Mybatis源码解析-MapperRegistry注册mapper接口

知识储备

SqlsessionFactory-mybatis持久层操作数据的根本，具体的解析是通过

SqlSessionFactoryBean

生成的，具体的形成可见>>>Spring mybatis源码篇章-SqlSessionFactoryBean

MapperInterface-mybatis的java接口类，用于service/controller层的调用，具体的解析是通过

MapperScannerConfigurer

扫描接口并封装成

MapperFactoryBean

来注册生成，可见>>>Spring mybatis源码篇章-sql mapper配置文件绑定mapper class类

保存mybatis的各种信息都是由

org.apache.ibatis.session.Configuration

类来维护的

MapperRegistry

由于mybatis与数据库进行通信需要保证其java访问类必须为接口类，所以我们必须了解其是怎么保存这些接口类的访问方式，入口由

Configuration#addMapper()

方法调用，如下述

MapperFactoryBean#checkConfig()

源码

@Override
protected void checkDaoConfig() {
super.checkDaoConfig();

notNull(this.mapperInterface, "Property 'mapperInterface' is required");

Configuration configuration = getSqlSession().getConfiguration();
if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
try {
//只关注此处的添加mapper接口
configuration.addMapper(this.mapperInterface);
} catch (Throwable t) {
throw new IllegalArgumentException(t);
} finally {
ErrorContext.instance().reset();
}
}
}

MapperRegistry#addMapper()-创建mapper访问代理

直接阅读源码

public <T> void addMapper(Class<T> type) {
if (type.isInterface()) {
if (hasMapper(type)) {
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
//将mapper接口包装成mapper代理
knownMappers.put(type, new MapperProxyFactory<T>(type));
//解析接口上的注解或者加载mapper配置文件生成mappedStatement
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}

看下关于mapper的代理如何生成

MapperProxyFactory

内部属性概览

//被代理类
private final Class<T> mapperInterface;
//支持对被代理类进行缓存
private Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();

创建代理类

@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
//采用JDK自带的Proxy代理模式生成
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}

public T newInstance(SqlSession sqlSession) {
//MapperProxy为InvocationHandler的实现类
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
//真实生成代理
return newInstance(mapperProxy);
}

接下来阅读下MapperProxy是怎么运用反射调用mapper接口类的方法

MapperProxy

直接看对JDK的proxy代理的实现方法

@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);
}
}
//尝试从缓存中获取，也就是看到的methodCache
final MapperMethod mapperMethod = cachedMapperMethod(method);
//通过MapperMethod对象调用方法
return mapperMethod.execute(sqlSession, args);
}

private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
//注意此处的传的参数为mapper接口类、method对象、Configuration对象
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}

需要观察下MapperMethod是如何操作数据的

MapperMethod

构造函数

//Sql指令类
private final SqlCommand command;
//方法签名类
private final MethodSignature method;

public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
this.command = new SqlCommand(config, mapperInterface, method);
this.method = new MethodSignature(config, method);
}

主要方法execute()

public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//CURD操作，对持久层返回的结果集进行处理
if (SqlCommandType.INSERT == command.getType()) {
//获取method方法上的带有@Param的参数，默认返回0,1,2,3...
Object param = method.convertArgsToSqlCommandParam(args);
//最终是通过sqlSession接口对象获取结果集，注意此处的command.getName()为mappedStatementId
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()) {
//查询语句的各种情况应对
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());
}
return result;
}

SqlCommand是MapperMethod的静态内部类，主要通过mapperInterface和method从Configuration中获取MappedStatement对象保存其id和type属性，供sqlsession持久层接口调用

MethodSignature是MappedMethod的静态内部类，主要对method对象的返回类型、参数等进行归类

MapperMethod的作用是处理SqlSession接口调用CRUD操作后产生的结果集

MapperRegistry#getMapper()-获取mapper代理类

源码如下

@SuppressWarnings("unchecked")
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 {
//获取mapper代理类
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}

那么获取代理类这个操作是如何被调用的呢？这其实是bean工厂对其里面所有的beanDefinition进行实例化调用的，这点可查看>>>Spring源码情操陶冶-AbstractApplicationContext#finishBeanFactoryInitialization。而针对

FactoryBean

接口的实例化会调用其中的

getObject()

方法，所以我们看下

MapperFactoryBean#getObject()

方法

public T getObject() throws Exception {
//通过sqlSessionTemplate调用Configuration#getMapper()方法间接调用MapperRegistry#getMapper()
return getSqlSession().getMapper(this.mapperInterface);
}

小结

本文是对

MapperFactoryBean#checkDaoConfig()

方法的补充，通过对Mybatis的

MapperRegistry

的分析我们可以得出以下结论：

mapperInterface的内部方法数据持久层访问是通过JDK的代理来完成的

MapperMethod是对上述代理的method方法的真实处理，主要是对sqlSession的返回结果集进行对应的整理输出，具体读者可自行查阅分析

sqlSession的CRUD肯定牵扯到MappedStatement对象的使用，后续我们着重分析下SqlSessionTemplate的源码加深我们对mybatis的理解