定义Nvl函数

import org.apache.hadoop.hive.ql.exec.UDFArgumentException;
 import org.apache.hadoop.hive.ql.exec.UDFArgumentLengthException;
 import org.apache.hadoop.hive.ql.exec.UDFArgumentTypeException;
 import org.apache.hadoop.hive.ql.metadata.HiveException;
 import org.apache.hadoop.hive.ql.udf.generic.GenericUDF;
 import org.apache.hadoop.hive.ql.udf.generic.GenericUDFUtils;
 import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector;

 /**
  * 自定义null值处理函数
  */
 public class Nvl extends GenericUDF {
  private GenericUDFUtils.ReturnObjectInspectorResolver returnOIResolver;
  private ObjectInspector[] argumentOIs;

  public ObjectInspector initialize(ObjectInspector[] arguments)
    throws UDFArgumentException {
   argumentOIs = arguments;
   //检查参数个数
   if (arguments.length != 2) {
    throw new UDFArgumentLengthException(
      "The operator 'NVL' accepts 2 arguments.");
   }
   returnOIResolver = new GenericUDFUtils.ReturnObjectInspectorResolver(true);
   //检查参数类型
   if (!(returnOIResolver.update(arguments[0]) && returnOIResolver
     .update(arguments[1]))) {
    throw new UDFArgumentTypeException(2,
      "The 1st and 2nd args of function NLV should have the same type, "
        + "but they are different: \"" + arguments[0].getTypeName()
        + "\" and \"" + arguments[1].getTypeName() + "\"");
   }
   return returnOIResolver.get();
  }

  public Object evaluate(DeferredObject[] arguments) throws HiveException {
   Object retVal = returnOIResolver.convertIfNecessary(arguments[0].get(), argumentOIs[0]);
   if (retVal == null) {
    retVal = returnOIResolver.convertIfNecessary(arguments[1].get(),
      argumentOIs[1]);
   }
   return retVal;
  }

  public String getDisplayString(String[] children) {
   StringBuilder sb = new StringBuilder();
   sb.append("if ");
   sb.append(children[0]);
   sb.append(" is null ");
   sb.append("returns");
   sb.append(children[1]);
   return sb.toString();
  }
 }

 2)添加jar到类路径
  ...
 3)注册函数
  $hive>CREATE TEMPORARY FUNCTION nvl AS 'org.apache.hadoop.hive.ql.udf.generic.GenericUDFNvl';