为什么java BufferedImage类处理大图直接抛出内存溢出的异常?

public static void mergeImage(int rows,int cols,File[] imgFiles,int classroomId) throws IOException {  
		
		HttpServletRequest request = ServletActionContext.getRequest();
	    int chunks = rows * cols;  
	    int chunkWidth, chunkHeight;  
	    int type;  
	    //读入小图  
	    /*File[] imgFiles = new File[chunks];  
	    for (int i = 0; i < chunks; i++) {  
	        imgFiles[i] = new File("D:/QRCode/" + i + ".png");  
	    }  */
	    //创建BufferedImage  
	    BufferedImage[] buffImages = new BufferedImage[chunks];  
	    for (int i = 0; i < chunks; i++) { 
	        buffImages[i] = ImageIO.read(imgFiles[i]);
	    }  
	    type = buffImages[0].getType();  
	    chunkWidth = buffImages[0].getWidth();  
	    chunkHeight = buffImages[0].getHeight();  
	  
	    //设置拼接后图的大小和类型  
	    BufferedImage finalImg = new BufferedImage(chunkWidth * cols, chunkHeight * rows, type);  
	  
	    //写入图像内容  
	    int num = 0;  
	    for (int i = 0; i < rows; i++) {  
	        for (int j = 0; j < cols; j++) {  
	            finalImg.createGraphics().drawImage(buffImages[num], chunkWidth * j, chunkHeight * i, null);  
	            num++;  
	        }  
	    } 
	    String separator = File.separator;
	    String path = request.getSession().getServletContext().getRealPath("/QrCode");
	    path += separator+classroomId+".png";
	    //输出拼接后的图像  
	    ImageIO.write(finalImg, "png", new File(path));  
	  
	}