文章内容
一、Mybatis核心成员
- Configuration MyBatis所有的配置信息都保存在Configuration对象之中，配置文件中的大部分配置都会存储到该类中
- SqlSession 作为MyBatis工作的主要顶层API，表示和数据库交互时的会话，完成必要数据库增删改查功能
- Executor MyBatis执行器，是MyBatis 调度的核心，负责SQL语句的生成和查询缓存的维护
- StatementHandler 封装了JDBC Statement操作，负责对JDBC statement 的操作，如设置参数等
- ParameterHandler 负责对用户传递的参数转换成JDBC Statement 所对应的数据类型
- 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(也可以说是数据表列类型)之间的映射和转换
- MappedStatement MappedStatement维护一条<select|update|delete|insert>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
以上主要成员在一次数据库操作中基本都会涉及，在SQL操作中重点需要关注的是SQL参数什么时候被设置和结果集怎么转换为JavaBean对象的，这两个过程正好对应StatementHandler和ResultSetHandler类中的处理逻辑。
二、MyBatis启动原理和源码解析
1、SqlSessionFactory与SqlSession
对于mybatis 的使用，从表面上来看，都是通过SqlSession去执行sql语句。那么就先看看是怎么获取SqlSession的吧：
1）读取mybatis的配置文件
首先，SqlSessionFactoryBuilder去读取mybatis的配置文件，然后build一个DefaultSqlSessionFactory。
String resource = "mybatis.xml";
// 加载mybatis的配置文件（它也加载关联的映射文件）
InputStream inputStream = null;
try {
inputStream = Resources.getResourceAsStream(resource);
} catch (IOException e) {
e.printStackTrace();
}
// 构建sqlSession的工厂
sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
2）创建SqlSessionFactory
源码如下，首先会创建SqlSessionFactory建造者对象，然后由它进行创建SqlSessionFactory。这里用到的是建造者模式，建造者模式最简单的理解就是不手动new对象，而是由其他类来进行对象的创建。
// SqlSessionFactoryBuilder类
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
return build(parser.parse()); // 开始进行解析了 :)
} 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.
}
}
}
通过以上步骤，咱们已经得到SqlSession对象了。SqlSession咱们也拿到了，咱们可以调用SqlSession中一系列的select…, insert…, update…, delete…方法轻松自如的进行CRUD操作了。 就这样？ 那咱配置的映射文件去哪儿了？ 别急， 咱们接着往下看。
2、利器之MapperProxy
在mybatis中，通过MapperProxy动态代理咱们的dao， 也就是说， 当咱们执行自己写的dao里面的方法的时候，其实是对应的mapperProxy在代理。那么，咱们就看看怎么获取MapperProxy对象吧：
1）通过SqlSession从Configuration中获取
通过SqlSession从Configuration中获取。源码如下：
public class DefaultSqlSession implements SqlSession {
private final Configuration configuration;
// sql最终由Executor执行
private final Executor executor;
/**
* 解析时会将Mapper保存到configuration中
*/ @Override
public <T> T getMapper(Class<T> type) {
return configuration.<T>getMapper(type, this);
}
// MapperMethod中最终调用SqlSession中的相关方法
@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();
}
}
}
2）Configuration类
SqlSession把包袱甩给了Configuration, 接下来就看看Configuration。源码如下：
public class Configuration {
protected final MapperRegistry mapperRegistry = new MapperRegistry(this);
/**
* mapperRegistry保存所有解析好的Mapper接口信息
* @param type
* @param sqlSession
* @return
*/ public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}
}
3）MapperRegistry类
Configuration不要这烫手的山芋，接着甩给了MapperRegistry， 那咱看看MapperRegistry。 源码如下：
public class MapperRegistry {
private final Configuration config;
private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();
/**
* @param type
* @param sqlSession
* @return
*/ @SuppressWarnings("unchecked")
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//MapperProxyFactory为工厂类，为Mapper接口生成代理类
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
//关键在这儿
return mapperProxyFactory.**newInstance**(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
}
4）生成Mapper接口的代理类
MapperProxyFactory通过JDK动态代理生成Mapper接口的代理类。咱们看看源码：
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();
/**
* 别人虐我千百遍，我待别人如初恋
* @param mapperProxy
* @return
*/ @SuppressWarnings("unchecked")
protected T **newInstance**(MapperProxy<T> mapperProxy) {
//动态代理我们写的dao接口
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);
}
}
这里是关键，通过以上的动态代理，咱们就可以方便地使用dao接口啦：
UserMapper userMapper = sqlSession.getMapper(UserMapper .class);
User insertUser = new User();
别急，还没完， 还没看具体是怎么执行sql语句的呢。
5）MapperProxy类
MapperProxy继承了InvocationHandler：
public class MapperProxy<T> implements InvocationHandler, Serializable {
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 (method.isDefault()) { // 默认方法
if (privateLookupInMethod == null) {
return invokeDefaultMethodJava8(proxy, method, args);
} else {
return invokeDefaultMethodJava9(proxy, method, args);
}
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
private MapperMethod cachedMapperMethod(Method method) {
return methodCache.computeIfAbsent(method,
k -> new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
}
}
6）MapperMethod类
MapperMethod：Mapper接口中每一个接口方法对应一个MapperMethod
public class MapperMethod {
private final SqlCommand command;
private final MethodSignature method;
public **MapperMethod**(Class<?> mapperInterface, Method method, Configuration config) {
this.command = new SqlCommand(config, mapperInterface, method);
this.method = new MethodSignature(config, mapperInterface, method);
}
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);
if (method.returnsOptional()
&& (result == null || !method.getReturnType().equals(result.getClass()))) {
result = Optional.ofNullable(result);
}
}
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;
}
private <E> Object **executeForMany**(SqlSession sqlSession, Object[] args) {
List<E> result;
Object param = method.convertArgsToSqlCommandParam(args);
if (method.hasRowBounds()) {
RowBounds rowBounds = method.extractRowBounds(args);
// 调用sqlSession的方法
result = sqlSession.selectList(command.getName(), param, rowBounds);
} else {
result = sqlSession.selectList(command.getName(), param);
}
// issue #510 Collections & arrays support
if (!method.getReturnType().isAssignableFrom(result.getClass())) {
if (method.getReturnType().isArray()) {
return convertToArray(result);
} else {
return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
}
}
return result;
}
public static class SqlCommand {
private final String name;
private final SqlCommandType type;
public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
final String methodName = method.getName();
final Class<?> declaringClass = method.getDeclaringClass();
// 从configuration中读取xml配置中的sql封装成MappedStatement
MappedStatement ms = **resolveMappedStatement**(mapperInterface, methodName, declaringClass,
configuration);
if (ms == null) {
if (method.getAnnotation(Flush.class) != null) {
name = null;
type = SqlCommandType.FLUSH;
} else {
throw new BindingException("Invalid bound statement (not found): "
+ mapperInterface.getName() + "." + methodName);
}
} else {
name = ms.getId();
type = ms.getSqlCommandType();
if (type == SqlCommandType.UNKNOWN) {
throw new BindingException("Unknown execution method for: " + name);
}
}
}
private MappedStatement **resolveMappedStatement**(Class<?> mapperInterface, String methodName,
Class<?> declaringClass, Configuration configuration) {
String **statementId** = mapperInterface.getName() + "." + methodName;
if (configuration.hasStatement(statementId)) {
return configuration.getMappedStatement(statementId);
} else if (mapperInterface.equals(declaringClass)) {
return null;
}
// ...
}
}
}
3、Excutor
接下来，咱们才要真正去看sql的执行过程了。上面，咱们拿到了MapperProxy, 每个MapperProxy对应一个dao接口， 那么咱们在使用的时候，MapperProxy是怎么做的呢？ 源码奉上：
1）MapperProxy
/**
* MapperProxy在执行时会触发此方法
*/@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);
//主要交给MapperMethod自己去管
return mapperMethod.execute(sqlSession, args);
}
2）MapperMethod
/**
* 根据命令类型先判断CRUD类型，然后根据类型去选择到底执行sqlSession中的哪个方法
* @param sqlSession
* @param args
* @return
*/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 {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
} 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;
}
既然又回到SqlSession了， 那么咱们就看看SqlSession的CRUD方法了，为了省事，还是就选择其中的一个方法来做分析吧。这儿，咱们选择了selectList方法：
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
//CRUD实际上是交给Excetor去处理
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，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();
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 {
//JDBC中的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
return resultSetHandler.<E> handleResultSets(ps);
}
到此， 一次sql的执行流程就完了。