WICKET-6963 Use singletons for PanelMarkupSourcingStrategy (#503)

(cherry picked from commit 2e3e95f)

Author: theigl

wicket-core/src/main/java/org/apache/wicket/markup/html/border/BorderPanel.java

@@ -92,7 +92,7 @@ public BorderPanel(final String id, final IModel<?> model)
 	@Override
 	protected IMarkupSourcingStrategy newMarkupSourcingStrategy()
 	{
-		return new PanelMarkupSourcingStrategy(true);
+		return PanelMarkupSourcingStrategy.get(true);
 	}
 
 	/**

wicket-core/src/main/java/org/apache/wicket/markup/html/form/FormComponentPanel.java

@@ -146,7 +146,7 @@ public boolean checkRequired()
 	@Override
 	protected IMarkupSourcingStrategy newMarkupSourcingStrategy()
 	{
-		return new PanelMarkupSourcingStrategy(false);
+		return PanelMarkupSourcingStrategy.get(false);
 	}
 
 	@Override

wicket-core/src/main/java/org/apache/wicket/markup/html/panel/Panel.java

@@ -81,7 +81,7 @@ public Panel(final String id, final IModel<?> model)
 	@Override
 	protected IMarkupSourcingStrategy newMarkupSourcingStrategy()
 	{
-		return new PanelMarkupSourcingStrategy(false);
+		return PanelMarkupSourcingStrategy.get(false);
 	}
 
 	/**

wicket-core/src/main/java/org/apache/wicket/markup/html/panel/PanelMarkupSourcingStrategy.java

@@ -33,9 +33,28 @@
  */
 public class PanelMarkupSourcingStrategy extends AssociatedMarkupSourcingStrategy
 {
+	private static final PanelMarkupSourcingStrategy PANEL_INSTANCE = new PanelMarkupSourcingStrategy(
+		false);
+	private static final PanelMarkupSourcingStrategy BORDER_INSTANCE = new PanelMarkupSourcingStrategy(
+		true);
+
 	// False for Panel and true for Border components.
 	private final boolean allowWicketComponentsInBodyMarkup;
 
+	/**
+	 * @param allowWicketComponentsInBodyMarkup
+	 *            {@code false} for Panel and {@code true} for Border components. If Panel then the
+	 *            body markup should only contain raw markup, which is ignored (removed), but no
+	 *            Wicket Component. With Border components, the body markup will be associated with
+	 *            the Body Component.
+	 * 
+	 * @return A singleton of the strategy
+	 */
+	public static PanelMarkupSourcingStrategy get(final boolean allowWicketComponentsInBodyMarkup)
+	{
+		return allowWicketComponentsInBodyMarkup ? BORDER_INSTANCE : PANEL_INSTANCE;
+	}
+
 	/**
 	 * Constructor.
 	 *