Creating a scalar udt in java

Chapman Flack edited this page Jan 30, 2016 · 4 revisions

Creating a scalar (or, base) user-defined type

This text assumes that you have some familiarity with how scalar types are created and added to the PostgreSQL type system. For more info on that topic, please read this chapter in the PostgreSQL docs.

Creating new scalar type using Java functions is very similar to how they are created using C functions from an SQL perspective but of course very different when looking at the actual implementation. Java stipulates that the mapping between a Java class and a corresponding SQL type should be done using the interfaces java.sql.SQLData, java.sql.SQLInput, and java.sql.SQLOutput and that is what PL/Java is using. In addition, the PostgreSQL type system stipulates that each type must have a textual representation.

Let us create a type called javatest.complex (similar to the complex type used in the PostgreSQL documentation). The name of the corresponding Java class will be org.postgresql.pljava.example.ComplexScalar.

The Java code for the scalar type

Prerequisites for the Java implementation

The java class for a scalar UDT must implement the java.sql.SQLData interface. In addition, it must also implement a method static T parse(String stringRepresentation, String typeName) where T will be the name of the class--that is, parse will create and return an instance of the class--and the java.lang.String toString() method. The toString() method must return something that the parse() method can parse.

package org.postgresql.pljava.example;

import java.io.IOException;
import java.io.StreamTokenizer;
import java.io.StringReader;
import java.sql.SQLData;
import java.sql.SQLException;
import java.sql.SQLInput;
import java.sql.SQLOutput;
import java.util.logging.Logger;

import org.postgresql.pljava.annotation.Function;
import org.postgresql.pljava.annotation.SQLType;
import org.postgresql.pljava.annotation.BaseUDT;

import static org.postgresql.pljava.annotation.Function.Effects.IMMUTABLE;
import static
       org.postgresql.pljava.annotation.Function.OnNullInput.RETURNS_NULL;

@BaseUDT(schema="javatest", name="complex",
         internalLength=16, alignment=BaseUDT.Alignment.DOUBLE)
public class ComplexScalar implements SQLData
{
   private double m_x;
   private double m_y;
   private String m_typeName;

   @Function(effects=IMMUTABLE, onNullInput=RETURNS_NULL)
   public static ComplexScalar parse(String input, String typeName)
   throws SQLException
   {
      try
      {
         StreamTokenizer tz = new StreamTokenizer(new StringReader(input));
         if(tz.nextToken() == '('
         && tz.nextToken() == StreamTokenizer.TT_NUMBER)
         {
            double x = tz.nval;
            if(tz.nextToken() == ','
            && tz.nextToken() == StreamTokenizer.TT_NUMBER)
            {
               double y = tz.nval;
               if(tz.nextToken() == ')')
               {
                  return new ComplexScalar(x, y, typeName);
               }
            }
         }
         throw new SQLException("Unable to parse complex from string \""
	    + input + '"');
      }
      catch(IOException e)
      {
         throw new SQLException(e.getMessage());
      }
   }

   public ComplexScalar()
   {
   }

   public ComplexScalar(double x, double y, String typeName)
   {
      m_x = x;
      m_y = y;
      m_typeName = typeName;
   }

   @Override
   public String getSQLTypeName()
   {
      return m_typeName;
   }

   @Function(effects=IMMUTABLE, onNullInput=RETURNS_NULL)
   @Override
   public void readSQL(SQLInput stream, String typeName) throws SQLException
   {
      m_x = stream.readDouble();
      m_y = stream.readDouble();
      m_typeName = typeName;
   }

   @Function(effects=IMMUTABLE, onNullInput=RETURNS_NULL)
   @Override
   public void writeSQL(SQLOutput stream) throws SQLException
   {
      stream.writeDouble(m_x);
      stream.writeDouble(m_y);
   }

   @Function(effects=IMMUTABLE, onNullInput=RETURNS_NULL)
   @Override
   public String toString()
   {
      s_logger.info(m_typeName + " toString");
      StringBuffer sb = new StringBuffer();
      sb.append('(');
      sb.append(m_x);
      sb.append(',');
      sb.append(m_y);
      sb.append(')');
      return sb.toString();
   }

   /* Meaningful code that actually does something with this type was
    * intentionally left out.
    */
}

The class itself is annotated with @BaseUDT, giving its SQL schema and name, and the length and alignment needed for its internal, stored form.

Because the compiler knows the class is a BaseUDT, it already expects the parse, toString, readSQL, and writeSQL methods to be present, and will generate the correct SQL to declare them as functions to PostgreSQL. The @Function annotations are only there to declare the immutability and on-null-input behavior for those methods, because those values are not the defaults when declaring a function.

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