SpringMVC 优雅实现 301 跳转

###前言
对于做站的人来说，SEO 是一件挺重要的事情，所以当我们的网页请求规则发生改变时，为了减少丢失搜索引擎的权重，我们就很有必要对以前老的 URL 做 301 跳转了。
在 HTTP 协议中，重定向分为两种：一种是 301 永久重定向，另外一种是 302 临时跳转。
###分析与实现
需求已定，下面就是分析如何实现了，最容易想到的就是，在`Controller`的方法中加入如下代码：
```java
response.setStatus(301);
response.setHeader("Location", "xxx");
```
这样做代码显得非常糟糕，首先，每个方法中都必须添加`HttpResponse`参数，其次，代码显得有些重复。

这个时候你应该想到`SpringMVC`自带的重定向功能了，比如下面用法：
```java
return "redirect:/xxx";
```
是的，或许你开始想到的就是上面的做法，但不幸的是，**`它默认是 302 临时跳转`**。

既然`SpringMVC`自带的跳转不行，那么我们为何不去看看它的实现呢？
`视图解析器`是`SpringMVC`中一个重要的组件，这里我们不对它做过多的讲解，直接看看它的源码，通常我们在配置文件中配置的视图解析器是：
> org.springframework.web.servlet.view.InternalResourceViewResolver

它的源码我们不看，因为它继承了`UrlBasedViewResolver`，我们直接看这个类的源码：
```java
public class UrlBasedViewResolver extends AbstractCachingViewResolver implements Ordered {

/**
	 * Prefix for special view names that specify a redirect URL (usually
	 * to a controller after a form has been submitted and processed).
	 * Such view names will not be resolved in the configured default
	 * way but rather be treated as special shortcut.
	 */
	public static final String REDIRECT_URL_PREFIX = "redirect:";

/**
	 * Prefix for special view names that specify a forward URL (usually
	 * to a controller after a form has been submitted and processed).
	 * Such view names will not be resolved in the configured default
	 * way but rather be treated as special shortcut.
	 */
	public static final String FORWARD_URL_PREFIX = "forward:";

/**
	 * Overridden to implement check for "redirect:" prefix.
	 * <p>Not possible in {@code loadView}, since overridden
	 * {@code loadView} versions in subclasses might rely on the
	 * superclass always creating instances of the required view class.
	 * @see #loadView
	 * @see #requiredViewClass
	 */
	@Override
	protected View createView(String viewName, Locale locale) throws Exception {
		// If this resolver is not supposed to handle the given view,
		// return null to pass on to the next resolver in the chain.
		if (!canHandle(viewName, locale)) {
			return null;
		}
		// Check for special "redirect:" prefix.
		if (viewName.startsWith(REDIRECT_URL_PREFIX)) {
			String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());
			RedirectView view = new RedirectView(redirectUrl, isRedirectContextRelative(), isRedirectHttp10Compatible());
			return applyLifecycleMethods(viewName, view);
		}
		// Check for special "forward:" prefix.
		if (viewName.startsWith(FORWARD_URL_PREFIX)) {
			String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());
			return new InternalResourceView(forwardUrl);
		}
		// Else fall back to superclass implementation: calling loadView.
		return super.createView(viewName, locale);
	}
```
上面我只贴了关键部分的代码，它的`转发(forard)`与`重定向(redirect)`实现都在`createView`方法中。

前面说了`SpringMVC`默认的重定向是 302 跳转，看上面源码可知，重定向视图就是`RedirectView`这个类，我们查看它的源码可以知道它默认就是 302 跳转：
```java
protected void sendRedirect(HttpServletRequest request, HttpServletResponse response,
                            String targetUrl, boolean http10Compatible) throws IOException {

String encodedRedirectURL = response.encodeRedirectURL(targetUrl);
	if (http10Compatible) {
		HttpStatus attributeStatusCode = (HttpStatus) request.getAttribute(View.RESPONSE_STATUS_ATTRIBUTE);
		if (this.statusCode != null) {
			response.setStatus(this.statusCode.value());
			response.setHeader("Location", encodedRedirectURL);
		}
		else if (attributeStatusCode != null) {
			response.setStatus(attributeStatusCode.value());
			response.setHeader("Location", encodedRedirectURL);
		}
		else {
                        //看这里的注释
			// Send status code 302 by default.
			response.sendRedirect(encodedRedirectURL);
		}
	}
	else {
		HttpStatus statusCode = getHttp11StatusCode(request, response, targetUrl);
		response.setStatus(statusCode.value());
		response.setHeader("Location", encodedRedirectURL);
	}
}
```

好了，到了这里我们要做的就是自定义视图解析器了，需要注意的是，如果你是直接继承`UrlBasedViewResolver`类，启动是会报错的，如下：
```java
Caused by: java.lang.IllegalArgumentException: Property 'viewClass' is required
	at org.springframework.web.servlet.view.UrlBasedViewResolver.initApplicationContext(UrlBasedViewResolver.java:405)
	at org.springframework.context.support.ApplicationObjectSupport.initApplicationContext(ApplicationObjectSupport.java:120)
	at org.springframework.web.context.support.WebApplicationObjectSupport.initApplicationContext(WebApplicationObjectSupport.java:76)
	at org.springframework.context.support.ApplicationObjectSupport.setApplicationContext(ApplicationObjectSupport.java:74)
	at org.springframework.context.support.ApplicationContextAwareProcessor.invokeAwareInterfaces(ApplicationContextAwareProcessor.java:119)
```
就是说`viewClass`属性是必须的，我们参见`InternalResourceViewResolver`类源码：
```java
/**
 * Sets the default {@link #setViewClass view class} to {@link #requiredViewClass}:
 * by default {@link InternalResourceView}, or {@link JstlView} if the JSTL API
 * is present.
 */
public InternalResourceViewResolver() {
	Class<?> viewClass = requiredViewClass();
	if (InternalResourceView.class == viewClass && jstlPresent) {
		viewClass = JstlView.class;
	}
	setViewClass(viewClass);
}
```
它在构造方法中调用了`setViewClass`来设置`viewClass`，如果你选择继承`UrlBasedViewResolver`类的话，可以参考`InternalResourceViewResolver`类的做法，但是既然我们一般都是用的`InternalResourceViewResolver`视图解析器，为何我们不直接继承它呢？

所以我的做法是选择继承`InternalResourceViewResolver`类，这样一来只需要将`springmvc.xml`中的视图解析器类改成我们自定义的即可：
```xml
<bean
	class="com.bddy.view.CustomViewResolver">
	<property name="prefix" value="/WEB-INF/views/"></property>
	<property name="suffix" value=".jsp"></property>
</bean>
```

自定义视图解析器：
```java
public class CustomViewResolver extends InternalResourceViewResolver {

private static final String REDIRECT_301_URL_PREFIX = "redirect 301:";

@Override
	protected View createView(String viewName, Locale locale) throws Exception {
		if (!canHandle(viewName, locale)) {
			return null;
		}
		if (viewName.startsWith(REDIRECT_301_URL_PREFIX)) {
			String redirectUrl = viewName.substring(REDIRECT_301_URL_PREFIX.length());
			RedirectView view = new RedirectView(redirectUrl, isRedirectContextRelative(), isRedirectHttp10Compatible(), false);
                        //设置 301 状态码
			view.setStatusCode(HttpStatus.MOVED_PERMANENTLY);
			return (View) getApplicationContext().getAutowireCapableBeanFactory().initializeBean(view, viewName);
		}
		return super.createView(viewName, locale);
	}
}
```

最后在我们的`Controller`中语法如下：
```java
@RequestMapping("/{id}.htm")
public String film(@PathVariable("id") Integer id) {
    return "redirect 301:/xzj/{id}.htm";
}
```

大功告成！

SpringMVC 优雅实现 301 跳转

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