed if the internal state cannot be represented   * by a primitive.   *    * The internal state can also contains fields with types like   * ArrayList<String> and HashMap<String,Double> if needed.   */  public static class UDAFAvgState {    private long mCount;    private double mSum;  }  /**   * The actual class for doing the aggregation. Hive will automatically look   * for all internal classes of the UDAF that implements UDAFEvaluator.   */  public static class UDAFExampleAvgEvaluator implements UDAFEvaluator {    UDAFAvgState state;    public UDAFExampleAvgEvaluator() {      super();      state = new UDAFAvgState();      init();    }    /**     * Reset the state of the aggregation.	 * 重置聚合过程的状态     */    public void init() {      state.mSum = 0;      state.mCount = 0;    }    /**     * Iterate through one row of original data.     *      * The number and type of arguments need to the same as we call this UDAF     * from Hive command line.     *      * This function should always return true.	 * 在原始值的一行数据上进行迭代     * 参数的个数和类型需与hive命令行中调用该UDF的参数相同。	 * 这个函数应当总是返回true     */    public boolean iterate(Double o) {      if (o != null) {        state.mSum += o;        state.mCount++;      }      return true;    }    /**     * Terminate a partial aggregation and return the state. If the state is a     * primitive, just return primitive Java classes like Integer or String.     */	//Hive需要部分聚集结果的时候会调用该方法     //会返回一个封装了聚集计算当前状态的对象    public UDAFAvgState terminatePartial() {      // This is SQL standard - average of zero items should be null.      return state.mCount == 0 ? null : state;    }    /**     * Merge with a partial aggregation.     *      * This function should always have a single argument which has the same     * type as the return value of terminatePartial().     */	//合并两个部分聚集值会调用这个方法    public boolean merge(UDAFAvgState o) {      if (o != null) {        state.mSum += o.mSum;        state.mCount += o.mCount;      }      return true;    }    /**     * Terminates the aggregation and return the final result.	 * 终止聚合过程，返回最终结果     */	//Hive需要最终聚集结果时候会调用该方法    public Double terminate() {      // This is SQL standard - average of zero items should be null.      return state.mCount == 0 ? null : Double.valueOf(state.mSum          / state.mCount);    }  }  private UDAFExampleAvg() {    // prevent instantiation  }}

add jar UDFExample.jar //注册jarcreate temporary function my_add as ‘org.apache.hadoop.hive.contrib.udf.example. UDFExampleAdd‘;  // UDF只是为这个Hive会话临时定义的create temporary function my_avg as ‘org.apache.hadoop.hive.contrib.udaf.example. UDAFExampleAvg‘; 

registerUDF("my_add", UDFExampleAdd.class, false);registerUDAF("my_avg", UDAFExampleAvg.class);