Mybatis系列(一)---SQL 执行流程分析

本文主要通过源码详细分析了 Mybatis 框架中 SQL 语句的执行流程,包括加载解析核心配置文件,创建SqlSessionFactory对象,创建SqlSession对象,执行 SQL 操作。

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1. Mybatis工作流程

1.1 概述

1.读取mybatis全局配置文件:将定义好的mybatis全局配置文件进行读取,并包装成为一个InputStream对象
2.解析配置文件:由SqlSessionFactoryBuilder类的bulid方法驱动,对包装好的XML文件进行解析。很容易看到,其具体的解析任务是交给XMLConfigBuilder对象完成.
3.创建SqlSessionFactory对象
4.创建SqlSession的对象
5.执行SQL操作

Mybatis底层自定义了Executor执行器接口操作数据库,Executor接口有两个实现,一个是基本执行器BaseExecutor、一个是缓存执行器CachingExecutor
Mybatis底层封装了 Mapped Statement对象,它包装了mybatis配置信息及sql映射信息等。mapper.xml文件中一个sql对应一个Mapped Statement对象,sql的id即是Mapped statement的id。

Mapped Statement对sql执行输入参数进行定义,包括HashMap、基本类型、pojo,Executor通过 Mapped Statement在执行sql前将输入的java对象映射至sql中,
输入参数映射就是jdbc编程中对preparedStatement设置参数。
Mapped Statement对sql执行输出结果进行定义,包括HashMap、基本类型、pojo,Executor通过 Mapped Statement在执行sql后将输出结果映射至java对象中,
输出结果映射过程相当于jdbc编程中对结果的解析处理过程。

1.2 实例代码

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@Test
public void testMybaits() throws IOException {
// 1. mybatis核心配置文件 以流的形式加载进来
String resources = "mybatis-config.xml";
InputStream resourceAsStream = Resources.getResourceAsStream(resources);
// 2. 解析配置文件 根据配置文件创建SqlSessionFactory
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
// 3. 用SqlSessionFactory创建SqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();
// 直接执行SQL操作或者获取mapper对象都在操作
User user = sqlSession.selectOne("com.illusory.i.shiro.mapper.UserMapper.findUserByName", "张三");
System.out.println(user);
// 4. SqlSession获取mapper
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
// 5. 执行CRUD操作
User userByName = mapper.findUserByName("username");
}

2.原理分析

2.1 读取mybatis全局配置文件

将定义好的mybatis全局配置文件进行读取,并包装称为一个InputStream对象。

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// 1. mybatis核心配置文件 以流的形式加载进来
String resources = "mybatis-config.xml";
InputStream resourceAsStream = Resources.getResourceAsStream(resources);

Resources.class是 Mybatis 提供的一个加载资源文件的工具类。

  • getResourceAsStream(String resource)
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//Resources类
/*
* Returns a resource on the classpath as a Stream object
*
* @param resource The resource to find
* @return The resource
* @throws java.io.IOException If the resource cannot be found or read
*/
public static InputStream getResourceAsStream(String resource) throws IOException {
return getResourceAsStream(null, resource);
}
  • getResourceAsStream()
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/*
* Returns a resource on the classpath as a Stream object
*
* @param loader The classloader used to fetch the resource
* @param resource The resource to find
* @return The resource
* @throws java.io.IOException If the resource cannot be found or read
*/
public static InputStream getResourceAsStream(ClassLoader loader, String resource) throws IOException {
InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader);
if (in == null) {
throw new IOException("Could not find resource " + resource);
}
return in;
}

获取到自身的 ClassLoader 对象,然后交给 ClassLoade r(lang包下的)来加载:

  • getResourceAsStream()
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//ClassLoaderWrapper 
/*
* Get a resource from the classpath, starting with a specific class loader
*
* @param resource - the resource to find
* @param classLoader - the first class loader to try
* @return the stream or null
*/
public InputStream getResourceAsStream(String resource, ClassLoader classLoader) {
return getResourceAsStream(resource, getClassLoaders(classLoader));
}

/*
* Try to get a resource from a group of classloaders
*
* @param resource - the resource to get
* @param classLoader - the classloaders to examine
* @return the resource or null
*/
InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) {
for (ClassLoader cl : classLoader) {
if (null != cl) {

// try to find the resource as passed
InputStream returnValue = cl.getResourceAsStream(resource);

// now, some class loaders want this leading "/", so we'll add it and try again if we didn't find the resource
if (null == returnValue) {
returnValue = cl.getResourceAsStream("/" + resource);
}

if (null != returnValue) {
return returnValue;
}
}
}
return null;
}

值的注意的是,它返回了一个InputStream对象。

2.2 解析配置文件

由SqlSessionFactoryBuilder类的bulid方法驱动,对包装好的XML文件进行解析。很容易看到,其具体的解析任务是交给XMLConfigBuilder对象完成.

  • SqlSessionFactory.build()
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public SqlSessionFactory build(InputStream inputStream) {
return build(inputStream, null, null);
}
  • SqlSessionFactoryBuilder.build()
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public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
inputStream.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}

首先通过 Document 对象来解析,然后返回 InputStream 对象,然后交给 XMLConfigBuilder 构造成org.apache.ibatis.session.Configuration 对象,

2.3 创建方法构造成SqlSessionFactory对象

将前面解析配置文件构造出来的Configuration对象交给SqlSessionFactoryBuilder.build()方法构造成SqlSessionFactory。

  • build方法如下:
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public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}

最终返回的是DefaultSqlSessionFactory对象

2.4 创建SqlSession

SqlSession 完全包含了面向数据库执行 SQL 命令所需的所有方法。你可以通过 SqlSession 实例来直接执行已映射的 SQL 语句。

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// 3. 用SqlSessionFactory创建SqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();
  • DefaultSqlSessionFactory.openSession()
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@Override
public SqlSession openSession() {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}

最终也是返回的一个DefaultSqlSession对象。

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private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
//根据Configuration的Environment属性来创建事务工厂
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
//通过事务工厂创建事务,默认level=null autoCommit=false
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
//创建执行器 真正执行sql语句的对象
final Executor executor = configuration.newExecutor(tx, execType);
//根据执行器返回对象 SqlSess
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();
}
}

构建步骤:

Environment–>TransactionFactory+autoCommit+tx-level–>Transaction+ExecType–>Executor+Configuration+autoCommit–>SqlSession

其中,Environment是Configuration中的属性。

2.5 执行SQL操作

SQL语句的执行才是MyBatis的重要职责,该过程就是通过封装JDBC进行操作,然后使用Java反射技术完成JavaBean对象到数据库参数之间的相互转换,
这种映射关系就是有TypeHandler对象来完成的,在获取数据表对应的元数据时,会保存该表所有列的数据库类型,大致逻辑如下所示:

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User user = sqlSession.selectOne("com.illusory.i.shiro.mapper.UserMapper.findUserByName", "张三");
System.out.println(user);

调用selectOne方法进行SQL查询,selectOne方法最后调用的是selectList,在selectList中,会查询
configuration中存储的MappedStatement对象,mapper文件中一个sql语句的配置对应一个MappedStatement对象,然后调用执行器进行查询操作。

  • DefaultSqlSession.selectOne();
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@Override
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
List<T> list = this.<T>selectList(statement, parameter);
if (list.size() == 1) {
return list.get(0);
} else if (list.size() > 1) {
throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
} else {
return null;
}
}
  • DefaultSqlSession.selectList();
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@Override
public <E> List<E> selectList(String statement, Object parameter) {
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}

@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}

执行器在query操作中,优先会查询缓存是否命中,命中则直接返回,否则从数据库中查询。

  • CachingExecutor.query()
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@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject);
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
Cache cache = ms.getCache();
if (cache != null) {
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
//BaseExecutor.query()
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
  • BaseExecutor.query()
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@SuppressWarnings("unchecked")
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
if (closed) {
throw new ExecutorException("Executor was closed.");
}
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
queryStack++;
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
queryStack--;
}
if (queryStack == 0) {
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
return list;
}
  • BaseExecutor.queryFromDatabase()
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private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
/**
* 先往localCache中插入一个占位对象,这个地方
*/
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}
/* 往缓存中写入数据,也就是缓存查询结果 */
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}

protected Connection getConnection(Log statementLog) throws SQLException {
Connection connection = transaction.getConnection();
if (statementLog.isDebugEnabled()) {
return ConnectionLogger.newInstance(connection, statementLog, queryStack);
} else {
return connection;
}
}

最后的doQuery由SimpleExecutor代理来完成,该方法中有2个子流程,一个是SQL参数的设置,另一个是SQL查询操作和结果集的封装。

  • SimpleExecutor.doQuery()方法如下:
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@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
/* 子流程1: SQL查询参数的设置 */
stmt = prepareStatement(handler, ms.getStatementLog());
/* 子流程2: SQL查询操作和结果集封装 */
return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}

子流程1 SQL查询参数的设置

首先获取数据库connection连接,然后准备statement,然后就设置SQL查询中的参数值。打开一个connection连接,在使用完后不会close,
而是存储下来,当下次需要打开连接时就直接返回。

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// SimpleExecutor类
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
/* 获取Connection连接 */
Connection connection = getConnection(statementLog);

/* 准备Statement */
stmt = handler.prepare(connection, transaction.getTimeout());

/* 设置SQL查询中的参数值 */
handler.parameterize(stmt);
return stmt;
}

子流程2 SQL查询结果集的封装

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// SimpleExecutor类
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
// 执行查询操作
ps.execute();
// 执行结果集封装
return resultSetHandler.<E> handleResultSets(ps);
}

// DefaultReseltSetHandler类
public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(mappedStatement.getId());

final List<Object> multipleResults = new ArrayList<Object>();

int resultSetCount = 0;
/**
* 获取第一个ResultSet,同时获取数据库的MetaData数据,包括数据表列名、列的类型、类序号等。
* 这些信息都存储在了ResultSetWrapper中了
*/
ResultSetWrapper rsw = getFirstResultSet(stmt);

List<ResultMap> resultMaps = mappedStatement.getResultMaps();
int resultMapCount = resultMaps.size();
validateResultMapsCount(rsw, resultMapCount);
while (rsw != null && resultMapCount > resultSetCount) {
ResultMap resultMap = resultMaps.get(resultSetCount);
handleResultSet(rsw, resultMap, multipleResults, null);
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}

String[] resultSets = mappedStatement.getResultSets();
if (resultSets != null) {
while (rsw != null && resultSetCount < resultSets.length) {
ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
if (parentMapping != null) {
String nestedResultMapId = parentMapping.getNestedResultMapId();
ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
handleResultSet(rsw, resultMap, null, parentMapping);
}
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
}

return collapseSingleResultList(multipleResults);
}

ResultSetWrapper 是 ResultSet 的包装类,调用 getFirstResultSet 方法获取第一个 ResultSet,同时获取数据库的 MetaData 数据,
包括数据表列名、列的类型、类序号等,这些信息都存储在 ResultSetWrapper 类中了。然后调用handleResultSet 方法来来进行结果集的封装。

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// DefaultResultSetHandler类
private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
try {
if (parentMapping != null) {
handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
} else {
if (resultHandler == null) {
DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
multipleResults.add(defaultResultHandler.getResultList());
} else {
handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
}
}
} finally {
// issue #228 (close resultsets)
closeResultSet(rsw.getResultSet());
}
}

这里调用handleRowValues方法进行结果值的设置

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// DefaultResultSetHandler类
public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
if (resultMap.hasNestedResultMaps()) {
ensureNoRowBounds();
checkResultHandler();
handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
} else {
// 封装数据
handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
}
}

private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
throws SQLException {
DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
skipRows(rsw.getResultSet(), rowBounds);
while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
Object rowValue = getRowValue(rsw, discriminatedResultMap);
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
}

private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
final ResultLoaderMap lazyLoader = new ResultLoaderMap();
// createResultObject为新创建的对象,数据表对应的类
Object rowValue = createResultObject(rsw, resultMap, lazyLoader, null);
if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
final MetaObject metaObject = configuration.newMetaObject(rowValue);
boolean foundValues = this.useConstructorMappings;
if (shouldApplyAutomaticMappings(resultMap, false)) {
// 这里把数据填充进去,metaObject中包含了resultObject信息
foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues;
}
foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
foundValues = lazyLoader.size() > 0 || foundValues;
rowValue = (foundValues || configuration.isReturnInstanceForEmptyRow()) ? rowValue : null;
}
return rowValue;
}

private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
boolean foundValues = false;
if (autoMapping.size() > 0) {
// 这里进行for循环调用,因为user表中总共有7列,所以也就调用7次
for (UnMappedColumnAutoMapping mapping : autoMapping) {
// 这里将esultSet中查询结果转换为对应的实际类型
final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
if (value != null) {
foundValues = true;
}
if (value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive)) {
// gcode issue #377, call setter on nulls (value is not 'found')
metaObject.setValue(mapping.property, value);
}
}
}
return foundValues;
}

mapping.typeHandler.getResult会获取查询结果值的实际类型,比如我们user表中id字段为int类型,那么它就对应Java中的Integer类型,
然后通过调用statement.getInt(“id”)来获取其int值,其类型为Integer。metaObject.setValue方法会把获取到的Integer值设置到Java类中的对应字段。

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// MetaObject类
public void setValue(String name, Object value) {
PropertyTokenizer prop = new PropertyTokenizer(name);
if (prop.hasNext()) {
MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
if (metaValue == SystemMetaObject.NULL_META_OBJECT) {
if (value == null && prop.getChildren() != null) {
// don't instantiate child path if value is null
return;
} else {
metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory);
}
}
metaValue.setValue(prop.getChildren(), value);
} else {
objectWrapper.set(prop, value);
}
}

metaValue.setValue方法最后会调用到Java类中对应数据域的set方法,这样也就完成了SQL查询结果集的Java类封装过程

3. MyBatis缓存

MyBatis提供查询缓存,用于减轻数据库压力,提高性能。MyBatis提供了一级缓存和二级缓存。

3.1 一级缓存

一级缓存是 SqlSession 级别的缓存,每个 SqlSession 对象都有一个哈希表用于缓存数据,不同 SqlSession 对象之间缓存不共享。
同一个 SqlSession 对象对象执行2遍相同的 SQL 查询,在第一次查询执行完毕后将结果缓存起来,这样第二遍查询就不用向数据库查询了,
直接返回缓存结果即可。MyBatis默认开启一级缓存的。
简单说就是SQL语句作为key,查询结果作为value,根据key去查找value,如果查询语句相同就能直接返回value。

3.2 二级缓存

二级缓存是mapper 级别的缓存,二级缓存是跨 SqlSession 的,多个 SqlSession 对象可以共享同一个二级缓存。不同的 SqlSession 对象执行两次相同的 SQL 语句,
第一次会将查询结果进行缓存,第二次查询直接返回二级缓存中的结果即可。MyBatis 默认不开启二级缓存的,可以在配置文件中使用如下配置来开启二级缓存:

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<settings>
<setting name="cacheEnabled" value="true"/>
</settings>

​ 当SQL语句进行更新操作(删除/添加/更新)时,会清空对应的缓存,保证缓存中存储的都是最新的数据。

4. 参考

https://www.cnblogs.com/dongying/p/4142476.html

http://www.mybatis.org/mybatis-3/zh/getting-started.html

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