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本文主要研究下hibernate的hbm2ddl

SchemaManagementTool

hibernate-core-5.0.12.Final-sources.jar!/org/hibernate/tool/schema/spi/SchemaManagementTool.java

public interface SchemaManagementTool extends Service {
    public SchemaCreator getSchemaCreator(Map options);
    public SchemaDropper getSchemaDropper(Map options);
    public SchemaMigrator getSchemaMigrator(Map options);
    public SchemaValidator getSchemaValidator(Map options);
}

这个tool定义了create、drop、migrate、validate四个功能。

SchemaCreatorImpl

hibernate-core-5.0.12.Final-sources.jar!/org/hibernate/tool/schema/internal/SchemaCreatorImpl.java

public class SchemaCreatorImpl implements SchemaCreator {

    @Override
    public void doCreation(Metadata metadata, boolean createNamespaces, List<Target> targets) throws SchemaManagementException {
        doCreation( metadata, createNamespaces, targets.toArray( new Target[ targets.size() ] ) );
    }
    //......
}

主要逻辑在doCreation里头，里头按如下顺序创建：

• 创建catalog/schema
• 创建before table auxiliary objects
• 创建sequences
• 创建tables
  • 创建indexes
  • 创建uniques
• 创建foreign keys
• 创建after table auxiliary objects

它们又主要借助dialect的各种exporter来实现

Dialect

hibernate-core-5.0.12.Final-sources.jar!/org/hibernate/dialect/Dialect.java

public abstract class Dialect implements ConversionContext {
    //......
    // DDL support ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    private StandardTableExporter tableExporter = new StandardTableExporter( this );
    private StandardSequenceExporter sequenceExporter = new StandardSequenceExporter( this );
    private StandardIndexExporter indexExporter = new StandardIndexExporter( this );
    private StandardForeignKeyExporter foreignKeyExporter = new StandardForeignKeyExporter( this );
    private StandardUniqueKeyExporter uniqueKeyExporter = new StandardUniqueKeyExporter( this );
    private StandardAuxiliaryDatabaseObjectExporter auxiliaryObjectExporter = new StandardAuxiliaryDatabaseObjectExporter( this );
    //......
}

这里定义了table、sequence、index、foreign key、unique key、auxiliary database object的exporter

StandardTableExporter

hibernate-core-5.0.12.Final-sources.jar!/org/hibernate/tool/schema/internal/StandardTableExporter.java

public class StandardTableExporter implements Exporter<Table> {
   //.....
    @Override
    public String[] getSqlCreateStrings(Table table, Metadata metadata) {
        final QualifiedName tableName = new QualifiedNameParser.NameParts(
                Identifier.toIdentifier( table.getCatalog(), table.isCatalogQuoted() ),
                Identifier.toIdentifier( table.getSchema(), table.isSchemaQuoted() ),
                table.getNameIdentifier()
        );

        final JdbcEnvironment jdbcEnvironment = metadata.getDatabase().getJdbcEnvironment();
        StringBuilder buf =
                new StringBuilder( tableCreateString( table.hasPrimaryKey() ) )
                        .append( ' ' )
                        .append(
                                jdbcEnvironment.getQualifiedObjectNameFormatter().format(
                                        tableName,
                                        jdbcEnvironment.getDialect()
                                )
                        )
                        .append( " (" );


        boolean isPrimaryKeyIdentity = table.hasPrimaryKey()
                && table.getIdentifierValue() != null
                && table.getIdentifierValue().isIdentityColumn( metadata.getIdentifierGeneratorFactory(), dialect );
        // this is the much better form moving forward as we move to metamodel
        //boolean isPrimaryKeyIdentity = hasPrimaryKey
        //              && table.getPrimaryKey().getColumnSpan() == 1
        //              && table.getPrimaryKey().getColumn( 0 ).isIdentity();

        // Try to find out the name of the primary key in case the dialect needs it to create an identity
        String pkColName = null;
        if ( table.hasPrimaryKey() ) {
            Column pkColumn = (Column) table.getPrimaryKey().getColumns().iterator().next();
            pkColName = pkColumn.getQuotedName( dialect );
        }

        final Iterator columnItr = table.getColumnIterator();
        boolean isFirst = true;
        while ( columnItr.hasNext() ) {
            final Column col = (Column) columnItr.next();
            if ( isFirst ) {
                isFirst = false;
            }
            else {
                buf.append( ", " );
            }
            String colName = col.getQuotedName( dialect );

            buf.append( colName ).append( ' ' );

            if ( isPrimaryKeyIdentity && colName.equals( pkColName ) ) {
                // to support dialects that have their own identity data type
                if ( dialect.getIdentityColumnSupport().hasDataTypeInIdentityColumn() ) {
                    buf.append( col.getSqlType( dialect, metadata ) );
                }
                buf.append( ' ' )
                        .append( dialect.getIdentityColumnSupport().getIdentityColumnString( col.getSqlTypeCode( metadata ) ) );
            }
            else {
                buf.append( col.getSqlType( dialect, metadata )  );

                String defaultValue = col.getDefaultValue();
                if ( defaultValue != null ) {
                    buf.append( " default " ).append( defaultValue );
                }

                if ( col.isNullable() ) {
                    buf.append( dialect.getNullColumnString() );
                }
                else {
                    buf.append( " not null" );
                }

            }

            if ( col.isUnique() ) {
                String keyName = Constraint.generateName( "UK_", table, col );
                UniqueKey uk = table.getOrCreateUniqueKey( keyName );
                uk.addColumn( col );
                buf.append(
                        dialect.getUniqueDelegate()
                                .getColumnDefinitionUniquenessFragment( col )
                );
            }

            if ( col.getCheckConstraint() != null && dialect.supportsColumnCheck() ) {
                buf.append( " check (" )
                        .append( col.getCheckConstraint() )
                        .append( ")" );
            }

            String columnComment = col.getComment();
            if ( columnComment != null ) {
                buf.append( dialect.getColumnComment( columnComment ) );
            }
        }
        if ( table.hasPrimaryKey() ) {
            buf.append( ", " )
                    .append( table.getPrimaryKey().sqlConstraintString( dialect ) );
        }

        buf.append( dialect.getUniqueDelegate().getTableCreationUniqueConstraintsFragment( table ) );

        applyTableCheck( table, buf );

        buf.append( ')' );

        if ( table.getComment() != null ) {
            buf.append( dialect.getTableComment( table.getComment() ) );
        }

        applyTableTypeString( buf );

        List<String> sqlStrings = new ArrayList<String>();
        sqlStrings.add( buf.toString() );

        applyComments( table, tableName, sqlStrings );

        applyInitCommands( table, sqlStrings );

        return sqlStrings.toArray( new String[ sqlStrings.size() ] );
    }
}

上面的sql生成具体还根据传入的dialect来实现不同数据库的语句的区别。

小结

要实现一个ddl的功能，一个要有不同的dialect来屏蔽不同数据库的区别，另外一个就是需要创建索引、序列、主键、外键等相关的对象，其他的就是字段类型的映射。