org.hsqldb.sample
Class TriggerSample

java.lang.Object
  org.hsqldb.sample.TriggerSample

All Implemented Interfaces:

Trigger

public class TriggerSample
extends 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

Field Summary

Fields inherited from interface org.hsqldb.Trigger

DELETE_AFTER, DELETE_AFTER_ROW, DELETE_BEFORE_ROW, INSERT_AFTER, INSERT_AFTER_ROW, INSERT_BEFORE_ROW, UPDATE_AFTER, UPDATE_AFTER_ROW, UPDATE_BEFORE_ROW

Constructor Summary

TriggerSample()

Method Summary

void	fire(int typ, String trn, String tn, Object[] or, Object[] nr) A sample HSQLDB Trigger interface implementation.
static String	getForEachSpec(int type)
static String	getOperationSpec(int type)
static String	getQueueSpec(int qs)
static String	getTriggerDDL(String trn, int typ, String tab, int qs, String impl)
static String	getTriggerDescriptor(String trn, int typ, String tab)
static String	getWhenSpec(int type)
static void	main(String[] args)

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

TriggerSample

public TriggerSample()

Method Detail

fire

public void fire(int typ,
                 String trn,
                 String tn,
                 Object[] or,
                 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 String getWhenSpec(int type)

getOperationSpec

public static String getOperationSpec(int type)

getQueueSpec

public static String getQueueSpec(int qs)

getForEachSpec

public static String getForEachSpec(int type)

getTriggerDDL

public static String getTriggerDDL(String trn,
                                   int typ,
                                   String tab,
                                   int qs,
                                   String impl)
                            throws SQLException

Throws:

SQLException

getTriggerDescriptor

public static String getTriggerDescriptor(String trn,
                                          int typ,
                                          String tab)

main

public static void main(String[] args)
                 throws SQLException

Throws:

SQLException