Swing Form = JNI = Delphi DLL = Delphi Form works but small issue

Question

Hi all
I'm trying to call a delphi dll from a java form.
This is the procedure
swing button action => jni => call procedure in delphi dll => create delphi formThis works for me but the problem is when click the button it create the delphi form by calling the dll but it's stucked for ever(frame not the whole form). Never return to do something else. Means that it's stay remain in clicked postion.
I do not know whether the problem exist with the swings' single threaded behavior. I have used another thread to call natives. But the problem persist.
So can any one suggest any answer.
Wish U all HAPPY NEW YEAR!
Here is the compact code
native calls
public class LoadLib
    public native void createForm();
    public native void forward();
    static
        System.loadLibrary("testjni");
}calling class
import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.Container;
import java.awt.event.ActionListener;
public class RUNClient extends JFrame {
     JFrame frame = new JFrame();
     JFrame frame2 = new JFrame();
     LoadLib lib = new LoadLib();
     JButton b1 = new JButton("OK");
     JButton b2 = new JButton("Forwad");
     private RUNClient() {
          frame.setLayout(null);
          frame.add(b1);
          frame.add(b2);
          b1.setBounds(110, 80, 80, 20);
          b2.setBounds(110, 50, 80, 20);
          b1.addActionListener(new ActionListener() {
               public void actionPerformed(ActionEvent e) {
                    lib.createForm();     // create form in dll
                    // after this call frame is stucked
          b2.addActionListener(new ActionListener() {
               public void actionPerformed(ActionEvent e) {
                    lib.forward();          // call another native to do some in created form
                    // this cannot be called because frame is stuck
          // other frame operation
     public static void main(String args[]) {
          new RUNClient();
}delphi code
library testjni;
uses
  JNI,Unit1;
  var
  f : TForm1;
procedure Java_LoadLib_createForm(PEnv: PJNIEnv; Obj: JObject); {$IFDEF WIN32} stdcall; {$ENDIF} {$IFDEF LINUX} cdecl; {$ENDIF}
begin
try
  f := TForm1.Create(nil);
  f.ShowModal;    
   finally
     f.Free;
     f.Release;
   end;
end;
procedure Java_LoadLib_forward(PEnv: PJNIEnv; Obj: JObject); {$IFDEF WIN32} stdcall; {$ENDIF} {$IFDEF LINUX} cdecl; {$ENDIF}
begin
Form1.Top := Form1.Top - 50;
end;
exports
  Java_LoadLib_createForm,
  Java_LoadLib_forward;
begin
end.

Answer 1

Hi deshan,
To realize the problem you should know principles of Java GUI (SWING or AWT) implementation in JVM:
1) Java GUI is implemented in a single thread with EventQueue static object, which is created with JVM when Java Application GUI starts.
2) Any painting or event calls are done on this thread (the same approach was used by Microsoft in .NET GUI).
3) While your code handles an event EventQueue waits for return from your event method. Some calls to GUI components can stick the whole Java GUI (this is the problem of SUN Java architecture, I did not have this in Microsoft Java). To avoid this problem you should use InvokeLater method that works like PostMessage to the main thread in Windows Application. InvokeLater generate an event that is posted to EventQueue and your code passed to InvokeLater method will be executed asynchronously in GUI Thread.
4) If you call any Java (SWING or AWT) component from the thread other than EventQueue thread you must synchronize these calls with locks by the object got from getTreeLock() method of the component you call. But you should be accurate with locks because they can cause deadlocks.
Your code does not return from Click Event where you try to create some GUI component but EventQueue is still waiting for “return” and the main GUI thread is suspended.