mybatis-plus分页插件失效探究解决
作者：子瞻
网上推荐
网上基本上都是推荐配置如下：
@Bean
public MybatisPlusInterceptor mybatisPlusInterceptor() {
MybatisPlusInterceptor mybatisPlusInterceptor = new MybatisPlusInterceptor();
mybatisPlusInterceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));
return mybatisPlusInterceptor;
}但是，仅仅这么做，就能达到我们的预期吗？
分页插件无效原因
其结局就是分页插件没有效果，原因是为什么呢？😔
图1
图2
通过对比上面两张图可以发现，图一DefaultSqlSession.selectList()底层调用Plugin.invoke();图二DefaultSqlSession.selectList()底层调用CachingExecutor.query()。
其中，图一是分页插件生效的调用链，图二是分页插件失效的调用链。
也就是说，分页插件失效的原因是，mybatis-plusPlugin类没有为分页插件拦截器生成Executor代理。
解决方案
具体应该怎么做呢？像下面这样，在构建SqlSessionFactory时，需要在MybatisSqlSessionFactoryBean显示设置Plugin。
@Bean(name = "defaultSqlSessionFactory")
public SqlSessionFactory defaultSqlSessionFactory(){
MybatisSqlSessionFactoryBean bean = new MybatisSqlSessionFactoryBean();
bean.setDataSource(dataSource);
//设置拦截器
bean.setPlugins(mybatisPlusInterceptor);
SqlSessionFactory sqlSessionFactory = bean.getObject();
//设置自动提交
sqlSessionFactory.openSession(true);
return sqlSessionFactory;
}那么，为分页插件生成代理类是在什么时机生成呢？先公布答案：
//设置自动提交 sqlSessionFactory.openSession(true);
调用链如下
图3
咱再看细节：DefaultSqlSessionFactory.openSessionFromDataSource()详情：
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
//这步很关键，创建执行者实例
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}Configuration.newExecutor()详情：
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
executor = new SimpleExecutor(this, transaction);
}
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
//对上面的executor进行代理（目的是把插件和执行器封装为代理类）
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}MybatisPlusInterceptor.pluginAll();
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}通过上面的重点code展示，我们大致了解了部分重要节点中分页插件代理类生成的逻辑。接下来我们继续了解具体分页插件工作的效果。
图4
public boolean willDoQuery(){
if (countMs != null) {
countSql = countMs.getBoundSql(parameter);
} else {
countMs = buildAutoCountMappedStatement(ms);
//生成查询count SQL
String countSqlStr = autoCountSql(page, boundSql.getSql());
PluginUtils.MPBoundSql mpBoundSql = PluginUtils.mpBoundSql(boundSql);
//构建BoundSql
countSql = new BoundSql(countMs.getConfiguration(), countSqlStr, mpBoundSql.parameterMappings(), parameter);
PluginUtils.setAdditionalParameter(countSql, mpBoundSql.additionalParameters());
}
//查询 count 数值
List<Object> result = executor.query(countMs, parameter, rowBounds, resultHandler, cacheKey, countSql);
}接下来，PaginationInnerInterceptor.beforeQuery(）生成分页sql；
最终MybatisPlusInterceptor.intercept()里面的executor.query(）执行分页sql。
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