Java文件上传分析

只要做过java web的人一定都接触过文件上传,但是中间或多或者肯定遇到过问题,那么下面就来分析下文件上传中遇到的各种坑~

##struts2表单上传文件
struts是通过默认拦截器实现的:

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<interceptor name="fileUpload" class="org.apache.struts2.interceptor.FileUploadInterceptor"/>

struts上传文件大小默认最大为2M(default.properties):

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struts.multipart.maxSize=2097152

当然我们也可以自定义设置(struts.properties):

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struts.multipart.maxSize=209715200

也可以在xml进行配置:

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<constant name="struts.multipart.maxSize" value="209715200">
</constant>

前端实现:

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<form action="xxx" enctype="multipart/form-data" method="post">
...
</form>

后台action实现:

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private File file;
private String fileFileName;
private String fileContentType

后台通过这样的写法,struts框架会自动赋值给这三个属性

##spring mvc表单上传文件
前端同上
后台实现:

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private MultiFile file;

上述两种都是通过form表单提交的,那么如何通过ajax直接上传文件呢?

##spring mvc+ajax上传文件
后台实现同上,但是如果不另加配置,则直接报415错误,需要加如下配置:

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<bean class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter">
<property name="messageConverters">
<list>
<ref bean="byteArrayConverter" />
</list>
</property>
</bean>
<bean id="byteArrayConverter" class="org.springframework.http.converter.ByteArrayHttpMessageConverter">
</bean>

进一步联想,上传文件是不是只能是post方法呢?
我们知道,浏览器是不支持RESTful风格中put和delete的
通过测试,使用put提交,直接报415错误,通过源码(org.springframework.web.filter.HiddenHttpMethodFilter)可以看到:

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@Override
protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
throws ServletException, IOException {
HttpServletRequest requestToUse = request;
if ("POST".equals(request.getMethod()) && request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) == null) {
String paramValue = request.getParameter(this.methodParam);
if (StringUtils.hasLength(paramValue)) {
requestToUse = new HttpMethodRequestWrapper(request, paramValue);
}
}
filterChain.doFilter(requestToUse, response);
}

从源码可以看出,filter中限制了只能用POST方法提交,那么如果一定要用PUT方法进行上传文件呢?
看了下HiddenHttpMethodFilter注释:

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/**
* {@link javax.servlet.Filter} that converts posted method parameters into HTTP methods,
* retrievable via {@link HttpServletRequest#getMethod()}. Since browsers currently only
* support GET and POST, a common technique - used by the Prototype library, for instance -
* is to use a normal POST with an additional hidden form field ({@code _method})
* to pass the "real" HTTP method along. This filter reads that parameter and changes
* the {@link HttpServletRequestWrapper#getMethod()} return value accordingly.
*
* <p>The name of the request parameter defaults to {@code _method}, but can be
* adapted via the {@link #setMethodParam(String) methodParam} property.
*
* <p><b>NOTE: This filter needs to run after multipart processing in case of a multipart
* POST request, due to its inherent need for checking a POST body parameter.</b>
* So typically, put a Spring {@link org.springframework.web.multipart.support.MultipartFilter}
* <i>before</i> this HiddenHttpMethodFilter in your {@code web.xml} filter chain.
*
* @author Arjen Poutsma
* @author Juergen Hoeller
* @since 3.0
*/

MultipartFileter:

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/**
* Set the bean name of the MultipartResolver to fetch from Spring's
* root application context. Default is "filterMultipartResolver".
*/
public void setMultipartResolverBeanName(String multipartResolverBeanName) {
this.multipartResolverBeanName = multipartResolverBeanName;
}

也就是说,我们可以通过在web.xml中注册一个MultipartFilter,一定要在HiddenHttpMethodFilter之前启动,这样的话就能实现

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<filter>
<filter-name>MultipartFilter</filter-name>
<filter-class>org.springframework.web.multipart.support.MultipartFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>MultipartFilter</filter-name>
<servlet-name>dispatcher</servlet-name>
</filter-mapping>

<filter>
<filter-name>HiddenHttpMethodFilter</filter-name>
<filter-class>org.springframework.web.filter.HiddenHttpMethodFilter</filter-class>
</filter>

同时配置:

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<bean id="filterMultipartResolver"
class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
<property name="maxUploadSize" value="209715200"/>
<property name="defaultEncoding" value="UTF-8"/>
<property name="resolveLazily" value="true"/>
</bean>

前端:

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function upload() {
var form = new FormData(document.getElementById("xx"));
form.append("_method", 'put');
$.ajax({
url: url,
type: 'post',
data: form,
processData: false,
contentType: false,
success: function (data) {
...
},
error: function (e) {
...
}
});
...
}

通过java文件上传分析,可以得出,框架为我们做了很多默认配置,为我们省去了很多工作,当然,也有限制,如果我们想挣脱限制,就要找准入口~

参考:
SpringMVC实现RESTful带有文件上传的put

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