5.18. File Download¶

Table of Contents

Overview
How to use

5.18.1. Overview ¶

This chapter explains the functionality to download a file from server to client, using Spring.

It is recommended to use Spring MVC View for rendering the files.

: Note

It is not recommended to include file rendering logic in controller class.

This is because, it deviates from the role of a controller. Moreover, a View can be easily changed when it is isolated from the controller.

Overview of File Download process is given below.

DispatchServlet sends file download request to the controller.
Controller fetches file display information.
Controller selects View.
File rendering is performed in View.

In order to perform file rendering in a Spring based Web application;
this guideline recommends implementation of custom view.
To implement custom view, org.springframework.web.servlet.View interface
is provided in Spring framework.

For PDF files
org.springframework.web.servlet.view.document.AbstractPdfView class of Spring is used as a subclass
when rendering PDF files using model information.

For Excel files
org.springframework.web.servlet.view.document.AbstractExcelView class of Spring is used as a subclass
when rendering Excel files using model information.

For file formats other than those specified above, various types of View implementations are provided in Spring.
For technical details on View, refer to Spring Reference View technologies.

org.terasoluna.gfw.web.download.AbstractFileDownloadView provided by common library is the
abstract class to download arbitrary files.
Define this class as a subclass to render files in formats other than PDF or Excel.

5.18.2. How to use ¶

5.18.2.1. Downloading PDF files ¶

For rendering PDF files, it is necessary to create a class that
inherits org.springframework.web.servlet.view.document.AbstractPdfView provided by Spring.
The procedure to download a PDF file using controller is explained below.

5.18.2.1.1. Implementation of Custom View ¶

Implementation of class that inherits AbstractPdfView

@Component  // (1)
public class SamplePdfView extends AbstractPdfView {  // (2)

  @Override
  protected void buildPdfDocument(Map<String, Object> model,
          Document document, PdfWriter writer, HttpServletRequest request,
          HttpServletResponse response) throws Exception {  // (3)

      document.add(new Paragraph((Date) model.get("serverTime")).toString());
  }
}

Sr. No.	Description
(1)	In this example, this class comes under the scope of component scanning by using `@Component` annotation. It will also come under the scope of `org.springframework.web.servlet.view.BeanNameViewResolver` which is described later.
(2)	Inherit AbstractPdfView.
(3)	Execute buildPdfDocument method.

AbstractPdfView uses iText for PDF rendering.

Therefore, it is necessary to add itext definition to pom.xml of Maven.

<dependencies>
    <!-- omitted -->
    <dependency>
        <groupId>com.lowagie</groupId>
        <artifactId>itext</artifactId>
        <exclusions>
            <exclusion>
                <artifactId>xml-apis</artifactId>
                <groupId>xml-apis</groupId>
            </exclusion>
            <exclusion>
                <artifactId>bctsp-jdk14</artifactId>
                <groupId>org.bouncycastle</groupId>
            </exclusion>
            <exclusion>
                <artifactId>jfreechart</artifactId>
                <groupId>jfree</groupId>
            </exclusion>
            <exclusion>
                <artifactId>dom4j</artifactId>
                <groupId>dom4j</groupId>
            </exclusion>
            <exclusion>
                <groupId>org.swinglabs</groupId>
                <artifactId>pdf-renderer</artifactId>
            </exclusion>
            <exclusion>
                <groupId>org.bouncycastle</groupId>
                <artifactId>bcprov-jdk14</artifactId>
            </exclusion>
        </exclusions>
   </dependency>
   <dependency>
        <groupId>org.bouncycastle</groupId>
        <artifactId>bcprov-jdk14</artifactId>
        <version>1.38</version>
   </dependency>
</dependencies>

: Note

Spring IO Platform defines itext version.

5.18.2.1.2. Definition of ViewResolver ¶

org.springframework.web.servlet.view.BeanNameViewResolver is a class, that selects View to be executed using bean name stored in Spring context.

When using BeanNameViewResolver, it is recommended to define such that BeanNameViewResolver is executed before

ViewResolver for JSP (InternalResourceViewResolver)
ViewResolver for Tiles (TilesViewResolver)

which are generally used.

Note

Spring Framework provides various types of ViewResolver and it allows chaining of multiple ViewResolver. Therefore, some unintended View may get selected under certain conditions.

It is possible to avoid such a situation by setting appropriate priority order in ViewResolver. Method to set priority order differs depending on definition method of ViewResolver.

When defining ViewResolver using <mvc:view-resolvers> element added from Spring Framework 4.1, definition order of ViewResolver specified in child element will be the priority order. (executed sequentially from top)
When specifying ViewResolverusing <bean>element in a conventional way, set priority order in orderproperty. (It is executed starting from smallest setting value).

bean definition file

 <mvc:view-resolvers>
     <mvc:bean-name /> <!-- (1) (2) -->
     <mvc:jsp prefix="/WEB-INF/views/" />
 </mvc:view-resolvers>

Sr. No.	Description
(1)	Define `BeanNameViewResolver` using `<mvc:bean-name>` element added from Spring Framework 4.1.
(2)	Define `<mvc:bean-name>`element right at the top so that it has a higher priority than the generally used `ViewResolver`(`ViewResolver`for JSP).

Tip

<mvc:view-resolvers> element is an XML element added from Spring Framework 4.1. If <mvc:view-resolvers> element is used, it is possible to define ViewResolver in a simple way.

Example of definition when <bean> element used in a conventional way is given below.

<bean class="org.springframework.web.servlet.view.BeanNameViewResolver">
    <property name="order" value="0"/>
</bean>

<bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
    <property name="prefix" value="/WEB-INF/views/" />
    <property name="suffix" value=".jsp" />
    <property name="order" value="1" />
</bean>

In orderproperty, specify a value that is lesser than InternalResourceViewResolver to ensure that it gets a high priority.

5.18.2.1.3. Specifying View in controller ¶

With the help of BeanNameViewResolver, by returning “samplePDFView” in Controller,

a view named “samplePDFView” gets used from the BeanIDs stored in Spring Context.

Java source code

@RequestMapping(value = "home", params= "pdf", method = RequestMethod.GET)
public String homePdf(Model model) {
        model.addAttribute("serverTime", new Date());
        return "samplePdfView";   // (1)
}

Sr. No.	Description
(1)	With “samplePdfView” as the return value of method, SamplePdfView class stored in Spring context is executed.

Following PDF file can be opened after executing the above procedure.

Picture - FileDownload PDF

5.18.2.2. Downloading Excel files ¶

For rendering EXCEL files, it is necessary to create a class that
inherits org.springframework.web.servlet.view.document.AbstractExcelView provided by Spring.
The procedure to download an EXCEL file using controller is explained below.

5.18.2.2.1. Implementation of Custom View ¶

Implementation of class that inherits AbstractExcelView

@Component  // (1)
public class SampleExcelView extends AbstractExcelView {  // (2)

    @Override
    protected void buildExcelDocument(Map<String, Object> model,
            HSSFWorkbook workbook, HttpServletRequest request,
            HttpServletResponse response) throws Exception {  // (3)
        HSSFSheet sheet;
        HSSFCell cell;

        sheet = workbook.createSheet("Spring");
        sheet.setDefaultColumnWidth(12);

        // write a text at A1
        cell = getCell(sheet, 0, 0);
        setText(cell, "Spring-Excel test");

        cell = getCell(sheet, 2, 0);
        setText(cell, (Date) model.get("serverTime")).toString());
    }
}

Sr. No.	Description
(1)	In this example, this class comes under the scope of component scanning by using `@Component` annotation. It will also come under the scope of `org.springframework.web.servlet.view.BeanNameViewResolver` which is described earlier.
(2)	Inherit AbstractExcelView.
(3)	Execute buildExcelDocument method.

AbstractExcelView uses Apache POI to render EXCEL file.

Therefore, it is necessary to add POI definition to the pom.xml file of Maven.

<dependencies>
    <!-- omitted -->
    <dependency>
        <groupId>org.apache.poi</groupId>
        <artifactId>poi</artifactId>
        <version>${org.apache.poi.poi.version}</version>
    </dependency>
</dependencies>

<properties>
    <!-- omitted -->
    <org.apache.poi.poi.version>3.9</org.apache.poi.poi.version>
</properties>

5.18.2.2.2. Definition of ViewResolver ¶

Settings are same as that for PDF file rendering. For details, refer to Definition of ViewResolver.

5.18.2.2.3. Specifying View in controller ¶

With the help of BeanNameViewResolver, by returning “sampleExcelView” in Controller,

a view named “sampleExcelView” gets used from the BeanIDs stored in Spring Context.

Java source

@RequestMapping(value = "home", params= "excel", method = RequestMethod.GET)
public String homeExcel(Model model) {
        model.addAttribute("serverTime", new Date());
        return "sampleExcelView";  // (1)
}

Sr. No.	Description
(1)	With “sampleExcelView”as the return value of method, SampleExcelView class stored in Spring context is executed.

EXCEL file can be opened as shown below after executing the above procedures.

Picture - FileDownload EXCEL

5.18.2.3. Downloading arbitrary files ¶

To download files in formats other than PDF or EXCEL,
class that inherits org.terasoluna.gfw.web.download.AbstractFileDownloadView provided by common library can be implemented.
Following steps should be implemented in AbstractFileDownloadView to render files in other format.

Fetch InputStream in order to write to the response body.
Set information in HTTP header.

How to implement file download using controller is explained below.

5.18.2.3.1. Implementation of Custom View ¶

The example of text file download is given below.

Implementation of class that inherits AbstractFileDownloadView

@Component  // (1)
public class TextFileDownloadView extends AbstractFileDownloadView {  // (2)

   @Override
   protected InputStream getInputStream(Map<String, Object> model,
           HttpServletRequest request) throws IOException {  // (3)
       Resource resource = new ClassPathResource("abc.txt");
       return resource.getInputStream();
   }

   @Override
   protected void addResponseHeader(Map<String, Object> model,
           HttpServletRequest request, HttpServletResponse response) {  // (4)
       response.setHeader("Content-Disposition",
               "attachment; filename=abc.txt");
       response.setContentType("text/plain");

   }
}

Sr. No.	Description
(1)	In this example, this class comes under the scope of component scanning by using `@Component` annotation. It will also come under the scope of `org.springframework.web.servlet.view.BeanNameViewResolver` which is described earlier.
(2)	Inherit AbstractFileDownloadView.
(3)	Execute getInputStream method. InputStream to be downloaded should be returned.
(4)	Execute addResponseHeader method. Set Content-Disposition or ContentType as per the file to be downloaded.

5.18.2.3.2. Definition of ViewResolver ¶

Settings are same as that of PDF file rendering. For details, refer to Definition of ViewResolver.

5.18.2.3.3. Specifying View in controller ¶

With the help of BeanNameViewResolver, by returning “textFileDownloadView” in Controller,

a view named “textFileDownloadView” gets used from the BeanIDs stored in Spring Context.