swt canvas.redraw doesn't work for clicking button

the code is :































































import org.eclipse.swt.SWT;































































import org.eclipse.swt.events.PaintEvent;































































import org.eclipse.swt.events.PaintListener;































































import org.eclipse.swt.events.SelectionAdapter;































































import org.eclipse.swt.events.SelectionEvent;































































import org.eclipse.swt.layout.GridData;































































import org.eclipse.swt.layout.GridLayout;































































import org.eclipse.swt.widgets.Button;































































import org.eclipse.swt.widgets.Canvas;































































import org.eclipse.swt.widgets.Display;































































import org.eclipse.swt.widgets.Shell;































































public class caseButton {































































private Shell shell;































































public static void main(String[] args) {































































try {































































caseButton window = new caseButton();































































window.open();































































} catch (Exception e) {































































e.printStackTrace();































































}































































}

/**































































* Open the window.































































*/































































public void open() {































































Display display = Display.getDefault();































































createContents();































































shell.open();































































shell.layout();































































while (!shell.isDisposed()) {































































if (!display.readAndDispatch()) {































































display.sleep();































































}































































}































































}































































/**































































* Create contents of the window.































































*/































































protected void createContents() {































































shell = new Shell();































































shell.setText("SWT Application");































































GridLayout gridLayout = new GridLayout();































































gridLayout.numColumns = 1;































































gridLayout.marginWidth = 0;































































gridLayout.marginHeight = 0;































































gridLayout.verticalSpacing = 4;































































gridLayout.makeColumnsEqualWidth=false;































































shell.setLayout(gridLayout);

GridData gridData=new GridData();































































gridData.verticalSpan = 0;































































gridData.horizontalSpan = 3;































































gridData.verticalAlignment = GridData.FILL;































































gridData.grabExcessVerticalSpace= true;































































gridData.horizontalAlignment = GridData.FILL;































































gridData.grabExcessHorizontalSpace = true;

final Button button = new Button(shell, SWT.PUSH);































































button.setText("draw Rectangle");

final Canvas canvas = new Canvas(shell, SWT.NONE);































































canvas.setLayoutData(gridData);

canvas.addPaintListener(new PaintListener() {































































public void paintControl(PaintEvent e) {































































e.gc.drawRoundRectangle(10, 10, 180, 80, 10, 10);































































}































































});































































button.addSelectionListener(new SelectionAdapter() {































































@Override































































public void widgetSelected(SelectionEvent e) {































































System.out.println("buttton_start ");































































canvas.redraw(); // it can' work































































System.out.println("buttton_end ");

}































































});































































}

}

the question is : when I click this button, it will print "buttton_start" and "buttton_end", the "canvas.redraw()" can't work. I don't know the reason , who can help me? thanks