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由JComponent生成BufferedImage

小米 — Fri, 14 Apr 2006 15:41:00 GMT

最近在弄swing，需要由JComponent生成BufferedImage，在CSDN上发现一个好例子。下面是范例：

Rectangle rect = comp.getBounds();
BufferedImage bufImage = new BufferedImage(rect.width,
rect.height,
BufferedImage.TYPE_INT_RGB);
Graphics g = bufImage.getGraphics();
g.translate(-rect.x, -rect.y);
comp.paint(g);

这样，JComponent中的图像就保存到BufferedImage中了。
原文的链接：http://dev.csdn.net/article/13/13531.shtm

小米 2006-04-14 23:41 发表评论

如何将BufferedImage实例保存为BMP文件

小米 — Mon, 04 Jul 2005 09:33:00 GMT

摘要: 要将BufferedImage实例保存为BMP文件，就需要知道BMP文件的格式，可以参考我转载的文章：《BMP文件格式》。下面是我的将BufferedImage实例保存为24位色BMP文件的实现。 ... 阅读全文

小米 2005-07-04 17:33 发表评论

关闭JBuilder2005的Smart MemberInsight功能

小米 — Wed, 15 Jun 2005 08:02:00 GMT

      JBuilder 2005中多了一个Smart MemberInsight功能，这个功能会智能的匹配需要调用的方法或者获取的属性。例如有一个赋值操作，左边是一个List，右边是用一个方法获取值，那么当输入"."时，JBuilder 2005会自动的找到所有返回的值为List的方法。
      不过，这实在是一个不怎么讨好的方法。因为很多时候并不是只有一层方法调用的，例如，当需要用一个方法获取一个实例，然后再用这个实例的方法获取需要的值时，这个功能就显得很碍手了。
      终于，我还是决定关闭这个“智能”的功能。打开"Tools"——>"Preferences"，在"Editor"的子项"CodeInsight"里，展开"MemberInsight"，把"Default to Smart MemberInsight"这一项去掉。如下图所示：

小米 2005-06-15 16:02 发表评论

用JFreeChart画柱状图的范例

小米 — Tue, 14 Jun 2005 10:40:00 GMT

这个范例说明如何用JFreeChart画简单的柱状图，下面是一个JSP的简单范例：

<%@ page contentType="text/html; charset=GB2312" %>
<%@ page import="java.awt.*, java.text.*, java.util.*" %>
<%@ page import="org.jfree.chart.*" %>
<%@ page import="org.jfree.chart.axis.*" %>
<%@ page import="org.jfree.chart.labels.StandardCategoryItemLabelGenerator" %>
<%@ page import="org.jfree.chart.plot.*" %>
<%@ page import="org.jfree.chart.renderer.*" %>
<%@ page import="org.jfree.chart.servlet.ServletUtilities" %>
<%@ page import="org.jfree.data.DefaultCategoryDataset" %>
<%@ page import="org.jfree.ui.TextAnchor" %>

<%
  //The data for the bar chart
  double[] data = {85, 156, 179.5, 211, 123};
  //The labels for the bar chart
  String[] labels = {"Mon", "Tue", "Wed", "Thu", "Fri"};

  DefaultCategoryDataset dataset = new DefaultCategoryDataset();
  for (int i = 0; i < data.length; i++) {
    dataset.addValue(data[i], null, labels[i]);
  }

  JFreeChart chart = ChartFactory.createBarChart3D("Weekly Server Load", "Work Week 25", "MBytes", dataset, PlotOrientation.VERTICAL, false, false, false);
  chart.setBackgroundPaint(new Color(0xE1E1E1));

  CategoryPlot plot = chart.getCategoryPlot();

  // 设置Y轴显示整数
  NumberAxis rangeAxis = (NumberAxis) plot.getRangeAxis();
  rangeAxis.setStandardTickUnits(NumberAxis.createIntegerTickUnits());

  CategoryAxis domainAxis = plot.getDomainAxis();
  //设置距离图片左端距离
  domainAxis.setLowerMargin(0.05);

  BarRenderer3D renderer = new BarRenderer3D();
  //设置柱的颜色
  renderer.setSeriesPaint(0, new Color(0xff00));
  plot.setRenderer(renderer);

  String filename = ServletUtilities.saveChartAsPNG(chart, 300, 280, null, session);
  String graphURL = request.getContextPath() + "/displayChart?filename=" + filename;
%>
<html>
<body topmargin="5" leftmargin="5" rightmargin="0">
<div style="font-size:18pt; font-family:verdana; font-weight:bold">
    3D Bar Chart
div>
<br>
<img src="<%= graphURL %>" border=0>
body>
html>

画出来的图：

和ChartDirector画出来的图做一个比较：

小米 2005-06-14 18:40 发表评论

用ChartDirector在JSP中画统计图

小米 — Tue, 14 Jun 2005 09:46:00 GMT

      以前一直是用JFreeChart画统计图的，不过JFreeChart画出来的图形不够精细，看起来有些模糊，今天在网上看到另外一个工具ChartDirector，这是一个商业版本的工具，不过也可以免费使用，只是在画出来的图形下面都有一条它的广告条。
      下面是它的一个柱状图的例子：

      范例程序：

<%@page import="ChartDirector.*" %>
<%
//The data for the bar chart
double[] data = {85, 156, 179.5, 211, 123};

//The labels for the bar chart
String[] labels = {"Mon", "Tue", "Wed", "Thu", "Fri"};

//Create a XYChart object of size 300 x 280 pixels
XYChart c = new XYChart(300, 280);

//Set the plotarea at (45, 30) and of size 200 x 200 pixels
c.setPlotArea(45, 30, 200, 200);

//Add a title to the chart
c.addTitle("Weekly Server Load");

//Add a title to the y axis
c.yAxis().setTitle("MBytes");

//Add a title to the x axis
c.xAxis().setTitle("Work Week 25");

//Add a bar chart layer with green (0x00ff00) bars using the given data
c.addBarLayer(data, 0xff00).set3D();

//Set the labels on the x axis.
c.xAxis().setLabels(labels);

//output the chart
String chart1URL = c.makeSession(request, "chart1");

//include tool tip for the chart
String imageMap1 = c.getHTMLImageMap("", "", "title='{xLabel}: {value} MBytes'")
    ;
%>
<html>
<body topmargin="5" leftmargin="5" rightmargin="0">
<div style="font-size:18pt; font-family:verdana; font-weight:bold">
    3D Bar Chart
div>
<hr color="#000080">
<a href="viewsource.jsp?file=<%=request.getServletPath()%>">
    <font size="2" face="Verdana">View Chart Source Codefont>
a>
div>
<br>
<img src='<%=response.encodeURL("getchart.jsp?"+chart1URL)%>'
    usemap="#map1" border="0">
<map name="map1"><%=imageMap1%>map>
body>
html>

如果要在柱的顶部显示数值，可以调用Layer的setDataLabelFormat方法设置，范例：layer.setDataLabelFormat("{value}");

其它的例子可以参考它的文档的说明。ChartDirector的网址：http://www.advsofteng.com

小米 2005-06-14 17:46 发表评论

用java.util.Timer定时执行任务

小米 — Thu, 09 Jun 2005 02:29:00 GMT

如果要在程序中定时执行任务，可以使用java.util.Timer这个类实现。使用Timer类需要一个继承了java.util.TimerTask的类。TimerTask是一个虚类，需要实现它的run方法，实际上是他implements了Runnable接口，而把run方法留给子类实现。
下面是我的一个例子：

class Worker extends TimerTask {
  public void run() {
    System.out.println("我在工作啦！");
  }
}

Timer类用schedule方法或者scheduleAtFixedRate方法启动定时执行，schedule重载了四个版本，scheduleAtFixedRate重载了两个。每个方法的实现都不同，下面是每个方法的说明：

schedule

public void schedule(TimerTask task,
                     long delay)

Schedules the specified task for execution after the specified delay.

Parameters:: task - task to be scheduled.; delay - delay in milliseconds before task is to be executed.
Throws:: IllegalArgumentException - if delay is negative, or delay + System.currentTimeMillis() is negative.; IllegalStateException - if task was already scheduled or cancelled, or timer was cancelled.

说明：该方法会在设定的延时后执行一次任务。

schedule

public void schedule(TimerTask task,
                     Date time)

Schedules the specified task for execution at the specified time. If the time is in the past, the task is scheduled for immediate execution.

Parameters:: task - task to be scheduled.; time - time at which task is to be executed.
Throws:: IllegalArgumentException - if time.getTime() is negative.; IllegalStateException - if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.

说明：该方法会在指定的时间点执行一次任务。

schedule

public void schedule(TimerTask task,
                     long delay,
                     long period)

Schedules the specified task for repeated fixed-delay execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals separated by the specified period.

In fixed-delay execution, each execution is scheduled relative to the actual execution time of the previous execution. If an execution is delayed for any reason (such as garbage collection or other background activity), subsequent executions will be delayed as well. In the long run, the frequency of execution will generally be slightly lower than the reciprocal of the specified period (assuming the system clock underlying Object.wait(long) is accurate).

Fixed-delay execution is appropriate for recurring activities that require "smoothness." In other words, it is appropriate for activities where it is more important to keep the frequency accurate in the short run than in the long run. This includes most animation tasks, such as blinking a cursor at regular intervals. It also includes tasks wherein regular activity is performed in response to human input, such as automatically repeating a character as long as a key is held down.

Parameters:: task - task to be scheduled.; delay - delay in milliseconds before task is to be executed.; period - time in milliseconds between successive task executions.
Throws:: IllegalArgumentException - if delay is negative, or delay + System.currentTimeMillis() is negative.; IllegalStateException - if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.

说明：该方法会在指定的延时后执行任务，并且在设定的周期定时执行任务。

schedule

public void schedule(TimerTask task,
                     Date firstTime,
                     long period)

Schedules the specified task for repeated fixed-delay execution, beginning at the specified time. Subsequent executions take place at approximately regular intervals, separated by the specified period.

Parameters:: task - task to be scheduled.; firstTime - First time at which task is to be executed.; period - time in milliseconds between successive task executions.
Throws:: IllegalArgumentException - if time.getTime() is negative.; IllegalStateException - if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.

说明：该方法会在指定的时间点执行任务，然后从该时间点开始，在设定的周期定时执行任务。特别的，如果设定的时间点在当前时间之前，任务会被马上执行，然后开始按照设定的周期定时执行任务。

scheduleAtFixedRate

public void scheduleAtFixedRate(TimerTask task,
                                long delay,
                                long period)

Schedules the specified task for repeated fixed-rate execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals, separated by the specified period.

In fixed-rate execution, each execution is scheduled relative to the scheduled execution time of the initial execution. If an execution is delayed for any reason (such as garbage collection or other background activity), two or more executions will occur in rapid succession to "catch up." In the long run, the frequency of execution will be exactly the reciprocal of the specified period (assuming the system clock underlying Object.wait(long) is accurate).

Fixed-rate execution is appropriate for recurring activities that are sensitive to absolute time, such as ringing a chime every hour on the hour, or running scheduled maintenance every day at a particular time. It is also appropriate for recurring activities where the total time to perform a fixed number of executions is important, such as a countdown timer that ticks once every second for ten seconds. Finally, fixed-rate execution is appropriate for scheduling multiple repeating timer tasks that must remain synchronized with respect to one another.

Parameters:: task - task to be scheduled.; delay - delay in milliseconds before task is to be executed.; period - time in milliseconds between successive task executions.
Throws:: IllegalArgumentException - if delay is negative, or delay + System.currentTimeMillis() is negative.; IllegalStateException - if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.

说明：该方法和schedule的相同参数的版本类似，不同的是，如果该任务因为某些原因（例如垃圾收集）而延迟执行，那么接下来的任务会尽可能的快速执行，以赶上特定的时间点。

scheduleAtFixedRate

public void scheduleAtFixedRate(TimerTask task,
                                Date firstTime,
                                long period)

Schedules the specified task for repeated fixed-rate execution, beginning at the specified time. Subsequent executions take place at approximately regular intervals, separated by the specified period.

Parameters:: task - task to be scheduled.; firstTime - First time at which task is to be executed.; period - time in milliseconds between successive task executions.
Throws:: IllegalArgumentException - if time.getTime() is negative.; IllegalStateException - if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.

说明：和上一个方法类似。

下面是我的一个测试片断：

  public static void main(String[] args) throws Exception {
    Timer timer = new Timer(false);
    timer.schedule(new Worker(), new Date(System.currentTimeMillis() + 1000));
  }

小米 2005-06-09 10:29 发表评论

JavaMail的简单实例

小米 — Thu, 02 Jun 2005 08:30:00 GMT

摘要: 利用JavaMail的API可以快速的实现发送邮件的功能。下面是我使用的一个简单的实例，实现了简单的文本邮件的发送。 import java.io.*;import java.util.*;import javax.activation.*;import javax.mail.*;... 阅读全文

小米 2005-06-02 16:30 发表评论

如何比较两个有可能为null的实例

小米 — Wed, 25 May 2005 09:00:00 GMT

以前在写程序的时候，碰到需要比较两个有可能为null的实例时，为了避免出现NullPointerException，经常用这样的一段代码来比较：

    Object obj1 = "abc";
    Object obj2 = "cde";

    if ((obj1 == null && obj2 == null) || (obj1 != null && obj1.equals(obj2))
        || (obj2 != null && obj2.equals(obj1))) {
      System.out.println("equals");
    }

这样的程序，读起来真是挺拗口。我一直没有想到什么好的方法解决这个问题，直到今天在看到JDK的AbstractList源码的equals方法的实现时，看到这一段：

if (!(o1==null ? o2==null : o1.equals(o2)))
return false;

原来用三元运算符可以很好的解决这个问题，呵呵，我前面的程序可以改写成：

    Object obj1 = "abc";
    Object obj2 = "cde";

    if (obj1 == null ? obj2 == null : obj1.equals(obj2))
      System.out.println("equals");

真是简洁多了！

小米 2005-05-25 17:00 发表评论

从SocketChannel对象池中获取的实例，使用时应注意的问题

小米 — Wed, 25 May 2005 07:02:00 GMT

      从对象池中获取的实例，因为并不清楚该channel是否已经设置成正确的状态，所以在使用时最好重新设置一遍。有以下几点需要注意：
      1.在使用阻塞IO时，需要把该channel设置成阻塞的，即需要调用SocketChannel.configureBlocking(true);
      2.在使用非阻塞IO时，需要把该channel设置成非阻塞的，即需要调用SocketChannel.configureBlocking(false);
      3.如果该channel注册了selector，那么在返回该实例到对象池中，需要把注册的selector清除，即需要调用Selector的close方法。

      下面是一段应用场景的例子：

        // 把命令输出
        channel.configureBlocking(true);
        PrintWriter writer = new PrintWriter(channel.socket().getOutputStream(), false);
        writer.write(command.endsWith("\n") ? command : command + "\n");
        writer.flush();

        channel.configureBlocking(false);

        // 创建Selector
        Selector selector = Selector.open();
        // 向Selector注册我们需要的READ事件
        SelectionKey skey = channel.register(selector, SelectionKey.OP_READ);

        boolean stop = false;
        int n = 0;
        int read = 0;
        ByteBuffer buffer = ByteBuffer.allocate(1024);

        // 轮询
        while (!stop) {
          // 获取Selector返回的时间值
          n = selector.select();

          // 当传回的值大于0事，读事件发生了
          if (n > 0) {
         // 处理发生的事件

          }
        }

        selector.close();

小米 2005-05-25 15:02 发表评论