Class TriggerSample

  • All Implemented Interfaces:
    Trigger

    public class TriggerSample
    extends java.lang.Object
    implements Trigger

    Sample code for use of triggers in hsqldb. This class org.hsqldb.sample package, but a typical implementation is in users's class hierarchy. SQL to invoke is:

    CREATE TRIGGER triggerSample BEFORE|AFTER INSERT|UPDATE|DELETE ON myTable [FOR EACH ROW] [QUEUE n] [NOWAIT] CALL "myPackage.trigClass"
    This will create a thread that will wait for its firing event to occur; when this happens, the trigger's thread runs the 'trigClass.fire' Note that this is still in the same Java Virtual Machine as the database, so make sure the fired method does not hang.

    There is a queue of events waiting to be run by each trigger thread. This is particularly useful for 'FOR EACH ROW' triggers, when a large number of trigger events occur in rapid succession, without the trigger thread getting a chance to run. If the queue becomes full, subsequent additions to it cause the database engine to suspend awaiting space in the queue. Take great care to avoid this situation if the trigger action involves accessing the database, as deadlock will occur. This can be avoided either by ensuring the QUEUE parameter makes a large enough queue, or by using the NOWAIT parameter, which causes a new trigger event to overwrite the most recent event in the queue. The default queue size is 1024.

    Ensure that "myPackage.trigClass" is present in the classpath which you use to start hsql.

    If the method wants to access the database, it must establish a JDBC connection.

    When the 'fire' method is called, it is passed the following arguments:

    fire (int type, String trigName, String tabName, Object oldRow[], Object[] newRow)

    where 'type' is one of the values enumerated in the Trigger interface and the 'oldRow'/'newRow' pair represents the rows acted on. The first length - 1 array slots contain column values and the final slot contains either null or the value of the internally assigned row identity, if the concerned table has no primary key. The final slot must _never_ be modified.

    The mapping of row classes to database types is specified in /doc/hsqlSyntax.html#Datatypes.

    To be done:

    1. Implement the "jdbc:default:connection: URL to provide transparent and portable access to internal connections for use in triggers and stored procedures.

    2. Implement declaritive column to trigger method argument mapping, conditional execution (WHEN clause), etc.

    3. Investigate and refine synchronous and asynchronous trigger models.

      Because certain combinations of trigger create parameters cause the individual triggered actions of a multirow update to run in different threads, it is possible for an 'after' trigger to run before its corresponding 'before' trigger; the acceptability and implications of this needs to be investigated, documented and the behaviour of the engine fully specified.

    4. Investigate and implement the SQL 200n specified execution stack under arbitrary triggered action and SQL-invoked routine call graphs.
    Since:
    1.7.0
    Author:
    Peter Hudson, boucherb@users
    • Constructor Detail

      • TriggerSample

        public TriggerSample()
    • Method Detail

      • fire

        public void fire​(int typ,
                         java.lang.String trn,
                         java.lang.String tn,
                         java.lang.Object[] or,
                         java.lang.Object[] nr)
        A sample HSQLDB Trigger interface implementation.

        This sample prints information about the firing trigger and records actions in an audit table.

        The techniques used here are simplified dramatically for demonstration purposes and are in no way recommended as a model upon which to build actual installations involving triggered actions.

        Specified by:
        fire in interface Trigger
        Parameters:
        typ - trigger type
        trn - trigger name
        tn - table name
        or - old row
        nr - new row
      • getWhenSpec

        public static java.lang.String getWhenSpec​(int type)
      • getOperationSpec

        public static java.lang.String getOperationSpec​(int type)
      • getQueueSpec

        public static java.lang.String getQueueSpec​(int qs)
      • getForEachSpec

        public static java.lang.String getForEachSpec​(int type)
      • getTriggerDDL

        public static java.lang.String getTriggerDDL​(java.lang.String trn,
                                                     int typ,
                                                     java.lang.String tab,
                                                     int qs,
                                                     java.lang.String impl)
                                              throws java.sql.SQLException
        Throws:
        java.sql.SQLException
      • getTriggerDescriptor

        public static java.lang.String getTriggerDescriptor​(java.lang.String trn,
                                                            int typ,
                                                            java.lang.String tab)
      • main

        public static void main​(java.lang.String[] args)
                         throws java.sql.SQLException
        Throws:
        java.sql.SQLException