19.2 編寫一個簡單的Servlet應用 · Java 開源項目中文文檔

## 19.2 編寫一個簡單的Servlet應用 ServletDemo1類實現了一個非常簡單的servlet，該servlet返回了一個PNG圖，PNG圖是使用JFreeChart生成的直方條形圖表。當運行該程序時，servlet在客戶端僅僅顯示一幅圖片，而沒有任何的HTML修飾，參見下圖19.1。 ![](img/jfc117021.png) 如圖19.1 瀏覽器中的servlet效果我們以這種方式顯示圖片，是沒有特殊意義，僅僅是為了： + 很好演示servlets的請求－響應交互特征； + 作為測試實例非常有用，我們會了解如何配置一個服務環境，如何讓頁面控件工作。我們可以瀏覽后面更復雜的實例，顯示使用HMTL表單如何請求不同的圖表，并且將產生的圖表的輸出植入到HTML中。下面是基本servlet的代碼。 ``` package demo; import java.io.IOException; import java.io.OutputStream; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import org.jfree.chart.ChartFactory; import org.jfree.chart.ChartUtilities; import org.jfree.chart.JFreeChart; import org.jfree.chart.plot.PlotOrientation; import org.jfree.data.category.DefaultCategoryDataset; /** * A basic servlet that returns a PNG image file generated by JFreeChart. This * class is described in the JFreeChart Developer Guide in the "Servlets" * chapter. */ public class ServletDemo1 extends HttpServlet { /** * Creates a new demo. */ public ServletDemo1() { // nothing required } /** * Processes a GET request. * * @param request * the request. * @param response * the response. * * @throws ServletException * if there is a servlet related problem. * @throws IOException * if there is an I/O problem. */ public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { OutputStream out = response.getOutputStream(); try { DefaultCategoryDataset dataset = new DefaultCategoryDataset(); dataset.addValue(10.0, "S1", "C1"); dataset.addValue(4.0, "S1", "C2"); dataset.addValue(15.0, "S1", "C3"); dataset.addValue(14.0, "S1", "C4"); dataset.addValue(-5.0, "S2", "C1"); dataset.addValue(-7.0, "S2", "C2"); dataset.addValue(14.0, "S2", "C3"); dataset.addValue(-3.0, "S2", "C4"); dataset.addValue(6.0, "S3", "C1"); dataset.addValue(17.0, "S3", "C2"); dataset.addValue(-12.0, "S3", "C3"); dataset.addValue(7.0, "S3", "C4"); dataset.addValue(7.0, "S4", "C1"); dataset.addValue(15.0, "S4", "C2"); dataset.addValue(11.0, "S4", "C3"); dataset.addValue(0.0, "S4", "C4"); dataset.addValue(-8.0, "S5", "C1"); dataset.addValue(-6.0, "S5", "C2"); dataset.addValue(10.0, "S5", "C3"); dataset.addValue(-9.0, "S5", "C4"); dataset.addValue(9.0, "S6", "C1"); dataset.addValue(8.0, "S6", "C2"); dataset.addValue(null, "S6", "C3"); dataset.addValue(6.0, "S6", "C4"); dataset.addValue(-10.0, "S7", "C1"); dataset.addValue(9.0, "S7", "C2"); dataset.addValue(7.0, "S7", "C3"); dataset.addValue(7.0, "S7", "C4"); dataset.addValue(11.0, "S8", "C1"); dataset.addValue(13.0, "S8", "C2"); dataset.addValue(9.0, "S8", "C3"); dataset.addValue(9.0, "S8", "C4"); dataset.addValue(-3.0, "S9", "C1"); dataset.addValue(7.0, "S9", "C2"); dataset.addValue(11.0, "S9", "C3"); dataset.addValue(-10.0, "S9", "C4"); JFreeChart chart = ChartFactory.createBarChart("Bar Chart", "Category", "Value", dataset, PlotOrientation.VERTICAL, true, true, false); response.setContentType("image/png"); ChartUtilities.writeChartAsPNG(out, chart, 400, 300); } catch (Exception e) { System.err.println(e.toString()); } finally { out.close(); } } } ``` 當一個客戶端（通常是一個web瀏覽器）發出一個請求時，Servlet引擎調用DoGet()方法，以響應這個請求，servlet執行下面幾步： + 為客戶端返回的輸出獲得一個輸出流引用 + 創建一個圖表； + 響應的內容類型設置為image/png，這告訴客戶端接受的數據類型是什么； + 一個圖表的PNG圖表beii寫進輸出流； + 輸出流關閉。