import org.apache.spark.sql.SparkSession
val spark = SparkSession
  .builder()
  .appName("...")
  .master("...")
  .config("spark.sql.extensions", "io.delta.sql.DeltaSparkSessionExtension")
  .config("spark.sql.catalog.spark_catalog", "org.apache.spark.sql.delta.catalog.DeltaCatalog")
  .getOrCreate()

A catalog implementation that will be used as the v2 interface to Spark's built-in v1 catalog: spark_catalog. This catalog shares its identifier namespace with the spark_catalog and must be consistent with it; for example, if a table can be loaded by the spark_catalog, this catalog must also return the table metadata. To delegate operations to the spark_catalog, implementations can extend 'CatalogExtension'.

 * A marker interface to provide a catalog implementation for Spark.
 * <p>
 * Implementations can provide catalog functions by implementing additional interfaces for tables,
 * views, and functions.
 * <p>
 * Catalog implementations must implement this marker interface to be loaded by
 * {@link Catalogs#load(String, SQLConf)}. The loader will instantiate catalog classes using the
 * required public no-arg constructor. After creating an instance, it will be configured by calling
 * {@link #initialize(String, CaseInsensitiveStringMap)}.
 * <p>
 * Catalog implementations are registered to a name by adding a configuration option to Spark:
 * {@code spark.sql.catalog.catalog-name=com.example.YourCatalogClass}. All configuration properties
 * in the Spark configuration that share the catalog name prefix,
 * {@code spark.sql.catalog.catalog-name.(key)=(value)} will be passed in the case insensitive
 * string map of options in initialization with the prefix removed.
 * {@code name}, is also passed and is the catalog's name; in this case, "catalog-name".

/**
   * List the tables in a namespace from the catalog.
   * <p>
   * If the catalog supports views, this must return identifiers for only tables and not views.
   *
   * @param namespace a multi-part namespace
   * @return an array of Identifiers for tables
   * @throws NoSuchNamespaceException If the namespace does not exist (optional).
   */
  Identifier[] listTables(String[] namespace) throws NoSuchNamespaceException;
  /**
   * Load table metadata by {@link Identifier identifier} from the catalog.
   * <p>
   * If the catalog supports views and contains a view for the identifier and not a table, this
   * must throw {@link NoSuchTableException}.
   *
   * @param ident a table identifier
   * @return the table's metadata
   * @throws NoSuchTableException If the table doesn't exist or is a view
   */
  Table loadTable(Identifier ident) throws NoSuchTableException;

/**
 * A thread-safe manager for [[CatalogPlugin]]s. It tracks all the registered catalogs, and allow
 * the caller to look up a catalog by name.
 *
 * There are still many commands (e.g. ANALYZE TABLE) that do not support v2 catalog API. They
 * ignore the current catalog and blindly go to the v1 `SessionCatalog`. To avoid tracking current
 * namespace in both `SessionCatalog` and `CatalogManger`, we let `CatalogManager` to set/get
 * current database of `SessionCatalog` when the current catalog is the session catalog.
 */
// TODO: all commands should look up table from the current catalog. The `SessionCatalog` doesn't
//       need to track current database at all.
private[sql]
class CatalogManager(
    conf: SQLConf,
    defaultSessionCatalog: CatalogPlugin,
    val v1SessionCatalog: SessionCatalog) extends Logging {

/**
   * Catalog for managing table and database states. If there is a pre-existing catalog, the state
   * of that catalog (temp tables & current database) will be copied into the new catalog.
   *
   * Note: this depends on the `conf`, `functionRegistry` and `sqlParser` fields.
   */
  protected lazy val catalog: SessionCatalog = {
    val catalog = new SessionCatalog(
      () => session.sharedState.externalCatalog,
      () => session.sharedState.globalTempViewManager,
      functionRegistry,
      conf,
      SessionState.newHadoopConf(session.sparkContext.hadoopConfiguration, conf),
      sqlParser,
      resourceLoader)
    parentState.foreach(_.catalog.copyStateTo(catalog))
    catalog
  }
  protected lazy val v2SessionCatalog = new V2SessionCatalog(catalog, conf)
  protected lazy val catalogManager = new CatalogManager(conf, v2SessionCatalog, catalog)

/**
 * A [[TableCatalog]] that translates calls to the v1 SessionCatalog.
 */
class V2SessionCatalog(catalog: SessionCatalog, conf: SQLConf)
  extends TableCatalog with SupportsNamespaces {
  import org.apache.spark.sql.connector.catalog.CatalogV2Implicits.NamespaceHelper
  import V2SessionCatalog._
  override val defaultNamespace: Array[String] = Array("default")
  override def name: String = CatalogManager.SESSION_CATALOG_NAME
  // This class is instantiated by Spark, so `initialize` method will not be called.
  override def initialize(name: String, options: CaseInsensitiveStringMap): Unit = {}
  override def listTables(namespace: Array[String]): Array[Identifier] = {
    namespace match {
      case Array(db) =>
        catalog
          .listTables(db)
          .map(ident => Identifier.of(Array(ident.database.getOrElse("")), ident.table))
          .toArray
      case _ =>
        throw new NoSuchNamespaceException(namespace)
    }
  }

protected def analyzer: Analyzer = new Analyzer(catalogManager, conf) {
...
protected def optimizer: Optimizer = {
    new SparkOptimizer(catalogManager, catalog, experimentalMethods) {
      override def earlyScanPushDownRules: Seq[Rule[LogicalPlan]] =
        super.earlyScanPushDownRules ++ customEarlyScanPushDownRules
      override def extendedOperatorOptimizationRules: Seq[Rule[LogicalPlan]] =
        super.extendedOperatorOptimizationRules ++ customOperatorOptimizationRules
    }
  }

 */
  def filter(conditionExpr: String): Dataset[T] = {
    filter(Column(sparkSession.sessionState.sqlParser.parseExpression(conditionExpr)))
  }

protected lazy val sqlParser: ParserInterface = {
    extensions.buildParser(session, new SparkSqlParser(conf))
  }

class DeltaCatalog(val spark: SparkSession) extends DelegatingCatalogExtension
  with StagingTableCatalog
  with SupportsPathIdentifier {
  def this() = {
    this(SparkSession.active)
  }
  ...

public abstract class DelegatingCatalogExtension implements CatalogExtension {
  private CatalogPlugin delegate;
  public final void setDelegateCatalog(CatalogPlugin delegate) {
    this.delegate = delegate;
  }

private def loadV2SessionCatalog(): CatalogPlugin = {
    Catalogs.load(SESSION_CATALOG_NAME, conf) match {
      case extension: CatalogExtension =>
        extension.setDelegateCatalog(defaultSessionCatalog)
        extension
      case other => other
    }
  }

private[sql] def v2SessionCatalog: CatalogPlugin = {
    conf.getConf(SQLConf.V2_SESSION_CATALOG_IMPLEMENTATION).map { customV2SessionCatalog =>
      try {
        catalogs.getOrElseUpdate(SESSION_CATALOG_NAME, loadV2SessionCatalog())
      } catch {
        case NonFatal(_) =>
          logError(
            "Fail to instantiate the custom v2 session catalog: " + customV2SessionCatalog)
          defaultSessionCatalog
      }
    }.getOrElse(defaultSessionCatalog)
  }

   首先得到配置项SQLConf.V2_SESSION_CATALOG_IMPLEMENTATION，也就是spark.sql.catalog.spark_catalog配置，
   如果spark配置了的话，就调用loadV2SessionCatalog加载该类，，否则就加载默认的v2SessionCatalog，也就是V2SessionCatalog实例

override def createTable(
      ident: Identifier,
      schema: StructType,
      partitions: Array[Transform],
      properties: util.Map[String, String]): Table = {
    if (DeltaSourceUtils.isDeltaDataSourceName(getProvider(properties))) {
      createDeltaTable(
        ident, schema, partitions, properties, sourceQuery = None, TableCreationModes.Create)
    } else {
      super.createTable(ident, schema, partitions, properties)
    }
  }
...
private def createDeltaTable(
      ident: Identifier,
      schema: StructType,
      partitions: Array[Transform],
      properties: util.Map[String, String],
      sourceQuery: Option[LogicalPlan],
      operation: TableCreationModes.CreationMode): Table = {
     ...
    val tableDesc = new CatalogTable(
      identifier = TableIdentifier(ident.name(), ident.namespace().lastOption),
      tableType = tableType,
      storage = storage,
      schema = schema,
      provider = Some("delta"),
      partitionColumnNames = partitionColumns,
      bucketSpec = maybeBucketSpec,
      properties = tableProperties.toMap,
      comment = Option(properties.get("comment")))
    // END: copy-paste from the super method finished.
    val withDb = verifyTableAndSolidify(tableDesc, None)
    ParquetSchemaConverter.checkFieldNames(tableDesc.schema.fieldNames)
    CreateDeltaTableCommand(
      withDb,
      getExistingTableIfExists(tableDesc),
      operation.mode,
      sourceQuery,
      operation,
      tableByPath = isByPath).run(spark)
    loadTable(ident)
      }
 override def loadTable(ident: Identifier): Table = {
    try {
      super.loadTable(ident) match {
        case v1: V1Table if DeltaTableUtils.isDeltaTable(v1.catalogTable) =>
          DeltaTableV2(
            spark,
            new Path(v1.catalogTable.location),
            catalogTable = Some(v1.catalogTable),
            tableIdentifier = Some(ident.toString))
        case o => o
      }
  }