MyBatis-底层源码解析-（详细），Java开发三年月薪才12K

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                } else {
                    /**
                     * 以下代码可以看出 url resource class 三个属性只能选择一个，否则就会报错
                     */
                    throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
                }
            }
        }
    }
}

}

## 1.4.2：mapperParser.parse();解析

**我的UserMapper.xml文件内容为：**

<?xml version="1.0" encoding="UTF-8"?> select * from `user` where userId = #{userId}

**UserMapper接口内容为：**

package com.mapper;

import com.entity.User;
import org.apache.ibatis.annotations.Select;

public interface UserMapper {
@Select(“select * from user where userId = 2”)
User findById(int userId);
}

**疑问？UserMapper.xml有<select id=“findById”，而在接口中的findById方法我又加了一个@Select注解；那么执行会选择哪一条Sql执行还是报错呢？**

**以UserMapper.xml为例子解析，可以看到 resource = mapper/UserMapper.xml**
![在这里插入图片描述](https://upload-images.jianshu.io/upload_images/25222111-04c09c0e358b0694?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

public class XMLMapperBuilder extends BaseBuilder {
public void parse() {
if (!configuration.isResourceLoaded(resource)) {
/**
* 解析 mapper.xml文件内容
/
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
/*
* 解析 mapper.xml的
* namespace指定的{UserMapper}接口的注解信息
*/
bindMapperForNamespace();
}

    parsePendingResultMaps();
    parsePendingCacheRefs();
    parsePendingStatements();
}

}

## 1.4.3：configurationElement(parser.evalNode("/mapper"));

**解析 mapper.xml文件内容**

public class XMLMapperBuilder extends BaseBuilder {
private void configurationElement(XNode context) {
try {
/**
* namespace属性
/
String namespace = context.getStringAttribute(“namespace”);
if (namespace == null || namespace.equals("")) {
/*
* 不指定 namespace会报错哦由此得知 namespace属性是必须指定的
/
throw new BuilderException(“Mapper’s namespace cannot be empty”);
}
builderAssistant.setCurrentNamespace(namespace);
/*
* 解析 cache-ref
/
cacheRefElement(context.evalNode(“cache-ref”));
/*
* 解析 cache
/
cacheElement(context.evalNode(“cache”));
/*
* 解析 parameterMap
/
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
/*
* 解析 resultMap
/
resultMapElements(context.evalNodes("/mapper/resultMap"));
/*
* 解析 sql
/
sqlElement(context.evalNodes("/mapper/sql"));
/*
* 解析 sql语句 select|insert|update|delete
* 重点分析这里，这里的解析会关联到 mapper接口的执行方法 sql语句映射
*/
buildStatementFromContext(context.evalNodes(“select|insert|update|delete”));
} catch (Exception e) {
throw new BuilderException(“Error parsing Mapper XML. The XML location is '” + resource + "’. Cause: " + e, e);
}
}
}

## 1.4.4：buildStatementFromContext(context.evalNodes(“select|insert|update|delete”));

**解析 sql语句 select|insert|update|delete；list参数内容是select|insert|update|delete的Sql语句**
**XMLStatementBuilder Sql语句的解析器**
![在这里插入图片描述](https://upload-images.jianshu.io/upload_images/25222111-e9b8bf020064dd10?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

## 1.4.5：statementParser.parseStatementNode();解析Sql语句

builderAssistant.addMappedStatement,并不是添加一个mapper.xml文件隐射的实例，<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">而是为每一个Sql语句创建一个实例</mark>

public class XMLStatementBuilder extends BaseBuilder {
private final MapperBuilderAssistant builderAssistant;
public void parseStatementNode() {
/**
* 此处省略一大推代码…
/
/*
*
* select * from user where userId = #{userId}
*
* 参数解析
*
* id：标签指定的id = findById
* sqlSource：Sql语句，Sql参数占位
* statementType：sql执行类型参考{@link StatementType}
* STATEMENT: 直接操作sql，不进行预编译 ${}
* PREPARED: 预处理，参数，进行预编译 #{}
* CALLABLE: 执行存储过程
* sqlCommandType：sql语句类型参考{@link SqlCommandType}
* UNKNOWN：未知，INSERT：新增，UPDATE：修改，DELETE：删除，SELECT：查询，FLUSH：刷新
*
* 其他参数可查看官网：https://mybatis.org/mybatis-3/zh/sqlmap-xml.html
*/
builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}
}

## 1.4.6：builderAssistant.addMappedStatement();

**创建一个MappedStatement实例添加到 Configuration.mappedStatements的Map集合中**

public class XMLStatementBuilder extends BaseBuilder {
public MappedStatement addMappedStatement() {
/**
* 此处省略一大推代码…
/
MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
/*
* 建造者模式
* 用于设置 MappedStatement的属性
* 此处省略一大推代码…
*/

    /**
     * 设置参数入参类型  parameterType属性
     * <select id="findById" parameterType="int" resultType="com.entity.User">
     *    select * from `user` where userId = #{userId}
     * </select>
     */
    ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
    if (statementParameterMap != null) {
        statementBuilder.parameterMap(statementParameterMap);
    }

    /**
     * 创建一个 {@link MappedStatement} 实例
     */
    MappedStatement statement = statementBuilder.build();

    /**
     * MappedStatement实例添加到 {@link #configuration.mappedStatements} Map集合中
     * MappedStatement 是对应一个Sql语句的实例对象
     *
     * configuration.mappedStatements 存放所有的MappedStatement实例，后面会详细介绍
     */
    configuration.addMappedStatement(statement);
    return statement;
}

}

## 以上流程执行完后，又回到<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">1.4.2：mapperParser.parse();解析</mark>，现在开始解析 namespace指定的接口的注解信息，并创建该接口的代理工厂对象，UserMapper接口。

![在这里插入图片描述](https://upload-images.jianshu.io/upload_images/25222111-380c8b0d4e8e7797?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

## 1.4.7：bindMapperForNamespace();

**开始解析接口注解，并添加一个<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">MapperProxyFactory</mark>代理工厂的对象到<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">configuration.mapperRegistry.knownMappers</mark>；key是<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">Mapper接口</mark>**

public class XMLMapperBuilder extends BaseBuilder {
private void bindMapperForNamespace() {
String namespace = builderAssistant.getCurrentNamespace();
if (namespace != null) {
Class<?> boundType = null;
try {
/**
* java 反射 Class.classForName
/
boundType = Resources.classForName(namespace);
} catch (ClassNotFoundException e) {
/*
* 这里并没有抛出异常，说明 namespace 可以指定一个不存在的接口
/
//ignore, bound type is not required 忽略，绑定类型不是必需的
}
if (boundType != null) {
if (!configuration.hasMapper(boundType)) {
// Spring may not know the real resource name so we set a flag
// to prevent loading again this resource from the mapper interface
// look at MapperAnnotationBuilder#loadXmlResource
configuration.addLoadedResource(“namespace:” + namespace);
/*
* 添加一个Mapper接口的代理工厂对象到configuration.mapperRegistry.knownMappers集合中
* 参考 {@link Configuration#mapperRegistry},
* {@link MapperRegistry#knownMappers}
*/
configuration.addMapper(boundType);
}
}
}
}
}

## 1.4.8：configuration.addMapper(boundType);

public class Configuration {
protected final MapperRegistry mapperRegistry = new MapperRegistry(this);
public void addMapper(Class type) {
/**
* mapperRegistry = {@link MapperRegistry} mapper接口注册器，存放所有的mapper接口信息
*/
mapperRegistry.addMapper(type);
}
}

**mapperRegistry.addMapper(type)；
为Mapper接口创建一个代理工厂，方便后期使用Mapper接口时创建代理类**
**解析mapper接口的注解信息**

public class MapperRegistry {
public void addMapper(Class type) {
if (type.isInterface()) {
if (hasMapper(type)) {
throw new BindingException(“Type " + type + " is already known to the MapperRegistry.”);
}
boolean loadCompleted = false;
try {
/**
* {@link MapperProxyFactory} 代理接口工厂
/
knownMappers.put(type, new MapperProxyFactory<>(type));
// It’s important that the type is added before the parser is run
// otherwise the binding may automatically be attempted by the
// mapper parser. If the type is already known, it won’t try.
/*
* {@link MapperAnnotationBuilder} mapper接口注解解析器
/
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
/ 开始解析 */
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}
}

## 1.4.9 parser.parse(); MapperAnnotationBuilder.parse()

**解析mapper接口的注解信息，parseStatement(method)主要在这个方法中完成**

public class MapperAnnotationBuilder {
public void parse() {
String resource = type.toString();
if (!configuration.isResourceLoaded(resource)) {
loadXmlResource();
configuration.addLoadedResource(resource);
assistant.setCurrentNamespace(type.getName());
/**
* 解析 @CacheNamespace 注解
/
parseCache();
/*
* 解析 CacheNamespaceRef 注解
/
parseCacheRef();
Method[] methods = type.getMethods();
for (Method method : methods) {
try {
// issue #237
if (!method.isBridge()) {
/*
* 解析Sql相关注解列如 @Select|@Update 之类的注解
*
* 重点关注
/
parseStatement(method);
}
} catch (IncompleteElementException e) {
configuration.addIncompleteMethod(new MethodResolver(this, method));
}
}
}
/*
* 解析待定方法
*/
parsePendingMethods();
}
}

## 1.5.0：parseStatement(method)；

**创建一个MappedStatement实例添加到 Configuration.mappedStatements的Map集合中**

public class MapperAnnotationBuilder {
private final MapperBuilderAssistant assistant;
void parseStatement(Method method) {
/**
* 此处省略一大推代码…
*/
assistant.addMappedStatement(
mappedStatementId,
sqlSource,
statementType,
sqlCommandType,
fetchSize,
timeout,
// ParameterMapID
null,
parameterTypeClass,
resultMapId,
getReturnType(method),
resultSetType,
flushCache,
useCache,
// TODO gcode issue #577
false,
keyGenerator,
keyProperty,
keyColumn,
// DatabaseID
null,
languageDriver,
// ResultSets
options != null ? nullOrEmpty(options.resultSets()) : null);
}
}

## 1.5.1：assistant.addMappedStatement();

**<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">这里可以参考1.4.6：builderAssistant.addMappedStatement();一模一样的操作</mark>**
<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">**都调用了configuration.addMappedStatement(statement);**</mark>
![在这里插入图片描述](https://upload-images.jianshu.io/upload_images/25222111-7a172e142e5239a8?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

**这里重点分析Configuration.addMappedStatement(statement);在做什么操作，并且解决 <mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">1.4.2留下的疑点；UserMapper.xml和UserMapper接口都有findById的Sql语句定义</mark>**

public class Configuration {
public void addMappedStatement(MappedStatement ms) {
mappedStatements.put(ms.getId(), ms);
}
}

**mappedStatements.put(ms.getId(), ms); 实际调用 Configuration.StrictMap.put()方法**
**<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">Configuration.StrictMap是一个重写的HashMap，put方法会先校验key是否存在</mark>**

public class Configuration {
/**
* mappedStatements Sql语句的对象
*
* Configuration.StrictMap 实现了HashMap
*/
protected final Map<String, MappedStatement> mappedStatements = new Configuration.StrictMap(“Mapped Statements collection”)
.conflictMessageProducer((savedValue, targetValue) ->
". please check " + savedValue.getResource() + " and " + targetValue.getResource());

protected static class StrictMap<V> extends HashMap<String, V> {
    @Override
    @SuppressWarnings("unchecked")
    public V put(String key, V value) {
        /**
         * key 是否存在 存在就抛出异常
         *
         * 由此得知，方法映射Sql语句只能定义一个，要么在 mapper.xml中定义，要么就注解定义
         */
        if (containsKey(key)) {
            throw new IllegalArgumentException(name + " already contains value for " + key
                    + (conflictMessageProducer == null ? "" : conflictMessageProducer.apply(super.get(key), value)));
        }
        if (key.contains(".")) {
            final String shortKey = getShortName(key);
            if (super.get(shortKey) == null) {
                super.put(shortKey, value);
            } else {
                super.put(shortKey, (V) new org.apache.ibatis.session.Configuration.StrictMap.Ambiguity(shortKey));
            }
        }
        return super.put(key, value);
    }
}

}

**debug调试，key已经存在，就会报错。由此得知，方法映射Sql语句只能定义一个，要么在 mapper.xml中定义，要么就注解定义**
![在这里插入图片描述](https://upload-images.jianshu.io/upload_images/25222111-a866e4089e7aa556?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

## 到此，MyBatis的启动流程就走完了。

## <mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">2：接下来就来看下是如何执行Sql查询的。</mark>

public class Main {
public static void main(String[] args) throws IOException {
String configName = “mybatis_config.xml”;
Reader reader = Resources.getResourceAsReader(configName);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);

    /**
     * 获取一个会话连接
     */
    SqlSession sqlSession = sqlSessionFactory.openSession();

    /**
     * 拿到 UserMapper 的代理类
     */
    UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

    /**
     * 执行Sql查询
     */
    User user = userMapper.findById(1);
    System.out.println(user);
}

}

> **输出结果：User{userId=1, username=‘张三’, sex=‘男’, age=12}**

**<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">一行代码的查询，底层既然走了那么多流程；</mark>**

## 流程图：

![在这里插入图片描述](https://upload-images.jianshu.io/upload_images/25222111-459d4ed3e065a0d5?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

## 2.1：sqlSessionFactory.openSession();打开会话连接

**调用DefaultSqlSessionFactory.openSessionFromDataSource();**

public class DefaultSqlSessionFactory implements SqlSessionFactory {
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
/**
* mybatis_config.xml配置的
* 数据源
*/
final Environment environment = configuration.getEnvironment();

        /**
         * transactionManager 配置的事务管理器工厂   type="JDBC"  {@link JdbcTransactionFactory}
         *<environments default="developmentss">
         *    <environment id="developmentss">
         *        <transactionManager type="JDBC"></transactionManager>
         *    </environment>
         *  </environments>
         */
        final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);

        /**
         * 创建事务管理器，由于上面指定的事务管理器工厂是 {@link JdbcTransactionFactory}
         * 所以创建的事务管理器是  {@link JdbcTransaction}
         *
         * @param level 事务隔离级别
         * @param autoCommit 是否自动提交事务
         */
        tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);

        /**
         * 创建Sql执行器
         *
         * @param execType 创建执行器类型 defaultExecutorType如果不指定 默认就是 SIMPLE
         *                 <settings>
         *                     <setting name="defaultExecutorType" value="SIMPLE"/>
         *                 </settings>
         */
        final Executor executor = configuration.newExecutor(tx, execType);

        /**
         * 创建一个默认的 SqlSession实例
         */
        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();
    }
}

}

## 2.2：configuration.newExecutor(tx, execType);创建执行器

public class Configuration {
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
if (ExecutorType.BATCH == executorType) {
/**
* 执行器不仅重用语句还会执行批量更新
/
executor = new BatchExecutor(this, transaction);
/*
* 执行器会重用预处理语句（PreparedStatement）
/
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
/*
* 普通的执行器也是默认的执行器
/
executor = new SimpleExecutor(this, transaction);
}
/*
* 如果开启了二级缓存 cacheEnabled，创建一个CachingExecutor缓存执行器
* cacheEnabled 默认为true
*
*
*
*/
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
}

## 2.3：sqlSession.getMapper(UserMapper.class);

**获取Mapper接口代理类实例**

public class DefaultSqlSession implements SqlSession {
@Override
public T getMapper(Class type) {
/**
* 用 Configuration 类的 getMapper方法
*/
return configuration.getMapper(type, this);
}
}

public class Configuration {
public T getMapper(Class type, SqlSession sqlSession) {
/**
* 调用 MapperRegistry Mapper接口注册器
*/
return mapperRegistry.getMapper(type, sqlSession);
}
}

public class MapperRegistry {
private final Configuration config;
private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    /**
     * knownMappers 从缓存中获取 MapperProxyFactory Mapper接口代理工厂
     * 如果没有找到就会抛出异常，
     * 说明获取Mapper接口代理实例时，需要事先定义好  -->  相当于Spring的扫包
     */
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
        throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
        /**
         * 创建代理实例
         */
        return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
        throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
}

}

## <mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">2.3.1：使用JDK代理创建Mapper接口代理</mark>

**InvocationHandler是MapperProxy**

public class MapperProxyFactory {

private final Class<T> mapperInterface;
/**
 * MapperMethod 缓存
 */
private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();

public MapperProxyFactory(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
}

@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
    /**
     * JDK 生产代理类
     */
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}

public T newInstance(SqlSession sqlSession) {
    /**
     * 代理类回调接口
     */
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
}

}

## 2.4：userMapper.findById(1); 调用Mapper接口方法Sql查询

**会走代理类 MapperProxy.invoke**

public class MapperProxy 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 (method.isDefault()) {
            return invokeDefaultMethod(proxy, method, args);
        }
    } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
    }
    /**
     * 根据方法全限名 获取一个MapperMethod实例，并且缓存
     *
     * 注意：这里的 methodCache 只是一个引用，缓存的所有对象都在 {@link MapperProxyFactory#methodCache}中
     */
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    /**
     * 开始执行
     */
    return mapperMethod.execute(sqlSession, args);
}

/**
 * 添加到缓存 methodCache {@link MapperProxyFactory#methodCache}中
 * computeIfAbsent   HashMap  存在就获取，不存在就新增
 * @param method
 * @return
 */
private MapperMethod cachedMapperMethod(Method method) {
    return methodCache.computeIfAbsent(method, k -> new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
}

}

## 2.5：mapperMethod.execute(sqlSession, args);

**执行Sql语句查询，由于我的返回结果是一个 User对象，所以会走到
result = sqlSession.selectOne(command.getName(), param);这一行，<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">查询一条记录</mark>**
**<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">实际走到 DefaultSqlSession.selectOne()</mark>**

public class MapperMethod {

private final org.apache.ibatis.binding.MapperMethod.SqlCommand command;
private final org.apache.ibatis.binding.MapperMethod.MethodSignature method;

public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
    this.command = new org.apache.ibatis.binding.MapperMethod.SqlCommand(config, mapperInterface, method);
    this.method = new org.apache.ibatis.binding.MapperMethod.MethodSignature(config, mapperInterface, method);
}
/**
* 开始执行Sql查询
*/
public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
        case INSERT: {
            // 省略代码...   执行 insert语句 <insert>/@Insert
            break;
        }
        case UPDATE: {
            // 省略代码...   执行 insert语句 <update>/@Update
            break;
        }
        case DELETE: {
            // 省略代码...   执行 delete语句 <delete>/@Delete
            break;
        }
        case SELECT:      // 执行 select语句 <select>/@Select
            if (method.returnsVoid() && method.hasResultHandler()) {
                // 返回类型是否为空 一般情况做Sql操作都要有返回结果的
                executeWithResultHandler(sqlSession, args);
                result = null;
            } else if (method.returnsMany()) {
                /**
                 * 是否返回多个结果集  {@link Collection}集合/数组
                 */
                result = executeForMany(sqlSession, args);
            } else if (method.returnsMap()) {
                /**
                 * 返回类型是否为 Map 集合
                 */
                result = executeForMap(sqlSession, args);
            } else if (method.returnsCursor()) {
                /**
                 * 返回类型是否是 游标  {@link org.apache.ibatis.cursor.Cursor}
                 */
                result = executeForCursor(sqlSession, args);
            } else {
                /**
                 * 将参数转换为Sql命令参数
                 */
                Object param = method.convertArgsToSqlCommandParam(args);
                /**
                 * 发起查询  调用的是 sqlSession中的方法
                 */
                result = sqlSession.selectOne(command.getName(), param);
                if (method.returnsOptional()
                        && (result == null || !method.getReturnType().equals(result.getClass()))) {
                    result = Optional.ofNullable(result);
                }
            }
            break;
        case FLUSH:  // 刷新
            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;
}

}

**DefaultSqlSession.selectOne()可以看出实际是调用<mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">selectList()</mark>，而且如果返回了多个结果集就会报错。错误信息如下**
**Expected one result (or null) to be returned by selectOne(), but found: 2**
![在这里插入图片描述](https://upload-images.jianshu.io/upload_images/25222111-51fde0f4a08cf0a4?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

## 2.6：DefaultSqlSession.selectList()

**查询多结果集**

public class DefaultSqlSession implements SqlSession {
/**
*
* @param statement 方法全限名比如：com.mapper.UserMapper.findById
* @param parameter 参数
* @param rowBounds 分页
* @param
* @return
/
@Override
public List selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
/*
* 根据方法全限名在 configuration.mappedStatements缓存集合中拿到方法对应的Sql Statement对象实例
/
MappedStatement ms = configuration.getMappedStatement(statement);
/*
* 使用执行器执行由当前设置的执行器执行
*
*
*
* 注意：cacheEnabled由于开启二级缓存默认为true，会先使用 {@link CachingExecutor} 缓存执行器查询
*/
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();
}
}
}

## 2.7：executor.query执行器查询

**执行器的创建查看 <mark style="box-sizing: border-box; outline: 0px; background-color: rgb(248, 248, 64); color: rgb(0, 0, 0); overflow-wrap: break-word;">2.2：configuration.newExecutor(tx, execType);创建执行器</mark>**

/**

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