onload事件在html文档中所有的节点都下载完成后执行，包括js,css,图片的资源完全下载后才执行。
如果js 设置了defer之后,js的解析执行在浏览器生成了html文档后执行，不包括图片的资源下载
如果js没有设置defer，将一个js放在head加载，会阻塞后面的内容加载。
如下面的例子
如果将defer=true去掉，直到x1.jscript和x2.jscript下载完成后，后面的alert('inline script');才会执行。
alert('win onload');会在图片加载完成后才会执行。
如果js和图片的资源是同一个host,js的加载会阻塞图片的加载，只有等js下载完成后才下载图片。
在firefox中是5个并行下载。
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"><html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>test defer and onload</title> <script type="text/javascript" src="http://localhost:9080/temp/x1.jscript?delaySec=10" defer=true></script> <script type="text/javascript" src="http://localhost:9080/temp/x2.jscript?delaySec=5" defer=true></script> <script type=text/javascript charset=utf-8 >alert('inline script'); window.onload = function(){ alert('win onload'); }</script></head><body> <img src="http://localhost:9080/temp/test.img?delaySec=10&x=1" alt="long loading"> <img src="http://localhost:9080/temp/test.img?delaySec=10&x=2" alt="long loading"> <img src="http://localhost:9080/temp/test.img?delaySec=10&x=3" alt="long loading"> <img src="http://localhost:9080/temp/test.img?delaySec=10&x=4" alt="long loading"> <img src="http://localhost:9080/temp/test.img?delaySec=10&x=5" alt="long loading"></body></html>.jscript通过一个java servlet来实现，模拟一个需要一个比较长时间的javascript文件下载
public class JSProcessor extends HttpServlet {protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {try {System.out.println("begin processing");String delaySec = req.getParameter("delaySec");Thread.currentThread().sleep(Integer.parseInt(delaySec)*1000);PrintWriter writer = resp.getWriter();writer.write("alert('done,delay + " + delaySec + "seconds')");System.out.println("end processing");} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {doGet(req, resp);}}.img也是通过一个java servlet来实现，模拟一个需要一个比较长时间的图片下载public class ImageProcessor extends HttpServlet {protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {try {resp.setContentType("image/jpeg;charset=GB2312");System.out.println("begin processing");String delaySec = req.getParameter("delaySec");Thread.currentThread().sleep(Integer.parseInt(delaySec)*1000);String imageFile = getServletContext().getRealPath("/blog_logo.jpg");System.out.println(imageFile);InputStream imageIn = new FileInputStream(new File(imageFile));JPEGImageDecoder decoder = JPEGCodec.createJPEGDecoder(imageIn);BufferedImage image = decoder.decodeAsBufferedImage();JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(resp.getOutputStream());encoder.encode(image);imageIn.close();System.out.println("end processing");} catch (Exception e) {// TODO Auto-generated catch blocke.printStackTrace();}}protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {doGet(req, resp);}}