在網上看了一下別人的解決方法，直接粗暴的獲取日期的月份，相減就給出答案，這不是我想要的，因為這個完全沒意義：

兩個日期跨年了怎麼辦，如2017-01-02 到2018-12-13相差幾個月？ 拿 1減去12嗎。。

後面自己實現了一下：

import org.joda.time.DateTime;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;
import java.util.Calendar;

@RestController
public class Test {
    @RequestMapping(value = "/", method = RequestMethod.GET)
    public void test() {

        DateTime startTime = new DateTime("2017-12-1");
        DateTime endTime = new DateTime("2018-7-29");

        Calendar c1 = Calendar.getInstance();
        Calendar c2 = Calendar.getInstance();

        c1.setTime(startTime.toDate());
        c2.setTime(endTime.toDate());
        int between = 0;


        while ((c1.get(Calendar.YEAR) != c2.get(Calendar.YEAR)) || (c1.get(Calendar.MONTH) != c2.get(Calendar.MONTH)) ) {
            startTime = startTime.plusMonths(1);
            c1.setTime(startTime.toDate());

            between = between + 1;
            System.out.println("startTime" + startTime);
            System.out.println("endTime" + endTime);

            System.out.println(c1.get(Calendar.YEAR) );
            System.out.println(c2.get(Calendar.YEAR));


            System.out.println(c1.get(Calendar.MONTH) );
            System.out.println(c2.get(Calendar.MONTH));
        }
        System.out.println(between); //7
    }