老子有钱下载官网
  咨询电话:15891232824

博马代理

MyBatis框架原理2:SqlSession运行过程

获取SqlSession对象

SqlSession session = sqlSessionFactory.openSession();

首先通过SqlSessionFactory的openSession方法获取SqlSession接口的实现类DefaultSqlSession对象。

public interface SqlSessionFactory { SqlSession openSession(); SqlSession openSession(boolean autoCommit); SqlSession openSession(Connection connection); SqlSession openSession(TransactionIsolationLevel level); SqlSession openSession(ExecutorType execType); SqlSession openSession(ExecutorType execType, boolean autoCommit); SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level); SqlSession openSession(ExecutorType execType, Connection connection); Configuration getConfiguration();}

SqlSessionFactory接口提供一系列重载的openSession方法,其参数如下:

boolean autoCommit:是否开启JDBC事务的自动提交,默认为false。Connection:提供连接。TransactionIsolationLevel:定义事务隔离级别。ExecutorType:定义执行器类型。

DefaultSqlSessionFactory对象调用覆写的openSession方法:

public SqlSession openSession() { return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false); }

得到一个定义了ExecutorType为configuration的默认执行器SIMPLE,事务隔离级别为null,JDBC事务自动提交为false的DefaultSqlSession对象。

获取MapperProxy代理对象

有了DefaultSqlSession对象,以查询一条数据为例,来看一下整个处理过程。

For example:

SqlSession session = sqlSessionFactory.openSession();try { BlogMapper mapper = session.getMapper(BlogMapper.class); Blog blog = mapper.selectBlog(101);} finally { session.close();}

MyBatis时序图:

根据MyBatis文档推荐的方法,调用Mapper接口中的方法实现对数据库的操作,上述例子中根据blog ID获取Blog对象。通过DefaultSqlSession对象的getMapper方法获取的是一个MapperProxy代理对象,这也是Mapper接口不用实现类的原因。当调用BlogMapper中的方法时,由于BlogMapper是一个JDK动态代理对象,它会运行invoke方法,代码如下:

@Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { //判断代理对象是否是一个类 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); } //生成MapperMethod对象 final MapperMethod mapperMethod = cachedMapperMethod(method); //执行execute方法 return mapperMethod.execute(sqlSession, args); } private MapperMethod cachedMapperMethod(Method method) { MapperMethod mapperMethod = methodCache.get(method); if (mapperMethod == null) { mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()); methodCache.put(method, mapperMethod); } return mapperMethod; }...

invoke方法判断代理的对象是否是一个类,由于代理对象是一个接口,所以通过cachedMapperMethod生成一个MappedMethod对象,然后执行execute方法,execute方法代码如下:

public Object execute(SqlSession sqlSession, Object[] args) { Object result; 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; }

因为这里是根据ID查询一个对象,所以最终调用了DefaultSqlSession的selectOne方法,selectOne方法又调用自身selectList方法,最终将查询操作委托给Executor:

@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; } }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 { //根据id获取MappedStatement对象 MappedStatement ms = configuration.getMappedStatement(statement); //wrapCollection方法处理集合参数 //委托Exector执行SQL 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(); } }

Executor执行过程

Executor(执行器),才是真正对JDBC操作的实例,它的结构如下:

CachingExecutor: SqlSession默认会调用CachingExecutor执行器的query方法,先从二级缓存获取数据,当无法从二级缓存获取数据时,则委托给BaseExcutor进行操作,CachingExecutor执行过程代码如下:

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { //判断是否有二级缓存 Cache cache = ms.getCache(); if (cache != null) { flushCacheIfRequired(ms); if (ms.isUseCache() && resultHandler == null) { ensureNoOutParams(ms, boundSql); @SuppressWarnings("unchecked") //从二级缓存获取数据 List<E> list = (List<E>) tcm.getObject(cache, key); //如果二级缓存没有数据则委托给BaseExcutor进行操作 if (list == null) { list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); tcm.putObject(cache, key, list); // issue #578 and #116 } return list; } } //如果没有二级缓存则委托给BaseExcutor进行操作 return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }

BaseExecutor是一个抽象类,查询操作时BaseExecutor首先从一级缓存获取数据,如果没有则由其子类来进行数据库操作,其query方法如下:

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++; //从一级缓存获取数据 list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; if (list != null) { handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else { //如果一级缓存没有数据,则从数据库获取 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; }

最后,我们例子中的查询操作交给了SimpleExecutor这个子类,可以看到SimpleExecutor直接调用了JDBC的代码,最终得到了我们查询的结果,其方法代码如下:

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); stmt = prepareStatement(handler, ms.getStatementLog()); return handler.<E>query(stmt, resultHandler); } finally { closeStatement(stmt); } }private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException { Statement stmt; Connection connection = getConnection(statementLog); stmt = handler.prepare(connection, transaction.getTimeout()); handler.parameterize(stmt); return stmt; }