公司目前数据源为主从模式：主库可读写，从库只负责读。使用spring-jdbc提供的AbstractRoutingDataSource结合ThreadLocal存储key，实现数据源动态切换。
最近项目加入数据源切换后，偶尔会报出read-only异常，百思不得其解......
<!--数据源-->
<bean id="dsCrm" class="cn.mwee.framework.commons.utils.datasource.RoutingDataSource">
<property name="targetDataSources">
<map key-type="java.lang.String">
<entry key="master" value-ref="dsCrm_master"/>
<entry key="slave1" value-ref="dsCrm_slave1"/>
<entry key="slave2" value-ref="dsCrm_slave2"/>
</map>
</property>
<!--默认走主库-->
<property name="defaultTargetDataSource" ref="dsCrm_master"/>
</bean>
RoutingDataSource类是对AbstractRoutingDataSource轻量封装实现determineCurrentLookupKey ：
public class RoutingDataSource extends AbstractRoutingDataSource {
@Override
protected Object determineCurrentLookupKey() {
return DBContext.getDBKey();
}
}
对应的业务代码如下，数据源切换在其他项目使用正常，代码迁移过来之后偶发报出read-only异常，数据库处于只读模式。写方法需要事物默认走主库，在该方法前也没有数据源的切换。
@Transactional(rollbackFor = Exception.class)
public DataResult settingMarketMsg(SettingMarketMsgRequest request) {
.....
}
@Slave
public DataResult detailMarketMsg(DetailMarketMsgRequest request) {
......
}
因为aop切面只会切入打上@Slave注解的方法并切为从库，方法返回会清除key。所以臆想着肯定不会有问题？思考N久。。。
最后在aop的配置中看到破绽：
1 @Component("dynamicDataSourceAspect")
2 public class DynamicDataSourceAspect {
3 public void setCrmDataSource(JoinPoint joinPoint) {
4 MethodSignature methodSignature = (MethodSignature) joinPoint.getSignature();
5 Method method = methodSignature.getMethod();
6 // 默认 master 优先
7 DBContext.setDBKey(DbKeyConstant.DS_MASTER);
8 if (method.isAnnotationPresent(Slave.class)) {
9 DBContext.setDBKey(DbKeyConstant.DS_SLAVE_1);
10 }
11 logger.info("Revert DataSource : {} > {}", DBContext.getDBKey(), joinPoint.getSignature());
12 }
13 public void clearCrmDataSource(JoinPoint joinPoint) {
14 logger.info("Clear DataSource : {} > {}", DBContext.getDBKey(), joinPoint.getSignature());
15 DBContext.clearDBKey();
16 }
17 }
aop的xml配置 :
1 <aop:aspectj-autoproxy proxy-target-class="true"/> 2 <aop:config> 3 <aop:aspect id="dynamicDataSourceAspect" ref="dynamicDataSourceAspect" order="3"> 4 <aop:pointcut id="dsAspect" 5 expression="@annotation(cn.mwee.framework.commons.utils.datasource.Slave)"/> 6 <aop:before pointcut-ref="dsAspect" method="setCrmDataSource"/> 7 <aop:after-returning pointcut-ref="dsAspect" method="clearCrmDataSource"/> 8 </aop:aspect> 9 </aop:config>
问题出在after-returning 即 方法正常返回之后执行，一旦执行异常就不会执行。此时线程如果没有被回收将一直持有该key。那线程持有该key，怎么跟上述【read-only异常】联系起来呢？
先看事物管理器配置：
<!--事务管理器-->
<bean id="crmTxManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
<property name="dataSource" ref="dsCrm"/>
</bean>
<tx:annotation-driven transaction-manager="crmTxManager"/>
DataSourceTransactionManager的源码事物开始doBegin部分:
1 /**
2 * This implementation sets the isolation level but ignores the timeout.
3 */
4 @Override
5 protected void doBegin(Object transaction, TransactionDefinition definition) {
6 DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
7 Connection con = null;
8
9 try {
10
11 // 第一次获取数据源，往后直接复用
12 if (txObject.getConnectionHolder() == null ||
13 txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
14 Connection newCon = this.dataSource.getConnection();
15 if (logger.isDebugEnabled()) {
16 logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
17 }
18 txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
19 }
20
21 txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
22 con = txObject.getConnectionHolder().getConnection();
23
24 Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
25 txObject.setPreviousIsolationLevel(previousIsolationLevel);
26
27 // Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
28 // so we don‘t want to do it unnecessarily (for example if we‘ve explicitly
29 // configured the connection pool to set it already).
30 if (con.getAutoCommit()) {
31 txObject.setMustRestoreAutoCommit(true);
32 if (logger.isDebugEnabled()) {
33 logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
34 }
35 con.setAutoCommit(false);
36 }
37
38 prepareTransactionalConnection(con, definition);
39 txObject.getConnectionHolder().setTransactionActive(true);
40
41 int timeout = determineTimeout(definition);
42 if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
43 txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
44 }
45
46 // Bind the connection holder to the thread.
47 if (txObject.isNewConnectionHolder()) {
48 TransactionSynchronizationManager.bindResource(getDataSource(), txObject.getConnectionHolder());
49 }
50 }
51
52 catch (Throwable ex) {
53 if (txObject.isNewConnectionHolder()) {
54 DataSourceUtils.releaseConnection(con, this.dataSource);
55 txObject.setConnectionHolder(null, false);
56 }
57 throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
58 }
59 }
AbstractRoutingDataSource获取数据源源码：
1 /**
2 * Retrieve the current target DataSource. Determines the
3 * {@link #determineCurrentLookupKey() current lookup key}, performs
4 * a lookup in the {@link #setTargetDataSources targetDataSources} map,
5 * falls back to the specified
6 * {@link #setDefaultTargetDataSource default target DataSource} if necessary.
7 * @see #determineCurrentLookupKey()
8 */
9 protected DataSource determineTargetDataSource() {
10 // 根据线程绑定的key获取数据源， 如果不存在则获取默认数据源
11 Assert.notNull(this.resolvedDataSources, "DataSource router not initialized");
12 Object lookupKey = determineCurrentLookupKey();
13 DataSource dataSource = this.resolvedDataSources.get(lookupKey);
14 if (dataSource == null && (this.lenientFallback || lookupKey == null)) {
15 dataSource = this.resolvedDefaultDataSource;
16 }
17 if (dataSource == null) {
18 throw new IllegalStateException("Cannot determine target DataSource for lookup key [" + lookupKey + "]");
19 }
20 return dataSource;
21 }
这样可以解释偶发报出read-only异常了。项目使用tomcat，其中tomcat工作线程池是复用。当tomcat工作线程响应请求，访问带有@Slave方法，数据源切换至从库。由于某种原因异常导致未进入after-returning线程绑定的key未清除。此时访问写方法并tomcat使用同一个工作线程响应请求，通过AbstractRoutingDataSource将获得只读库的数据源，因而会产生报出read-only异常。
问题偶发原因在于必须满足两点：
1、只读请求异常未切数据源
2、复用相同tomcat工作线程池。
找到问题症结之后，自然容易解决：将after-returning 修改为 after（不管是否异常，都执行)，每次必清空数据源即可。
<aop:config>
<aop:aspect id="dynamicDataSourceAspect" ref="dynamicDataSourceAspect" order="3">
<aop:pointcut id="dsAspect"
expression="@annotation(cn.mwee.framework.commons.utils.datasource.Slave)"/>
<aop:before pointcut-ref="dsAspect" method="setCrmDataSource"/>
<aop:after pointcut-ref="dsAspect" method="clearCrmDataSource"/>
</aop:aspect>
</aop:config>
参考链接 ：
@Transactional导致AbstractRoutingDataSource动态数据源无法切换的解决办法
原文地址：https://www.cnblogs.com/xiaoxing/p/10224617.html
时间： 2024-08-30 12:00:24