WebWork 整合 JFreeChart：在 Action 中輸出動態圖表圖片

WebWork (Struts 2 前身) 以攔截器鏈與結果類型機制支援多樣化視圖輸出。本文示範如何自訂 Result 類型 (ChartResult)，在 Action 中構建 JFreeChart 物件，直接輸出 PNG 圖片至 HTTP Response，無需暫存檔案，實現「Action 即圖表服務」的乾淨架構。

一、環境依賴 (WebWork 2.2 / JFreeChart 1.0.x)

• webwork-2.2.jar, xwork-1.0.jar, ognl-2.6.jar, commons-logging.jar
• jfreechart-1.0.0.jar, jcommon-1.0.0.jar
• web.xml 註冊 FilterDispatcher (WebWork 核心入口)

二、自訂 ChartResult (核心關鍵)

package pawpaw.test;

import com.opensymphony.webwork.ServletActionContext;
import com.opensymphony.xwork.ActionInvocation;
import com.opensymphony.xwork.Result;
import org.jfree.chart.JFreeChart;
import org.jfree.chart.ChartUtilities;
import javax.servlet.http.HttpServletResponse;
import java.io.OutputStream;

public class ChartResult implements Result {
    private JFreeChart chart;
    private int width = 400;
    private int height = 300;

    // WebWork 會自動注入同名參數
    public void setWidth(int width) { this.width = width; }
    public void setHeight(int height) { this.height = height; }
    public void setChart(JFreeChart chart) { this.chart = chart; }

    @Override
    public void execute(ActionInvocation invocation) throws Exception {
        JFreeChart chartToRender = chart;
        if (chartToRender == null) {
            // 從 ValueStack 取得 (Action 的 getter 回傳)
            chartToRender = (JFreeChart) invocation.getStack().findValue("chart");
        }
        if (chartToRender == null) {
            throw new NullPointerException("No chart found in Action or Result config");
        }

        HttpServletResponse response = ServletActionContext.getResponse();
        response.setContentType("image/png");
        try (OutputStream os = response.getOutputStream()) {
            ChartUtilities.writeChartAsPNG(os, chartToRender, width, height);
        }
    }
}

三、xwork.xml 註冊結果類型與 Action

    
    
        
        
            
        

        
            
                500
                350
            
        
    

四、Action 類別：構建 JFreeChart

package pawpaw.test;

import com.opensymphony.xwork.ActionSupport;
import org.jfree.chart.*;
import org.jfree.chart.axis.NumberAxis;
import org.jfree.chart.plot.XYPlot;
import org.jfree.chart.renderer.xy.StandardXYItemRenderer;
import org.jfree.data.xy.XYSeries;
import org.jfree.data.xy.XYSeriesCollection;
import java.awt.*;

public class ViewModerationChartAction extends ActionSupport {
    private JFreeChart chart;

    public String execute() throws Exception {
        // 1. 準備資料
        XYSeries series = new XYSeries("Random Walk");
        for (int i = 0; i <= 100; i++) {
            series.add(i, Math.random() * 100); // 模擬資料
        }
        XYSeriesCollection dataset = new XYSeriesCollection(series);

        // 2. 建立圖表
        NumberAxis xAxis = new NumberAxis("Raw Marks");
        NumberAxis yAxis = new NumberAxis("Moderated Marks");
        XYPlot plot = new XYPlot(dataset, xAxis, yAxis, 
            new StandardXYItemRenderer(StandardXYItemRenderer.LINES));
        
        chart = new JFreeChart(
            "Moderation Function", 
            JFreeChart.DEFAULT_TITLE_FONT, 
            plot, 
            false
        );

        // 3. 美化
        chart.setBackgroundPaint(Color.WHITE);
        Font titleFont = new Font("微軟正黑體", Font.BOLD, 18);
        chart.setTitle(new TextTitle("Moderation Function", titleFont));
        
        return SUCCESS;
    }

    // ChartResult 會透過此 getter 從 ValueStack 取得
    public JFreeChart getChart() { return chart; }
}

五、web.xml 註冊 JFreeChart 內建 Servlet (可選，用於暫存檔模式)

    DisplayChart
    org.jfree.chart.servlet.DisplayChart


    DisplayChart
    /servlet/DisplayChart

本方案不需此 Servlet，直接串流輸出更高效。內建 Servlet 適合「生成圖片 → 存暫存檔 → HTML <img src='/servlet/DisplayChart?file=xxx'>」的間接模式。

六、驗證與擴展

• 瀏覽 https://host:port/app/viewModerationChart.action 直接顯示 PNG 圖片
• 參數化：Action 可接收請求參數動態生成不同圖表
• 擴展輸出格式：ChartUtilities.writeChartAsJPEG、writeChartAsSVG (需 Batik)
• 整合快取：在 Result 中加入 Last-Modified/ETag 支援瀏覽器快取

七、遷移至 Struts 2 提示

Struts 2 繼承 WebWork 架構，Result 介面兼容，僅套件名變更 (org.apache.struts2)，配置檔改為 struts.xml，遷移成本極低。