使用Collections.sort() 对List 排序

一、问题

有一次线上系统运行过程中，爆出更新数据库死锁的异常，联系DBA排查原因：在批量更新库存时，发生死锁。

 

二、解决方案

对调用DAL API的参数List进行排序，保证获取资源的顺序一致，从而避免数据库死锁，于是就有了这篇blog。

 

三、利用java.List.Collections对List排序

    1. 待排序的POJO类，OfferSaleItemStatDAOUpdateParam.java：

public class OfferSaleItemStatDAOUpdateParam {
    /**
     * 排序条件1
     */
    private Long   offerId;
    /**
     * 排序条件2
     */
    private String specId;
    /**
     * 当前变化的订购数量，即detA值
     */
    private Long   orderAmount;

    .. 忽略get and set method ...
    public String toString() {
        return ToStringBuilder.reflectionToString(this);
    }

 

    2. 编写比较器OfferSaleItemStatDAOUpdateParamComparator，继承了Comparator接口：

public class OfferSaleItemStatDAOUpdateParamComparator implements Comparator<OfferSaleItemStatDAOUpdateParam> {

    /**
     * <pre>
     * 1. 先比较offerId
     * 2. 然后再比较specId
     * 
     * <pre>
     */
    @Override
    public int compare(OfferSaleItemStatDAOUpdateParam o1, OfferSaleItemStatDAOUpdateParam o2) {
        if (o1 == null || o1.getOfferId() == null || o2 == null || o2.getOfferId() == null) {
            return 0;
        }
        if (o1.getOfferId() < o2.getOfferId()) {
            return -1;
        } else if (o1.getOfferId() > o2.getOfferId()) {
            return 1;
        } else {
            String o1SpecId = (o1.getSpecId() == null) ? "" : o1.getSpecId();
            String o2SpecId = (o2.getSpecId() == null) ? "" : o2.getSpecId();

            int tempResult = o2SpecId.compareTo(o1SpecId);
            if (tempResult < 0) {
                return -1;
            } else if (tempResult > 0) {
                return 1;
            } else {
                return 0;
            }
        }
    }
}

 

   3. 测试结果，使用java.List.Collections对List排序

public class OfferSaleItemStatDAOUpdateParamComparatorTest extends JTester {
    @Test
    public void testCompare() {
        OfferSaleItemStatDAOUpdateParamComparator comparator = new OfferSaleItemStatDAOUpdateParamComparator();
        
        OfferSaleItemStatDAOUpdateParam p1 = new OfferSaleItemStatDAOUpdateParam();
        OfferSaleItemStatDAOUpdateParam p2 = new OfferSaleItemStatDAOUpdateParam();
        OfferSaleItemStatDAOUpdateParam p3 = new OfferSaleItemStatDAOUpdateParam();
        OfferSaleItemStatDAOUpdateParam p4 = new OfferSaleItemStatDAOUpdateParam();
        
        p1.setOfferId(2L);
        p1.setSpecId("bc");
        p2.setOfferId(1L);
        p2.setSpecId(null);
        p3.setOfferId(3L);
        p3.setSpecId("gdf");
        p4.setOfferId(3L);
        p4.setSpecId("ac");
        List<OfferSaleItemStatDAOUpdateParam>  ll = new ArrayList<OfferSaleItemStatDAOUpdateParam>();
        ll.add(p1);
        ll.add(p2);
        ll.add(p3);
        ll.add(p4);
        Collections.sort(ll, comparator);
        System.out.println(ll.get(0));
        System.out.println(ll.get(1));
        System.out.println(ll.get(2));
        System.out.println(ll.get(3));
    }
}

   

    输出结果，可以看出，先按照offerId（数字）进行排序，然后在按照specId（字母）排序：

OfferSaleItemStatDAOUpdateParam@a6d8cb6b[offerId=1,specId=<null>,orderAmount=<null>]
OfferSaleItemStatDAOUpdateParam@478e4327[offerId=2,specId=bc,orderAmount=<null>]
OfferSaleItemStatDAOUpdateParam@13a78071[offerId=3,specId=gdf,orderAmount=<null>]
OfferSaleItemStatDAOUpdateParam@59e184cb[offerId=3,specId=ac,orderAmount=<null>]