java代码判断图片文件格式， 不是根据文件后缀来判断。

public static final String TYPE_JPG = "jpg";
    public static final String TYPE_GIF = "gif";
    public static final String TYPE_PNG = "png";
    public static final String TYPE_BMP = "bmp";
    public static final String TYPE_UNKNOWN = "unknown";

/**
     * byte数组转换成16进制字符串
     * @param src
     * @return
     */
    public static String bytesToHexString(byte[] src){    
           StringBuilder stringBuilder = new StringBuilder();    
           if (src == null || src.length <= 0) {    
               return null;    
           }    
           for (int i = 0; i < src.length; i++) {    
               int v = src[i] & 0xFF;    
               String hv = Integer.toHexString(v);    
               if (hv.length() < 2) {    
                   stringBuilder.append(0);    
               }    
               stringBuilder.append(hv);    
           }    
           return stringBuilder.toString();    
       }
    

    /**
     * 根据文件流判断图片类型
     * @param fis
     * @return jpg/png/gif/bmp
     */
    public static String getPicType(FileInputStream fis) {
        //读取文件的前几个字节来判断图片格式
        byte[] b = new byte[4];
        try {
            fis.read(b, 0, b.length);
            String type = bytesToHexString(b).toUpperCase();
            if (type.contains("FFD8FF")) {
                return TYPE_JPG;
            } else if (type.contains("89504E47")) {
                return TYPE_PNG;
            } else if (type.contains("47494638")) {
                return TYPE_GIF;
            } else if (type.contains("424D")) {
                return TYPE_BMP;
            }else{
                return TYPE_UNKNOWN;
            }
        } catch (IOException e) {
            e.printStackTrace();
        }finally{
            if(fis != null){
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return null;
    }

//test code
System.out.println("图片格式1： " + getPicType(new FileInputStream(new File("D:\3_副本.png"))));
System.out.println("图片格式2： " + getPicType(new FileInputStream(new File("D:\3.png"))));