How to implement a listener for access events of a sqlite database in Java

问题

I'd like to display a 'indeterminate JProgressBar' in my Java application each time it accesses the SQLite database for reading / writing. I think the most important thing is to realize a trigger signal from the database. Then implement a listener to this trigger to display the progress bar.

I use SQLite with the Xerial JDBC driver with JDK 1.7. The application is 'stand alone'. The SQLite database is on the hdd. I have a good basic understanding of Java, but i'm no expert. Approaches with Hibernate / Spring etc. are way too much for me.

What i've found so far from an internet search (click header links for details):

Idea 1 (Manual approach):

Manually start the progress bar each time needed. Then execute Swingworker. Use its overridden done() method to stop the progress bar.

Pro:

Very easy approach

Con:

A lot of coding necessary for every event you need the progress bar

Idea 2 (Overriding methods):

Override the commit() and the close() method of java.sql.Connection and send a trigger signal everytime these methods are executed.

Pro:

Very easy approach
No extra coding necessary
Exact start and stop trigger for database access
JDBC approach

Con:

Parsing of commits necessary to find out if read or write access

Idea 3 (Man in the middle):

Use jdbctools as a middle layer for monitoring. Use JDBCLogger or JDBCCallFilter to detect accesses.

Pro:

No extra coding necessary
Exact start and stop trigger for database access

Con:

Little documentation / no examples
Outdated (2008)?

Idea 4 (Pooling):

Use ConnectionEventListener from javax.sqlto monitor database connection.

Pro:

Built-in Java SQL functionality

Con:

Needs a PooledConnection object which is a bit over the top for the task
No trigger for connection start

Idea 5 (use SQLite C interface):

Use 'Query Progress Callbacks' from SQLite C Interface which is intended for the use case.

Pro:

Native SQLite functionality

Con:

Because it is in 'C' it must be implemented in the JDBC driver to use this functionality. The Xerial driver doesn't seem to have it implemented
Limited to SQLite database

Summary: I'd tend to try 'idea 2' but i'd like to hear what the experts are saying first: Which approach would you prefer or how would you implement that kind of trigger signal?

回答1:

At first:

Idea 2 (Overriding methods) didn't work because the methods are static and cannot be overridden.
Idea 5 (use SQLite C interface) needs a Java implementation for 'Query Progress Callbacks'. There's already some work done but it has not been merged into the JDBC driver by Xerial yet. For more details see here.
Idea 3 & 4 seem to be feasable but make my project too complicated and harder to maintain.

So i came across to extend Idea 1:

The best triggers for a progress bar are the moments when you open and close a database connection.
These triggers can fire events which can be caught by an event listener that will open a dialog with a progress bar

At first you need to create some classes for the event mechanisms. You can find details about the idea of EventObjects here and here.

Db_Status.java:

public class Db_Status 
{
    public final static Db_Status CONNECTED = new Db_Status("connected");
    public final static Db_Status PENDING = new Db_Status("pending");
    public final static Db_Status CLOSED = new Db_Status("closed"); // Closed database connection
    public final static Db_Status IDLE = new Db_Status("idle"); // Initial status of database after app start
    public final static Db_Status ERROR = new Db_Status("error"); // Error while accessing / closing database

    private String status;

    public String toString() {
        return status;
    }

    private Db_Status(String statusstring) {
        status = statusstring;
    }

}

Db_ActionEvent.java:

public class Db_ActionEvent extends EventObject 
{
    private Db_Status status;

    public Db_ActionEvent(Object source, Db_Status DBstate) {
        super(source);
        status = DBstate;
    }
    public Db_Status Status() {
        return status;
    }

}

Db_ActionListener.java:

public interface Db_ActionListener {

    public void DbActionReceived(Db_ActionEvent event );

}

You must then implement the listener. This is the place for the reaction after the listener has caught a database event, here: ouput to console:

public class UI_DbProgressbar implements Db_ActionListener{
    public void DbActionReceived(Db_ActionEvent event) {
        if(event.Status() == Db_Status.PENDING)
        {
            System.out.println("DB is busy!" );
        }
        else if(event.Status() == Db_Status.CLOSED)
        {
            System.out.println("DB connection is closed!");
        }
        else if(event.Status() == Db_Status.CONNECTED)
        {
            System.out.println("DB is connected!");
        }
        else
        {
            System.out.println("DB idle!" );
        }
    }
}

Finally you must establish the trigger mechanism. You may have a central class that handles all of the database accesses. There you will have to

Create two central methods to open and close connections
Add some methods to control the listener

Adapt the constructor to add right from the creation of the class a listener to it.

public class Db_DataExchange{

//
// Adaptions to the constructor
//

public Db_DataExchange() 
{
    this.DBStateListeners = new ArrayList();

    // Your constructor code here ...

    UI_DbProgressbar listener = new UI_DbProgressbar();
    this.addDbActionListener(listener);
}

//
// Add some methods to control the listener
//

private Db_Status DBAccessStatus = Db_Status.IDLE;
private List DBStateListeners;

public synchronized void addDbActionListener(Db_ActionListener l) {
    DBStateListeners.add( l );
}

public synchronized void removeDbActionListener(Db_ActionListener l) {
    DBStateListeners.remove( l );
}

private synchronized void fireDbActionEvent(Db_Status AccessStatus) 
{
    DBAccessStatus = AccessStatus;
    Db_ActionEvent status = new Db_ActionEvent(this, DBAccessStatus);
    Iterator listeners = DBStateListeners.iterator();
    while(listeners.hasNext()) {
        ((Db_ActionListener) listeners.next()).DbActionReceived(status);
    }
}

//
// Create two central methods to open and close connections
//

/**
 * Your method to establish a jdbc database connection  
 */
public Connection EstablishDBConnection(String ConnectionURL){
    Connection c = null;

    try {
        // Load JDBC sqlite driver
        Class.forName(this.DBDriverName);

        // url - a database url of the form jdbc:subprotocol:subname
        c = DriverManager.getConnection(ConnectionURL);

        c.setAutoCommit(false);

        System.out.println("Database connected successfully!");
        // this ist the important trigger:
        this.fireDbActionEvent(Db_Status.CONNECTED);

        return c;

    } catch ( Exception e ) {
        System.err.println( e.getClass().getName() + ": " + e.getMessage() );
        this.fireDbActionEvent(Db_Status.ERROR);
        return c;
    }

}

/**
 * Your method to close a jdbc database connection 
 */
public boolean CloseDBConnection(Connection c){
    try 
    {
        c.close(); // finally close
        // this ist the important trigger:
        this.fireDbActionEvent(Db_Status.CLOSED);

        return true;

    } catch ( Exception e ) 
    {
        //System.err.println( e.getClass().getName() + ": " + e.getMessage() );
        logger.error("Error in {}: {}",e.getClass().getName(),e.getMessage());
        this.fireDbActionEvent(Db_Status.ERROR);

        return false;
    }
}

}

来源：https://stackoverflow.com/questions/36306324/how-to-implement-a-listener-for-access-events-of-a-sqlite-database-in-java

标签

java

database

sqlite

database-connection

listener