java上传图片前预览 控制图片尺寸大小 控制上传格式

原文：http://blog.sina.com.cn/s/blog_4d3d00180102v4yl.html

var maxH = 768;
var maxW = 1024;
function DrawImage(ImgD){
   var preW = 300;
   var preH = 400 ;
   var image=new Image();
   image.src=ImgD.src;
   if(image.width > maxW || image.height > maxH){
alert("图片尺寸过大，请选择" + maxW + "*" + maxH + "的图片！");
return;
   }
   if(image.width>0 && image.height>0){
    flag=true;
    if(image.width/image.height>= preW/preH){
     if(image.width>preW){ 
     ImgD.width=preW;
     ImgD.height=(image.height*preW)/image.width;
     }else{
     ImgD.width=image.width; 
     ImgD.height=image.height;
     }
     ImgD.alt=image.width+"×"+image.height;
     }
    else{
     if(image.height>preH){ 
     ImgD.height=preH;
     ImgD.width=(image.width*preH)/image.height;    
     }else{
     ImgD.width=image.width; 
     ImgD.height=image.height;
     }
     ImgD.alt=image.width+"×"+image.height;
     }
    }
   }
function checkFormat(filePath) {
var   i = filePath.lastIndexOf('.');
var   len = filePath.length;
var   str = filePath.substring(len,i+1);
var   extName = "JPG,GIF,PNG,JPEG,BMP";
if(extName.indexOf(str.toUpperCase()) < 0)  {    
  alert("请选择正确的图片文件!");  
  return false;  
}
return true;        
}  

function FileChange(Value){
if(checkFormat(Value)){
  flag=false;
  document.getElementByIdx_x("uploadimage").width=10;
  document.getElementByIdx_x("uploadimage").height=10;
  document.getElementByIdx_x("uploadimage").alt="";
  document.getElementByIdx_x("uploadimage").src=Value;
}
}