Allow for more than 2 nested components.

The VPOLL component can contain VEVENT with nested VALARMS to make 3 levels.

There is a limit check on the depth - it's set to 10 and the only reason for the moment is to block DOS attacks with extremely deep components.

src/main/java/net/fortuna/ical4j/data/DefaultContentHandler.java

@@ -14,6 +14,7 @@
 import java.net.URISyntaxException;
 import java.text.ParseException;
 import java.util.ArrayList;
+import java.util.LinkedList;
 import java.util.List;
 import java.util.function.Consumer;
 import java.util.function.Supplier;
@@ -38,9 +39,10 @@ public class DefaultContentHandler implements ContentHandler {
 
     private PropertyBuilder propertyBuilder;
 
-    private ComponentBuilder<CalendarComponent> componentBuilder;
-
-    private ComponentBuilder<Component> subComponentBuilder;
+    /**
+     * The current component builders.
+     */
+    private final LinkedList<ComponentBuilder<CalendarComponent>> components = new LinkedList<>();
 
     private Calendar calendar;
 
@@ -61,9 +63,21 @@ public DefaultContentHandler(Consumer<Calendar> consumer, TimeZoneRegistry tzReg
         this.componentFactorySupplier = componentFactorySupplier;
     }
 
+    public ComponentBuilder<CalendarComponent> getComponentBuilder() {
+        if (components.size() == 0) {
+            return null;
+        }
+        return components.peek();
+    }
+
+    public void endComponent() {
+        components.pop();
+    }
+
     @Override
     public void startCalendar() {
         calendar = new Calendar();
+        components.clear();
         propertiesWithTzId = new ArrayList<>();
     }
 
@@ -76,33 +90,38 @@ public void endCalendar() throws IOException {
 
     @Override
     public void startComponent(String name) {
-        if (componentBuilder != null) {
-            subComponentBuilder = new ComponentBuilder<>();
-            subComponentBuilder.factories(componentFactorySupplier.get()).name(name);
-        } else {
-            componentBuilder = new ComponentBuilder<>();
-            componentBuilder.factories(componentFactorySupplier.get()).name(name);
+        if (components.size() > 10) {
+            throw new RuntimeException("Components nested too deep");
         }
+
+        ComponentBuilder<CalendarComponent> componentBuilder =
+                new ComponentBuilder<>();
+        componentBuilder.factories(componentFactorySupplier.get()).name(name);
+        components.push(componentBuilder);
     }
 
     @Override
     public void endComponent(String name) {
-        assertComponent(componentBuilder);
+        assertComponent(getComponentBuilder());
+
+        final ComponentBuilder<CalendarComponent> componentBuilder =
+                getComponentBuilder();
 
-        if (subComponentBuilder != null) {
-            Component subComponent = subComponentBuilder.build();
-            componentBuilder.subComponent(subComponent);
+        DefaultContentHandler.this.endComponent();
 
-            subComponentBuilder = null;
+        final ComponentBuilder<CalendarComponent> parent =
+                getComponentBuilder();
+
+        if (parent != null) {
+            Component subComponent = componentBuilder.build();
+            parent.subComponent(subComponent);
         } else {
             CalendarComponent component = componentBuilder.build();
             calendar.getComponents().add(component);
             if (component instanceof VTimeZone && tzRegistry != null) {
                 // register the timezone for use with iCalendar objects..
                 tzRegistry.register(new TimeZone((VTimeZone) component));
             }
-
-            componentBuilder = null;
         }
     }
 
@@ -127,12 +146,8 @@ public void endProperty(String name) throws URISyntaxException, ParseException,
         }
         // replace with a constant instance if applicable..
         property = Constants.forProperty(property);
-        if (componentBuilder != null) {
-            if (subComponentBuilder != null) {
-                subComponentBuilder.property(property);
-            } else {
-                componentBuilder.property(property);
-            }
+        if (getComponentBuilder() != null) {
+            getComponentBuilder().property(property);
         } else if (calendar != null) {
             calendar.getProperties().add(property);
         }