Mybatis工作流程及其原理与解析

Mybatis简介：

MyBatis 是一款优秀的持久层框架，它支持定制化 SQL、存储过程以及高级映射。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以使用简单的 XML 或注解来配置和映射原生信息，将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。本文将通过debug的方式来了解其工作原理。

Mybatis核心类：

SqlSessionFactory：每个基于 MyBatis 的应用都是以一个 SqlSessionFactory 的实例为中心的。SqlSessionFactory 的实例可以通过 SqlSessionFactoryBuilder 获得。而 SqlSessionFactoryBuilder 则可以从 XML 配置文件或通过Java的方式构建出 SqlSessionFactory 的实例。SqlSessionFactory 一旦被创建就应该在应用的运行期间一直存在，建议使用单例模式或者静态单例模式。一个SqlSessionFactory对应配置文件中的一个环境（environment），如果你要使用多个数据库就配置多个环境分别对应一个SqlSessionFactory。

SqlSession：SqlSession是一个接口，它有2个实现类，分别是DefaultSqlSession(默认使用)以及SqlSessionManager。SqlSession通过内部存放的执行器（Executor）来对数据进行CRUD。此外SqlSession不是线程安全的，因为每一次操作完数据库后都要调用close对其进行关闭，官方建议通过try-finally来保证总是关闭SqlSession。

Executor：Executor（执行器）接口有两个实现类，其中BaseExecutor有三个继承类分别是BatchExecutor（重用语句并执行批量更新），ReuseExecutor（重用预处理语句prepared statements），SimpleExecutor（普通的执行器）。以上三个就是主要的Executor。通过下图可以看到Mybatis在Executor的设计上面使用了装饰者模式，我们可以用CachingExecutor来装饰前面的三个执行器目的就是用来实现缓存。

image

MappedStatement：MappedStatement就是用来存放我们SQL映射文件中的信息包括sql语句，输入参数，输出参数等等。一个SQL节点对应一个MappedStatement对象。

Mybatis工作流程：

image

阅读全文有惊喜哦！！！

下面将通过debug方式对Mybatis进行一步步解析。首先贴出我的mybatis-config.xml文件以及Mapper.xml文件。

PUBLIC "-//mybatis.org//DTD Config 3.0//EN"

"http://mybatis.org/dtd/mybatis-3-config.dtd">

PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"

"http://mybatis.org/dtd/mybatis-3-mapper.dtd">

select * from user

User where id = #{id}

insert into User (username,birthday,sex,address)

values (#{name},#{birthday},#{sex},#{address})

update User set username = #{username},birthday = #{birthday},

sex = #{sex},address = #{address} where id = #{id}

delete from User where id = #{id}

select * from User where sex = #{param1}

and username like #{param2}

and address = #{parma3}

select count(*) from user where username like #{username}

username like #{pattern}

and sex = #{sex}

and address = #{address}

where id in

{id}

image

第一步通过SqlSessionFactoryBuilder创建SqlSessionFactory：

首先在SqlSessionFactoryBuilder的build（）方法中可以看到MyBatis内部定义了一个类XMLConfigBuilder用来解析配置文件mybatis-config.xml。针对配置文件中的每一个节点进行解析并将数据存放到Configuration这个对象中，紧接着使用带有Configuration的构造方法发返回一个DefautSqlSessionFactory。

public SqlSessionFactory build(InputStream inputStream) {

return build(inputStream, null, null);

}

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {

try {

XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);

//解析mybatis-config.xml

return build(parser.parse());

} 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.

}

}

}

//返回SqlSessionFactory，默认使用的是实现类DefaultSqlSessionFactory

public SqlSessionFactory build(Configuration config) {

return new DefaultSqlSessionFactory(config);

}

public Configuration parse() {

if (parsed) {

throw new BuilderException("Each XMLConfigBuilder can only be used once.");

}

parsed = true;

//获取根节点configuration

parseConfiguration(parser.evalNode("/configuration"));

return configuration;

}

//开始解析mybatis-config.xml,并把解析后的数据存放到configuration中

private void parseConfiguration(XNode root) {

try {

//保存mybatis-config.xml中的标签setting,本例中开启全局缓存cacheEnabled，设置默认执行器defaultExecutorType=REUSE

Properties settings = settingsAsPropertiess(root.evalNode("settings"));

//issue #117 read properties first

//解析是否配置了外部properties，例如本例中配置的jdbc.propertis

propertiesElement(root.evalNode("properties"));

//查看是否配置了VFS，默认没有，本例也没有使用

loadCustomVfs(settings);

//查看是否用了类型别名，减少完全限定名的冗余，本例中使用了别名User代替了com.ctc.Model.User

typeAliasesElement(root.evalNode("typeAliases"));

//查看是否配置插件来拦截映射语句的执行，例如拦截Executor的Update方法，本例没有使用

pluginElement(root.evalNode("plugins"))

//查看是否配置了ObjectFactory，默认情况下使用对象的无参构造方法或者是带有参数的构造方法，本例没有使用

objectFactoryElement(root.evalNode("objectFactory"));

//查看是否配置了objectWrapperFatory,这个用来或者ObjectWapper，可以访问：对象，Collection，Map属性。本例没有使用

objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));

//查看是否配置了reflectorFactory,mybatis的反射工具，提供了很多反射方法。本例没有使用

reflectorFactoryElement(root.evalNode("reflectorFactory"));

//放入参数到configuration对象中

settingsElement(settings);

// read it after objectFactory and objectWrapperFactory issue #631

//查看数据库环境配置

environmentsElement(root.evalNode("environments"));

//查看是否使用多种数据库，本例没有使用

databaseIdProviderElement(root.evalNode("databaseIdProvider"));

//查看是否配置了新的类型处理器，如果跟处理的类型跟默认的一致就会覆盖。本例没有使用

typeHandlerElement(root.evalNode("typeHandlers"));

//查看是否配置SQL映射文件,有四种配置方式，resource，url，class以及自动扫包package。本例使用package

mapperElement(root.evalNode("mappers"));

} catch (Exception e) {

throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);

}

}

第二步通过SqlSessionFactory创建SqlSession：

@Override

public SqlSession openSession() {

return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);

}

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {

Transaction tx = null;

try {

//拿到前文从mybatis中解析到的数据库环境配置

final Environment environment = configuration.getEnvironment();

final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);

//拿到jdbc的事务管理器，有两种一种是jbc,一种的managed。本例使用的是JdbcTransaction

tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);

//从mybatis配置文件可以看到本例使用了REUSE，因此返回的是ReuseExecutor并把事务传入对象中

final Executor executor = configuration.newExecutor(tx, execType);

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();

}

}

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {

executorType = executorType == null ? defaultExecutorType : executorType;

executorType = executorType == null ? ExecutorType.SIMPLE : executorType;

Executor executor;

if (ExecutorType.BATCH == executorType) {

executor = new BatchExecutor(this, transaction);

} else if (ExecutorType.REUSE == executorType) {

executor = new ReuseExecutor(this, transaction);

} else {

executor = new SimpleExecutor(this, transaction);

}

if (cacheEnabled) {

executor = new CachingExecutor(executor);

}

executor = (Executor) interceptorChain.pluginAll(executor);

return executor;

}

//返回一个SqlSession，默认使用DefaultSqlSession

public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {

this.configuration = configuration;

this.executor = executor;

this.dirty = false;

this.autoCommit = autoCommit;

}

第三步通过SqlSession拿到Mapper对象的代理：

@Override

public T getMapper(Class type) {

return configuration.getMapper(type, this);

}

public T getMapper(Class type, SqlSession sqlSession) {

//前文解析Mybatis-config.xml的时候，在解析标签mapper就是用configuration对象的mapperRegistry存放数据

return mapperRegistry.getMapper(type, sqlSession);

}

@SuppressWarnings("unchecked")

public T getMapper(Class type, SqlSession sqlSession) {

//knownMapper是一个HashMap在存放mapperRegistry的过程中，以每个Mapper对象的类型为Key, MapperProxyFactory 为value保存。

//例如本例中保存的就是Key:com.ctc.mapper.UserMapper,value就是保存了key的MapperProxyFactory对象

final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) 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);

}

}

public T newInstance(SqlSession sqlSession) {

//生成一个mapperProxy对象，这个对象实现了InvocationHandler, Serializable。就是JDK动态代理中的方法调用处理器

final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);

return newInstance(mapperProxy);

}

public MapperProxy(SqlSession sqlSession, Class mapperInterface, Map methodCache) {

this.sqlSession = sqlSession;

this.mapperInterface = mapperInterface;

this.methodCache = methodCache;

}

@SuppressWarnings("unchecked")

protected T newInstance(MapperProxy mapperProxy) {

//通过JDK动态代理生成一个Mapper的代理，在本例中的就是UserMapper的代理类，它实现了UserMapper接口

return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);

}

第四步通过MapperProxy调用Maper中相应的方法：

@Override

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

//判断当前调用的method是不是Object中声明的方法，如果是的话直接执行。

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);

}

//把当前请求放入一个HashMap中，一旦下次还是同样的方法进来直接返回。

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;

}

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 {

//本次案例会执行selectOne

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;

}

@Override

public T selectOne(String statement, Object parameter) {

// Popular vote was to return null on 0 results and throw exception on too many.

List list = this.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 List 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();

}

}

//这边调用的是CachingExecutor类的query,还记得前文解析mybatis-config.xml的时候我们指定了REUSE但是因为在配置文件中开启了缓存

//所以ReuseExecutor被CachingExecotur装饰，新增了缓存的判断，最后还是会调用ReuseExecutor

@Override

public List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {

BoundSql boundSql = ms.getBoundSql(parameterObject);

CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);

return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);

}

@Override

public List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)

throws SQLException {

Cache cache = ms.getCache();

if (cache != null) {

flushCacheIfRequired(ms);

if (ms.isUseCache() && resultHandler == null) {

ensureNoOutParams(ms, parameterObject, boundSql);

@SuppressWarnings("unchecked")

List list = (List) tcm.getObject(cache, key);

if (list == null) {

//如果缓存中没有数据则查询数据库

list = delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);

//结果集放入缓存

tcm.putObject(cache, key, list); // issue #578 and #116

}

return list;

}

}

return delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);

}

MyBatis 是一款优秀的持久层框架，它支持定制化 SQL、存储过程以及高级映射。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以使用简单的 XML 或注解来配置和映射原生信息，将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。本文将通过debug的方式来了解其工作原理。

Mybatis核心类：

SqlSessionFactory：每个基于 MyBatis 的应用都是以一个 SqlSessionFactory 的实例为中心的。SqlSessionFactory 的实例可以通过 SqlSessionFactoryBuilder 获得。而 SqlSessionFactoryBuilder 则可以从 XML 配置文件或通过Java的方式构建出 SqlSessionFactory 的实例。SqlSessionFactory 一旦被创建就应该在应用的运行期间一直存在，建议使用单例模式或者静态单例模式。一个SqlSessionFactory对应配置文件中的一个环境（environment），如果你要使用多个数据库就配置多个环境分别对应一个SqlSessionFactory。

SqlSession：SqlSession是一个接口，它有2个实现类，分别是DefaultSqlSession(默认使用)以及SqlSessionManager。SqlSession通过内部存放的执行器（Executor）来对数据进行CRUD。此外SqlSession不是线程安全的，因为每一次操作完数据库后都要调用close对其进行关闭，官方建议通过try-finally来保证总是关闭SqlSession。

Executor：Executor（执行器）接口有两个实现类，其中BaseExecutor有三个继承类分别是BatchExecutor（重用语句并执行批量更新），ReuseExecutor（重用预处理语句prepared statements），SimpleExecutor（普通的执行器）。以上三个就是主要的Executor。通过下图可以看到Mybatis在Executor的设计上面使用了装饰者模式，我们可以用CachingExecutor来装饰前面的三个执行器目的就是用来实现缓存。

image

MappedStatement：MappedStatement就是用来存放我们SQL映射文件中的信息包括sql语句，输入参数，输出参数等等。一个SQL节点对应一个MappedStatement对象。

Mybatis工作流程：

image

下面将通过debug方式对Mybatis进行一步步解析。首先贴出我的mybatis-config.xml文件以及Mapper.xml文件。

PUBLIC "-//mybatis.org//DTD Config 3.0//EN"

"http://mybatis.org/dtd/mybatis-3-config.dtd">

PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"

"http://mybatis.org/dtd/mybatis-3-mapper.dtd">

select * from user

User where id = #{id}

insert into User (username,birthday,sex,address)

values (#{name},#{birthday},#{sex},#{address})

update User set username = #{username},birthday = #{birthday},

sex = #{sex},address = #{address} where id = #{id}

delete from User where id = #{id}

select * from User where sex = #{param1}

and username like #{param2}

and address = #{parma3}

select count(*) from user where username like #{username}

username like #{pattern}

and sex = #{sex}

and address = #{address}

where id in

{id}

image

第一步通过SqlSessionFactoryBuilder创建SqlSessionFactory：

首先在SqlSessionFactoryBuilder的build（）方法中可以看到MyBatis内部定义了一个类XMLConfigBuilder用来解析配置文件mybatis-config.xml。针对配置文件中的每一个节点进行解析并将数据存放到Configuration这个对象中，紧接着使用带有Configuration的构造方法发返回一个DefautSqlSessionFactory。

public SqlSessionFactory build(InputStream inputStream) {

return build(inputStream, null, null);

}

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {

try {

XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);

//解析mybatis-config.xml

return build(parser.parse());

} 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.

}

}

}

//返回SqlSessionFactory，默认使用的是实现类DefaultSqlSessionFactory

public SqlSessionFactory build(Configuration config) {

return new DefaultSqlSessionFactory(config);

}

public Configuration parse() {

if (parsed) {

throw new BuilderException("Each XMLConfigBuilder can only be used once.");

}

parsed = true;

//获取根节点configuration

parseConfiguration(parser.evalNode("/configuration"));

return configuration;

}

//开始解析mybatis-config.xml,并把解析后的数据存放到configuration中

private void parseConfiguration(XNode root) {

try {

//保存mybatis-config.xml中的标签setting,本例中开启全局缓存cacheEnabled，设置默认执行器defaultExecutorType=REUSE

Properties settings = settingsAsPropertiess(root.evalNode("settings"));

//issue #117 read properties first

//解析是否配置了外部properties，例如本例中配置的jdbc.propertis

propertiesElement(root.evalNode("properties"));

//查看是否配置了VFS，默认没有，本例也没有使用

loadCustomVfs(settings);

//查看是否用了类型别名，减少完全限定名的冗余，本例中使用了别名User代替了com.ctc.Model.User

typeAliasesElement(root.evalNode("typeAliases"));

//查看是否配置插件来拦截映射语句的执行，例如拦截Executor的Update方法，本例没有使用

pluginElement(root.evalNode("plugins"))

//查看是否配置了ObjectFactory，默认情况下使用对象的无参构造方法或者是带有参数的构造方法，本例没有使用

objectFactoryElement(root.evalNode("objectFactory"));

//查看是否配置了objectWrapperFatory,这个用来或者ObjectWapper，可以访问：对象，Collection，Map属性。本例没有使用

objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));

//查看是否配置了reflectorFactory,mybatis的反射工具，提供了很多反射方法。本例没有使用

reflectorFactoryElement(root.evalNode("reflectorFactory"));

//放入参数到configuration对象中

settingsElement(settings);

// read it after objectFactory and objectWrapperFactory issue #631

//查看数据库环境配置

environmentsElement(root.evalNode("environments"));

//查看是否使用多种数据库，本例没有使用

databaseIdProviderElement(root.evalNode("databaseIdProvider"));

//查看是否配置了新的类型处理器，如果跟处理的类型跟默认的一致就会覆盖。本例没有使用

typeHandlerElement(root.evalNode("typeHandlers"));

//查看是否配置SQL映射文件,有四种配置方式，resource，url，class以及自动扫包package。本例使用package

mapperElement(root.evalNode("mappers"));

} catch (Exception e) {

throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);

}

}

第二步通过SqlSessionFactory创建SqlSession：

@Override

public SqlSession openSession() {

return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);

}

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {

Transaction tx = null;

try {

//拿到前文从mybatis中解析到的数据库环境配置

final Environment environment = configuration.getEnvironment();

final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);

//拿到jdbc的事务管理器，有两种一种是jbc,一种的managed。本例使用的是JdbcTransaction

tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);

//从mybatis配置文件可以看到本例使用了REUSE，因此返回的是ReuseExecutor并把事务传入对象中

final Executor executor = configuration.newExecutor(tx, execType);

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();

}

}

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {

executorType = executorType == null ? defaultExecutorType : executorType;

executorType = executorType == null ? ExecutorType.SIMPLE : executorType;

Executor executor;

if (ExecutorType.BATCH == executorType) {

executor = new BatchExecutor(this, transaction);

} else if (ExecutorType.REUSE == executorType) {

executor = new ReuseExecutor(this, transaction);

} else {

executor = new SimpleExecutor(this, transaction);

}

if (cacheEnabled) {

executor = new CachingExecutor(executor);

}

executor = (Executor) interceptorChain.pluginAll(executor);

return executor;

}

//返回一个SqlSession，默认使用DefaultSqlSession

public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {

this.configuration = configuration;

this.executor = executor;

this.dirty = false;

this.autoCommit = autoCommit;

}

第三步通过SqlSession拿到Mapper对象的代理：

@Override

public T getMapper(Class type) {

return configuration.getMapper(type, this);

}

public T getMapper(Class type, SqlSession sqlSession) {

//前文解析Mybatis-config.xml的时候，在解析标签mapper就是用configuration对象的mapperRegistry存放数据

return mapperRegistry.getMapper(type, sqlSession);

}

@SuppressWarnings("unchecked")

public T getMapper(Class type, SqlSession sqlSession) {

//knownMapper是一个HashMap在存放mapperRegistry的过程中，以每个Mapper对象的类型为Key, MapperProxyFactory 为value保存。

//例如本例中保存的就是Key:com.ctc.mapper.UserMapper,value就是保存了key的MapperProxyFactory对象

final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) 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);

}

}

public T newInstance(SqlSession sqlSession) {

//生成一个mapperProxy对象，这个对象实现了InvocationHandler, Serializable。就是JDK动态代理中的方法调用处理器

final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);

return newInstance(mapperProxy);

}

public MapperProxy(SqlSession sqlSession, Class mapperInterface, Map methodCache) {

this.sqlSession = sqlSession;

this.mapperInterface = mapperInterface;

this.methodCache = methodCache;

}

@SuppressWarnings("unchecked")

protected T newInstance(MapperProxy mapperProxy) {

//通过JDK动态代理生成一个Mapper的代理，在本例中的就是UserMapper的代理类，它实现了UserMapper接口

return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);

}

第四步通过MapperProxy调用Maper中相应的方法：

@Override

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

//判断当前调用的method是不是Object中声明的方法，如果是的话直接执行。

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);

}

//把当前请求放入一个HashMap中，一旦下次还是同样的方法进来直接返回。

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;

}

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 {

//本次案例会执行selectOne

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;

}

@Override

public T selectOne(String statement, Object parameter) {

// Popular vote was to return null on 0 results and throw exception on too many.

List list = this.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 List 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();

}

}

//这边调用的是CachingExecutor类的query,还记得前文解析mybatis-config.xml的时候我们指定了REUSE但是因为在配置文件中开启了缓存

//所以ReuseExecutor被CachingExecotur装饰，新增了缓存的判断，最后还是会调用ReuseExecutor

@Override

public List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {

BoundSql boundSql = ms.getBoundSql(parameterObject);

CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);

return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);

}

@Override

public List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)

throws SQLException {

Cache cache = ms.getCache();

if (cache != null) {

flushCacheIfRequired(ms);

if (ms.isUseCache() && resultHandler == null) {

ensureNoOutParams(ms, parameterObject, boundSql);

@SuppressWarnings("unchecked")

List list = (List) tcm.getObject(cache, key);

if (list == null) {

//如果缓存中没有数据则查询数据库

list = delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);

//结果集放入缓存

tcm.putObject(cache, key, list); // issue #578 and #116

}

return list;

}

}

return delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);

}