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Lazy fetching with Streams
Applies to ✅ Open Source Edition ✅ Express Edition ✅ Professional Edition ✅ Enterprise Edition
jOOQ 3.7+ supports Java 8, and with Java 8, it supports java.util.stream.Stream
. This opens up a range of possibilities of combining the declarative aspects of SQL with the functional aspects of the new Stream
API. Much like the Cursors from the previous section, such a Stream
keeps an internal reference to a JDBC java.sql.ResultSet
, which means that the Stream
has to be treated like a resource. Here's an example of using such a stream:
// Obtain a Stream reference: try (Stream<BookRecord> stream = create.selectFrom(BOOK).stream()) { stream.forEach(Util::doThingsWithBook); }
A more sophisticated example would be using streams to transform the results and add business logic to it. For instance, to generate a DDL script with CREATE TABLE
statements from the INFORMATION_SCHEMA
of an H2 database:
create.select( COLUMNS.TABLE_NAME, COLUMNS.COLUMN_NAME, COLUMNS.TYPE_NAME) .from(COLUMNS) .orderBy( COLUMNS.TABLE_CATALOG, COLUMNS.TABLE_SCHEMA, COLUMNS.TABLE_NAME, COLUMNS.ORDINAL_POSITION) .fetch() // Eagerly load the whole ResultSet into memory first .stream() .collect(groupingBy( r -> r.getValue(COLUMNS.TABLE_NAME), LinkedHashMap::new, mapping( r -> new SimpleEntry( r.getValue(COLUMNS.COLUMN_NAME), r.getValue(COLUMNS.TYPE_NAME) ), toList() ))) .forEach( (table, columns) -> { // Just emit a CREATE TABLE statement System.out.println("CREATE TABLE " + table + " ("); // Map each "Column" type into a String containing the column specification, // and join them using comma and newline. Done! System.out.println( columns.stream() .map(col -> " " + col.getKey() + " " + col.getValue()) .collect(Collectors.joining(",\n")) ); System.out.println(");"); });
The above combination of SQL and functional programming will produce the following output:
CREATE TABLE CATALOGS( CATALOG_NAME VARCHAR ); CREATE TABLE COLLATIONS( NAME VARCHAR, KEY VARCHAR ); CREATE TABLE COLUMNS( TABLE_CATALOG VARCHAR, TABLE_SCHEMA VARCHAR, TABLE_NAME VARCHAR, COLUMN_NAME VARCHAR, ORDINAL_POSITION INTEGER, COLUMN_DEFAULT VARCHAR, IS_NULLABLE VARCHAR, DATA_TYPE INTEGER, CHARACTER_MAXIMUM_LENGTH INTEGER, CHARACTER_OCTET_LENGTH INTEGER, NUMERIC_PRECISION INTEGER, NUMERIC_PRECISION_RADIX INTEGER, NUMERIC_SCALE INTEGER, CHARACTER_SET_NAME VARCHAR, COLLATION_NAME VARCHAR, TYPE_NAME VARCHAR, NULLABLE INTEGER, IS_COMPUTED BOOLEAN, SELECTIVITY INTEGER, CHECK_CONSTRAINT VARCHAR, SEQUENCE_NAME VARCHAR, REMARKS VARCHAR, SOURCE_DATA_TYPE SMALLINT );
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