triggersample.java

一个用java写的开源的数据库系统

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// use the org.hsqldb.sample package here; but will be in your own package usually
package org.hsqldb.sample;

// peterhudson@users 20020130 - patch 478657 by peterhudson - new class

/**
 * <P>Sample code for use of triggers in hsqldb.
 *
 * SQL to invoke is:<p>
 * CREATE TRIGGER triggerSample BEFORE|AFTER INSERT|UPDATE|DELETE
 * ON myTable [FOR EACH ROW] [QUEUE n] [NOWAIT] CALL "myPackage.trigClass"<br>
 *
 * 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.<p>
 *
 * 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.<p>
 *
 * Ensure that "myPackage.trigClass" is present in the classpath which
 * you use to start hsql.<p>
 *
 * If the method wants to access the database, it must establish
 * a JDBC connection.<p>
 *
 * When the 'fire' method is called, it is passed the following arguments:<br>
 * fire (String trigName, String tabName, Object row[])<br>
 *
 * where 'row' represents the row acted on, with each column being
 * a member of the array. The mapping of row classes to database
 * types is specified in /doc/hsqlSyntax.html#Datatypes
 *
 *
 * For implementation at a later date:<br>
 * 1.   jdbc:default:connection: URL for JDBC trigger method connections to
 * the database.<br>
 * 2.   arguments to the trigger method.<br>
 * 3.   Because they run in different threads, it is possible for an
 * 'after' trigger to run before its corresponding 'before' trigger;
 * the acceptability of this needs to be investigated.
 *
 * @author Peter Hudson
 * @version 1.7.0
 */
public class TriggerSample implements org.hsqldb.Trigger {

    /**
     * fire method declaration
     * <P> This is a sample implementation that simply prints information
     * about the trigger firing.
     *
     * @param trigName
     * @param tabName
     * @param row
     */
    public void fire(String trigName, String tabName, Object row[]) {

        System.out.println(trigName + " trigger fired on " + tabName);
        System.out.print("col 0 value <");
        System.out.print(row[0]);
        System.out.println(">");

        // you can cast row[i] given your knowledge of what the table
        // format is.
    }
}

/**
 *
 *
 * test SQL
 * CREATE CACHED TABLE trig_test (int_field     integer)
 * CREATE TRIGGER ins_before BEFORE INSERT ON trig_test CALL "org.hsqldb.sample.TriggerSample"
 * CREATE TRIGGER ins_after  AFTER  INSERT ON trig_test CALL "org.hsqldb.sample.TriggerSample"
 * CREATE TRIGGER upd_before BEFORE UPDATE ON trig_test CALL "org.hsqldb.sample.TriggerSample"
 * CREATE TRIGGER upd_after  AFTER  UPDATE ON trig_test CALL "org.hsqldb.sample.TriggerSample"
 * CREATE TRIGGER upd_before_row BEFORE UPDATE ON trig_test FOR EACH ROW CALL "org.hsqldb.sample.TriggerSample"
 * CREATE TRIGGER upd_after_row  AFTER  UPDATE ON trig_test FOR EACH ROW CALL "org.hsqldb.sample.TriggerSample"
 * CREATE TRIGGER del_before BEFORE DELETE ON trig_test CALL "org.hsqldb.sample.TriggerSample"
 * CREATE TRIGGER del_after  AFTER  DELETE ON trig_test CALL "org.hsqldb.sample.TriggerSample"
 * CREATE TRIGGER del_before_row BEFORE DELETE ON trig_test FOR EACH ROW CALL "org.hsqldb.sample.TriggerSample"
 * CREATE TRIGGER del_after_row  AFTER  DELETE ON trig_test FOR EACH ROW CALL "org.hsqldb.sample.TriggerSample"
 * INSERT INTO trig_test VALUES (1)
 * INSERT INTO trig_test VALUES (2)
 * INSERT INTO trig_test VALUES (3)
 * UPDATE trig_test SET int_field = int_field + 3
 * DELETE FROM trig_test
 */