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JSR 356: Java API for WebSocket
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package javax.websocket;

import java.util.ServiceLoader;

/**
 * Provider class that allows the developer to get a reference to
 * the implementation of the WebSocketContainer.
 * The provider class uses the 
 * ServiceLoader 
 * to load an implementation of ContainerProvider. Specifically, the fully qualified classname
 * of the container implementation of ContainerProvider must be listed in the 
 * META-INF/services/javax.websocket.ContainerProvider file in the implementation JAR file.
 *
 * @author dannycoward
 */
public abstract class ContainerProvider {
 
    /** 
     * Obtain a new instance of a WebSocketContainer. The method looks for the
     * ContainerProvider implementation class in the order listed in the META-INF/services/javax.websocket.ContainerProvider 
     * file, returning the WebSocketContainer implementation from the ContainerProvider implementation
     * that is not {@code null}.
     * @return an implementation provided instance of type WebSocketContainer
     */
    public static WebSocketContainer getWebSocketContainer() {
         WebSocketContainer wsc = null;
        for (ContainerProvider impl : ServiceLoader.load(ContainerProvider.class)) {
            wsc = impl.getContainer();
            if (wsc != null) {
                return wsc;
            } 
        }
        if (wsc == null) {
            throw new RuntimeException("Could not find an implementation class.");
        } else {
            throw new RuntimeException("Could not find an implementation class with a non-null WebSocketContainer.");
        }
    }
 
    /**
     * Load the container implementation.
     * @return the implementation class
     */
    protected abstract WebSocketContainer getContainer();
}