使用Mybatis时,经常通过下面的代码获取SqlSession对象然后使用getMapper方法获取相应的mapper对象,通过mapper对象执行数据库的增删改查操作。
SqlSessionFactoryBuilder sqlSessionFactoryBuilder =null;
SqlSessionFactory sqlSessionFactory = null;
SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
sqlSessionFactory=sqlSessionFactoryBuilder.build(Resources.getResourceAsStream("mybatis-config.xml"));
SqlSession sqlSession = sqlSessionFactory.openSession();
UserMapper userMapper = session.getMapper(UserMapper.class);
user2Mapper.insertUser(user);
session.commit();
可是我们只定义了Mapper接口并没有实现接口,那么Mybatis是如何帮助我们完成数据库操作的呢?
首先是通过SqlSessionFactoryBuilder的build方法获取sqlSessionFactory ,在build方法中会通过XMLConfigBuilder解析全局xml文件并封装为Configuration对象,然后使用Configuration对象创建默认的DefaultSqlSessionFactory 。build方法代码如下:
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
reader.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
然后调用SqlSessionFactory的openSession方法获取SqlSession,openSession方法返回默认的DefaultSqlSession对象。openSession方法代码如下:
@Override
public SqlSession openSession() {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
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);
final Executor executor = configuration.newExecutor(tx, execType);//Excutor执行器,真正执行数据库操作的组件,默认是SimpleExcutor,开启缓存的情况下使用CachingExecutor(Mybatis默认开启缓存)
return new DefaultSqlSession(configuration, executor, autoCommit);//DefaultSqlSession实现了SqlSession接口
} 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();
}
}
获取到DefaultSqlSession对象后我们调用了getMapper方法,通过源码可以看DefaultSqlSession的getMapper方法中调用了Configuration的getMapper方法。DefaultSqlSession的getMapper方法代码如下:
@Override
public <T> T getMapper(Class<T> type) {
return configuration.<T>getMapper(type, this);
}
继续跟下去发现MapperRegistry的getMapper方法调用了MapperProxyFactory的newInstance方法,看名称可以知道在这里创建了Mapper接口的代理对象。MapperRegistry的getMapper方法代码如下:
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
进入newInstance方法中看看,可以看到Mybatis使用JDK动态代理为Mapper接口生产代理对象。newInstance方法代码如下:
@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);
}
既然是动态代理,那我们就去就看看代理类MapperProxy重写的invoke方法,这段代码的大意是:如果调用由Object继承而来的方法(如toString)就直接使用反射调用,否则使用MapperMehtod的execute方法。invoke方法源码如下:
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
查看execute源码,发现调用了SqlSession的是增删改查方法。execute方法代码如下:
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
if (SqlCommandType.INSERT == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
} else if (SqlCommandType.UPDATE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
} else if (SqlCommandType.DELETE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
} else if (SqlCommandType.SELECT == command.getType()) {
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);
}
} else if (SqlCommandType.FLUSH == command.getType()) {
result = sqlSession.flushStatements();
} else {
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;
}
查看DefaultSqlSession的selectOne方法,发现最终调用了selectList,并在selectList方法中调用了executor的query方法去查询。selectList方法代码如下:
@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();
}
}
总结
1.SqlSessionFactoryBuilder使用XMLConfigBuilder解析全局xml文件并保存在Configuration对象中,使用Configuration对象初始化DefaultSqlSessionFactory。
2.SqlSessionFactory主要负责创建默认的DefaultSqlSession(Excutor也在这里创建)。
3.使用getMapper方法时,使用JDK动态代理为Mapper接口创建代理对象。
4.具体完成数据库操作的组件是SqlSession中的Excutor。