Struts2 —— Result机制

Struts2将Result列为一个独立的层次，可以说是整个Struts2的Action层架构设计中的另外一个精华所在。Result之所以成为一个层次，其实是为了解决MVC框架中，如何从Control层转向View层这样一个问题而存在的。所以，接下来我们详细讨论一下Result的方方面面。

Result的职责

Result作为一个独立的层次存在，必然有其存在的价值，它也必须完成它所在的层次的职责。Result是为了解决如何从Control层转向View层这样一个问题而存在的，那么Result最大的职责，就是架起Action到View的桥梁。具体来说，我把这些职责大概分成以下几个方面：

封装跳转逻辑

Result 的首要职责，是封装Action层到View层的跳转逻辑。之前我们已经反复提到，Struts2的Action是一个与Web容器无关的POJO。所 以，在Action执行完毕之后，框架需要把代码的执行权重新交还给Web容器，并转向到相应的页面或者其他类型的View层。
而这个跳转逻辑，就由Result来完成。这样，好处也是显而易见的，对Action屏蔽任何Web容器的相关信息，使得每个层次更加清晰。

View 层的显示类型非常多，有最常见的JSP、当下非常流行的Freemarker/Velocity模板、Redirect到一个新的地址、文本流、图片流、 甚至是JSON对象等等。所以Result层的独立存在，就能够对这些显示类型进行区分，并封装合理的跳转逻辑。

以JSP转向为例，在Struts2自带的ServletDispatcherResult中就存在着核心的JSP跳转逻辑：

HttpServletRequest request = ServletActionContext.getRequest();
RequestDispatcher dispatcher = request.getRequestDispatcher(finalLocation);

....

dispatcher.forward(request, response);

"font-size: small;">"color: #ff00ff;">HttpServletRequest request = ServletActionContext.getRequest();
RequestDispatcher dispatcher = request.getRequestDispatcher(finalLocation);

....

dispatcher.forward(request, response);

再以Redirect重定向为例，在Struts2自带的ServletRedirectResult中，也同样存在着重定向的核心代码：

HttpServletResponse response = (HttpServletResponse) ctx.get(ServletActionContext.HTTP_RESPONSE);

....

response.sendRedirect(finalLocation);

"font-size: small;">"color: #ff00ff;">HttpServletResponse response = (HttpServletResponse) ctx.get(ServletActionContext.HTTP_RESPONSE);

....

response.sendRedirect(finalLocation);

由此可见，绝大多数的Result，都封装了与Web容器相关的跳转逻辑，由于这些逻辑往往需要和Servlet对象打交道，所以，遵循Struts2的基本原则，将它作为一个独立的层次，从而将Action从Web容器中解放出来。

准备显示数据

之 前提到，View层的展现方式很多，除了传统的JSP以外，还有类似Freemarker/Velocity这样的模板。根据模板显示的基本原理，需要将 预先定义好的模板（Template）和需要展示的数据（Model）组织起来，交给模板引擎，才能够正确显示。而这部分工作，就由Result层来完 成。

以Struts2自带的FreemarkerResult为例，在Result中，就存在着为模板准备数据的逻辑代码：

protected TemplateModel createModel() throws TemplateModelException {
    ServletContext servletContext = ServletActionContext.getServletContext();
    HttpServletRequest request = ServletActionContext.getRequest();
    HttpServletResponse response = ServletActionContext.getResponse();
    ValueStack stack = ServletActionContext.getContext().getValueStack();

    Object action = null;
    if(invocation!= null ) action = invocation.getAction(); //Added for NullPointException
    return freemarkerManager.buildTemplateModel(stack, action, servletContext, request, response, wrapper);
}

"font-size: small;">"color: #ff00ff;">protected TemplateModel createModel() throws TemplateModelException {
    ServletContext servletContext = ServletActionContext.getServletContext();
    HttpServletRequest request = ServletActionContext.getRequest();
    HttpServletResponse response = ServletActionContext.getResponse();
    ValueStack stack = ServletActionContext.getContext().getValueStack();

    Object action = null;
    if(invocation!= null ) action = invocation.getAction(); //Added for NullPointException
    return freemarkerManager.buildTemplateModel(stack, action, servletContext, request, response, wrapper);
}

有兴趣的读者可以顺着思路去看源码，看看这些Result到底是如何获取各种对象的值的。

控制输出行为

有 的时候，针对同一种类型的View展示，我们可能会有不同的输出行为。具体来说，可能有时候，我们需要对输出流指定特定的BufferSize、 Encoding等等。Result层，作为一个独立的层次，可以提供极大的扩展性，从而保证我们能够定义自己期望的输出类型。

以Struts2自带的HttpHeaderResult为例：

public void execute(ActionInvocation invocation) throws Exception {
    HttpServletResponse response = ServletActionContext.getResponse();

    if (status != -1) {
        response.setStatus(status);
    }

    if (headers != null) {
        ValueStack stack = ActionContext.getContext().getValueStack();

        for (Iterator iterator = headers.entrySet().iterator();
             iterator.hasNext();) {
            Map.Entry entry = (Map.Entry) iterator.next();
            String value = (String) entry.getValue();
            String finalValue = parse ? TextParseUtil.translateVariables(value, stack) : value;
            response.addHeader((String) entry.getKey(), finalValue);
        }
    }
}

"font-size: small;">"color: #ff00ff;">public void execute(ActionInvocation invocation) throws Exception {
    HttpServletResponse response = ServletActionContext.getResponse();

    if (status != -1) {
        response.setStatus(status);
    }

    if (headers != null) {
        ValueStack stack = ActionContext.getContext().getValueStack();

        for (Iterator iterator = headers.entrySet().iterator();
             iterator.hasNext();) {
            Map.Entry entry = (Map.Entry) iterator.next();
            String value = (String) entry.getValue();
            String finalValue = parse ? TextParseUtil.translateVariables(value, stack) : value;
            response.addHeader((String) entry.getKey(), finalValue);
        }
    }
}

我们可以在这里添加我们自定义的内容到HttpHeader中去，从而控制Http的输出。

Result的定义

让我们来看看Result的接口定义：

public interface Result extends Serializable {

    /**
     * Represents a generic interface for all action execution results, whether that be displaying a webpage, generating
     * an email, sending a JMS message, etc.
     */
    public void execute(ActionInvocation invocation) throws Exception;
}

"font-size: small;">"color: #ff00ff;">public interface Result extends Serializable {

    /**
     * Represents a generic interface for all action execution results, whether that be displaying a webpage, generating
     * an email, sending a JMS message, etc.
     */
    public void execute(ActionInvocation invocation) throws Exception;
}

这个接口定义非常简单，通过传入ActionInvocation，执行一段逻辑。我们来看看Struts2针对这个接口实现的一个抽象类，它规定了许多默认实现：

public abstract class StrutsResultSupport implements Result, StrutsStatics {

    private static final Log _log = LogFactory.getLog(StrutsResultSupport.class);

    /** The default parameter */
    public static final String DEFAULT_PARAM = "location";

    private boolean parse;
    private boolean encode;
    private String location;
    private String lastFinalLocation;

    public StrutsResultSupport() {
        this(null, true, false);
    }

    public StrutsResultSupport(String location) {
        this(location, true, false);
    }

    public StrutsResultSupport(String location, boolean parse, boolean encode) {
        this.location = location;
        this.parse = parse;
        this.encode = encode;
    }

    // setter method 省略

    /**
     * Implementation of the execute method from the Result interface. This will call
     * the abstract method {@link #doExecute(String, ActionInvocation)} after optionally evaluating the
     * location as an OGNL evaluation.
     *
     * @param invocation the execution state of the action.
     * @throws Exception if an error occurs while executing the result.
     */
    public void execute(ActionInvocation invocation) throws Exception {
        lastFinalLocation = conditionalParse(location, invocation);
        doExecute(lastFinalLocation, invocation);
    }

    /**
     * Parses the parameter for OGNL expressions against the valuestack
     *
     * @param param The parameter value
     * @param invocation The action invocation instance
     * @return The resulting string
     */
    protected String conditionalParse(String param, ActionInvocation invocation) {
        if (parse && param != null && invocation != null) {
            return TextParseUtil.translateVariables(param, invocation.getStack(),
                    new TextParseUtil.ParsedValueEvaluator() {
                        public Object evaluate(Object parsedValue) {
                            if (encode) {
                                if (parsedValue != null) {
                                    try {
                                        // use UTF-8 as this is the recommended encoding by W3C to
                                        // avoid incompatibilities.
                                        return URLEncoder.encode(parsedValue.toString(), "UTF-8");
                                    }
                                    catch(UnsupportedEncodingException e) {
                                        _log.warn("error while trying to encode ["+parsedValue+"]", e);
                                    }
                                }
                            }
                            return parsedValue;
                        }
            });
        } else {
            return param;
        }
    }

    /**
     * Executes the result given a final location (jsp page, action, etc) and the action invocation
     * (the state in which the action was executed). Subclasses must implement this class to handle
     * custom logic for result handling.
     *
     * @param finalLocation the location (jsp page, action, etc) to go to.
     * @param invocation    the execution state of the action.
     * @throws Exception if an error occurs while executing the result.
     */
    protected abstract void doExecute(String finalLocation, ActionInvocation invocation) throws Exception;
}

"font-size: small;">"color: #ff00ff;">public abstract class StrutsResultSupport implements Result, StrutsStatics {

    private static final Log _log = LogFactory.getLog(StrutsResultSupport.class);

    /** The default parameter */
    public static final String DEFAULT_PARAM = "location";

    private boolean parse;
    private boolean encode;
    private String location;
    private String lastFinalLocation;

    public StrutsResultSupport() {
        this(null, true, false);
    }

    public StrutsResultSupport(String location) {
        this(location, true, false);
    }

    public StrutsResultSupport(String location, boolean parse, boolean encode) {
        this.location = location;
        this.parse = parse;
        this.encode = encode;
    }

    // setter method 省略

    /**
     * Implementation of the execute method from the Result interface. This will call
     * the abstract method {@link #doExecute(String, ActionInvocation)} after optionally evaluating the
     * location as an OGNL evaluation.
     *
     * @param invocation the execution state of the action.
     * @throws Exception if an error occurs while executing the result.
     */
    public void execute(ActionInvocation invocation) throws Exception {
        lastFinalLocation = conditionalParse(location, invocation);
        doExecute(lastFinalLocation, invocation);
    }

    /**
     * Parses the parameter for OGNL expressions against the valuestack
     *
     * @param param The parameter value
     * @param invocation The action invocation instance
     * @return The resulting string
     */
    protected String conditionalParse(String param, ActionInvocation invocation) {
        if (parse && param != null && invocation != null) {
            return TextParseUtil.translateVariables(param, invocation.getStack(),
                    new TextParseUtil.ParsedValueEvaluator() {
                        public Object evaluate(Object parsedValue) {
                            if (encode) {
                                if (parsedValue != null) {
                                    try {
                                        // use UTF-8 as this is the recommended encoding by W3C to
                                        // avoid incompatibilities.
                                        return URLEncoder.encode(parsedValue.toString(), "UTF-8");
                                    }
                                    catch(UnsupportedEncodingException e) {
                                        _log.warn("error while trying to encode ["+parsedValue+"]", e);
                                    }
                                }
                            }
                            return parsedValue;
                        }
            });
        } else {
            return param;
        }
    }

    /**
     * Executes the result given a final location (jsp page, action, etc) and the action invocation
     * (the state in which the action was executed). Subclasses must implement this class to handle
     * custom logic for result handling.
     *
     * @param finalLocation the location (jsp page, action, etc) to go to.
     * @param invocation    the execution state of the action.
     * @throws Exception if an error occurs while executing the result.
     */
    protected abstract void doExecute(String finalLocation, ActionInvocation invocation) throws Exception;
}

很显然，这个默认实现是为那些类似JSP，Freemarker或者Redirect这样的页面跳转的Result而准备的一个基类，它规定了Result将要跳转到的具体页面的位置、是否需要解析参数，等等。

如果我们试图编写自定义的Result，我们可以实现Result接口，并在struts.xml中进行声明：

public class CustomerResult implements Result {

    public void execute(ActionInvocation invocation) throws Exception {
    // write your code here
}
}

"font-size: small;">"color: #ff00ff;">public class CustomerResult implements Result {

    public void execute(ActionInvocation invocation) throws Exception {
    // write your code here
}
}

<result-type name="customerResult" class="com.javaeye.struts2.CustomerResult"/>

<span style="font-size: small;"><span style="color: #ff00ff;"><result-type name="customerResult" class="com.javaeye.struts2.CustomerResult"/>
span>span>

常用的Result

接下来，大致介绍一下Struts2内部已经实现的Result，并看看他们是如何工作的。

dispatcher

<result-type name="dispatcher" class="org.apache.struts2.dispatcher.ServletDispatcherResult" default="true"/>

<span style="font-size: small;"><span style="color: #ff00ff;"><result-type name="dispatcher" class="org.apache.struts2.dispatcher.ServletDispatcherResult" default="true"/>
span>span>

dispatcher主要用于返回JSP，HTML等以页面为基础View视图，这个也是Struts2默认的Result类型。

在使用dispatcher时，唯一需要指定的，是JSP或者HTML页面的位置，这个位置将被用于定位返回的页面：

<result name="success">/index.jspresult>

<span style="font-size: small;"><span style="color: #ff00ff;"><result name="success">/index.jspresult>span>span>

而Struts2本身也没有对dispatcher做出什么特殊的处理，只是简单的使用Servlet API进行forward。

freemarker / velocity

<result-type name="freemarker" class="org.apache.struts2.views.freemarker.FreemarkerResult"/>
<result-type name="velocity" class="org.apache.struts2.dispatcher.VelocityResult"/>

<span style="font-size: small;"><span style="color: #ff00ff;"><result-type name="freemarker" class="org.apache.struts2.views.freemarker.FreemarkerResult"/>
<result-type name="velocity" class="org.apache.struts2.dispatcher.VelocityResult"/>
span>span>

随 着模板技术的越来越流行，使用Freemarker或者Velocity模板进行View层展示的开发者越来越多。Struts2同样为模板作为 Result做出了支持。由于模板的显示需要模板（Template）与数据（Model）的紧密配合，所以在Struts2中，这两个Result的主 要工作是为模板准备数据。

以Freemarker为例，我们来看看它是如何为模板准备数据的：

public void doExecute(String location, ActionInvocation invocation) throws IOException, TemplateException {
    this.location = location;
    this.invocation = invocation;
    this.configuration = getConfiguration();
    this.wrapper = getObjectWrapper();

    // 获取模板的位置
    if (!location.startsWith("/")) {
        ActionContext ctx = invocation.getInvocationContext();
        HttpServletRequest req = (HttpServletRequest) ctx.get(ServletActionContext.HTTP_REQUEST);
        String base = ResourceUtil.getResourceBase(req);
        location = base + "/" + location;
    }

    // 得到模板
    Template template = configuration.getTemplate(location, deduceLocale());
    // 为模板准备数据
    TemplateModel model = createModel();

    // 根据模板和数据进行输出
    // Give subclasses a chance to hook into preprocessing
    if (preTemplateProcess(template, model)) {
        try {
            // Process the template
            template.process(model, getWriter());
        } finally {
            // Give subclasses a chance to hook into postprocessing
            postTemplateProcess(template, model);
        }
    }
}

"font-size: small;">"color: #ff00ff;">public void doExecute(String location, ActionInvocation invocation) throws IOException, TemplateException {
    this.location = location;
    this.invocation = invocation;
    this.configuration = getConfiguration();
    this.wrapper = getObjectWrapper();

    // 获取模板的位置
    if (!location.startsWith("/")) {
        ActionContext ctx = invocation.getInvocationContext();
        HttpServletRequest req = (HttpServletRequest) ctx.get(ServletActionContext.HTTP_REQUEST);
        String base = ResourceUtil.getResourceBase(req);
        location = base + "/" + location;
    }

    // 得到模板
    Template template = configuration.getTemplate(location, deduceLocale());
    // 为模板准备数据
    TemplateModel model = createModel();

    // 根据模板和数据进行输出
    // Give subclasses a chance to hook into preprocessing
    if (preTemplateProcess(template, model)) {
        try {
            // Process the template
            template.process(model, getWriter());
        } finally {
            // Give subclasses a chance to hook into postprocessing
            postTemplateProcess(template, model);
        }
    }
}

从 源码中，我们可以看到，createModel()方法真正为模板准备需要显示的数据。而之前，我们已经看到过这个方法的源码，这个方法所准备的数据不仅 包含ValueStack中的数据，还包含了被封装过的HttpServletRequest，HttpSession等对象的数据。从而使得模板能够以 它特定的语法输出这些数据。

Velocity的Result也是类似，有兴趣的读者可以顺着思路继续深究源码。

redirect

<result-type name="chain" class="com.opensymphony.xwork2.ActionChainResult"/>
<result-type name="redirect" class="org.apache.struts2.dispatcher.ServletRedirectResult"/>
<result-type name="redirectAction" class="org.apache.struts2.dispatcher.ServletActionRedirectResult"/>

<span style="font-size: small;"><span style="color: #ff00ff;"><result-type name="chain" class="com.opensymphony.xwork2.ActionChainResult"/>
<result-type name="redirect" class="org.apache.struts2.dispatcher.ServletRedirectResult"/>
<result-type name="redirectAction" class="org.apache.struts2.dispatcher.ServletActionRedirectResult"/>span>span>

如 果你在Action执行完毕后，希望执行另一个Action，有2种方式可供选择。一种是forward，另外一种是redirect。有关 forward和redirect的区别，这里我就不再展开，这应该属于Java程序员的基本知识。在Struts2中，分别对应这两种方式的 Result，就是chain和redirect。

先来谈谈redirect，既然是重定向，那么源地址与目标地址之间是2个不同的 HttpServletRequest。所以目标地址将无法通过ValueStack等Struts2的特性来获取源Action中的数据。如果你需要对 目标地址传递参数，那么需要在目标地址url或者配置文件中指出：

window._bd_share_config={"common":{"bdSnsKey":{},"bdText":"","bdMini":"2","bdMiniList":false,"bdPic":"","bdStyle":"0","bdSize":"16"},"share":{}};with(document)0[(getElementsByTagName('head')[0]||body).appendChild(createElement('script')).src='http://bdimg.share.baidu.com/static/api/js/share.js?v=89860593.js?cdnversion='+~(-new Date()/36e5)];
				
			  
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