发布于2021-06-12 15:59 阅读(574) 评论(0) 点赞(17) 收藏(4)
目录
本章着重介绍MyBatis执行Sql的流程,关于在执行过程中缓存、动态SQl生成等细节不在本章中体现
还是以之前的查询作为列子:
- public class App {
- public static void main(String[] args) {
- String resource = "mybatis-config.xml";
- Reader reader;
- try {
- //将XML配置文件构建为Configuration配置类
- reader = Resources.getResourceAsReader(resource);
- // 通过加载配置文件流构建一个SqlSessionFactory DefaultSqlSessionFactory
- SqlSessionFactory sqlMapper = new SqlSessionFactoryBuilder().build(reader);
- // 数据源 执行器 DefaultSqlSession
- SqlSession session = sqlMapper.openSession();
- try {
- // 执行查询 底层执行jdbc
- //User user = (User)session.selectOne("com.tuling.mapper.selectById", 1);
-
- UserMapper mapper = session.getMapper(UserMapper.class);
- System.out.println(mapper.getClass());
- User user = mapper.selectById(1L);
- System.out.println(user.getUserName());
- } catch (Exception e) {
- e.printStackTrace();
- }finally {
- session.close();
- }
- } catch (IOException e) {
- e.printStackTrace();
- }
- }
- }
之前提到拿到sqlSession之后就能进行各种CRUD操作了,所以我们就从sqlSession.getMapper这个方法开始分析,看下整个Sql的执行流程是怎么样的。
- 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);
- //获取执行器,这边获得的执行器已经代理拦截器的功能(见下面代码)
- final Executor executor = configuration.newExecutor(tx, execType);
- //根据获取的执行器创建SqlSession
- 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();
- }
- }
-
- //interceptorChain生成代理类,具体参见Plugin这个类的方法
- 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);
- }
- executor = (Executor) interceptorChain.pluginAll(executor);
- return executor;
- }
Executor分成两大类,一类是CacheExecutor,另一类是普通Executor。
普通Executor又分为三种基本的Executor执行器,SimpleExecutor、ReuseExecutor、BatchExecutor。
作用范围:Executor的这些特点,都严格限制在SqlSession生命周期范围内。
CacheExecutor其实是封装了普通的Executor,和普通的区别是在查询前先会查询缓存中是否存在结果,如果存在就使用缓存中的结果,如果不存在还是使用普通的Executor进行查询,再将查询出来的结果存入缓存。
到此为止,我们已经获得了SqlSession,拿到SqlSession就可以执行各种CRUD方法了。
简单总结
SQL的具体执行流程见后续博客。
一些重要类总结:
进入sqlSession.getMapper方法,会发现调的是Configration对象的getMapper方法:
- public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
- //mapperRegistry实质上是一个Map,里面注册了启动过程中解析的各种Mapper.xml
- //mapperRegistry的key是接口的Class类型
- //mapperRegistry的Value是MapperProxyFactory,用于生成对应的MapperProxy(动态代理类)
- return mapperRegistry.getMapper(type, sqlSession);
- }
进入getMapper方法:
- public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
- final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
- //如果配置文件中没有配置相关Mapper,直接抛异常
- if (mapperProxyFactory == null) {
- throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
- }
- try {
- //关键方法
- return mapperProxyFactory.newInstance(sqlSession);
- } catch (Exception e) {
- throw new BindingException("Error getting mapper instance. Cause: " + e, e);
- }
- }
进入MapperProxyFactory的newInstance方法:
- public class MapperProxyFactory<T> {
-
- private final Class<T> mapperInterface;
- private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
-
- public MapperProxyFactory(Class<T> mapperInterface) {
- this.mapperInterface = mapperInterface;
- }
-
- public Class<T> getMapperInterface() {
- return mapperInterface;
- }
-
- public Map<Method, MapperMethod> getMethodCache() {
- return methodCache;
- }
-
- //生成Mapper接口的动态代理类MapperProxy,MapperProxy实现了InvocationHandler 接口
- @SuppressWarnings("unchecked")
- protected T newInstance(MapperProxy<T> mapperProxy) {
- return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
- }
-
- public T newInstance(SqlSession sqlSession) {
- final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
- return newInstance(mapperProxy);
- }
-
- }
获取Mapper的流程总结如下:
下面是动态代理类MapperProxy,调用Mapper接口的所有方法都会先调用到这个代理类的invoke方法(注意由于Mybatis中的Mapper接口没有实现类,所以MapperProxy这个代理对象中没有委托类,也就是说MapperProxy干了代理类和委托类的事情)。好了下面重点看下invoke方法。
- //MapperProxy代理类
- public class MapperProxy<T> implements InvocationHandler, Serializable {
-
- private static final long serialVersionUID = -6424540398559729838L;
- private final SqlSession sqlSession;
- private final Class<T> mapperInterface;
- private final Map<Method, MapperMethod> methodCache;
-
- public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
- this.sqlSession = sqlSession;
- this.mapperInterface = mapperInterface;
- this.methodCache = methodCache;
- }
-
- @Override
- public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
- try {
- if (Object.class.equals(method.getDeclaringClass())) {
- return method.invoke(this, args);
- } else if (isDefaultMethod(method)) {
- return invokeDefaultMethod(proxy, method, args);
- }
- } catch (Throwable t) {
- throw ExceptionUtil.unwrapThrowable(t);
- }
- //获取MapperMethod,并调用MapperMethod
- final MapperMethod mapperMethod = cachedMapperMethod(method);
- return mapperMethod.execute(sqlSession, args);
- }
MapperProxy的invoke方法非常简单,主要干的工作就是创建MapperMethod对象或者是从缓存中获取MapperMethod对象。获取到这个对象后执行execute方法。
所以这边需要进入MapperMethod的execute方法:这个方法判断你当前执行的方式是增删改查哪一种,并通过SqlSession执行相应的操作。(这边以sqlSession.selectOne这种方式进行分析~)
- public Object execute(SqlSession sqlSession, Object[] args) {
- Object result;
- //判断是CRUD那种方法
- switch (command.getType()) {
- case INSERT: {
- Object param = method.convertArgsToSqlCommandParam(args);
- result = rowCountResult(sqlSession.insert(command.getName(), param));
- break;
- }
- case UPDATE: {
- Object param = method.convertArgsToSqlCommandParam(args);
- result = rowCountResult(sqlSession.update(command.getName(), param));
- break;
- }
- case DELETE: {
- Object param = method.convertArgsToSqlCommandParam(args);
- result = rowCountResult(sqlSession.delete(command.getName(), param));
- break;
- }
- case SELECT:
- if (method.returnsVoid() && method.hasResultHandler()) {
- executeWithResultHandler(sqlSession, args);
- result = null;
- } else if (method.returnsMany()) {
- result = executeForMany(sqlSession, args);
- } else if (method.returnsMap()) {
- result = executeForMap(sqlSession, args);
- } else if (method.returnsCursor()) {
- result = executeForCursor(sqlSession, args);
- } else {
- Object param = method.convertArgsToSqlCommandParam(args);
- result = sqlSession.selectOne(command.getName(), param);
- }
- break;
- case FLUSH:
- result = sqlSession.flushStatements();
- break;
- default:
- throw new BindingException("Unknown execution method for: " + command.getName());
- }
- if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
- throw new BindingException("Mapper method '" + command.getName()
- + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
- }
- return result;
- }
详细流程图
https://www.processon.com/view/link/5efc23966376891e81f2a37e
sqlSession.selectOne方法会会调到DefaultSqlSession的selectList方法。这个方法获取了获取了MappedStatement对象,并最终调用了Executor的query方法。
- 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();
- }
- }
然后,通过一层一层的调用(这边省略了缓存操作的环节,会在后面的文章中介绍),最终会来到doQuery方法, 这儿咱们就随便找个Excutor看看doQuery方法的实现吧,我这儿选择了SimpleExecutor:
- 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();
- //内部封装了ParameterHandler和ResultSetHandler
- StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
- stmt = prepareStatement(handler, ms.getStatementLog());
- //StatementHandler封装了Statement, 让 StatementHandler 去处理
- return handler.<E>query(stmt, resultHandler);
- } finally {
- closeStatement(stmt);
- }
- }
接下来,咱们看看StatementHandler 的一个实现类 PreparedStatementHandler(这也是我们最常用的,封装的是PreparedStatement), 看看它使怎么去处理的:
- public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
- //到此,原形毕露, PreparedStatement, 这个大家都已经滚瓜烂熟了吧
- PreparedStatement ps = (PreparedStatement) statement;
- ps.execute();
- //结果交给了ResultSetHandler 去处理,处理完之后返回给客户端
- return resultSetHandler.<E> handleResultSets(ps);
- }
到此,整个调用流程结束。
这边结合获取SqlSession的流程,做下简单的总结:
以上是获得SqlSession的流程,下面总结下本博客中介绍的Sql的执行流程:
Executor组件有两个直接实现类,分别是BaseExecutor和CachingExecutor。CachingExecutor静态代理了BaseExecutor。Executor组件封装了Transction组件,Transction组件中又分装了Datasource组件。
Executor、StatementHandler 、ParameterHandler、ResultSetHandler,Mybatis的插件会对上面的四个组件进行动态代理。
StatementHandler:封装了JDBC Statement操作,负责对JDBC statement 的操作,如设置参数、将Statement结果集转换成List集合。
ParameterHandler:负责对用户传递的参数转换成JDBC Statement 所需要的参数,
ResultSetHandler:负责将JDBC返回的ResultSet结果集对象转换成List类型的集合;
TypeHandler:负责java数据类型和jdbc数据类型之间的映射和转换
MappedStatement:MappedStatement维护了一条节点的封装,
SqlSource:负责根据用户传递的parameterObject,动态地生成SQL语句,将信息封装到BoundSql对象中,并返回
BoundSql:表示动态生成的SQL语句以及相应的参数信息
Configuration:MyBatis所有的配置信息都维持在Configuration对象之中。
原文链接:https://blog.csdn.net/Xx__WangQi/article/details/117674835
作者:niceboty
链接:http://www.javaheidong.com/blog/article/222588/25bb21b51db54f547305/
来源:java黑洞网
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