作者：钟昕灵，叩丁狼教育高级讲师
相信只要是使用过MyBatis开发的同学,都有使用其中的Mapper接口来开发,因为确实是很方便,方便到我们只需要编写接口而不需要写实现类,就能够完成对数据库的CRUD操作,但是不知道大家有没有去思考过,如果我们真的只有Mapper接口的话,程序又是如何去完成实际的业务的呢?来看看下面的代码
cn.wolfcode.mybatis.mapper.UserMapper接口
public interface UserMapper {
void save(User u);
}
UserMapper.xml映射文件
INSERT INTO user (id, username, password) VALUES (NULL, #{username}, #{password})
UserServiceImpl业务方法
public void save(User u) throws IOException {
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("mybatis-config.xml"));
SqlSession session = factory.openSession();
UserMapper mapper = session.getMapper(UserMapper.class);
mapper.save(u);
session.commit();
session.close();
}
从上面的代码中可以看出,我们只需要给MyBatis提供Mapper接口和与之匹配的映射文件,就能够让MyBatis按照我们的需求执行到对应的SQL
这里的实现原理就是我们前面所讲过的 动态代理,接下来我们看一波源码
通过debug断点调试,我们可以依次看到下面的代码
DefaultSqlSession:
public T getMapper(Class type) {
return this.configuration.getMapper(type, this);
}
Configuration:
public T getMapper(Class type, SqlSession sqlSession) {
return this.mapperRegistry.getMapper(type, sqlSession);
}
MapperRegistry:
public T getMapper(Class type, SqlSession sqlSession) {
MapperProxyFactory mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
if(mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
} else {
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception var5) {
throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
}
}
}
该方法中最关键代码:mapperProxyFactory.newInstance(sqlSession);
MapperProxyFactory是一个创建MapperProxy的工厂类,调用其中的newInstance方法可以获取到一个代理对象,继续往下看
MapperProxyFactory:
protected T newInstance(MapperProxy mapperProxy) {
return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
MapperProxy mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
return this.newInstance(mapperProxy);
}
在该类中可以看到, Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
最终由JDK的动态代理,动态的为我们在内存中创建了一个代理对象
到此,我们已经看到了一部分真相,就是我们为mybatis提供Mapper接口,而mybatis使用JDK的动态代理为我们生成实现类
相信大家和我一样,还想继续了解一下,在这个代理类中具体为我们做了什么,那好,我们继续
如果大家了解JDK的动态代理的话,那么就应该知道我们现在最关心的应该是InvocationHandler的实现,从上面的代码中可以看到,它叫做MapperProxy
MapperProxy:
public class MapperProxy implements InvocationHandler, Serializable {
private static final long serialVersionUID = -6424540398559729838L;
private final SqlSession sqlSession;
private final Class mapperInterface;
private final Map methodCache;
public MapperProxy(SqlSession sqlSession, Class mapperInterface, Map methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if(Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else {
MapperMethod mapperMethod = this.cachedMapperMethod(method);
return mapperMethod.execute(this.sqlSession, args);
}
}
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = (MapperMethod)this.methodCache.get(method);
if(mapperMethod == null) {
mapperMethod = new MapperMethod(this.mapperInterface, method, this.sqlSession.getConfiguration());
this.methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
}
在invoke方法中可以看到,如果我们调用的是Object中的方法,不做任何处理,直接调用,否则执行:
mapperMethod.execute(this.sqlSession, args);
MapperMethod:
public Object execute(SqlSession sqlSession, Object[] args) {
Object param;
Object result;
if(SqlCommandType.INSERT == this.command.getType()) {
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.insert(this.command.getName(), param));
} else if(SqlCommandType.UPDATE == this.command.getType()) {
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.update(this.command.getName(), param));
} else if(SqlCommandType.DELETE == this.command.getType()) {
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.delete(this.command.getName(), param));
} else {
if(SqlCommandType.SELECT != this.command.getType()) {
throw new BindingException("Unknown execution method for: " + this.command.getName());
}
if(this.method.returnsVoid() && this.method.hasResultHandler()) {
this.executeWithResultHandler(sqlSession, args);
result = null;
} else if(this.method.returnsMany()) {
result = this.executeForMany(sqlSession, args);
} else if(this.method.returnsMap()) {
result = this.executeForMap(sqlSession, args);
} else {
param = this.method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(this.command.getName(), param);
}
}
if(result == null && this.method.getReturnType().isPrimitive() && !this.method.returnsVoid()) {
throw new BindingException("Mapper method '" + this.command.getName()
+ " attempted to return null from a method with a primitive return type (" + this.method.getReturnType() + ").");
} else {
return result;
}
}
主要将SQL分为两类执行,DML和SQL
如果是DML,调用SQLSession中对应的方法执行,并且使用rowCountResult方法根据方法的返回值和受影响的行数做处理
如果是查询,则要根据方法的返回值的类型来执行不同的方法
如果Collection系的集合获取数组来接收,使用selectList方法执行查询
如果使用Map集合,调用selectMap方法执行查询
否则,调用selectOne执行查询
相信,源码看到这里,大家心里应该很清楚MyBatis中Mapper接口的使用原理了
最后总结一下:
MapperProxyFactory中,使用JDK的动态代理生成Mapper接口的代理代理类
由动态处理器MapperProxy中调用MapperMethod中的方法处理执行SQL
最后,在MapperMethod中根据执行的方法返回值决定调用SqlSession中的对应方法执行SQL
