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网上基本上都是举荐配置如下:
@Bean
public MybatisPlusInterceptor mybatisPlusInterceptor() {MybatisPlusInterceptor mybatisPlusInterceptor = new MybatisPlusInterceptor();
mybatisPlusInterceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));
return mybatisPlusInterceptor;
}
然而,仅仅这么做,就能达到咱们的预期吗?其终局就是分页插件没有成果,起因是为什么呢?😔
图 1
图 2
通过比照下面两张图能够发现,图一 DefaultSqlSession.selectList() 底层调用 Plugin.invoke(); 图二 DefaultSqlSession.selectList() 底层调用 CachingExecutor.query()。其中,图一是分页插件失效的调用链,图二是分页插件生效的调用链。
也就是说,分页插件生效的起因是,mybatis-plusPlugin 类没有为分页插件拦截器生成 Executor 代理。具体应该怎么做呢?像上面这样,在构建 SqlSessionFactory 时,须要在 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();
// 设置主动提交
sqlSessionFactory.openSession(true);
return sqlSessionFactory;
}
那么,为分页插件生成代理类是在什么机会生成呢?先颁布答案:
// 设置主动提交
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);
// 这步很要害,创立执行者实例
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);
}
// 对下面的 executor 进行代理(目标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;
}
通过下面的重点 code 展现,咱们大抵理解了局部重要节点中分页插件代理类生成的逻辑。接下来咱们持续理解具体分页插件工作的成果。
图 4
public boolean willDoQuery(){if (countMs != null) {countSql = countMs.getBoundSql(parameter);
} else {countMs = buildAutoCountMappedStatement(ms);
// 生成查问 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());
}
// 查问 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() 外面的 executor.query()执行分页 sql。