This patch updates the ImageViewer example to use Multiple Threading.

When importing an image into the database, the example now fires off a
new
Thread, which imports the image in the background. This also means that
the application doesn't freeze on the user, and they can still browse
the
images in the database, while the upload is running.

This now makes the ImageViewer a true example on how to use Threads (the
threadtest class is just that - a test).

Peter

Author: bmomjian

src/interfaces/jdbc/example/ImageViewer.java

@@ -26,6 +26,9 @@
  * It's only if you use the non jdbc facilities, do you have to take this into
  * account.
  *
+ * Note: For PostgreSQL 6.4, the driver is now Thread safe, so this example
+ * application has been updated to use multiple threads, especially in the
+ * image storing and retrieving methods.
  */
 
 public class ImageViewer implements ItemListener
@@ -42,8 +45,15 @@ public class ImageViewer implements ItemListener
   // This is a simple component to display our image
   public class imageCanvas extends Canvas
   {
+    // holds the image
     private Image image;
 
+    // holds the background buffer
+    private Image bkg;
+    
+    // the size of the buffer
+    private Dimension size;
+    
     public imageCanvas()
     {
       image=null;
@@ -71,14 +81,35 @@ public void update(Graphics g)
       paint(g);
     }
 
-    public void paint(Graphics g)
+    /**
+     * Paints the image, using double buffering to prevent screen flicker
+     */
+    public void paint(Graphics gr)
     {
+      Dimension s = getSize();
+      
+      if(size==null || bkg==null || !s.equals(size)) {
+	size = s;
+	bkg = createImage(size.width,size.height);
+      }
+      
+      // now set the background
+      Graphics g = bkg.getGraphics();
       g.setColor(Color.gray);
-      g.fillRect(0,0,getSize().width,getSize().height);
+      g.fillRect(0,0,s.width,s.height);
 
+      // now paint the image over the background
       if(image!=null)
 	g.drawImage(image,0,0,this);
+      
+      // dispose the graphics instance
+      g.dispose();
+      
+      // paint the image onto the component
+      gr.drawImage(bkg,0,0,this);
+      
     }
+    
   }
 
   public ImageViewer(Frame f,String url,String user,String password) throws ClassNotFoundException, FileNotFoundException, IOException, SQLException
@@ -197,9 +228,8 @@ public void close()
   }
 
   /**
-   * This imports an image into the database.
-   *
-   * This is the most efficient method, using the large object extension.
+   * This imports an image into the database, using a Thread to do this in the
+   * background.
    */
   public void importImage()
   {
@@ -209,51 +239,92 @@ public void importImage()
     String dir = d.getDirectory();
     d.dispose();
 
-    // Now the real import stuff
-    if(name!=null && dir!=null) {
-      try {
-	System.out.println("Importing file");
-	// A temporary buffer - this can be as large as you like
-	byte buf[] = new byte[2048];
-	
-	// Open the file
-	System.out.println("Opening file "+dir+"/"+name);
-	FileInputStream fis = new FileInputStream(new File(dir,name));
-	
-	// Gain access to large objects
-	System.out.println("Gaining LOAPI");
-	
-	// Now create the large object
-	System.out.println("creating blob");
-	int oid = lom.create();
-	
-	System.out.println("Opening "+oid);
-	LargeObject blob = lom.open(oid);
+    // now start the true importer
+    Thread t = new importer(db,name,dir);
+    //t.setPriority(Thread.MAX_PRIORITY);
+    t.start();
+  }
+  
+  /**
+   * This is an example of using a thread to import a file into a Large Object.
+   * It uses the Large Object extension, to write blocks of the file to the
+   * database.
+   */
+  class importer extends Thread
+  {
+    String name,dir;
+    Connection db;
+    
+    public importer(Connection db,String name,String dir) {
+      this.db = db;
+      this.name = name;
+      this.dir = dir;
+    }
+    
+    public void run() {
+      
+      // Now the real import stuff
+      if(name!=null && dir!=null) {
+	Statement stat = null;
 
-	// Now copy the file into the object.
-	//
-	// Note: we dont use write(buf), as the last block is rarely the same
-	// size as our buffer, so we have to use the amount read.
-	System.out.println("Importing file");
-	int s,t=0;
-	while((s=fis.read(buf,0,buf.length))>0) {
-	  System.out.println("Block s="+s+" t="+t);t+=s;
-	  blob.write(buf,0,s);
+	try {
+	  // fetch the large object manager
+	  LargeObjectManager lom = ((postgresql.Connection)db).getLargeObjectAPI();
+	  
+	  System.out.println("Importing file");
+	  // A temporary buffer - this can be as large as you like
+	  byte buf[] = new byte[2048];
+	  
+	  // Open the file
+	  System.out.println("Opening file "+dir+"/"+name);
+	  FileInputStream fis = new FileInputStream(new File(dir,name));
+	  
+	  // Gain access to large objects
+	  System.out.println("Gaining LOAPI");
+	  
+	  // Now create the large object
+	  System.out.println("creating blob");
+	  int oid = lom.create();
+	  
+	  System.out.println("Opening "+oid);
+	  LargeObject blob = lom.open(oid);
+	  
+	  // Now copy the file into the object.
+	  //
+	  // Note: we dont use write(buf), as the last block is rarely the same
+	  // size as our buffer, so we have to use the amount read.
+	  System.out.println("Importing file");
+	  int s,t=0;
+	  while((s=fis.read(buf,0,buf.length))>0) {
+	    System.out.println("Block s="+s+" t="+t);t+=s;
+	    blob.write(buf,0,s);
+	  }
+	  
+	  // Close the object
+	  System.out.println("Closing blob");
+	  blob.close();
+	  
+	  // Now store the entry into the table
+	  
+	  // As we are a different thread to the other window, we must use
+	  // our own thread
+	  stat = db.createStatement();
+	  stat.executeUpdate("insert into images values ('"+name+"',"+oid+")");
+	  
+	  // Finally refresh the names list, and display the current image
+	  ImageViewer.this.refreshList();
+	  ImageViewer.this.displayImage(name);
+	} catch(Exception ex) {
+	  label.setText(ex.toString());
+	} finally {
+	  // ensure the statement is closed after us
+	  try {
+	    if(stat != null)
+	      stat.close();
+	  } catch(SQLException se) {
+	    System.err.println("closing of Statement failed");
+	  }
 	}
-	
-	// Close the object
-	System.out.println("Closing blob");
-	blob.close();
-	
-	// Now store the entry into the table
-	stat.executeUpdate("insert into images values ('"+name+"',"+oid+")");
-	stat.close();
-	
-	// Finally refresh the names list, and display the current image
-	refreshList();
-	displayImage(name);
-      } catch(Exception ex) {
-	label.setText(ex.toString());
       }
     }
   }
@@ -364,7 +435,7 @@ public static void main(String args[])
     }
 
     try {
-      Frame frame = new Frame("PostgreSQL ImageViewer v6.3 rev 1");
+      Frame frame = new Frame("PostgreSQL ImageViewer v6.4 rev 1");
       frame.setLayout(new BorderLayout());
       ImageViewer viewer = new ImageViewer(frame,args[0],args[1],args[2]);
       frame.pack();