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<>前言
在JPA中，使用save方法时是这样的：如果我们save的对象指定了主键，那么会根据主键先进行一次查询，如果查询记录不存在则执行insert语句，如果查询记录存在则执行update语句。
<>问题现象
业务场景是这样的：当某个用户钱包流水发生变化时，我们会先查询用户钱包是否存在（新注册的用户一开始没有钱包），如果存在则直接更新钱包余额，并添加一条流水，如果用户钱包不存在，则新生成钱包，再更新钱包余额，最后添加流水。
当同一个用户同时产生多条流水时，现在的流程则可能出现问题。
<>伪代码
// 查询用户钱包是否存在 User u = select(userId); if(u == null){ // 不存在则生成钱包，初始化钱包余额为0 jpa
.save(u); } // 添加流水 addFlow(u); // 更新钱包余额 update(u);
<>问题分析
问题就发生在当第一个线程save成功后，第二个线程再执行时，save就会变成update，并且会覆盖第一个线程所执行的操作。
<>解决方法
只要让自定义save方法，就是insert操作就可以了，当第2个线程save时，会报主键冲突，然后第2个线程再重试一次，再次查询钱包是否存在时，就可以查询到了，然后就不用再生成钱包了，直接更新余额即可（更新余额幂等性）。
