package javax.jms;

/** A client uses a <CODE>MessageConsumer</CODE> object to receive messages 
 * from a destination.  A <CODE>MessageConsumer</CODE> object is created by 
 * passing a <CODE>Destination</CODE> object to a message-consumer creation
 * method supplied by a session.
 *
 * <P><CODE>MessageConsumer</CODE> is the parent interface for all message 
 * consumers.
 *
 * <P>A message consumer can be created with a message selector. A message
 * selector allows 
 * the client to restrict the messages delivered to the message consumer to 
 * those that match the selector.
 *
 * <P>A client may either synchronously receive a message consumer's messages 
 * or have the consumer asynchronously deliver them as they arrive.
 *
 * <P>For synchronous receipt, a client can request the next message from a 
 * message consumer using one of its <CODE>receive</CODE> methods. There are 
 * several variations of <CODE>receive</CODE> that allow a 
 * client to poll or wait for the next message.
 *
 * <P>For asynchronous delivery, a client can register a 
 * <CODE>MessageListener</CODE> object with a message consumer. 
 * As messages arrive at the message consumer, it delivers them by calling the 
 * <CODE>MessageListener</CODE>'s <CODE>onMessage</CODE> method.
 *
 * <P>It is a client programming error for a <CODE>MessageListener</CODE> to 
 * throw an exception.
 *
 * @see         javax.jms.QueueReceiver
 * @see         javax.jms.TopicSubscriber
 * @see         javax.jms.Session
 */

public interface MessageConsumer
{

   /** Gets this message consumer's message selector expression.
    *  
    * @return this message consumer's message selector, or null if no
    *         message selector exists for the message consumer (that is, if 
    *         the message selector was not set or was set to null or the 
    *         empty string)
    *  
    * @exception JMSException if the JMS provider fails to get the message
    *                         selector due to some internal error.
    */

   String getMessageSelector() throws JMSException;

   /** Gets the message consumer's <CODE>MessageListener</CODE>.
    *  
    * @return the listener for the message consumer, or null if no listener
    * is set
    *  
    * @exception JMSException if the JMS provider fails to get the message
    *                         listener due to some internal error.
    * @see javax.jms.MessageConsumer#setMessageListener
    */

   MessageListener getMessageListener() throws JMSException;

   /** Sets the message consumer's <CODE>MessageListener</CODE>.
    * 
    * <P>Setting the message listener to null is the equivalent of 
    * unsetting the message listener for the message consumer.
    *
    * <P>The effect of calling <CODE>MessageConsumer.setMessageListener</CODE>
    * while messages are being consumed by an existing listener
    * or the consumer is being used to consume messages synchronously
    * is undefined.
    *  
    * @param listener the listener to which the messages are to be 
    *                 delivered
    *  
    * @exception JMSException if the JMS provider fails to set the message
    *                         listener due to some internal error.
    * @see javax.jms.MessageConsumer#getMessageListener
    */

   void setMessageListener(MessageListener listener) throws JMSException;

   /** Receives the next message produced for this message consumer.
    *  
    * <P>This call blocks indefinitely until a message is produced
    * or until this message consumer is closed.
    *
    * <P>If this <CODE>receive</CODE> is done within a transaction, the 
    * consumer retains the message until the transaction commits.
    *  
    * @return the next message produced for this message consumer, or 
    * null if this message consumer is concurrently closed
    *  
    * @exception JMSException if the JMS provider fails to receive the next
    *                         message due to some internal error.
    * 
    */

   Message receive() throws JMSException;

   /** Receives the next message that arrives within the specified
    * timeout interval.
    *  
    * <P>This call blocks until a message arrives, the
    * timeout expires, or this message consumer is closed.
    * A <CODE>timeout</CODE> of zero never expires, and the call blocks 
    * indefinitely.
    *
    * @param timeout the timeout value (in milliseconds)
    *
    * @return the next message produced for this message consumer, or 
    * null if the timeout expires or this message consumer is concurrently 
    * closed
    *
    * @exception JMSException if the JMS provider fails to receive the next
    *                         message due to some internal error.
    */

   Message receive(long timeout) throws JMSException;

   /** Receives the next message if one is immediately available.
    *
    * @return the next message produced for this message consumer, or 
    * null if one is not available
    *  
    * @exception JMSException if the JMS provider fails to receive the next
    *                         message due to some internal error.
    */

   Message receiveNoWait() throws JMSException;

   /** Closes the message consumer.
    *
    * <P>Since a provider may allocate some resources on behalf of a
    * <CODE>MessageConsumer</CODE> outside the Java virtual machine, clients 
    * should close them when they
    * are not needed. Relying on garbage collection to eventually reclaim
    * these resources may not be timely enough.
    *
    * <P>This call blocks until a <CODE>receive</CODE> or message listener in 
    * progress has completed. A blocked message consumer <CODE>receive</CODE> 
    * call 
    * returns null when this message consumer is closed.
    *  
    * @exception JMSException if the JMS provider fails to close the consumer
    *                         due to some internal error.
    */

   void close() throws JMSException;
}
