聊聊hibernate的hbm2ddl的实现-白红宇

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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);}复制代码

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
    
      targets) throws SchemaManagementException {        doCreation( metadata, createNamespaces, targets.toArray( new Target[ targets.size() ] ) );    }    //......}复制代码

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 );    //......}复制代码

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
    
      sqlStrings = new ArrayList
     
      ();        sqlStrings.add( buf.toString() );        applyComments( table, tableName, sqlStrings );        applyInitCommands( table, sqlStrings );        return sqlStrings.toArray( new String[ sqlStrings.size() ] );    }}复制代码
     
    
    
      {   //.....    @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

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。