我们都知道Java程序之所以被广大程序员青睐，很大的一个原因是因为Java有GC(垃圾收集)，不用程序员花很大的精力来解决内存释放和泄漏问题。而这些问题总是C/C++程序员需要花很大精力来认真地面对的。
问题总是双面的，GC给我们带来了很大的快乐，释放了程序员很多的精力和时间，但是在某些时候也会给我们带来一些小小的麻烦。Java里的Object并非交给GC去释放就可高枕无忧了，下面从JDK1.4的Demo中Java2D的Memory Monitor说起。
先看看Memory Monitor单独运行的效果。
可以看到因为有个While循环，GC释放内存有一定的时间，在这个时间中间，内存消耗的很厉害。峰值达到923K。程序代码的如下：
//SysGcMain.java Created on 9:15:59
package com.gx2.system;
/**
* @author Frank Gao
@version 1.00
* Copy right by GX2 Studio 2003
* Copyright (c) 2003 Sun Microsystems, Inc. All
Rights Reserved.
*
* Redistribution and use in source……以后省略
*/
/*
* @(#)MemoryMonitor.java
1.32 03/01/23
*/
import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import java.awt.geom.Line2D;
import java.awt.geom.Rectangle2D;
import java.util.Date;
import javax.swing.*;
import javax.swing.border.EtchedBorder;
import javax.swing.border.TitledBorder;
/**
* Tracks Memory allocated & used, displayed in graph form.
*/
public class SysGcMain extends JPanel {
static JCheckBox dateStampCB = new JCheckBox("Output Date Stamp");
public Surface surf;
JPanel controls;
boolean doControls;
JTextField tf;
public SysGcMain() {
setLayout(new BorderLayout());
setBorder(new TitledBorder(new EtchedBorder(), "Memory Monitor"));
add(surf = new Surface());
controls = new JPanel();
controls.setPreferredSize(new Dimension(135,80));
Font font = new Font("serif", Font.PLAIN, 10);
JLabel label = new JLabel("Sample Rate");
label.setFont(font);
label.setForeground(Color.black);
controls.add(label);
tf = new JTextField("1000");
tf.setPreferredSize(new Dimension(45,20));
controls.add(tf);
controls.add(label = new JLabel("ms"));
label.setFont(font);
label.setForeground(Color.black);
controls.add(dateStampCB);
dateStampCB.setFont(font);
addMouseListener(new MouseAdapter() {
public void mouseClicked(MouseEvent e) {
removeAll();
if ((doControls = !doControls)) {
surf.stop();
add(controls);
} else {
try {
surf.sleepAmount = Long.parseLong(tf.getText().trim());
} catch (Exception ex) {}
surf.start();
add(surf);
}
validate();
repaint();
}
});
}
public class Surface extends JPanel implements Runnable {
public Thread thread;
public long sleepAmount = 1000;
private int w, h;
private BufferedImage bimg;
private Graphics2D big;
private Font font = new Font("Times New Roman", Font.PLAIN, 11);
private Runtime r = Runtime.getRuntime();
private int columnInc;
private int pts[];
private int ptNum;
private int ascent, descent;
private float freeMemory, totalMemory;
private Rectangle graphOutlineRect = new Rectangle();
private Rectangle2D mfRect = new Rectangle2D.Float();
private Rectangle2D muRect = new Rectangle2D.Float();
private Line2D graphLine = new Line2D.Float();
private Color graphColor = new Color(46, 139, 87);
private Color mfColor = new Color(0, 100, 0);
private String usedStr;
public Surface() {
setBackground(Color.black);
addMouseListener(new MouseAdapter() {
public void mouseClicked(MouseEvent e) {
if (thread == null) start(); else stop();
}
});
}
public Dimension getMinimumSize() {
return getPreferredSize();
}
public Dimension getMaximumSize() {
return getPreferredSize();
}
public Dimension getPreferredSize() {
return new Dimension(135,80);
}
public void paint(Graphics g) {
if (big == null) {
return;
}
big.setBackground(getBackground());
big.clearRect(0,0,w,h);
float freeMemory = (float) r.freeMemory();
float totalMemory = (float) r.totalMemory();
// .. Draw allocated and used strings ..
big.setColor(Color.green);
big.drawString(String.valueOf((int) totalMemory/1024) + "K allocated",
4.0f, (float) ascent+0.5f);
usedStr = String.valueOf(((int) (totalMemory - freeMemory))/1024)
+ "K used";
big.drawString(usedStr, 4, h-descent);
// Calculate remaining size
float ssH = ascent + descent;
float remainingHeight = (float) (h - (ssH*2) - 0.5f);
float blockHeight = remainingHeight/10;
float blockWidth = 20.0f;
float remainingWidth = (float) (w - blockWidth - 10);
// .. Memory Free ..
big.setColor(mfColor);
int MemUsage = (int) ((freeMemory / totalMemory) * 10);
int i = 0;
for ( ; i
mfRect.setRect(5,(float) ssH+i*blockHeight,
blockWidth,(float) blockHeight-1);
big.fill(mfRect);
}
// .. Memory Used ..
big.setColor(Color.green);
for ( ; i
{
muRect.setRect(5,(float) ssH+i*blockHeight,
blockWidth,(float) blockHeight-1);
big.fill(muRect);
}
// .. Draw History Graph ..
big.setColor(graphColor);
int graphX = 30;
int graphY = (int) ssH;
int graphW = w - graphX - 5;
int graphH = (int) remainingHeight;
graphOutlineRect.setRect(graphX, graphY, graphW, graphH);
big.draw(graphOutlineRect);
int graphRow = graphH/10;
// .. Draw row ..
for (int j = graphY; j
graphLine.setLine(graphX,j,graphX+graphW,j);
big.draw(graphLine);
}
// .. Draw animated column movement ..
int graphColumn = graphW/15;
if (columnInc == 0) {
columnInc = graphColumn;
}
for (int j = graphX+columnInc; j
graphLine.setLine(j,graphY,j,graphY+graphH);
big.draw(graphLine);
}
--columnInc;
if (pts == null) {
pts = new int[graphW];
ptNum = 0;
} else if (pts.length != graphW) {
int tmp[] = null;
if (ptNum
tmp = new int[pt
