onload事件在html文檔中所有的節點都下載完成後執行，包括js,css,圖片的資源完全下載後才執行。
如果js 設置了defer之後,js的解析執行在瀏覽器生成了html文檔後執行，不包括圖片的資源下載
如果js沒有設置defer，將一個js放在head加載，會阻塞後面的內容加載。
如下面的例子
如果將defer=true去掉，直到x1.jscript和x2.jscript下載完成後，後面的alert('inline script');纔會執行。
alert('win onload');會在圖片加載完成後纔會執行。
因爲瀏覽器對資源加載的並行下載有限制,如果js和圖片的資源是同一個host,js的加載會阻塞圖片的加載，只有等js下載完成後纔開始下載圖片。
在firefox中對同一種資源的並行下載是5個。
<!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>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 block
e.printStackTrace();
}
}
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}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 block
e.printStackTrace();
}
}
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}
