从上文<MyBatis框架中Mapper映射配置的使用及原理解析(六) MapperRegistry> 中我们知道DefaultSqlSession的getMapper方法，最后是通过MapperRegistry对象获得Mapper实例：
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) { //说明这个Mapper接口没有注册
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try { //生成一个MapperProxy对象
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
从代码中我们看到试图从一个叫knownMappers的变量取出MapperProxyFactory。
我们看看这个knownMapper在MapperRegistry中的定义：
private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();
有getMapper方法，那么必然后addMapper方法：
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 {
knownMappers.put(type, new MapperProxyFactory<T>(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.
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}
在<MyBatis框架中Mapper映射配置的使用及原理解析(四) 解析Mapper接口映射xml文件> 文章中，解析玩xml后，绑定命名空间bindMapperForNamespace()方法
//绑定到命名空间
private void bindMapperForNamespace() {
String namespace = builderAssistant.getCurrentNamespace();
if (namespace != null) {
Class<?> boundType = null;
try {
boundType = Resources.classForName(namespace);
} catch (ClassNotFoundException e) {
//ignore, bound type is not required
}
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);
configuration.addMapper(boundType);
}
}
}
}
我们看到
configuration.addMapper(boundType);
正是调用MapperRegistry.addMapper方法
public <T> void addMapper(Class<T> type) {
mapperRegistry.addMapper(type);
}
我们在回过头来看getMapper是如何获得Mapper对象的：
1.先获取MapperProxyFactory
2.再调用MapperProxyFactory对象的newInstance方法获得Mapper。
我们看MapperProxyFactory代码：
public T newInstance(SqlSession sqlSession) {
//创建一个Mapperxy对象，这个方法实现了JDK动态代理中的InvocationHandler接口
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
protected T newInstance(MapperProxy<T> mapperProxy) {
//mapperInterface，说明Mapper接口被代理了，这样子返回的对象就是Mapper接口的子类，方法被调用时会被mapperProxy拦截,也就是执行mapperProxy.invoke()方法
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
这里就是返回的一个代理类实例MapperProxy。
package org.apache.ibatis.binding; import java.io.Serializable;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.util.Map; import org.apache.ibatis.reflection.ExceptionUtil;
import org.apache.ibatis.session.SqlSession; /**
* @author Clinton Begin
* @author Eduardo Macarron
*/
public class MapperProxy<T> implements InvocationHandler, Serializable { private static final long serialVersionUID = -6424540398559729838L;
private final SqlSession sqlSession;
//Mapper接口
private final Class<T> mapperInterface;
/*
* Mapper接口中的每个方法都会生成一个MapperMethod对象, methodCache维护着他们的对应关系
*/
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;
}
//这里会拦截Mapper接口的所有方法
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (Object.class.equals(method.getDeclaringClass())) { //如果是Object中定义的方法，直接执行。如toString(),hashCode()等
try {
return method.invoke(this, args);//
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
final MapperMethod mapperMethod = cachedMapperMethod(method); //其他Mapper接口定义的方法交由mapperMethod来执行
return mapperMethod.execute(sqlSession, args);
}
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;
}
}
要使用Java的动态代理就必须得实现InvocationHandler接口:
@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 (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
首先判断代理对象是一个接口还是一个类，显然我们没有对mapper接口进行任何实现，那么它将执行
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
生成一个MapperMethod对象，接着调用其execute方法，把sqlSession和参数传递进去，执行Mapper方法。