Spring 文件上传

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为了使用文件上传，你需要创建一个带文件上传域（upload field）的（HTML）表单，让Spring将文件绑定到你的表单上（如下所示）：
<html>    <head>        <title>Upload a file please</title>    </head>    <body>        <h1>Please upload a file</h1>        <form method="post" action="upload.form" enctype="multipart/form-data">            <input type="file" name="file"/>            <input type="submit"/>        </form>    </body></html>
在上面这个表单里有一个input元素，这个元素的名字（“file”）和服务器端处理这个表单的bean（在下面将会提到）中类型为byte[]的属性名相同。在这个表单里我们也声明了编码参数（enctype="multipart/form-data"）以便让浏览器知道如何对这个文件上传表单进行编码（千万不要忘记这么做！）。
和其它不能自动转为字符串类型或者基本类型（primitive type）的属性一样，为了将上传的二进制数据存成bean的属性，必须通过ServletRequestDatabinder注册一个属性编辑器。 Spring中内置了几个这样的编辑器，它们可以处理文件，然后将结果存成bean的属性。比如StringMultipartEditor可以将文件转换成一个字符串（使用用户声明的字符集）。 ByteArrayMultipartEditor可以以将文件转换为byte数组。他们的功能和CustomDateEditor相似。
为了使用（HTML）表单上传文件，需要声明一个解析器，一个控制器，再将文件上传的URL映射到控制器来处理这个请求。下面是这几个bean的声明。
<beans>    <bean id="multipartResolver"        class="org.springframework.web.multipart.commons.CommonsMultipartResolver"/>    <bean id="urlMapping" class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">        <property name="mappings">            <value>                /upload.form=fileUploadController            </value>        </property>    </bean>    <bean id="fileUploadController" class="examples.FileUploadController">        <property name="commandClass" value="examples.FileUploadBean"/>        <property name="formView" value="fileuploadform"/>        <property name="successView" value="confirmation"/>    </bean></beans>
下面的代码定义了控制器和用来存放文件的那个bean。
public class FileUploadController extends SimpleFormController {    protected ModelAndView onSubmit(        HttpServletRequest request,        HttpServletResponse response,        Object command,        BindException errors) throws ServletException, IOException {         // cast the bean        FileUploadBean bean = (FileUploadBean) command;         let's see if there's content there        byte[] file = bean.getFile();        if (file == null) {             // hmm, that's strange, the user did not upload anything        }         // well, let's do nothing with the bean for now and return        return super.onSubmit(request, response, command, errors);    }    protected void initBinder(HttpServletRequest request, ServletRequestDataBinder binder)        throws ServletException {        // to actually be able to convert Multipart instance to byte[]        // we have to register a custom editor        binder.registerCustomEditor(byte[].class, new ByteArrayMultipartFileEditor());        // now Spring knows how to handle multipart object and convert them    }}public class FileUploadBean {    private byte[] file;    public void setFile(byte[] file) {        this.file = file;    }    public byte[] getFile() {        return file;    }}
FileUploadBean用一个byte[]类型的属性来存放文件。通常控制器注册一个自定义的编辑器以便让Spring知道如何将解析器找到的multipart对象转换成bean指定的属性。在下面这个例子里，上传的文件被绑定为（表单支持的）对象（form backing）的String属性：
public class FileUploadController extends SimpleFormController {    protected ModelAndView onSubmit(        HttpServletRequest request,        HttpServletResponse response,        Object command,        BindException errors) throws ServletException, IOException {         // cast the bean        FileUploadBean bean = (FileUploadBean) command;         let's see if there's content there        String file = bean.getFile();        if (file == null) {             // hmm, that's strange, the user did not upload anything        }         // well, let's do nothing with the bean for now and return        return super.onSubmit(request, response, command, errors);    }    protected void initBinder(HttpServletRequest request, ServletRequestDataBinder binder)        throws ServletException {        // to actually be able to convert Multipart instance to a String        // we have to register a custom editor        binder.registerCustomEditor(String.class, new StringMultipartFileEditor());        // now Spring knows how to handle multipart object and convert them    }}public class FileUploadBean {    private String file;    public void setFile(String file) {        this.file = file;    }    public String getFile() {        return file;    }}
如果仅仅是处理一个文本文件的上传，上面这个例子的做法还是合理的。但如果上传的是一张图片，那段代码就会出问题。
最后的解决方法就是将表单支持对象（form backing）的相关属性设成MultipartFile类型。这样的话，没有类型转换的需要，我们也就不需要声明任何属性编辑器（PropertyEditor）。
public class FileUploadController extends SimpleFormController {    protected ModelAndView onSubmit(        HttpServletRequest request,        HttpServletResponse response,        Object command,        BindException errors) throws ServletException, IOException {         // cast the bean        FileUploadBean bean = (FileUploadBean) command;         let's see if there's content there        MultipartFile file = bean.getFile();        if (file == null) {             // hmm, that's strange, the user did not upload anything        }         // well, let's do nothing with the bean for now and return        return super.onSubmit(request, response, command, errors);    }}public class FileUploadBean {    private MultipartFile file;    public void setFile(MultipartFile file) {        this.file = file;    }    public MultipartFile getFile() {        return file;    }}