Drag and Drop Subsystem for the JavaTM 2 Platform Standard Edition 5.0

This document has these sections:

1.0 Requirements

This specification defines the API for the drag and drop facilities for the Java 2 Platform.

The primary requirements that this specification addresses, are:

  1. Provision of a platform-independent drag and drop facility for Java GUI clients implemented through AWT and Swing classes.
  2. Integration with platform dependent drag and drop facilities, permitting Java clients to be able to participate in DnD operation with native applications using:
    • OLE (Win32) DnD
    • XDND protocol
    • CDE/Motif dynamic protocol
    • MacOS
    • OS/2
  3. Support for 100% pure JavaOS/Java implementation.
  4. Leverages the existing java.awt.datatransfer.* package to enable the transfer of data, described by an extensible data type system based on the MIME standard.
  5. Does not preclude the use of "accessibility" features where available.
  6. Extensible to support diverse input devices.

The specification derives from the previous work mentioned above, but incorporates significant differences from that original work as a result of the advent of the JavaBeansTM event model, lightweight (Swing) components, and an increasing understanding of the cross-platform integration and interoperability issues.

2.0 API

The following sections cover the drag and drop API.

2.1 Overview

Drag and drop is a direct manipulation gesture found in many graphical user interface systems that provides a mechanism to transfer information between two entities logically associated with presentation elements in the GUI. Normally driven by a physical gesture of a human user using an appropriate input device, drag and drop provides both a mechanism to enable continuous feedback regarding the possible outcome of any subsequent data transfer to the user during navigation over the presentation elements in the GUI, and the facilities to provide for any subsequent data negotiation and transfer.

A typical drag and drop operation can be decomposed into the following states (not entirely sequentially):

The remainder of this document details the proposed API changes to support this model.

2.2 Drag Gesture Recognition

The gesture(s) that can initiate a drag and drop operation vary, not only per platform, but also per Component, and per device. Therefore a mechanism is required in order to encapsulate these dependencies, thus making the task of the author of a Component that wishes to initiate a drag and drop operation much simpler.

2.2.1 DragGestureRecognizer

The DragGestureRecognizer is an abstract base class for all device/platform/Component specific drag and drop gesture recognizers, and is defined as:

public abstract DragGestureRecognizer {
    protected DragGestureRecognizer(
        DragSource  ds,
        Component c,
        int  srcActions,
        DragGestureListener dgl
    );
    public Component getComponent();
    public void      setComponent(Component c);
    public int  getSourceActions();
    public void setSourceActions(int actions);
    public java.awt.InputEvent getTriggerEvent();
    public void resetRecognizer();
    public void addDragGestureListener(
        DragGestureListener dgl
    ) throws TooManyListenerExceptions;
    public void removeDragGestureListener(
       DragGestureListener dgl
    );
    protected abstract void registerListeners();
    protected abstract void unregisterListeners();
    protected void fireDragGestureRecognized(
       int dragAction
    );
    protected void appendEvent(InputEvent awtie);
}

An appropriate concrete subclasses of DragGestureRecognizer for a particular may be obtained in a variety of ways; from a DragSource instance, from the Toolkit, or by other means. Concrete implementation subclasses are obtained through standard APIs' by specifying a Class reference to an abstract DragGestureRecognizer superclass, an instance of a concrete subclass of this actual parameter is instantiated and returned to the requestor.

Once a DragGestureRecognizer instance is associated with a Component and a DragSource it registers its own particular set of EventListeners' with the target Component in order to monitor the appropriate events being delivered to that Component to detect an initiating gesture. (Using registerListeners and unregisterListeners to add/remove these monitoring EventListeners').

Note that a DragGestureRecognizer may throw either an IllegalStateException or an IllegalArgumentException if either the Component or DragSource specified is either not in the correct state for, or is not interoperable with, that DragGestureRecognizer.

When a concrete DragGestureRecognizer instance detects a drag-initiating user gesture on the Component it is associated with, it will fire a DragGestureEvent to the DragGestureListener registered on its unicast event source for DragGestureListener events. This DragGestureListener is responsible for causing the associated DragSource to start the drag and drop operation (if appropriate).

The implementation provides (at least) an abstract subclass for recognizing mouse device gestures MouseDragGestureRecognizer. Other abstract subclasses may be provided by the platform to support other input devices or particular Component class semantics. Concrete superclasses of this MouseDragGestureRecognizer that encapsulate platform dependent mouse based gestures are available from the Toolkit object via its createDragGestureRecognizer(Class adgrc, DragSource ds, Component c, int sa, DragGestureListener dgl) method. This Toolkit API provides platform dependent concrete implementations that extend particular platform independent abstract definitions (classes).

The MouseDragGestureRecognizer abstract class is defined as:

public abstract   MouseDragGestureRecognizer
       extends    DragGestureRecognizer
          implements MouseListener, MouseMotionListener {

    public MouseDragGestureRecognizer(
        DragSource  ds,
        Component   c,
        int  sa,
        DragGestureListener dsl
    );   
    //...
}

The DragGestureListener is defined as:

public interface DragGestureListener extends EventListener {
    void dragGestureRecognized(DragGestureEvent dge);
}

Usually the dragGestureRecognized() method will simply, via the DragGestureEvent's convenience API startDrag, start a drag and drop operation on the associated DragSource.

Note that per Component (class or instance) behavior that may effect the initiating gesture would usually be implemented in this DragGestureListener method, or in the DragGestureRecognizer subclass where appropriate or possible.

The DragGestureEvent is defined as:

publc class DragGestureEvent extends EventObject {
        public DragGestureEvent(DragGestureRecognizer dgr,
             int  dragAction,
             java.util.List  events
     );
    public DragGestureRecognizer getSourceAsDragGestureRecognizer();
    public Component  getComponent();
    public DragSource getDragSource();
    public java.util.Iterator iterator();
    public Object[] toArray();
    public Object[] toArray(Object[] array);
    public int getDragAction();
    public startDrag(Cursor             dragCursor,
                     Transferable       t,
                     DragSourceListener dsl
    );
    public startDrag(Cursor             dragCursor,
                     Image              dragImage,
                     Point              imageOffset,
                     Transferable       t,
                     DragSourceListener dsl
    );
    //...
}

The DragGestureEvent encapsulates all the information regarding the nature of the gesture that has just been recognized, including:

2.3 Drag Source

The DragSource is the entity responsible for the initiation of the drag and drop operation:

2.3.1 The DragSource definition

The DragSource and associated constant interfaces are defined as follows:

The DnDConstants class defines the operations that may be applied to the subject of the transfer:

public final class java.awt.dnd.DnDConstants {
     public static int ACTION_NONE = 0x0;
     public static int ACTION_COPY = 0x1;
     public static int ACTION_MOVE = 0x2;
     public static int ACTION_COPY_OR_MOVE= ACTION_COPY | ACTION_MOVE;
     public static int ACTION_REFERENCE = 0x40000000;
}

public class java.awt.dnd.DragSource {
     public static Cursor     DefaultCopyDrop;
     public static Cursor     DefaultMoveDrop;
     public static Cursor     DefaultLinkDrop;
     public static Cursor     DefaultCopyNoDrop;
     public static Cursor     DefaultMoveNoDrop;
     public static Cursor     DefaultLinkNoDrop;
     public static DragSource getDefaultDragSource();
     public static boolean isDragImageSupported();
     public void startDrag(DragGestureEvent trigger,
                        Cursor             dragCursor,
                        Image              dragImage,
                        Point              dragImageOffset,
                        Transferable       transferable,
                        DragSourceListener dsl,
                        FlavorMap          fm)
                  throws InvalidDnDOperationException;
     protected DragSourceContext  createDragSourceContext(
                        DragSourceContextPeer dscp,
                        DragGestureEvent      trigger,
                        Cursor                dragCursor,
                        Image                 dragImage,
                        Point                 dragImageOffset,
                        Transferable          transferable,
                        DragSourceListener    dsl
      );
      public FlavorMap getFlavorMap();
      public DragGestureRecongizer createDragGestureRecognizer(
              Class               abstractRecognizerClass,
              Component           c,
              int                 srcActions,
              DragGestureListener dgl
     );
     public DragGestureRecongizer createDefaultDragGestureRecognizer(
              Component           c,
              int                 srcActions,
              DragGestureListener dgl
     );
     //...
}

The DragSource may be used in a number of scenarios:

A controlling object, shall obtain a DragSource instance prior to a user's gesture, effecting an associated Component, in order to process the operation. Once obtained a DragGestureRecognizer should be obtained and used to associate the DragSource with a Component.

The initial interpretation of the user's gesture, and the subsequent starting of the drag operation are the responsibility of the implementing Component, this is usually implemented by a DragGestureRecognizer.

When a gesture occurs, the DragSource's startDrag method shall be invoked in order to cause processing of the user's navigational gestures and delivery of drag and drop protocol notifications. A DragSource shall only permit a single drag and drop operation to be current at any one time, and shall reject any further startDrag requests by throwing an IllegalDnDOperationException until such time as the extant operation is complete.

In order to start a drag operation the caller of the startDrag method shall provide the following parameters:

As stated above, the primary role of the startDrag method is to initiate a Drag on behalf of the user. In order to accomplish this, the startDrag method must create a DragSourceContext instance to track the operation itself, and more importantly it must initiate the operation itself in the underlying platform implementation. In order to accomplish this, the DragSource must first obtain a DragSourceContextPeer from the underlying system (usually via an invocation of java.awt.Toolkit.createDragSourceContextPeer method) and subsequently associate this newly created DragSourceContextPeer (which provides a platform independent interface to the underlying systems capabilities) with a DragSourceContext.The startDrag method invokes the createDragSourceContext method to instantiate an appropriate DragSourceContext and associate the DragSourceContextPeer with that.

If the drag and drop system is unable to initiate a drag operation for some reason the startDrag method shall throw a java.awt.dnd.InvalidDnDOperationException to signal such a condition. Typically this exception is thrown when the underlying platform system is either not in a state to initiate a drag, or the parameters specified are invalid.

Note that during the drag neither the set of operations the source exposed at the start of the drag operation may change for the duration of the operation, in other words the operation(s) and are constant for the duration of the operation with respect to the DragSource.

The getFlavorMap method is used by the underlying system to obtain a FlavorMap object in order to map the DataFlavors exposed by the Transferable to data type names of the underlying DnD platform. [see later for details of the FlavorMap]

A "private" FlavorMap may be provided to the startDrag() method of the DragSource, or null, in which case the "default" FlavorMap for that DragSource class or instance is used.

2.3.2 The DragSourceContext Definition

As a result of a DragSource's startDrag method being successfully invoked an instance of the DragSourceContext class is created. This instance is responsible for tracking the state of the operation on behalf of the DragSource and dispatching state changes to the DragSourceListener.

The DragSourceContext class is defined as follows:

public class DragSourceContext implements DragSourceListener {
        public DragSourceContext(
                DragSourceContextPeer dscp,
                DragGestureEvent  trigger,
                Cursor  dragCursor,
                Image  dragImage,
                Point  dragOffset,
                Transferable  transferable,
                DragSourceListener  dsl
        );
        public DragSource  getDragSource();
        public Component  getComponent();
        public DragGestureEvent getTrigger();
        public Image     getDragImage();
        public Point     getDragImageOffset();
        public void transferablesFlavorsChanged();
        public int getSourceActions();
        public Cursor getCursor();
        pbulic void   setCursor(Cursor Cursor)
                    throws InvalidDnDOperationException;
        public void addDragSourceListener(DragSourceListener dsl)
                    throws TooManyListenersException;
        public void removeDragSourceListener(DragSourceListener dsl);
        protected updateCurrentCursor(int dropOperation,
                                   int targetActions,
                                   int status
        );
        // values for status parameter above.
        protected static final int DEFAULT = 0;
        protected static final int ENTER   = 1;
        protected static final int OVER    = 2;
        protected static final int CHANGED = 3;

        //...
}

Note that the DragSourceContext itself implements DragSourceListener, this is to allow the platform peer, the DragSourceContextPeer instance, created by the DragSource, to notify the DragSourceContext of changes in state in the ongoing operation, and thus allows the DragSourceContext to interpose itself between the platform and the DragSourceListener provided by the initiator of the operation.

The state machine the platform exposes, with respect to the source, or initiator of the Drag and Drop operation is detailed below:

Following context describes this graphic

Notifications of changes in state with respect to the initiator during a drag and drop operation, as illustrated above, are delivered from the DragSourceContextPeer, to the appropriate DragSourceContext, which delegates notifications, via a unicast JavaBeans compliant EventListener subinterface, to an arbitrary object that implements DragSourceListener registered with the DragSource via startDrag.

The primary responsibility of the DragSourceListener is to monitor the progress of the user's navigation during the drag and drop operation and provide the "Drag-Over" effects feedback to the user. Typically this is accomplished via changes to the "Drag Cursor".

Every drag operation has 2 logical cursor states (Drag Cursors) associated with it:

The state of the Cursor may be modified by calling the setCursor method of the DragSourceContext.

2.3.3 The DragSourceListener Definition

The DragSourceListener interface is defined as follows:

public interface java.awt.dnd.DragSourceListener
        extends java.util.EventListener {
        void dragEnter  (DragSourceDragEvent dsde);
        void dragOver   (DragSourceDragEvent dsde);
        void dropActionChanged (DragSourceDragEvent dsde);
        void dragExit   (DragSourceEvent     dse);
        void dragDropEnd  (DragSourceDropEvent dsde);
}

As the drag operation progresses, the DragSourceListener's dragEnter, dragOver, and dragExit methods shall be invoked as a result of the user's navigation of the logical "Drag" cursor's location intersecting the geometry of GUI Component(s) with associated DropTarget(s). [See below for details of the DropTarget's protocol interactions].

The DragSourceListener's dragEnter method is invoked when the following conditions are true:

The DropTarget's registered DropTargetListener dragEnter method is invoked and returns successfully.

The registered DropTargetListener invokes the DropTargetDragEvent's acceptDrag method to accept the drag based upon interrogation of the source's potential drop actions and available data types (DataFlavors).

The DragSourceListener's dragOver method is invoked when the following conditions are true:

The DragSourceListener's dragExit method is invoked when one of the following conditions is true:

The DragSourceListener's dropActionChanged() method is invoked when the state of the input device(s), typically the mouse buttons or keyboard modifiers, that the user is interacting with in order to perform the drag operation, changes.

The dragDropEnd() method is invoked to signify that the operation is completed. The getDropSuccess method of the DragSourceDropEvent can be used to determine the termination state. The getDropAction method returns the operation that the DropTarget selected (via the DropTargetDropEvent acceptDrop parameter) to apply to the drop operation.

Once this method is complete the current DragSourceContext and the associated resources are invalid.

2.3.4 The DragSourceEvent Definition

The DragSourceEvent class is the root Event class for all events pertaining to the DragSource, and is defined as follows:

public class   java.awt.dnd.DragSourceEvent extends java.util.EventObject {
        public DragSourceEvent(DragSourceContext dsc);
        public DragSourceContext getDragSourceContext();
        //...
};

An instance of this event is passed to the DragSourceListener dragExit method.

2.3.5 The DragSourceDragEvent Definition

The DragSourceDragEvent class is defined as follows:


public class java.awt.dnd.DragSourceDragEvent extends DragSourceEvent {
        public int getTargetActions();
        public int getUserAction();
        public int getGestureModifiers();
        public int getGestureModifiersEx();
        public int getDropAction();
}

An instance of the above class is passed to a DragSourceListener's dragEnter, dragOver, and dragGestureChanged methods.

The getDragSourceContext method returns the DragSourceContext associated with the current drag and drop operation.

The getUserAction method returns the action that is currently selected by the user's gesture.

The getTargetActions method returns the drop action selected by the current drop target if this drop action is supported by the drag source or DnDConstants.ACTION_NONE if this drop action is not supported by the drag source.

The logical intersection of these two results and the set of drop actions supported by the drag source defines the actual effect of a drop and is returned via getDropAction.

The getGestureModifiers method returns the current state of the input device modifiers, usually the mouse buttons and keyboard modifiers, associated with the user's gesture.

The getGestureModifiersEx method returns the current state of the input device extended modifiers associated with the user's gesture.

2.3.6 The DragSourceDropEvent Definition

The DragSourceDropEvent class is defined as follows:

public public class java.awt.dnd.DragSourceDropEvent
                        extends java.util.EventObject {
       public DragSourceDropEvent(DragSourceContext dsc);
       public DragSourceDropEvent(DragSourceContext dsc,
                                int  action, 
                                boolean success);
       public boolean getDropSuccess();
       public int getDropAction();
}

An instance of the above class is passed to a DragSourceListener's dragDropEnd method. This event encapsulates the termination state of the drag and drop operation for the DragSource.

If the drop occurs, then the participating DropTarget will signal the success or failure of the data transfer via the DropTargetContext's dropComplete method, this status is made available to the initiator via the getDropSuccess method. The operation that the destination DropTarget selected to perform on the subject of the drag (passed by the DropTarget's acceptDrop method) is returned via the getDropAction method.

If the drag operation was aborted for any reason prior to a drop occurring, for example if the user ends the gesture outside a DropTarget, or if the DropTarget invokes rejectDrop, the getDropSuccess method will return false, otherwise true.

2.4 Drop Target

2.4.1 java.awt.Component additions for DropTarget (de)registration.

The java.awt.Component class has two additional methods added to allow the (dis)association with a DropTarget. In particular:

public class java.awt.Component /* ... */ {
        //...
        public synchronized void setDropTarget(DropTarget dt);
        public synchronized DropTarget getDropTarget(DropTarget df);
        //...
}

To associate a DropTarget with a Component one may invoke either; DropTarget.setCompononent or Component.setDropTarget methods. Thus conforming implementations of both methods are required to guard against mutual recursive invocations.

To disassociate a DropTarget with a Component one may invoke either; DropTarget.setCompononent(null) or Component.setDropTarget(null) methods.

Conformant implementations of both setter methods in DropTarget and Component should be implemented in terms of each other to ensure proper maintenance of each other's state.

The setDropTarget method throws IllegalArgumentException if the DropTarget actual parameter is not suitable for use with this class/instance of Component. It may also throw UnsupportedOperationException if, for instance, the Component does not support external setting of a DropTarget.

2.4.2 The DropTarget Definition

A DropTarget encapsulates all of the platform-specific handling of the Drag and Drop protocol with respect to the role of the recipient or destination of the operation.

A single DropTarget instance may typically be associated with any arbitrary instance of java.awt.Component. Establishing such a relationship exports the associated Components geometry to the client desktop as being receptive to drag and drop operations when the coordinates of the logical cursor intersects that visible geometry.

The DropTarget class is defined as follows:

public class java.awt.dnd.DropTarget
       implements DropTargetListener, Serializable {

        public DropTarget(Component          c,
                       int                actions,
                       DropTargetListener dsl,
                       boolean            isActive,
                       FlavorMap          fm
        );
        public DropTarget();
        public DropTarget(Component c);
        public DropTarget(Component c, DropTargetListener dsl);
        public Component getComponent();
        public void      setComponent(Component c);
        public DropTargetContext getDropTargetContext();
        public void addDropTargetListener(DropTargetListener dte)
                  throws TooManyListenersException;
        public void removeDropTargetListener(DropTargetListener dte);
        public void setActive(boolean active);
        public boolean isActive();
        public FlavorMap getFlavorMap();
        public void      setFlavorMap(FlavorMap fm);
        public void setDefaultActions(int actions);
        public int  getDefaultActions();
        protected DropTargetContext createDropTargetContext();
        public void addNotify(ComponentPeer cp);
        public void removeNotify(ComponentPeer cp);
        // ...
}

The setComponent method throws IllegalArgumentException if the Component actual parameter is not appropriate for use with this class/instance of DropTarget, and may also throw UnsupportedOperationException if the Component specified disallows the external setting of a DropTarget.

The addDropTargetListener and removeDropTargetListener methods allow the unicast DropTargetListener to be changed.

The setActive and isActive methods allow the DropTarget to be made active or otherwise and for its current state to be determined.

The getFlavorMap methods is used to obtain the FlavorMap associated with this DropTarget for the purposes of mapping any platform dependent type names to/from their corresponding platform independent DataFlavors.

The setFlavorMap method allows a new FlavorMap to be assigned to the DropTarget, a parameter of null causes a "default" FlavorMap to be installed for the DropTarget.

The createDropTargetContext method is only invoked to provide the underlying platform dependent peer with an instantiation of a new DropTargetContext as a Drag operation initially encounters the Component associated with the DropTarget. If no DropTargetContext is currently associated with a DropTarget, a permitted side-effect of an invocation of getDropTargetContext is to instantiate a new DropTargetContext.

The addNotify and removeNotify methods are only called from Component to notify the DropTarget of the Component's (dis)association with its ComponentPeer.

Note that the DropTarget itself implements DropTargetListener, this is to allow the platform peer, the DropTargetContextPeer instance, created by the platform, to notify the DropTarget of changes in state in the ongoing operation, and thus allows the DropTarget to interpose itself between the platform and the DropTargetListener registered with the DropTarget.

2.4.3 The DropTargetContext Definition

As the logical cursor associated with an ongoing drag and drop operation first intersects the visible geometry of a Component with an associated DropTarget, the DropTargetContext associated with the DropTarget is the interface, through which, access to, and control over state of the recipient protocol is achieved from the DropTargetListener.

A DropTargetContext is created by a DropTarget, via it's createDropTargetContext method, as a side effect of a call to a DropTarget's getDropTargetContext method, if no <>DropTargetContext currently exists for that DropTarget.

The DropTargetContext interface is defined as follows:

public class DropTargetContext {
        public DropTarget getDropTarget();
        public Component getComponent();
        public void  dropComplete(boolean success)
                                throws InvalidDnDOperationException;
        public void acceptDrag(int dropAction);
        public void rejectDrag();
        public void acceptDrop(int dropAction);
        public void rejectDrop();
        public void addNotify(DropTargetContextPeer dtcp);
        public void removeNotify();
        protected Transferable createTransferableProxy(Transferable t,
                   boolean isLocal
        );
        protected void setTargetActions(int actions);
        protected int  getTargetActions();
        protected DataFlavor[] getCurrentDataFlavors();
        protected List getCurrentDataFlavorsAsList();
        protected boolean isDataFlavorSupported(DataFlavor df);
        protected Transferable getTransferable();
        // ...
}

Most of the access and control methods are protected, since public access to their state is usually achieved by calling through a particular DropTargetEvent subclass that delegates the request to the DropTargetContext.

The getDropTarget() method return the DropTarget that created this DropTargetContext.

The getComponent method returns the Component associated with the DropTarget that created this DropTargetContext.

The acceptDrag method is delegated from the similar method on DropTargetDragEvent and is invoked from one of the DropTargetListener's methods; dragEnter(), dragOver or dropActionChanged to signify that the recipient is prepared to accept a drop with the operation specified, which is usually the user's currently selected action.

The rejectDrag method is delegated from the similar method on DropTargetDragEvent and is invoked from one of the DropTargetListener's methods; dragEnter, dragOver or dropActionChanged to signify that the recipient is unable to accept a drop with the user's currently selected action.

The acceptDrop method is delegated from the similar method on DropTargetDropEvent and is invoked from the DropTargetListener's drop method to signify that the recipient is prepared to accept a drop with the operation specified, which is usually the user's currently selected action.

The rejectDrop method is delegated from the similar method on DropTargetDropEvent and is invoked from the DropTargetListener's drop method to signify that the recipient is unable to accept a drop with the user's currently selected action. This terminates a drag and drop operation without a data transfer.

The dropComplete method signals to the originating DragSource that the DropTargetListener has completed the transfer(s) that comprise the subject of the drag and drop operation and that the operation is complete. The success (or failure) of the transfer(s) and the subsequent application of the operation specified is signaled by the value of the actual parameter.

The getDataFlavors method returns an array of the DataFlavors available from the DragSource.

The getTransferable method returns a Transferable (not necessarily the one the DragSource registered, it may be a proxy, and certainly shall be in the inter-JVM case) to enable data transfers to occur via its getTransferData method. Note that it is illegal to invoke getTransferable without first invoking an acceptDrop.

The addNotify and removeNotify methods are exclusively called by the underlying platform's DropTargetContextPeer in order to notify the DropTargetContext that a drag and drop operation is occurring/ceasing on the DropTargetContext and associated DropTarget.

The createTransferableProxy method enables a DropTargetContext implementation to interpose a Transferable between the DropTargetListener and the Transferable provided by the caller, which is typically the underlying platform DropTargetContextPeer.

2.4.4 The DropTargetListener Definition

Providing the appropriate "Drag-under" feedback semantics, and processing of any subsequent Drop, is enabled through the DropTargetListener asssociated with a DropTarget.

The DropTargetListener determines the appropriate "Drag-under" feedback and its response to the DragSource regarding drop eligibility by inspecting the sources suggested actions, the data types available and the data itself.

A particular DropTargetListener instance may be associated with a DropTarget via addDropTargetListener and removed via removeDropTargetListener methods.

public interface java.awt.dnd.DropTargetListener
        extends java.util.EventListener {
        void dragEnter            (DropTargetDragEvent dtde);
        void dragOver             (DropTargetDragEvent dtde);
        void dropActionChanged    (DropTargetDragEvent dtde);
        void dragExit             (DropTargetDragEvent dtde);
        void drop                 (DropTargetDropEvent dtde);
}

The following context describes this graphic.

The dragEnter method of the DropTargetListener is invoked when the hotspot of the logical "Drag" Cursor intersects a visible portion of the DropTarget's associated Component's geometry. The DropTargetListener, upon receipt of this notification, shall interrogate the operations or actions, and the types of the data (DataFlavors) and the data itself as supplied by the DragSource to determine the appropriate actions and "Drag-under" feedback to respond with invocation of either acceptDrag or rejectDrag.

The dragOver method of the DropTargetListener is invoked while the hotspot of the logical "Drag" Cursor, in motion, continues to intersect a visible portion of the DropTarget's associated Component's geometry. The DropTargetListener, upon receipt of this notification, shall interrogate the operation "actions" the types of the data and the data itself as supplied by the DragSource to determine the appropriate "actions" and "Drag-under" feedback to respond with an invocation of either acceptDrag or rejectDrag.

The dragExit method of the DropTargetListener is invoked when the hotspot of the logical "Drag" Cursor ceases to intersect a visible portion of the DropTarget's associated Component's geometry, or immediately prior to a drop notification. The DropTargetListener, upon receipt of this notification, shall undo any "Drag-under" feedback effects it has previously applied. Note that the DropTargetContext associated with the DropTarget is invalidated as a side-effect.

The drop method of the DropTargetListener is invoked as a result of the user terminating their drag gesture while intersecting. The DropTargetListener, upon receipt of this notification, shall perform the operation specified by the return value of the getSourceActions method on the DropTargetDropEvent object, upon the Transferable object returned from the getTransferable method, and subsequently invoke the dropComplete method of the associated DropTargetContext to signal the success, or otherwise, of the operation.

2.4.5 The DropTargetDragEvent and DropTargetDropEvent Definitions

The DropTargetEvent and DropTargetDragEvent are defined as follows:

public abstract class java.awt.dnd.DropTargetEvent
          extends java.util.EventObject {

        public DropTargetContext getDropTargetContext();
        //...
}

A DropTargetEvent is passed to the DropTargetListener's dragExit method.

public class   java.awt.dnd.DropTargetDragEvent
                         extends java.awt.dnd.DropTargetEvent {
        public Transferable getTransferable();
        public Point  getLocation();
        public int   getSourceActions();
        public getDropAction();
        public DataFlavor[] getCurrentDataFlavors();
        public List  getCurrentDataFlavorsAsList();
        public boolean isDataFlavorSupported();
        public void acceptDrag(int operation);
        public void rejectDrag();
        //...
}

A DropTargetDragEvent is passed to the DropTargetListener's dragEnter, dragOver and dropActionChanged methods.

The getLocation method return the current co-ordinates, relative to the associated Component's origin, of the hotspot of the logical "Drag" cursor.

The getSourceActions method returns the current "actions", or operations (ACTION_MOVE, ACTION_COPY, or ACTION_REFERENCE) the DragSource associates with the current drag and drop gesture.

The return value of the getDropAction method is the action that is selected by the user's gesture.

The getCurrentDataFlavors, getCurrentDataFlavorsAsList and isDataFlavorSupported methods are provided in order for the recipient to interrogate the list of types available from the source.

The getTransferable method is provided in order for the recipient to interrogate the data available from the source. Note that getTransferable on the DropTargetDragEvent instance should only be called within the respective DropTargetListener's method, and all the necessary data should be retrieved from the returned Transferable before that method returns.

The DropTargetDropEvent is defined as follows:

public class java.awt.dnd.DropTargetDropEvent
          extends java.awt.dnd.DropTargetEvent {

        public Point      getLocation();
        public int getSourceActions();
        public int getDropAction();
        public void acceptDrop(int dropAction);
        public void rejectDrop();       
        public boolean isLocalTransfer();
        public DataFlavor[] getCurrentDataFlavors();
        public List  getCurrentDataFlavorsAsList();
        public boolean isDataFlavorSupported(DataFlavor df);
        public Transferable getTransferable();
        public void dropComplete(boolean success);
        //...
}

A DropTargetDropEvent is passed to the DropTargetListener's drop method, as the drop occurs. The DropTargetDropEvent provides the DropTargetListener with access to the data associated with the operation, via the Transferable returned from the getTransferable method.

The return value of the getSourceActions method is defined to be the action(s) defined by the source at the time at which the drop occurred.

The return value of the getDropAction method is the action that is selected by the user's gesture.

The return value of the getLocation method is defined to be the location at which the drop occurred.

The getCurrentDataFlavors, getCurrentDataFlavorsAsList, and isDataFlavorSupported, methods are provided in order for the recipient to interrogate the list of types available from the source for subsequent transfer via the getTransferData method of the Transferable.

A typical implementation of the drop method will inspect the actions and the DataFlavors' available to determine if a successful exchange can occur or not.

When an exchange may occur, a DropTargetListener.drop implementation shall invoke acceptDrop with the selected operation as an actual parameter, prior to any invocation of getTransferable. Calling getTransferable prior to acceptDrop shall result in an InvalidDnDOperationException.

The rejectDrop shall be called to reject the drop operation. Once called no further interaction can occur between the two participants, therefore it is typical to return from the drop method immediately after calling this.

The isLocalTransfer method shall return true, if the source of the drag and drop operation resides within the same physical JVM as the recipient of the drop notification, and false otherwise.

This distinction is significant to the recipient when it receives object references back from invoking Transferable.getTransferData in the local case, since in this case the object reference it receives is the same object reference held by the source (i.e it is not a copy, proxy or distinct object) thus the recipient must treat such a shared object reference differently in the local case as follows:

The dropComplete method signals the end of the associated drag and drop operation, and indicates the success (or failure) of the transfers performed by the recipient. Invoking this method results in the DragSourceListener's dragDropEnd method being called with the appropriate state available from it's DragSourceDropEvent. Failure to invoke this method will result in the drag and drop operation failing to terminate properly.

2.4.6 Autoscrolling Support

Many GUI Components present a scrollable "viewport" over a (potentially) large dataset. During a drag and drop operation it is desirable to be able to "autoscroll" such "viewports" to allow a user to navigate over such a dataset, scrolling to locate a particular member (initially not visible through the "viewport") that they wish to drop the subject of the operation upon.

Components that are scrollable provide drag "autoscrolling" support to their DropTarget by implementing the following interface:

public interface Autoscroll {
        Insets getAutoscrollInsets();
        void autoScrollContent(Point cursorLocn);
}

An implementing DropTarget shall periodically call the autoscroll method of its associated Component (if present), passing the current logical cursor location in Component co-ordinates, when the following conditions are met:

Should any of the above conditions cease to be valid, autoscrolling shall terminate until the next triggering condition occurs.

Both the initial delay prior to autoscrolling commencing, the interval between autoscrolling notifications, and the pixel hysteresis value are externally configurable and can be queried from the Toolkit.getDesktopProperty method.

2.5 Data Transfer Phase

In the case where a valid drop occurs, the DropTargetListener's drop method is responsible for undertaking the transfer of the data associated with the gesture. The DropTargetDropEvent provides a means to obtain a Transferable object that represent that data object(s) to be transferred.

From the drop method, the DropTargetListener shall initially either rejectDrop (immediately returning thereafter) or acceptDrop specifying the selected operation from those returned by getSourceActions.

Subsequent to an acceptDrop, but not before, getTransferable may be invoked, and any data transfers performed via the returned Transferable's getTransferData method. Finally, once the destination of the drop has completed the transfer(s) of the objects from the source it shall signal the success, or immediate failure, of the transfer(s) via an invocation of DropTargetContext.dropComplete.

Upon returning from the DropTargetContext.dropComplete method the Transferable and DragSourceContext instances are no longer guaranteed to be valid and all references to them shall be discarded by the recipient to allow them to be subsequently garbage collected.

When using the ACTION_REFERENCE operation the source and destination should take care to agree upon the object and the associated semantics of the transfer. Typically in intra-JVM transfers a live object reference would be passed between source and destination, but in the case of inter-JVM transfers, or transfers between native and Java applications, live object references do not make sense, so some other ‘reference' type should be exchanged such as a URI for example. Both the DragSource and DropTarget can detect if the transfer is intra-JVM or not.

2.5.1 FlavorMap and SystemFlavorMap

All the target DnD platforms represent their transfer data types using a similar mechanism, however the representations do differ. The Java platform uses MIME types encapsulated within a DataFlavor to represent its data types. Unfortunately in order to permit the transfer of data between Java and platform native applications the existence of these platform names need to be exposed, thus a mechanism is required in order to create an extensible (platform independent) mapping between these platform dependent type names, their representations, and the Java MIME based DataFlavors.

The implementation will provide a mechanism to externally specify a mapping between platform native data types (strings) and MIME types (strings) used to construct DataFlavors. This external mapping will be used by the underlying platform specific implementation code in order to expose the appropriate DataFlavors (MIME types), exported by the source, to the destination, via the underlying platform DnD mechanisms.

Both the DragSource and DropTarget classes provide access for the underlying system to map platform dependent names to and from DataFlavors.

public interface java.awt.datatransfer.FlavorMap {
        java.util.Map getNativesForFlavors(DataFlavor[] dfs);
        java.util.Map getFlavorsForNatives(String[] natives);
}

The getNativesForFlavors method takes an array of DataFlavors and returns a Map object containing zero or more keys of type DataFlavor, from the actual parameter dfs, with associated values of type String, which correspond to the platform-dependent type name for that MIME type.

The getFlavorsForNatives method takes an array of String types and returns a Map object containing zero or more keys of type String, from the actual parameter natives, with associated values of type DataFlavor, which correspond to the platform-independent type for that platform-dependent type name.

The Map object returned by both methods may be mutable but is not required to be.

If NULL is passed to either of these methods they should return their current map of all keys and values known to the implementation at the time of the call.

For example on Win32 the Clipboard Format Name for simple text is "CF_TEXT" (actually it is the integer 1) and on Motif it is the X11 Atom named "STRING", the MIME type one may use to represent this would be "text/plain;charset=us-ascii". Therefore a platform portable FlavorMap would map between these names; CF_TEXT on win32 and STRING on Motif/X11.

Typically, as implemented in the SystemFlavorMap these mappings are held in an external persistent configuration format (a properties file or URL) and are loaded from the platform to configure the FlavorMap appropriately for a given platform.

The SystemFlavorMap class is provided to implement a simple, platform configurable mechanism for specifying a system-wide set of common mappings, and is defined as follows:

public class  java.awt.datatransfer.SystemFlavorMap implements FlavorMap, FlavorTable {
        public static FlavorMap getSystemFlavorMap(); 
        public synchronized Map getNativesForFlavors(DataFlavor[] dfs); 
        public synchronized Map getFlavorsForNatives(String[] natives); 
        public static String encodeDataFlavor(DataFlavor flav); 
        public static String encodeJavaMIMEType(String mimeType);
        public static boolean isJavaMIMEType(String mimeStr); 
        public static String decodeJavaMIMEType(String nat);
        public static DataFlavor decodeDataFlavor(String nat); 
        //...
}

The SystemFlavorMap class provides a simple implementation, using a properties file (see java.awt.Properties), of a persistent platform FlavorMap. Using the value of the AWT property "AWT.flavorMapFileURL" (see Toolkit.getProperty) or the default file location of System.getProperty("java.home") + File.separator + "lib" + File.separator + "flavormap.properties", this class creates the appropriate Maps from the properties found therein.

In addition the class provides several static convenience functions used to encode and decode Java MimeTypes to and from a platform dependent namespace. The syntax of the properties file is:

{ <platform_type_name> ‘=' <IETF_MIME_RFC_conformant_specification> <nl> } *

The default implementations of DragSource and DropTarget return the SystemFlavorMap from their getFlavorMap method, unless they have been provided with an overriding implementation.

2.5.2 Transferring Data across the JVM boundary

Since one of the primary goals of this API is to allow drag and drop of data between Java and native applications this has some significant consequences upon the method and mechanism of the actual data encoding and exchange when data is passed across the boundary of the Java Virtual Machine.

Since one of the participants in such an exchange may be a native application, and thus has no knowledge of the Java type system, the drag and drop system cannot just exchange a Java object reference since the other participant may have no knowledge of, or capability to manipulate such a type.

When any exchange occurs, it can only do so, regardless of the implementation of the participants, if and only if both participants are agreed upon a familiar data type and encoding. Thus, sadly the burden of the exchange is born mostly by the participants themselves.

What this means in practical terms is that for "native" data formats, such as platform-dependent image, document, or other "Content-Types" the encoding and decoding of their associated external data format is the responsibility of the source and destination of the transfer.

The drag and drop system shall expose the external representation of such "native" data types across the boundary of the Java Virtual Machine as encapsulated within a java.io.InputStream or a subclass thereof.

This means that any DataFlavor with a representation class that extends java.io.InputStream can be transferred, and will be exposed for transfer, across the Java Virtual Machine boundary.

To implement the exchange of such a native data type, a developer would define a DataFlavor with a MIME "Content-Type" that describes the nature of the "native" data type, with a representation class that extends a java.io.InputStream class that encodes the data encapsulated into a stream of bytes.

In particular such InputStream subclasses shall implement the following semantics:

Provision of this method (or inheriting a superclasses implementation) enables the drag and drop system (associated with the DragSource) to automatically extract an encoded stream of the encapsulated data from the Transferable, thus enabling the transfer of the data across the JVM boundary as a simple byte stream to the requestor of the particular DataFlavor.

2.5.3 Transferring lists of files across the JVM boundary.

A typical subject of a Drag and Drop transfer is a list of one or more platform dependent filenames. In order to ease the programming task of developers either producing or consuming such lists the drag and drop system treats them as a special case.

If a DataFlavor is specified with a MIME "Content-Type" of application/x-java-file-list;class=java.util.List the drag and drop system will expect the list elements to be a homogeneous list of objects of type java.io.File. Thus a source shall, if supporting the transfer of a list of files, construct such a List of File objects when such a DataFlavor is requested, and a recipient shall expect such a List of File objects if it requests such a valid DataFlavor from its source. This special case provides a simple mechanism for the transfer of lists of files between source and target.

2.5.4 Transferring java.rmi.Remote references across the JVM boundary.

It is possible to Drag and Drop object references between JVMs' by using the facilities of the RMI mechanism. The Drag and Drop system will automatically arrange for the transfer of any object reference that adheres to these requirements:

If these conditions are met, then if an appropriate DataFlavor is requested, then the object returned to the requestor (if it is in a difference JVM from the source) will be an RMI reference to an instance of the Remote object subinterface specified as the representation class of the DataFlavor.

3.0 Issues

3.0.1 What are the implications of the various platform protocol engines?

Due to limitations of particular underlying platform drag and drop and window system implementations, the interaction of a drag operation, and the event delivery semantics to AWT Components is platform dependent. Therefore during a drag operation a DragSource may process platform Window System Events pertaining to that drag to the exclusion of normal event processing.

Due to interactions between the single-threaded design center of the platform native DnD systems, and the native window system event dispatching implementations in AWT, "callbacks" into DropTargetListener and DragSourceListener will occur either on, or synchronized with the AWT system event dispatch thread. This behavior is highly undesirable for security reasons but is an implementation, not architectural feature, and is unavoidable.

3.0.2 Inter/Intra VM transfers?

To enable intra-JVM Drag and Drop Transfers the existing DataFlavor class will be extended to enable it to represent the type of a "live" object reference, as opposed to a Serialized (persistent) representation of one. Such objects may be transferred between source and destination within the same JVM and ClassLoader context.

The MIME Content-Type shall be application/x-java-local-objectref.

3.0.3 Lifetime of the Transferable(s)?

Transferable objects, their associated DataFlavors', and the objects that encapsulate the underlying data specified as the operand(s) of a drag and drop operation shall remain valid until, at least, the DragSourceListener associated with the DragSource controlling the operation, receives a dragDropEnd. The lifetime of the subject(s) of the operation, transferred between source and target is implementation defined beyond that point in time.

3.0.4 Implications of ACTION_MOVE semantics on source objects exposed via Transferable?

The "source" of a successful Drag and Drop (ACTION_MOVE) operation is required to delete/relinquish all references to the object(s) that are the subject of the Transferable immediately after transfer has been successfully completed. That is before returning from the DragSourceListener.dragDropEnd notification.

3.0.5 Semantics of ACTION_REFERENCE operation.

As a result of significant input from developers to an earlier version of the specification an additional operation/action tag; ACTION_REFERENCE was added to include existing platform Drag and Drop"Link" semantics.

It is believed that Reference, or Link, semantics are already sufficiently poorly specified for the platform native Drag and Drop to render it essentially useless even between native applications, thus between native and platform independent Java applications it is not recommended.

For Java to Java usage the required semantic; within the same JVM/ClassLoader, is defined such that the destination shall obtain a Java object reference to the subject(s) of the transfer. Between Java JVM's or ClassLoaders, the semantic is implementation defined, but could be implemented through transferring either a URL from the source to the destination or an RMI Remote reference.

Appendix A : DropTargetPeer definition

Although not a normative part of this specification this definition is included for clarity:

public interface DropTargetPeer {
        void addDropTarget(DropTarget dt);
        void removeDropTarget(DropTarget dt);
}

Appendix B : DragSourceContextPeer definition

Although not a normative part of this specification this definition is included for clarity:

public interface DragSourceContextPeer {
    void startDrag(DragSourceContext dsc,
                   Cursor            c,
                   Image             di,
                   Point             ioff
    ) throws InvalidDnDOperationException; 
    Cursor getCursor();
    void   setCursor(Cursor c) throws InvalidDnDOperationException;
    void transferablesFlavorsChanged();
}

Appendix C : DropTargetContextPeer definition

Although not a normative part of this specification this definition is included for clarity:

public interface DropTargetContextPeer {
    int  getTargetActions();
    void setTargetActions(int actions); 
    DropTarget getDropTarget(); 
    DataFlavor[] getTransferDataFlavors(); 
    Transferable getTransferable() throws InvalidDnDOperationException;
    boolean isTransferableJVMLocal();
    void acceptDrag(int dragAction);
    void rejectDrag(); 
    void acceptDrop(int dropAction);
    void rejectDrop();
    void dropComplete(boolean success);
}

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