SpringMVC的View模块用什么实现？

参考：https://blog.csdn.net/andy_zhang2007/article/details/99644770

接口View定义了如下两个方法 :

1. String getContentType()：返回Content-Type字符串。如果不能提前确定，返回null。

2. render(@Nullable Map<String, ?> model, HttpServletRequest request, HttpServletResponse response)：使用指定的数据模型model渲染页面。如果没有模型数据,model可以是null或者空对象。

为了提供更多便利，Spring MVC对接口View提供了抽象实现AbstractView，封装了一些通用逻辑。框架某处，或者开发人员如果要实现一个View,继承AbstractView做自己的扩展定制即可。

如上是Spring框架自身提供的一些View实现，从此清单可以看出，绝大多数继承自AbstractView。

View接口：

package org.springframework.web.servlet;

import java.util.Map;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.lang.Nullable;


public interface View {

    /**
     * Name of the HttpServletRequest attribute that contains the response status code.
     * Note: This attribute is not required to be supported by all View implementations.
     * @since 3.0
     */
    String RESPONSE_STATUS_ATTRIBUTE = View.class.getName() + ".responseStatus";

    /**
     * Name of the HttpServletRequest attribute that contains a Map with path variables.
     * The map consists of String-based URI template variable names as keys and their corresponding
     * Object-based values -- extracted from segments of the URL and type converted.
     * Note: This attribute is not required to be supported by all View implementations.
     * @since 3.1
     */
    String PATH_VARIABLES = View.class.getName() + ".pathVariables";

    /**
     * The org.springframework.http.MediaType selected during content negotiation,
     * which may be more specific than the one the View is configured with. For example:
     * "application/vnd.example-v1+xml" vs "application/*+xml".
     * @since 3.2
     */
    String SELECTED_CONTENT_TYPE = View.class.getName() + ".selectedContentType";


    /**
     * Return the content type of the view, if predetermined.
     * Can be used to check the view's content type upfront,
     * i.e. before an actual rendering attempt.
     * @return the content type String (optionally including a character set),
     * or null if not predetermined
     */
    @Nullable
    default String getContentType() {
        return null;
    }

    /**
     * Render the view given the specified model.
     * The first step will be preparing the request: In the JSP case, this would mean
     * setting model objects as request attributes. The second step will be the actual
     * rendering of the view, for example including the JSP via a RequestDispatcher.
     * @param model a Map with name Strings as keys and corresponding model
     * objects as values (Map can also be null in case of empty model)
     * @param request current HTTP request
     * @param response he HTTP response we are building
     * @throws Exception if rendering failed
     */
    void render(@Nullable Map<String, ?> model, HttpServletRequest request, HttpServletResponse response)
            throws Exception;

}

抽象基类AbstractView：

package org.springframework.web.servlet.view;

// 省略 import 行

public abstract class AbstractView extends WebApplicationObjectSupport implements View, BeanNameAware {

    /** Default content type. Overridable as bean property. */
    public static final String DEFAULT_CONTENT_TYPE = "text/html;charset=ISO-8859-1";

    /** Initial size for the temporary output byte array (if any). */
    private static final int OUTPUT_BYTE_ARRAY_INITIAL_SIZE = 4096;


    @Nullable
    private String contentType = DEFAULT_CONTENT_TYPE;

    // 如果要将 RequestContext 作为一个属性放到数据模型中，就使用该变量
    // 指定相应的属性名称。缺省值为 null，表示不把 RequestContext 放到
    // 数据模型中
    @Nullable
    private String requestContextAttribute;

    // 静态属性，在当前 View 对象创建时被填充内容
    private final Map<String, Object> staticAttributes = new LinkedHashMap<>();

    // 是否暴露路径变量到模型，缺省使用 true
    private boolean exposePathVariables = true;

    // 是否将所有 Spring IoC 容器中的 bean 作为请求属性暴露，属性名称使用 bean 名称。
    // 缺省为 false。
    // 如果将该值设置为 true，则所有 bean 会作为请求属性暴露，在 JSP 2.0
    // 中，通过 ${...}这种方式就能经可以访问到 bean 。
    // 一旦开启该功能，请求属性的覆盖的优先级为 :
    // model 属性 > bean > 自定义的 request/session 属性
    // 跟该属性功能类似的属性是 exposedContextBeanNames,但二者不同
    private boolean exposeContextBeansAsAttributes = false;

   // 要暴露到请求属性空间的 bean 的名称清单
   // 这个属性 exposedContextBeanNames 和 exposeContextBeansAsAttributes=true 的
   // 区别是 exposedContextBeanNames 仅仅暴露部分bean，而
   // exposeContextBeansAsAttributes=true  暴露所有 bean
    @Nullable
    private Set<String> exposedContextBeanNames;

    // 记录当前View的 bean 名称，方便跟踪使用，框架构造View对象时会设置该值
    @Nullable
    private String beanName;



    /**
     * Set the content type for this view.
     * Default is "text/html;charset=ISO-8859-1".
     * May be ignored by subclasses if the view itself is assumed
     * to set the content type, e.g. in case of JSPs.
     */
    public void setContentType(@Nullable String contentType) {
        this.contentType = contentType;
    }

    /**
     * Return the content type for this view.
     */
    @Override
    @Nullable
    public String getContentType() {
        return this.contentType;
    }

    /**
     * Set the name of the RequestContext attribute for this view.
     * Default is none.
     */
    public void setRequestContextAttribute(@Nullable String requestContextAttribute) {
        this.requestContextAttribute = requestContextAttribute;
    }

    /**
     * Return the name of the RequestContext attribute, if any.
     */
    @Nullable
    public String getRequestContextAttribute() {
        return this.requestContextAttribute;
    }

    /**
     * 使用 CSV 格式字符串添加静态属性
     * Set static attributes as a CSV string.
     * Format is: attname0={value1},attname1={value1}
     * "Static" attributes are fixed attributes that are specified in
     * the View instance configuration. "Dynamic" attributes, on the other hand,
     * are values passed in as part of the model.
     */
    public void setAttributesCSV(@Nullable String propString) throws IllegalArgumentException {
        if (propString != null) {
            StringTokenizer st = new StringTokenizer(propString, ",");
            while (st.hasMoreTokens()) {
                String tok = st.nextToken();
                int eqIdx = tok.indexOf('=');
                if (eqIdx == -1) {
                    throw new IllegalArgumentException(
                            "Expected '=' in attributes CSV string '" + propString + "'");
                }
                if (eqIdx >= tok.length() - 2) {
                    throw new IllegalArgumentException(
                            "At least 2 characters ([]) required in attributes CSV string '" + propString + "'");
                }
                String name = tok.substring(0, eqIdx);
                String value = tok.substring(eqIdx + 1);

                // Delete first and last characters of value: { and }
                value = value.substring(1);
                value = value.substring(0, value.length() - 1);

                addStaticAttribute(name, value);
            }
        }
    }

    /**
     * 通过 Properties 对象形式添加静态属性
     * Set static attributes for this view from a
     * java.util.Properties object.
     * "Static" attributes are fixed attributes that are specified in
     * the View instance configuration. "Dynamic" attributes, on the other hand,
     * are values passed in as part of the model.
     * This is the most convenient way to set static attributes. Note that
     * static attributes can be overridden by dynamic attributes, if a value
     * with the same name is included in the model.
     * Can be populated with a String "value" (parsed via PropertiesEditor)
     * or a "props" element in XML bean definitions.
     * @see org.springframework.beans.propertyeditors.PropertiesEditor
     */
    public void setAttributes(Properties attributes) {
        CollectionUtils.mergePropertiesIntoMap(attributes, this.staticAttributes);
    }

    /**
     *  通过 Map 对象形式添加静态属性
     * Set static attributes for this view from a Map. This allows to set
     * any kind of attribute values, for example bean references.
     * "Static" attributes are fixed attributes that are specified in
     * the View instance configuration. "Dynamic" attributes, on the other hand,
     * are values passed in as part of the model.
     * Can be populated with a "map" or "props" element in XML bean definitions.
     * @param attributes a Map with name Strings as keys and attribute objects as values
     */
    public void setAttributesMap(@Nullable Map<String, ?> attributes) {
        if (attributes != null) {
            attributes.forEach(this::addStaticAttribute);
        }
    }

    /**
     *  返回静态属性
     * Allow Map access to the static attributes of this view,
     * with the option to add or override specific entries.
     * Useful for specifying entries directly, for example via
     * "attributesMap[myKey]". This is particularly useful for
     * adding or overriding entries in child view definitions.
     */
    public Map<String, Object> getAttributesMap() {
        return this.staticAttributes;
    }

    /**
     * 添加一个静态属性
     * Add static data to this view, exposed in each view.
     * "Static" attributes are fixed attributes that are specified in
     * the View instance configuration. "Dynamic" attributes, on the other hand,
     * are values passed in as part of the model.
     * Must be invoked before any calls to render.
     * @param name the name of the attribute to expose
     * @param value the attribute value to expose
     * @see #render
     */
    public void addStaticAttribute(String name, Object value) {
        this.staticAttributes.put(name, value);
    }

    /**
     *  返回静态属性
     * Return the static attributes for this view. Handy for testing.
     * Returns an unmodifiable Map, as this is not intended for
     * manipulating the Map but rather just for checking the contents.
     * @return the static attributes in this view
     */
    public Map<String, Object> getStaticAttributes() {
        return Collections.unmodifiableMap(this.staticAttributes);
    }

    /**
     * Specify whether to add path variables to the model or not.
     * Path variables are commonly bound to URI template variables through the @PathVariable
     * annotation. They're are effectively URI template variables with type conversion applied to
     * them to derive typed Object values. Such values are frequently needed in views for
     * constructing links to the same and other URLs.
     * Path variables added to the model override static attributes (see #setAttributes(Properties))
     * but not attributes already present in the model.
     * By default this flag is set to true. Concrete view types can override this.
     * @param exposePathVariables true to expose path variables, and false otherwise
     */
    public void setExposePathVariables(boolean exposePathVariables) {
        this.exposePathVariables = exposePathVariables;
    }

    /**
     * Return whether to add path variables to the model or not.
     */
    public boolean isExposePathVariables() {
        return this.exposePathVariables;
    }

    /**
     * Set whether to make all Spring beans in the application context accessible
     * as request attributes, through lazy checking once an attribute gets accessed.
     * This will make all such beans accessible in plain ${...}
     * expressions in a JSP 2.0 page, as well as in JSTL's c:out
     * value expressions.
     * Default is "false". Switch this flag on to transparently expose all
     * Spring beans in the request attribute namespace.
     * NOTE: Context beans will override any custom request or session
     * attributes of the same name that have been manually added. However, model
     * attributes (as explicitly exposed to this view) of the same name will
     * always override context beans.
     * @see #getRequestToExpose
     */
    public void setExposeContextBeansAsAttributes(boolean exposeContextBeansAsAttributes) {
        this.exposeContextBeansAsAttributes = exposeContextBeansAsAttributes;
    }

    /**
     * Specify the names of beans in the context which are supposed to be exposed.
     * If this is non-null, only the specified beans are eligible for exposure as
     * attributes.
     * If you'd like to expose all Spring beans in the application context, switch
     * the #setExposeContextBeansAsAttributes "exposeContextBeansAsAttributes"
     * flag on but do not list specific bean names for this property.
     */
    public void setExposedContextBeanNames(String... exposedContextBeanNames) {
        this.exposedContextBeanNames = new HashSet<>(Arrays.asList(exposedContextBeanNames));
    }

    /**
     * Set the view's name. Helpful for traceability.
     * Framework code must call this when constructing views.
     */
    @Override
    public void setBeanName(@Nullable String beanName) {
        this.beanName = beanName;
    }

    /**
     * Return the view's name. Should never be null,
     * if the view was correctly configured.
     */
    @Nullable
    public String getBeanName() {
        return this.beanName;
    }


    /**
     * Prepares the view given the specified model, merging it with static
     * attributes and a RequestContext attribute, if necessary.
     * Delegates to renderMergedOutputModel for the actual rendering.
     * @see #renderMergedOutputModel
     * 这里参数 model 是控制器返回的动态属性
     */
    @Override
    public void render(@Nullable Map<String, ?> model, HttpServletRequest request,
            HttpServletResponse response) throws Exception {

        if (logger.isDebugEnabled()) {
            logger.debug("View " + formatViewName() +
                    ", model " + (model != null ? model : Collections.emptyMap()) +
                    (this.staticAttributes.isEmpty() ? "" : ", static attributes " + this.staticAttributes));
        }

       // 合并静态属性和动态属性到最终使用的数据模型  mergedModel
       // 静态属性和动态属性同名时，采用动态属性的值
        Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);
        
        // 渲染数据模型到结果页面前准备响应对象
        prepareResponse(request, response);
       
       // 渲染数据模型到结果页面
        renderMergedOutputModel(mergedModel, getRequestToExpose(request), response);
    }

    /**
     * Creates a combined output Map (never null) that includes dynamic values and static attributes.
     * Dynamic values take precedence over static attributes.
     */
    protected Map<String, Object> createMergedOutputModel(@Nullable Map<String, ?> model,
            HttpServletRequest request, HttpServletResponse response) {

       // 获取路径变量名称值对儿到  Map pathVars
        @SuppressWarnings("unchecked")
        Map<String, Object> pathVars = (this.exposePathVariables ?
                (Map<String, Object>) request.getAttribute(View.PATH_VARIABLES) : null);

       //合并静态属性，路径变量，动态属性 
        // Consolidate static and dynamic model attributes.
        int size = this.staticAttributes.size();
        size += (model != null ? model.size() : 0);
        size += (pathVars != null ? pathVars.size() : 0);

        Map<String, Object> mergedModel = new LinkedHashMap<>(size);
        mergedModel.putAll(this.staticAttributes);
        if (pathVars != null) {
            mergedModel.putAll(pathVars);
        }
        if (model != null) {
            mergedModel.putAll(model);
        }

        // Expose RequestContext?
        if (this.requestContextAttribute != null) {
            mergedModel.put(this.requestContextAttribute, createRequestContext(request, response, mergedModel));
        }

        return mergedModel;
    }

    /**
     * Create a RequestContext to expose under the specified attribute name.
     * The default implementation creates a standard RequestContext instance for the
     * given request and model. Can be overridden in subclasses for custom instances.
     * @param request current HTTP request
     * @param model combined output Map (never null),
     * with dynamic values taking precedence over static attributes
     * @return the RequestContext instance
     * @see #setRequestContextAttribute
     * @see org.springframework.web.servlet.support.RequestContext
     */
    protected RequestContext createRequestContext(
            HttpServletRequest request, HttpServletResponse response, Map<String, Object> model) {

        return new RequestContext(request, response, getServletContext(), model);
    }

    /**
     * Prepare the given response for rendering. 渲染视图前准备响应对象
     * The default implementation applies a workaround for an IE bug
     * when sending download content via HTTPS.
     * @param request current HTTP request
     * @param response current HTTP response
     */
    protected void prepareResponse(HttpServletRequest request, HttpServletResponse response) {
        // 缺省实现实际上什么都不做
        // 因为 generatesDownloadContent() 缺省返回 false
        if (generatesDownloadContent()) {
          // 如果当前是下载场景，添加如下头部  
            response.setHeader("Pragma", "private");
            response.setHeader("Cache-Control", "private, must-revalidate");
        }
    }

    /**
     * Return whether this view generates download content
     * (typically binary content like PDF or Excel files).
     * The default implementation returns false. Subclasses are
     * encouraged to return true here if they know that they are
     * generating download content that requires temporary caching on the
     * client side, typically via the response OutputStream.
     * @see #prepareResponse
     * @see javax.servlet.http.HttpServletResponse#getOutputStream()
     */
    protected boolean generatesDownloadContent() {
        return false;
    }

    /**
     * Get the request handle to expose to #renderMergedOutputModel, i.e. to the view.
     * The default implementation wraps the original request for exposure of Spring beans
     * as request attributes (if demanded).
     * @param originalRequest the original servlet request as provided by the engine
     * @return the wrapped request, or the original request if no wrapping is necessary
     * @see #setExposeContextBeansAsAttributes
     * @see #setExposedContextBeanNames
     * @see org.springframework.web.context.support.ContextExposingHttpServletRequest
     */
    protected HttpServletRequest getRequestToExpose(HttpServletRequest originalRequest) {
        if (this.exposeContextBeansAsAttributes || this.exposedContextBeanNames != null) {
            WebApplicationContext wac = getWebApplicationContext();
            Assert.state(wac != null, "No WebApplicationContext");
            return new ContextExposingHttpServletRequest(originalRequest, wac, this.exposedContextBeanNames);
        }
        return originalRequest;
    }

    /**
     * 此方法为抽象方法，必须有抽象子类提供，这里的参数 model 是经过合并静态属性，动态属性，路径变量
     * 之后最重要应用到视图的数据模型，除非开发人员有其他自定义属性要渲染，否则直接使用该数据模型
     * 渲染视图即可
     * Subclasses must implement this method to actually render the view.
     * The first step will be preparing the request: In the JSP case,
     * this would mean setting model objects as request attributes.
     * The second step will be the actual rendering of the view,
     * for example including the JSP via a RequestDispatcher.
     * @param model combined output Map (never null),
     * with dynamic values taking precedence over static attributes
     * @param request current HTTP request
     * @param response current HTTP response
     * @throws Exception if rendering failed
     */
    protected abstract void renderMergedOutputModel(
            Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception;


    /**
     * Expose the model objects in the given map as request attributes.
     * Names will be taken from the model Map.
     * 工具方法 : 将指定数据模型中的属性添加为请求属性
     * This method is suitable for all resources reachable by javax.servlet.RequestDispatcher.
     * @param model a Map of model objects to expose
     * @param request current HTTP request
     */
    protected void exposeModelAsRequestAttributes(Map<String, Object> model,
            HttpServletRequest request) throws Exception {

        model.forEach((name, value) -> {
            if (value != null) {
                request.setAttribute(name, value);
            }
            else {
                request.removeAttribute(name);
            }
        });
    }

    /**
     * 工具方法 : 为当前 View 创建一个临时的   OutputStream 对象
     * Create a temporary OutputStream for this view.
     * This is typically used as IE workaround, for setting the content length header
     * from the temporary stream before actually writing the content to the HTTP response.
     */
    protected ByteArrayOutputStream createTemporaryOutputStream() {
        return new ByteArrayOutputStream(OUTPUT_BYTE_ARRAY_INITIAL_SIZE);
    }

    /**
     * Write the given temporary OutputStream to the HTTP response.
     * 工具方法 : 将指定的临时 OutputStream baos 中的数据写入到 response
     * @param response current HTTP response
     * @param baos the temporary OutputStream to write
     * @throws IOException if writing/flushing failed
     */
    protected void writeToResponse(HttpServletResponse response, ByteArrayOutputStream baos) throws IOException {
        // Write content type and also length (determined via byte array).
        response.setContentType(getContentType());
        response.setContentLength(baos.size());

        // Flush byte array to servlet output stream.
        ServletOutputStream out = response.getOutputStream();
        baos.writeTo(out);
        out.flush();
    }

    /**
     * 工具方法 : 结合考虑请求属性 View.SELECTED_CONTENT_TYPE 和 #getContentType 返回值设置响应的 Content Type
     * Set the content type of the response to the configured
     * #setContentType(String) content type unless the
     * View#SELECTED_CONTENT_TYPE request attribute is present and set
     * to a concrete media type.
     */
    protected void setResponseContentType(HttpServletRequest request, HttpServletResponse response) {
        MediaType mediaType = (MediaType) request.getAttribute(View.SELECTED_CONTENT_TYPE);
        if (mediaType != null && mediaType.isConcrete()) {
            response.setContentType(mediaType.toString());
        }
        else {
            response.setContentType(getContentType());
        }
    }

    @Override
    public String toString() {
        return getClass().getName() + ": " + formatViewName();
    }

    protected String formatViewName() {
        return (getBeanName() != null ? "name '" + getBeanName() + "'" : "[" + getClass().getSimpleName() + "]");
    }

}