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Struts2(9)之ValueStack值栈

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概述

ValueStack 是 Struts 的一个接口,字面意义为值栈,OgnlValueStack 是 ValueStack 的实现类,客户端发起一个请求 Struts2 架构会创建一个 Action 实例同时创建一个 OgnlValueStack 值栈实例,OgnlValueStack 贯穿整个 Action 的生命周期,Struts2 中使用 OGNL 将请求 Action 的参数封装为对象存储到值栈中,并通过 OGNL 表达式读取值栈中对象的属性值。

ValueStack 其实类似一个数据中转站,Struts2 中的数据都保存在值栈中。

值栈的内部结构

ValueStack 中有两个主要的区域:

  • root:其实就是一个 ArrayList
  • context:其实就是一个 Map

context 中放置了 Web 开发中常用对象的引用,例如:

  • request:原生 Servlet 请求对象。
  • session:会话对象。
  • application:ServletContext 对象
  • parameters:请求参数对象。
  • attr:依次在 requestsessionapplication 寻找匹配值。

所说的操作值栈,通常指的是操作 ValueStack 中的 root 区域。
requestsessionapplication 中存取值就相当于操作 ValueStack 的 context 区域。

值栈和 ActionContext 的关系

首先请求时会经过核心过滤器,查看核心过滤器的 doFilter 方法:

// org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter#doFilter
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {

    HttpServletRequest request = (HttpServletRequest) req;
    HttpServletResponse response = (HttpServletResponse) res;

    try {
        if (excludedPatterns != null && prepare.isUrlExcluded(request, excludedPatterns)) {
            chain.doFilter(request, response);
        } else {
            prepare.setEncodingAndLocale(request, response);
            // <1>
            prepare.createActionContext(request, response);
            prepare.assignDispatcherToThread();
            request = prepare.wrapRequest(request);
            ActionMapping mapping = prepare.findActionMapping(request, response, true);
            if (mapping == null) {
                boolean handled = execute.executeStaticResourceRequest(request, response);
                if (!handled) {
                    chain.doFilter(request, response);
                }
            } else {
                execute.executeAction(request, response, mapping);
            }
        }
    } finally {
        prepare.cleanupRequest(request);
    }
}

而创建 ActionContext 就在 <1> 处,查看 prepare.createActionContext 方法:

// org.apache.struts2.dispatcher.ng.PrepareOperations#createActionContext
public ActionContext createActionContext(HttpServletRequest request, HttpServletResponse response) {
    ActionContext ctx;
    Integer counter = 1;
    Integer oldCounter = (Integer) request.getAttribute(CLEANUP_RECURSION_COUNTER);
    if (oldCounter != null) {
        counter = oldCounter + 1;
    }

    ActionContext oldContext = ActionContext.getContext();
    if (oldContext != null) {
        // detected existing context, so we are probably in a forward
        ctx = new ActionContext(new HashMap<String, Object>(oldContext.getContextMap()));
    } else {
        // <2>
        ValueStack stack = dispatcher.getContainer().getInstance(ValueStackFactory.class).createValueStack();
        stack.getContext().putAll(dispatcher.createContextMap(request, response, null));
        // <3>
        ctx = new ActionContext(stack.getContext());
    }
    request.setAttribute(CLEANUP_RECURSION_COUNTER, counter);
    ActionContext.setContext(ctx);
    return ctx;
}

<2> 处创建了值栈对象 stack ,接着在 <3> 处将 stack.getContext() 传给了 ActionContext 来创建 ActionContext 实例,而 stack.getContext() 中拥有对值栈的引用,所以这部分执行完后在 ActionContext 中是直接可以取到值栈的。

结论:ActionContext 之所以能访问 Servlet 的 API ,是因为在其内部有值栈的引用,而值栈的 context 部分又拥有对 Servlet 常用对象(requestsessionservletContext)的引用。

ValueStack 的获得

通过上一节,已经知道是可以通过 ActionContext 获取到值栈的引用的。接着看核心过滤器的 `doFilter 方法:

// org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter#doFilter
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {

    HttpServletRequest request = (HttpServletRequest) req;
    HttpServletResponse response = (HttpServletResponse) res;

    try {
        if (excludedPatterns != null && prepare.isUrlExcluded(request, excludedPatterns)) {
            chain.doFilter(request, response);
        } else {
            prepare.setEncodingAndLocale(request, response);
            prepare.createActionContext(request, response);
            prepare.assignDispatcherToThread();
            request = prepare.wrapRequest(request);
            ActionMapping mapping = prepare.findActionMapping(request, response, true);
            if (mapping == null) {
                boolean handled = execute.executeStaticResourceRequest(request, response);
                if (!handled) {
                    chain.doFilter(request, response);
                }
            } else {
                    // <1>
                execute.executeAction(request, response, mapping);
            }
        }
    } finally {
        prepare.cleanupRequest(request);
    }
}

看到 <1> 处,这行用来开始执行 Action,查看 executeAction 方法:

// org.apache.struts2.dispatcher.ng.ExecuteOperations#executeAction
public void executeAction(HttpServletRequest request, HttpServletResponse response, ActionMapping mapping) throws ServletException {
    dispatcher.serviceAction(request, response, mapping);
}

接着看到 serviceAction 方法:

// org.apache.struts2.dispatcher.Dispatcher#serviceAction
public void serviceAction(HttpServletRequest request, HttpServletResponse response, ActionMapping mapping)
        throws ServletException {

    Map<String, Object> extraContext = createContextMap(request, response, mapping);

    // If there was a previous value stack, then create a new copy and pass it in to be used by the new Action
    ValueStack stack = (ValueStack) request.getAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY);
    boolean nullStack = stack == null;
    if (nullStack) {
        ActionContext ctx = ActionContext.getContext();
        if (ctx != null) {
            stack = ctx.getValueStack();
        }
    }
    if (stack != null) {
        extraContext.put(ActionContext.VALUE_STACK, valueStackFactory.createValueStack(stack));
    }

    String timerKey = "Handling request from Dispatcher";
    try {
        UtilTimerStack.push(timerKey);
        String namespace = mapping.getNamespace();
        String name = mapping.getName();
        String method = mapping.getMethod();

        ActionProxy proxy = getContainer().getInstance(ActionProxyFactory.class).createActionProxy(
                namespace, name, method, extraContext, true, false);

        request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, proxy.getInvocation().getStack());

        // if the ActionMapping says to go straight to a result, do it!
        if (mapping.getResult() != null) {
            Result result = mapping.getResult();
            result.execute(proxy.getInvocation());
        } else {
            proxy.execute();
        }

        // If there was a previous value stack then set it back onto the request
        if (!nullStack) {
            // <2>
            request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, stack);
        }
    } catch (ConfigurationException e) {
        logConfigurationException(request, e);
        sendError(request, response, HttpServletResponse.SC_NOT_FOUND, e);
    } catch (Exception e) {
        if (handleException || devMode) {
            sendError(request, response, HttpServletResponse.SC_INTERNAL_SERVER_ERROR, e);
        } else {
            throw new ServletException(e);
        }
    } finally {
        UtilTimerStack.pop(timerKey);
    }
}

直接看 <2> 处,当值栈不为空时,将值栈的引用放入了 request 域。

结论:除了通过 ActionContext 获得值栈,我们还可以通过 request 获取到值栈。

在 Action 中我们可以通过如下代码获取值栈:

// 获取值栈方式 1 、通过 ActionContext
ValueStack valueStack1 = ActionContext.getContext().getValueStack();

// 获取值栈方式 2 、通过 request
// STRUTS_VALUESTACK_KEY = "struts.valueStack";
ValueStack valueStack2 = (ValueStack)ServletActionContext.getRequest().getAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY);

System.out.println(valueStack1 == valueStack2); // true

操作值栈

方法一:在 Action 中提供属性的 get 方法

默认情况下,Struts2 会将访问的 Action 对象压入值栈,所以在 Action 中提供的属性会随之存入值栈:

// Action
import com.opensymphony.xwork2.ActionSupport;

public class Test1Action extends ActionSupport {
    private String name;

    private Integer age;

    public String getName() {
        return name;
    }

    public Integer getAge() {
        return age;
    }

    @Override
    public String execute() throws Exception {
        this.name = "张三";
        this.age = 19;
        return super.execute();
    }
}
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
    <title>Test</title>
</head>
<body>
<s:property value="name"/>
<s:property value="age"/>
</body>
</html>

image.png

方法二:手动调用值栈方法

我们已经知道了如何在 Action 中获取值栈,当然也可以在 Action 中操作值栈:

// Action
package com.zze.action;

import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionSupport;

public class Test2Action extends ActionSupport {
    @Override
    public String execute() throws Exception {
        ActionContext.getContext().getValueStack().set("name","张三");
        ActionContext.getContext().getValueStack().set("age",20);
        return super.execute();
    }
}
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
    <title>Test</title>
</head>
<body>
<s:property value="name"/>
<s:property value="age"/>
</body>
</html>

image.png

查看值栈中的数据

Struts2 为方便我们调试,给我们提供了一个标签,我们用这个标签可以直接查看到值栈中的数据:

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
    <title>Test</title>
</head>
<body>
<s:debug/>
</body>
</html>

image.png

使用标签获取值栈中的数据

已经知道如何操作值栈,现在我们看一下如何在页面中获取到值栈中的数据。

Struts2 为简易我们在页面中获取值栈数据的操作,给我们提供了一些标签,看如下示例:

准备

// com.zze.bean.User
import java.util.Date;

public class User {
    private String name;
    private Integer age;
    private Date birthday;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", birthday=" + birthday +
                '}';
    }
}
<!-- struts.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE struts PUBLIC
        "-//Apache Software Foundation//DTD Struts Configuration 2.3//EN"
        "http://struts.apache.org/dtds/struts-2.3.dtd">
<struts>
    <constant name="struts.devMode" value="true"/>
    <package name="test" extends="struts-default" namespace="/">
        <action name="*" class="com.zze.action.{1}Action">
            <result>/index.jsp</result>
        </action>
    </package>
</struts>

例 1:root 中获取 JavaBean 对象

// com.zze.action.Test1Action
import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionSupport;
import com.zze.bean.User;

public class Test1Action extends ActionSupport {
    @Override
    public String execute() throws Exception {
        User user = new User();
        user.setName("张三");
        user.setAge(29);
        // 将 user 压入栈顶
        ActionContext.getContext().getValueStack().push(user);
        return super.execute();
    }
}
<!-- index.jsp -->
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
    <title>Test</title>
</head>
<body>
<%--可直接访问栈顶对象属性--%>
<s:property value="name"/>
<s:property value="age"/>

<s:debug/>
</body>
</html>

image.png

例 2:root 中获取 JavaBean 对象集合

// com.zze.action.Test2Action
import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionSupport;
import com.zze.bean.User;

import java.util.ArrayList;
import java.util.List;

public class Test2Action extends ActionSupport {
    @Override
    public String execute() throws Exception {
        User user1 = new User();
        user1.setName("张三");
        user1.setAge(29);
        User user2 = new User();
        user2.setName("李四");
        user2.setAge(30);

        List<User> userList = new ArrayList<>();
        userList.add(user1);
        userList.add(user2);

        ActionContext.getContext().getValueStack().set("userList",userList);
        return super.execute();
    }
}
<!-- index.jsp -->
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
    <title>Test</title>
</head>
<body>
<s:property value="userList[0].name"/>
<s:property value="userList[0].age"/>
<s:property value="userList[1].name"/>
<s:property value="userList[1].age"/>
<s:debug/>
</body>
</html>

image.png

例3:context中获取域字段

// com.zze.action.Test3Action
package com.zze.action;

import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionSupport;

public class Test3Action extends ActionSupport {

    @Override
    public String execute() throws Exception {
        ActionContext.getContext().put("name","张三");
        ActionContext.getContext().getSession().put("name","李四");
        ActionContext.getContext().getApplication().put("name","王五");
        return super.execute();
    }
}
<!-- index.jsp -->
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
    <title>Test</title>
</head>
<body>
<s:property value="#request.name"/>
<s:property value="#session.name"/>
<s:property value="#application.name"/>
<s:debug/>
</body>
</html>

image.png

获取 root 区域中数据直接使用对象属性名即可,如果是 map 则使用 key;获取 context 中属性需在 key 前加上 #

EL 表达式获取值栈数据

获取值栈数据的方式除了上面通过 Struts2 提供的标签的方式,还可以通过 EL 表达式获取,例如:

// com.zze.action.Test4Action
import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionSupport;
import com.zze.bean.User;

public class Test4Action extends ActionSupport {

    @Override
    public String execute() throws Exception {
        User user = new User();
        user.setName("托马斯");
        ActionContext.getContext().getValueStack().push(user);
        return super.execute();
    }
}
<!-- index.jsp -->
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
    <title>Test</title>
</head>
<body>
${name}

<s:debug/>
</body>
</html>

image.png

原理分析

我们知道,EL 表达式本来就只能获取 11 个隐式对象 中的数据,为什么在这里还能获取值栈中的数据呢?当然是 Struts2 做了手脚,依旧从核心过滤器开始查看源码:

// org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter#doFilter
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {

    HttpServletRequest request = (HttpServletRequest) req;
    HttpServletResponse response = (HttpServletResponse) res;

    try {
        if (excludedPatterns != null && prepare.isUrlExcluded(request, excludedPatterns)) {
            chain.doFilter(request, response);
        } else {
            prepare.setEncodingAndLocale(request, response);
            prepare.createActionContext(request, response);
            prepare.assignDispatcherToThread();
            // <1>
            request = prepare.wrapRequest(request);
            ActionMapping mapping = prepare.findActionMapping(request, response, true);
            if (mapping == null) {
                boolean handled = execute.executeStaticResourceRequest(request, response);
                if (!handled) {
                    chain.doFilter(request, response);
                }
            } else {
                execute.executeAction(request, response, mapping);
            }
        }
    } finally {
        prepare.cleanupRequest(request);
    }
}

看到第 <1> 处,通过 prepare.wrapRequest(request) 将原生 request 进行了包装,查看 wrapRequest 方法:

// org.apache.struts2.dispatcher.ng.PrepareOperations#wrapRequest
public HttpServletRequest wrapRequest(HttpServletRequest oldRequest) throws ServletException {
    HttpServletRequest request = oldRequest;
    try {
        // Wrap request first, just in case it is multipart/form-data
        // parameters might not be accessible through before encoding (ww-1278)
        request = dispatcher.wrapRequest(request);
        ServletActionContext.setRequest(request);
    } catch (IOException e) {
        throw new ServletException("Could not wrap servlet request with MultipartRequestWrapper!", e);
    }
    return request;
}

继续进入 dispatcher.wrapRequest 方法:

// org.apache.struts2.dispatcher.Dispatcher#wrapRequest
public HttpServletRequest wrapRequest(HttpServletRequest request) throws IOException {
    // don't wrap more than once
    if (request instanceof StrutsRequestWrapper) {
        return request;
    }

    String content_type = request.getContentType();
    if (content_type != null && content_type.contains("multipart/form-data")) {
        MultiPartRequest mpr = getMultiPartRequest();
        LocaleProvider provider = getContainer().getInstance(LocaleProvider.class);
        request = new MultiPartRequestWrapper(mpr, request, getSaveDir(), provider, disableRequestAttributeValueStackLookup);
    } else {
        // <2>
        request = new StrutsRequestWrapper(request, disableRequestAttributeValueStackLookup);
    }

    return request;
}

如果不是文件上传类的请求,将会执行第 <2> 处,也就是说普通情况下请求 Action 该方法返回的 request 就是 StrutsRequestWrapper 的实例,查看该类:

// org.apache.struts2.dispatcher.StrutsRequestWrapper
package org.apache.struts2.dispatcher;

import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.util.ValueStack;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;

import static org.apache.commons.lang3.BooleanUtils.isTrue;

public class StrutsRequestWrapper extends HttpServletRequestWrapper {

    private static final String REQUEST_WRAPPER_GET_ATTRIBUTE = "__requestWrapper.getAttribute";
    private final boolean disableRequestAttributeValueStackLookup;


    public StrutsRequestWrapper(HttpServletRequest req) {
        this(req, false);
    }


    public StrutsRequestWrapper(HttpServletRequest req, boolean disableRequestAttributeValueStackLookup) {
        super(req);
        this.disableRequestAttributeValueStackLookup = disableRequestAttributeValueStackLookup;
    }


    public Object getAttribute(String key) {
        if (key == null) {
            throw new NullPointerException("You must specify a key value");
        }

        if (disableRequestAttributeValueStackLookup || key.startsWith("javax.servlet")) {
            return super.getAttribute(key);
        }

        ActionContext ctx = ActionContext.getContext();
        Object attribute = super.getAttribute(key);

        if (ctx != null && attribute == null) {
            boolean alreadyIn = isTrue((Boolean) ctx.get(REQUEST_WRAPPER_GET_ATTRIBUTE));

            if (!alreadyIn && !key.contains("#")) {
                try {
                    // If not found, then try the ValueStack
                    ctx.put(REQUEST_WRAPPER_GET_ATTRIBUTE, Boolean.TRUE);
                    ValueStack stack = ctx.getValueStack();
                    if (stack != null) {
                        attribute = stack.findValue(key);
                    }
                } finally {
                    ctx.put(REQUEST_WRAPPER_GET_ATTRIBUTE, Boolean.FALSE);
                }
            }
        }
        return attribute;
    }
}

可以看到这个类其实就是将 getAttribute 方法进行了重写,当通过该方法获取一个值时,如果通过原生 request 未获取到,则继续从值栈中寻找这个 key 对应的值并返回。

而我们通过 EL 表达式获取值实际上也会调用 request.getAttribute 方法,此时 Struts2 对该方法进行了包装增强,这就是使用 EL 能获取到值栈数据的原因。

# Struts2  

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