velocitylayoutform.java

这是我自己开发的一个MVC框架

package dark.web.frame.velocity.form;

import java.io.IOException;
import java.io.StringWriter;
import java.io.UnsupportedEncodingException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.velocity.Template;
import org.apache.velocity.app.Velocity;
import org.apache.velocity.context.Context;
import org.apache.velocity.exception.MethodInvocationException;
import org.apache.velocity.exception.ParseErrorException;
import org.apache.velocity.exception.ResourceNotFoundException;

/**
 * <p>Title:            </p>
 * <p>Description:      根据org.apache.velocity.servlet.VelocityLayoutServlet修改
 * 						以使dwf中的Form支持Velocity的Layout管理</p>
 * <p>Copyright:        Copyright (c) 2005</p>
 * <p>Company:          DIS</p>
 * <p>Create Time:      2005-3-4 3:02:46</p>
 * @author             <a href="mailto:[e-mail]">darkhe</a>
 * @version            1.0
 */
public abstract class VelocityLayoutForm extends VelocityForm
{
	/**
	 * The context/parameter key used to specify an alternate
	 * layout to be used for a request instead of the default layout.
	 */
	public static String KEY_LAYOUT = "layout";
		
	/**
	 * The context key that will hold the content of the screen.
	 * 
	 * This key ($screen_content) must be present in the layout
	 * template for the current screen to be rendered.
	 */
	public static String KEY_SCREEN_CONTENT = "screen_content";	
	
	/**
	 * The context key that holds the {@link Throwable} that
	 * broke the rendering of the requested screen.
	 */
	public static String KEY_ERROR_CAUSE = "error_cause";

	/**
	 * The context key that holds the stack trace of the error that
	 * broke the rendering of the requested screen.
	 */
	public static String KEY_ERROR_STACKTRACE = "stack_trace";

	/**
	 * The context key that holds the {@link MethodInvocationException} 
	 * that broke the rendering of the requested screen.
	 *
	 * If this value is placed in the context, then $error_cause
	 * will hold the error that this invocation exception is wrapping.
	 */
	public static String KEY_ERROR_INVOCATION_EXCEPTION = "invocation_exception";
		
	private String errorTemplate;
	private String layoutDir;
	private String defaultLayout;
	
	/**
	  * Overrides VelocityViewServlet to check the request for 
	  * an alternate layout
	  *
	  * @param request client request
	  * @param response client response
	  * @return the Context to fill
	  */
	protected Context createContext(
		HttpServletRequest request,
		HttpServletResponse response)
	{

		Context ctx = super.createContext(request, response);

		// check if an alternate layout has been specified 
		// by way of the request parameters
		String layout = request.getParameter(KEY_LAYOUT);
		if (layout != null)
		{
			// let the template know what its new layout is
			ctx.put(KEY_LAYOUT, layout);
		}
		return ctx;
	}

	/**
	 * Overrides VelocityViewServlet.mergeTemplate to do a two-pass 
	 * render for handling layouts
	 */
	protected void mergeTemplate(
		Template template,
		Context context,
		HttpServletResponse response)
		throws
			ResourceNotFoundException,
			ParseErrorException,
			MethodInvocationException,
			IOException,
			UnsupportedEncodingException,
			Exception
	{
		//
		// this section is based on Tim Colson's "two pass render"
		//
		// Render the screen content
		StringWriter sw = new StringWriter();
		template.merge(context, sw);
		// Add the resulting content to the context
		context.put(KEY_SCREEN_CONTENT, sw.toString());

		// Check for an alternate layout
		//
		// we check after merging the screen template so the screen 
		// can overrule any layout set in the request parameters
		// by doing #set( $layout = "MyLayout.vm" )
		Object obj = context.get(KEY_LAYOUT);
		String layout = (obj == null) ? null : obj.toString();
		if (layout == null)
		{
			// no alternate, use default
			layout = defaultLayout;
		}
		else
		{
			// make it a full(er) path
			layout = layoutDir + layout;
		}

		try
		{
			//load the layout template
			template = getTemplate(layout);
		}
		catch (Exception e)
		{
			Velocity.error(
				"VelocityLayoutCommand: Can't load layout \""
					+ layout
					+ "\": "
					+ e);

			// if it was an alternate layout we couldn't get...
			if (!layout.equals(defaultLayout))
			{
				// try to get the default layout
				// if this also fails, let the exception go
				template = getTemplate(defaultLayout);
			}
		}

		// Render the layout template into the response
		super.mergeTemplate(template, context, response);
	}

	/**
	 * Overrides VelocityViewServlet to display user's custom error template
	 */
	protected void error(
		HttpServletRequest request,
		HttpServletResponse response,
		Exception e)
		throws ServletException
	{
		try
		{
			// get a velocity context
			Context ctx = createContext(request, response);

			Throwable cause = e;

			// if it's an MIE, i want the real cause and stack trace!
			if (cause instanceof MethodInvocationException)
			{
				// put the invocation exception in the context
				ctx.put(KEY_ERROR_INVOCATION_EXCEPTION, e);
				// get the real cause
				cause = ((MethodInvocationException) e).getWrappedThrowable();
			}

			// add the cause to the context
			ctx.put(KEY_ERROR_CAUSE, cause);

			// grab the cause's stack trace and put it in the context
			StringWriter sw = new StringWriter();
			cause.printStackTrace(new java.io.PrintWriter(sw));
			ctx.put(KEY_ERROR_STACKTRACE, sw.toString());

			// retrieve and render the error template
			Template et = getTemplate(errorTemplate);
			mergeTemplate(et, ctx, response);

		}
		catch (Exception e2)
		{
			// d'oh! log this
			Velocity.error(
				"VelocityLayoutCommand: "
					+ " Error during error template rendering - "
					+ e2);
			// then punt the original to a higher authority
			super.error(request, response, e);
		}
	}

	/**
	 * @return
	 */
	public String getDefaultLayout()
	{
		return defaultLayout;
	}

	/**
	 * @param string
	 */
	public void setDefaultLayout(String string)
	{
		defaultLayout = string;
	}

	/**
	 * @return
	 */
	public String getErrorTemplate()
	{
		return errorTemplate;
	}

	/**
	 * @return
	 */
	public String getLayoutDir()
	{
		return layoutDir;
	}

	/**
	 * @param string
	 */
	public void setErrorTemplate(String string)
	{
		errorTemplate = string;
	}

	/**
	 * @param string
	 */
	public void setLayoutDir(String string)
	{
		layoutDir = string;
	}
}