开篇
这篇文章的主要目的是为了讲清楚Mybatis的整个执行流程，会通过源码、流程图等多个维度进行说明，相关的细节由于涉及面比较广这里暂时先不详细展开。
整体的思路先让大家有个宏观的概念，由了这个主轴以后我们再针对主轴上每个节点再进行细节分析，顺藤摸瓜，让我们先找到这个贯穿的藤。
Mybatis的使用模板
首先我们使用Mybatis的时候都有一般固定的套路：
- 定义map接口，定义对外查询sql的接口。
- 配置xml文件，定义查询sql语句。
- 引入xml文件，引入xml文件进行解析。
- 调用map接口，调用查询接口提供查询服务。
1、新建一个com.kang.mapper的包，定义map接口，这里以UserMapper为例
package com.kang.mapper;
import java.util.List;
import com.kang.pojo.User;
public interface UserMapper {
//根据用户id查询用户信息
public User findUserById(int id) throws Exception;
//查询用户列表
public List<User> findUserByUsername(String username) throws Exception;
//添加用户信息
public void insertUser(User user)throws Exception;
}
2、配置xml文件 UserMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.kang.mapper.UserMapper">
<!-- 注意这里的 namespace必须对应着map接口的全类名-->
<select id="findUserById" parameterType="int" resultType="user">
select * from user where id = #{id}
</select>
<select id="findUserByUsername" parameterType="java.lang.String"
resultType="user">
select * from user where username like '%${value}%'
</select>
<insert id="insertUser" parameterType="user">
<selectKey keyProperty="id" order="AFTER" resultType="java.lang.Integer">
select LAST_INSERT_ID()
</selectKey>
insert into user(username,birthday,sex,address)
values(#{username},#{birthday},#{sex},#{address})
</insert>
</mapper>
3、在SqlMapConfig.xml中加入映射文件
<!-- 加载 映射文件 -->
<mappers>
<mapper resource="map/UserMapper.xml" />
</mappers>
4、调用方法
//获取session
SqlSession session = sqlSessionFactory.openSession();
//获取mapper接口的代理对象
UserMapper userMapper = session.getMapper(UserMapper.class);
//调用代理对象方法
User user = userMapper.findUserById(27);
System.out.println(user);
//关闭session
session.close();
System.out.println("---------执行完毕-----------");
Mybatis执行过程
Mybatis执行过程流程图
Mybatis执行过程时序图
Mybatis执行过程源码分析
step1-DefaultSqlSession获取代理类
UserMapper userMapper = session.getMapper(UserMapper.class)
--------------------DefaultSqlSession.java------------------------------
public <T> T getMapper(Class<T> type) {
//最后会去调用MapperRegistry.getMapper
return configuration.<T>getMapper(type, this);
}
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}
step2-MapperRegistry类生成代理类
从代码可以看出来通过addMapper方法添加knownMappers，这个方法是在解析xml配置中被调用。
------------------------MapperRegistry.java-----------------------------------------
//返回代理类
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);
}
}
//我们在解析XML文件的时候添加了如何添加一个映射
public <T> void addMapper(Class<T> type) {
//mapper必须是接口！才会添加
if (type.isInterface()) {
if (hasMapper(type)) {
//如果重复添加了，报错
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
knownMappers.put(type, new MapperProxyFactory<T>(type));
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
//如果加载过程中出现异常需要再将这个mapper从mybatis中删除,这种方式比较丑陋吧，难道是不得已而为之？
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}
step3-MapperProxyFactory生成代理类MapperProxy
--------------------MapperProxyFactory.java------------------------------------------
/**
* 映射器代理工厂
*/
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
private 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;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
//用JDK自带的动态代理生成映射器
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);
}
}
step4-MapperProxy通过invoke调用真正的接口
MapperProxy的invoke当中执行method.invoke(this, args)调用真正的SQL查询接口，method就是查询的SQL接口的MapperMethod封装。
---------------------MapperProxy.java-----------------------
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 {
//代理以后，所有Mapper的方法调用时，都会调用这个invoke方法
//并不是任何一个方法都需要执行调用代理对象进行执行，如果这个方法是Object中通用的方法（toString、hashCode等）无需执行
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
//这里优化了，去缓存中找MapperMethod
final MapperMethod mapperMethod = cachedMapperMethod(method);
//执行
return mapperMethod.execute(sqlSession, args);
}
//去缓存中找MapperMethod
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
//找不到才去new
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
}
step5-MapperMethod的execute过程
MapperMethod我们暂时只关注result = sqlSession.selectOne(command.getName(), param)过程，command.getName()返回的是在Mybatis的mapper.xml文件对应的接口文件当中定义的。
--------------------------MapperMethod.java---------------------------------
//执行
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//可以看到执行时就是4种情况，insert|update|delete|select，分别调用SqlSession的4大类方法
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()) {
//如果有结果处理器
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
//如果结果有多条记录
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
//如果结果是map
result = executeForMap(sqlSession, args);
} else {
//否则就是一条记录
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;
}
step6-DefaultSqlSession的执行过程
selectList当中通过executor.query()方法调用进入到executor当中执行。
------------------------DefaultSqlSession.java-----------------------------------
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
//转而去调用selectList,很简单的，如果得到0条则返回null，得到1条则返回1条，得到多条报TooManyResultsException错
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;
}
}
@Override
public <E> List<E> selectList(String statement, Object parameter) {
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
//核心selectList
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
//根据statement id找到对应的MappedStatement
MappedStatement ms = configuration.getMappedStatement(statement);
//转而用执行器来查询结果,注意这里传入的ResultHandler是null
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();
}
}
step7-BaseExecutor的执行过程
在executor的执行过程中，首先调用的是BaseExecutor的接口，Executor的类关系图参加文章末尾。
--------------------------BaseExecutor.java------------------------------------------
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler)
throws SQLException {
//得到绑定sql
BoundSql boundSql = ms.getBoundSql(parameter);
//创建缓存Key
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
//查询
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler)
throws SQLException {
//得到绑定sql
BoundSql boundSql = ms.getBoundSql(parameter);
//创建缓存Key
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
//查询
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
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());
//如果已经关闭，报错
if (closed) {
throw new ExecutorException("Executor was closed.");
}
//先清局部缓存，再查询.但仅查询堆栈为0，才清。为了处理递归调用
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
//加一,这样递归调用到上面的时候就不会再清局部缓存了
queryStack++;
//先根据cachekey从localCache去查
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
//若查到localCache缓存，处理localOutputParameterCache
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
//从数据库查
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
//清空堆栈
queryStack--;
}
if (queryStack == 0) {
//延迟加载队列中所有元素
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
// issue #601
//清空延迟加载队列
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
//如果是STATEMENT，清本地缓存
clearLocalCache();
}
}
return list;
}
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds,
ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
//先向缓存中放入占位符？？？
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
//最后删除占位符
localCache.removeObject(key);
}
//加入缓存
localCache.putObject(key, list);
//如果是存储过程，OUT参数也加入缓存
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}
step8-SimpleExecutor的执行过程
我们这里暂且以SimpleExecutor为例进行分析，其他还有ReuseExecutor和BatchExecutor。通过handler.<E>query(stmt, resultHandler)调用SimpleStatementHandler。
在StatementHandler的创建过程中长链路调用过程中我们创建了ParameterHandler。
----------------------SimpleExecutor.java---------------------------------------
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();
//新建一个StatementHandler
//这里看到ResultHandler传入了,这个StatementHandler 是RoutingStatementHandler
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
//准备语句，transaction获取connection，connection创建statement。
stmt = prepareStatement(handler, ms.getStatementLog());
//StatementHandler.query
return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement,
Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
//创建路由选择语句处理器
StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement,
parameterObject, rowBounds, resultHandler, boundSql);
//插件在这里插入
statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
return statementHandler;
}
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds,
ResultHandler resultHandler, BoundSql boundSql) {
//根据语句类型，委派到不同的语句处理器(STATEMENT|PREPARED|CALLABLE)
switch (ms.getStatementType()) {
case STATEMENT:
delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case PREPARED:
delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case CALLABLE:
delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
default:
throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
}
}
public SimpleStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter,
RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
}
protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject,
RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
this.configuration = mappedStatement.getConfiguration();
this.executor = executor;
this.mappedStatement = mappedStatement;
this.rowBounds = rowBounds;
this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
this.objectFactory = configuration.getObjectFactory();
if (boundSql == null) { // issue #435, get the key before calculating the statement
generateKeys(parameterObject);
boundSql = mappedStatement.getBoundSql(parameterObject);
}
this.boundSql = boundSql;
//生成parameterHandler
this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject,
boundSql);
//生成resultSetHandler
this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds,
parameterHandler, resultHandler, boundSql);
}
step9-StatementHandler的执行过程
这里我们以SimpleStatementHandler为例进行说明，BaseStatementHandler的具体实现类还包括 CallableStatementHandler和PreparedStatementHandler。
---------------------------------SimpleStatementHandler.java---------------------------
//select-->结果给ResultHandler
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
String sql = boundSql.getSql();
//这里感觉调用jdbc的接口完成查询,待研究
statement.execute(sql);
//先执行Statement.execute，然后交给ResultSetHandler.handleResultSets
return resultSetHandler.<E>handleResultSets(statement);
}
step10-ResultSetHandler的执行过程
ResultSetHandler的实现类只有DefaultResultSetHandler。
---------------------------DefaultResultSetHandler.java---------------------------
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;
ResultSetWrapper rsw = getFirstResultSet(stmt);
// 处理 getResultMaps
List<ResultMap> resultMaps = mappedStatement.getResultMaps();
//一般resultMaps里只有一个元素
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++;
}
//处理 getResulSets
String[] resultSets = mappedStatement.getResulSets();
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);
}
类关系图
参考文档
Mybatis中Mapper动态代理的实现原理
Mybatis3.3.x技术内幕（十一）：执行一个Sql命令的完整流程
