java如何根据二进制流确定图片的类型

public enum ImageType {
	PNG('P','N','G','-',"PNG"),
	JPG('J','F','I','F',"JPG"),
	GIF('G','I','F','-',"GIF");
	private char begin, mid, end, last;
	private String type;
	private ImageType(char begin, char mid, char end, char last, String type){
		this.begin = begin;
		this.mid = mid;
		this.end = end;
		this.last = last;
		this.type = type;
	}
	
	/**
	 * 获取图片类型
	 * @param data
	 * @return
	 */
	public static ImageType getImageType(byte[] data) {
		// Png:
		if (PNG.getBegin() == data[1] && PNG.getMid() == data[2] && PNG.getEnd() == data[3]) {
			return PNG;
		}
		// Gif:
		if (GIF.getBegin() == data[0] && GIF.getMid() == data[1] && GIF.getEnd() == data[2]) {
			return GIF;
		}
		// JPG:
		if (JPG.getBegin() == data[6] && JPG.getMid() == data[7] && JPG.getEnd() == data[8]
		        && JPG.getLast() == data[9]) {
			return JPG;
		}
		return null;
	}
}

//把JPG图片的后缀改成txt
public static void main(String[] args) throws Exception {
		FileInputStream fs = new FileInputStream("d:\\Koala.txt");
		byte[] data = new byte[10];//读10个长度就够了
		fs.read(data);
		System.out.println(ImageType.getImageType(data));
		fs.close();
	}

JPG

public enum FileType2 {
	FILE;
	private static final Map<String, String> FILE_TYPES;
	
	static {
		FILE_TYPES = new HashMap<String, String>();
		FILE_TYPES.put("PNG", "PNG");
		FILE_TYPES.put("JFIF", "JPG");
		FILE_TYPES.put("GIF", "GIF");
		FILE_TYPES.put("PDF", "PDF");
	}

	/**
	 * 从集合中取得相应类型
	 * 
	 * @param target
	 * @return
	 */
	public static String getType(String target) {
		if (isEmpty(target)) {
			return null;
		}
		String result = null;
		for (String key : FILE_TYPES.keySet()) {
			if (!contains(target, key)) continue;
			result = FILE_TYPES.get(key);
			break;
		}
		return result;
	}

	/**
	 * 是否包含
	 * 
	 * @param str
	 * @param searchStr
	 * @return
	 */
	private static boolean contains(String str, String searchStr) {
		if (str == null || searchStr == null) {
			return false;
		}
		return str.indexOf(searchStr) >= 0;
	}

	/**
	 * 是否为空
	 * 
	 * @param str
	 * @return
	 */
	private static boolean isEmpty(String str) {
		return str == null || str.length() == 0;
	}
}

FileInputStream fs = new FileInputStream("d:\\Koala.JPG");
		byte[] data = new byte[10];// 读10个长度就够了
		fs.read(data);
		StringBuilder str = new StringBuilder("");
		for (int i = 0; i < data.length; i++) {
			str.append((char) data[i]);
//			System.out.println(i + ":" + (char) data[i]);
		}
		System.out.println("------------------");
		System.out.println(FileType2.getType(str.toString()));
//		System.out.println("文件类型:"+ImageType.getImageType(data).getType());
		fs.close();