Struts2 檔案上傳下載完整實戰：攔截器配置、多檔案、類型限制、StreamResult 下載

一、檔案上傳核心機制

Struts2 透過 fileUpload 攔截器 封裝 Apache Commons FileUpload，自動將表單 multipart/form-data 解析為 Action 屬性。

前端表單必要條件

• method="post"
• enctype="multipart/form-data"
• <input type="file" name="file1">

Action 接收屬性命名規約

二、struts.xml 完整配置

<package name="upload" extends="struts-default">
    <action name="fileUploadAction" class="com.example.FileUploadAction">
        <interceptor-ref name="defaultStack">
            <!-- 限制副檔名 -->
            <param name="fileUpload.allowedExtensions">txt,jpg,pdf</param>
            <!-- 限制 MIME 類型（與副檔名取交集） -->
            <param name="fileUpload.allowedTypes">text/plain,image/jpeg,application/pdf</param>
        </interceptor-ref>
        <result name="success">/upload/success.jsp</result>
        <result name="input">/upload/error.jsp</result> <!-- 驗證失敗導向 -->
    </action>
</package>

全域檔案大小限制

<constant name="struts.multipart.maxSize" value="31457280" /> <!-- 30MB -->

三、Action 實作：儲存上傳檔案

public class FileUploadAction extends ActionSupport {
    private File file1;
    private String file1FileName;
    private String file1ContentType;
    // Getter/Setter 省略

    @Override
    public String execute() {
        String savePath = ServletActionContext.getServletContext().getRealPath("/upload");
        File destDir = new File(savePath);
        if (!destDir.exists()) destDir.mkdirs();
        
        File destFile = new File(destDir, file1FileName);
        FileUtils.copyFile(file1, destFile); // Commons IO
        return SUCCESS;
    }
}

四、錯誤頁面顯示驗證訊息

<%@ taglib uri="/struts-tags" prefix="s" %>
<body>
    <s:fielderror /> <!-- 自動顯示 fileUpload 攔截器產生的錯誤 -->
</body>

五、檔案下載：StreamResult 實作

struts.xml 配置

<package name="download" extends="struts-default">
    <action name="down_*" class="com.example.DownAction" method="{1}">
        <result name="list">/download/list.jsp</result>
        <result name="download" type="stream">
            <param name="contentType">application/octet-stream</param>
            <param name="inputName">inputStream</param> <!-- 對應 getInputStream() -->
            <param name="contentDisposition">attachment;filename="${downloadFileName}"</param>
            <param name="bufferSize">4096</param>
        </result>
    </action>
</package>

列表頁 list.jsp

<c:forEach var="fileName" items="${fileNames}" varStatus="vs">
    <tr>
        <td>${vs.count}</td>
        <td>${fileName}</td>
        <td>
            <c:url var="url" value="down_download">
                <c:param name="fileName" value="${fileName}" />
            </c:url>
            <a href="${url}">下載</a>
        </td>
    </tr>
</c:forEach>

DownAction 核心程式碼

public class DownAction extends ActionSupport {
    private String fileName;
    // Setter 處理中文編碼
    public void setFileName(String fileName) {
        try {
            this.fileName = new String(fileName.getBytes("ISO-8859-1"), "UTF-8");
        } catch (UnsupportedEncodingException e) {
            throw new RuntimeException(e);
        }
    }

    public String list() {
        String path = ServletActionContext.getServletContext().getRealPath("/upload");
        File dir = new File(path);
        String[] names = dir.list();
        ActionContext.getContext().put("fileNames", names);
        return "list";
    }

    public String download() { return "download"; }

    // StreamResult 會自動呼叫 getInputStream()
    public InputStream getInputStream() {
        return ServletActionContext.getServletContext().getResourceAsStream("/upload/" + fileName);
    }

    // 下載時顯示的檔名（瀏覽器另存為對話框）
    public String getDownloadFileName() {
        try {
            return URLEncoder.encode(fileName, "UTF-8");
        } catch (UnsupportedEncodingException e) {
            throw new RuntimeException(e);
        }
    }
}

六、關鍵細節總結

• 上傳目錄：需手動建立 webapp/upload 並確保寫入權限
• 中文檔名：上傳、下載雙向編碼處理（ISO-8859-1 → UTF-8、URLEncoder）
• 大檔案：調整 struts.multipart.maxSize 並考慮分片上傳
• 安全性：嚴格限制 allowedExtensions/allowedTypes，防止惡意腳本上傳