两个Long类型相乘溢出怎么解决

public static void main(String[] args) throws Exception {
        long a = Long.MAX_VALUE;
        long b = Long.MAX_VALUE;

        BigDecimal ba = new BigDecimal(String.valueOf(a));
        BigDecimal bb = new BigDecimal(String.valueOf(b));
        BigDecimal bc = ba.multiply(bb);
        System.out.println(String.valueOf(a));
        System.out.println(String.valueOf(b));
        System.out.println(bc);
    }

9223372036854775807
9223372036854775807
85070591730234615847396907784232501249

两个Long类型相乘溢出怎么解决