Custom Types in Hibernate

Hibernate is a great ORM tool that can be customized to fit almost any need. In this topic, I will show how you can store fields of arbitrary type. For example, let's take an array of strings (String []) and a POJO object that we want to store in the database:

1. public class StringArrayContainer { private Integer id ; private String [ ] tags ; // . }
2. public class StringArrayContainer { private Integer id ; private String [ ] tags ; // . }
3. public class StringArrayContainer { private Integer id ; private String [ ] tags ; // . }
4. public class StringArrayContainer { private Integer id ; private String [ ] tags ; // . }
5. public class StringArrayContainer { private Integer id ; private String [ ] tags ; // . }

In order for a Hibernate to save a field of type “array of strings” to the database, it is necessary (and sufficient) to write a class that implements the UserType interface and also refer to it in the mapping (hbm.xml file).

1. public class StringArrayCustomType implements UserType {
2. // TODO: write the implementation.
3. }

Let us write the implementation of the class StringArrayCustomType method after method.

• sqlTypes - an array of column types in the database. In our case, it is CLOB.

  1. private static final int [ ] SQL_TYPES = new int [ ] { Types . CLOB } ;
  2. @ Override
  3. public int [ ] sqlTypes ( ) {
  4. return SQL_TYPES ;
  5. }

• returnedClass - the type of the object returned by the nullSafeGet method. We have an array of strings.

  1. @ Override
  2. public Class returnedClass ( ) {
  3. return String [ ] . class ;
  4. }

• equals - checks for equal two values. We consider our array unchanged (even in Java we can change the contents of the array, but in this case we administratively prohibit it ;-)), so we simply check for equality of the link.

  1. @ Override
  2. public boolean equals ( Object x, Object y ) throws HibernateException {
  3. return x == y ;
  4. }

• hashCode - we delegate the generation of hashCode to the array itself.

  1. @ Override
  2. public int hashCode ( Object x ) throws HibernateException {
  3. return x. hashCode ( ) ;
  4. }

• nullSafeGet - restores the field value when reading from the database.

  1. @ Override
  2. public Object nullSafeGet ( ResultSet rs, String [ ] names, Object owner ) throws HibernateException, SQLException {
  3. String value = ( String ) Hibernate. TEXT . nullSafeGet ( rs, names [ 0 ] ) ;
  4. if ( value == null ) {
  5. return null ;
  6. } else {
  7. String [ ] array = StringUtils. split ( value, '' ) ;
  8. for ( int i = 0 ; i < array. length ; i ++ ) {
  9. array [ i ] = WhitespaceEscapeUtil. unescape ( array [ i ] ) ;
  10. }
  11. return array ;
  12. }
  13. }

• nullSafeSet - encodes the value of the field to write to the database.

  1. @ Override
  2. public void nullSafeSet ( PreparedStatement st, Object value, int index ) throws HibernateException, SQLException {
  3. if ( value == null ) {
  4. Hibernate. TEXT . nullSafeSet ( st, null , index ) ;
  5. } else {
  6. String [ ] array = ( String [ ] ) value ;
  7. String [ ] copy = new String [ array. length ] ;
  8. for ( int i = 0 ; i < array. length ; i ++ ) {
  9. copy [ i ] = WhitespaceEscapeUtil. escape ( array [ i ] ) ;
  10. }
  11. Hibernate. TEXT . nullSafeSet ( st, StringUtils. join ( copy, '' ) , index ) ;
  12. }
  13. }

• deepCopy - performs a full copy of the object. We are not interested because our array does not change.

  1. @ Override
  2. public Object deepCopy ( Object value ) throws HibernateException {
  3. return value ;
  4. }

• isMutable - returns true if the object can change.

  1. @ Override
  2. public boolean isMutable ( ) {
  3. return false ;
  4. }

• disassemble - restores an object from a view suitable for storage in a second-level cache.

  1. @ Override
  2. public Serializable disassemble ( Object value ) throws HibernateException {
  3. return ( Serializable ) value ;
  4. }

• assemble - converts an object into a form suitable for storage in a second-level cache.

  1. @ Override
  2. public Object assemble ( Serialable cached, Object owner ) throws HibernateException {
  3. return cached ;
  4. }

• replace - copies changes from the new value to the old one. We are not interested because our array does not change.

  1. @ Override
  2. public Object replace ( Object original, Object target, Object owner ) throws HibernateException {
  3. return original ;
  4. }

Finally, update the mapping file (hbm.xml):

1. <class name = "StringArrayContainer" table = "containers" >
2. <id name = "id" >
3. <generator class = "native" />
4. </ id >
5. <property name = "tags" column = "tags" type = "StringArrayCustomType" />
6. </ class >

And enjoy storing an array of strings.
')
PS In the next article we will fix a special criterion for filtering by this field.
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