MyBATIS插件原理第二篇 Mapper运行原理

我们目前在MyBATIS中，我们知道MyBATIS的Mapper是一个接口，而不是一个实体类。在Java中接口是没有办法运行的。那么它是怎么运行的呢？

有了第一篇的基础，我们可以大胆的想象——它是通过动态代理运行，没有错真实的情况就是这样的。

让我们看看mybatis是怎么实现这个动态代理的：

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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;
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 {
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);
}

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;
}

}

这便是mybaitis处理对象的方法，我们可以看到invoke方法。我们知道一旦mapper是一个代理对象，那么它就会运行到invoke方法里面，invoke首先判断是否一个类，显然这里mapper是一个接口，不是类所以判定失败。那么跟着就会生成MapperMethod对象，它是通过cachedMapperMethod方法对其初始化的，然后执行execute方法，把sqlSession和当前运行的参数传递进去。

于是让我们看看这个execute方法的源码：

package org.apache.ibatis.binding;

public class MapperMethod {

private final SqlCommand command;
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);
}

public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
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()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
<span style="color:#FF0000;">result = executeForMany(sqlSession, args);//我们主要看看这个方法</span>
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
} else if (SqlCommandType.FLUSH == command.getType()) {
result = sqlSession.flushStatements();
} 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;
}
 ........
//方法还是很多，我们不需要全看就看一个很常用的查询返回多条记录的
private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
    List<E> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<E>selectList(command.getName(), param, rowBounds);
    } else {
      <span style="color:#FF0000;">result = sqlSession.<E>selectList(command.getName(), param);</span>
    }
    // issue #510 Collections & arrays support
    if (!method.getReturnType().isAssignableFrom(result.getClass())) {
      if (method.getReturnType().isArray()) {
        return convertToArray(result);
      } else {
        return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
      }
    }
    return result;
  }
.......
 }

好这里我们看到，MapperMethod 类采用命令模式运行，然后根据上下文跳转可能跳转到许多方法中取，我们不需要全部明白，我们可以看到里面的executeForMany方法，再看看它的实现，实际上它最后就是通过sqlSession对象去运行对象的SQL而已。

至此，相信大家已经了解了MyBATIS为什么只用mappper接口便能够运行sql，因为mapperd的xml文件的命名空间对应的便是这个接口全路径，那么它根据全路径和方法名，便能够绑定起来，通过动态代理技术，让这个接口跑起来。