java利用自带的ReentrantLock锁结合mysql的for update 防并发

java防并发处理思路：

这个并发问题困扰小杰好久了，主要是锁能锁住，但是spring的session没有及时更新，导致下面的进程进来时，还是查不到上次更新后的数据。

解决方案：

1、加上ReentrantLock锁

2、检查订单的数据，double check下是否上次已经执行过了。在sql的最后加上 for update

下面是加锁代码：
/**
* 返回根据产品和设施所加的锁
* 
* @param facilityId
*            设施ID
* @param productId
*            产品ID
* 
* @return ReentrantLock
*/
private ReentrantLock getInventoryTransactionLock(String facilityId,
String productId) {
String key = facilityId + "_" + productId;
try {
java.security.MessageDigest md = java.security.MessageDigest
.getInstance("MD5");
md.update(key.getBytes());
byte[] result = md.digest();
key = new String(result, 0, 16);
} catch (java.security.NoSuchAlgorithmException e) {
key = facilityId + "_" + productId;
}

ReentrantLock lock = locks.get(key);
if (lock == null) {
synchronized (_lock) {
lock = locks.get(key);
if (lock == null) {
lock = new ReentrantLock();
locks.put(key, lock);
}
}
}
return lock;
}

下面是加锁代码：

/**

* 返回根据产品和设施所加的锁

* @param facilityId

* 设施ID

* @param productId

* 产品ID

* @return ReentrantLock

private ReentrantLock getInventoryTransactionLock(String facilityId,

String productId) {

String key = facilityId + "_" + productId;

try {

java.security.MessageDigest md = java.security.MessageDigest

.getInstance("MD5");

md.update(key.getBytes());

byte[] result = md.digest();

key = new String(result, 0, 16);

} catch (java.security.NoSuchAlgorithmException e) {

key = facilityId + "_" + productId;

}

ReentrantLock lock = locks.get(key);

if (lock == null) {

synchronized (_lock) {

lock = locks.get(key);

if (lock == null) {

lock = new ReentrantLock();

locks.put(key, lock);

}

return lock;

}

// 加锁
Lock lock = this.getInventoryTransactionLock(fromFacilityId, productId);
lock.lock();
// double check 代码
public boolean checkOrderProductOutNumber(Session session, String orderGoodsId,
String productId, BigDecimal amount) {
String sql = "select og.goods_number,ifnull(sum(-iid.quantity_on_hand_diff),0) as out_num" +
" from ecshop.ecs_order_goods og " +
" left join romeo.product_mapping pm ON og.goods_id = pm.ecs_goods_id and og.style_id = pm.ecs_style_id" +
" left join romeo.inventory_item_detail iid ON convert(og.rec_id using utf8) = iid.order_goods_id" +
" left join romeo.inventory_item ii ON ii.inventory_item_id = iid.inventory_item_id" +
" where og.rec_id = '"+orderGoodsId+"' and pm.product_id = '"+productId+"'" +
   " group by og.rec_id for update";

最后关闭锁
if (lock != null) {
lock.unlock();
}

// 加锁

Lock lock = this.getInventoryTransactionLock(fromFacilityId, productId);

lock.lock();

// double check 代码

public boolean checkOrderProductOutNumber(Session session, String orderGoodsId,

String productId, BigDecimal amount) {

String sql = "select og.goods_number,ifnull(sum(-iid.quantity_on_hand_diff),0) as out_num" +

" from ecshop.ecs_order_goods og " +

" left join romeo.product_mapping pm ON og.goods_id = pm.ecs_goods_id and og.style_id = pm.ecs_style_id" +

" left join romeo.inventory_item_detail iid ON convert(og.rec_id using utf8) = iid.order_goods_id" +

" left join romeo.inventory_item ii ON ii.inventory_item_id = iid.inventory_item_id" +

" where og.rec_id = '"+orderGoodsId+"' and pm.product_id = '"+productId+"'" +

" group by og.rec_id for update";

最后关闭锁

if (lock != null) {

lock.unlock();

}

大功搞定，困恼了小杰2年的问题，终于搞定了，之前一直想着怎样让session更新，却忘了mysql自带的for update 功能。

希望能帮助到需要的人。

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