MyBatis从入门到精通(八)—源码剖析之Mapper代理方式细节

具体代码如下：

//前三步都相同InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);SqlSession sqlSession = sqlSessionFactory.openSession();//这⾥不再调⽤SqlSession的api,⽽是获得了接⼝对象，调⽤接⼝中的⽅法。UserMapper mapper = sqlSession.getMapper(UserMapper.class);//模拟ids的数据List<Integer> ids = new ArrayList<Integer>();ids.add(1);ids.add(2);List<User> userList = mapper.findByIds(ids);

思考⼀个问题，通常的Mapper接⼝我们都没有实现的⽅法却可以使⽤，是为什么呢？
答案很简单：动态代理

public class Configuration {      protected final MapperRegistry mapperRegistry = new MapperRegistry(this);}public class MapperRegistry {  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();}

开始之前介绍⼀下MyBatis初始化时对接⼝的处理：MapperRegistry是Configuration中的⼀个属性，它内部维护⼀个HashMap⽤于存放mapper接⼝的⼯⼚类，每个接⼝对应⼀个⼯⼚类。mappers中可以配置接⼝的包路径，或者某个具体的接⼝类。

<mappers>  <mapper class="com.zjq.mapper.UserMapper"/>  <package name="com.zjq.mapper"/></mappers>

当解析mappers标签时，它会判断解析到的是mapper配置⽂件时，会再将对应配置⽂件中的增删改查标签 封装成MappedStatement对象，存⼊mappedStatements中。(上⽂介绍了)当判断解析到接⼝时，会建此接⼝对应的MapperProxyFactory对象，存⼊HashMap中，key =接⼝的字节码对象，value =此接⼝对应MapperProxyFactory对象。

源码剖析-getmapper()

进⼊ sqlSession.getMapper(UserMapper.class )中

public interface SqlSession extends Closeable {      //调用configuration中的getMapper      <T> T getMapper(Class<T> type);}public class DefaultSqlSession implements SqlSession {  @Override  public <T> T getMapper(Class<T> type) {    //调用configuration 中的getMapper    return configuration.<T>getMapper(type, this);  }}public class Configuration {  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {    //调用MapperRegistry 中的getMapper    return mapperRegistry.getMapper(type, sqlSession);  }}public class MapperRegistry {  @SuppressWarnings("unchecked")  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {    //从 MapperRegistry 中的 HashMap 中拿 MapperProxyFactory    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);    if (mapperProxyFactory == null) {      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");    }    try {      //调用MapperProxyFactory的newInstance通过动态代理⼯⼚⽣成实例      return mapperProxyFactory.newInstance(sqlSession);    } catch (Exception e) {      throw new BindingException("Error getting mapper instance. Cause: " + e, e);    }  }}public class MapperProxyFactory<T> {  public T newInstance(SqlSession sqlSession) {    //创建了 JDK动态代理的Handler类    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);    //调⽤了重载⽅法    return newInstance(mapperProxy);  }     @SuppressWarnings("unchecked")  protected T newInstance(MapperProxy<T> mapperProxy) {    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);  }}//MapperProxy 类，实现了 InvocationHandler 接⼝public class MapperProxy<T> implements InvocationHandler, Serializable {  //省略部分源码  private final SqlSession sqlSession;  private final Class<T> mapperInterface;  private final Map<Method, MapperMethod> methodCache;      //构造，传⼊了 SqlSession，说明每个session中的代理对象的不同的！    public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {    this.sqlSession = sqlSession;    this.mapperInterface = mapperInterface;    this.methodCache = methodCache;  }     //省略部分源码}

源码剖析-invoke()

在动态代理返回了示例后，我们就可以直接调⽤mapper类中的⽅法了，但代理对象调⽤⽅法，执⾏是在MapperProxy中的invoke⽅法中。

public class MapperProxy<T> implements InvocationHandler, Serializable {   @Override  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {    try {      //如果是Object定义的⽅法，直接调⽤      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);    }    // 获得 MapperMethod 对象    final MapperMethod mapperMethod = cachedMapperMethod(method);    //重点在这：最终调⽤了MapperMethod执⾏的⽅法    return mapperMethod.execute(sqlSession, args);  }}

进⼊execute⽅法：

public class MapperMethod {   public Object execute(SqlSession sqlSession, Object[] args) {    Object result;    //判断mapper中的⽅法类型，最终调⽤的还是SqlSession中的⽅法 switch(command.getType())     switch (command.getType()) {      case INSERT: { //转换参数 Object param = method.convertArgsToSqlCommandParam(args); //执⾏INSERT操作 //转换rowCount result = rowCountResult(sqlSession.insert(command.getName(), param)); break;      }      case UPDATE: { //转换参数 Object param = method.convertArgsToSqlCommandParam(args); // 转换 rowCount result = rowCountResult(sqlSession.update(command.getName(), param)); break;      }      case DELETE: { //转换参数 Object param = method.convertArgsToSqlCommandParam(args); // 转换 rowCount result = rowCountResult(sqlSession.delete(command.getName(), param)); break;      }      case SELECT: //⽆返回，并且有ResultHandler⽅法参数，则将查询的结果，提交给 ResultHandler 进⾏处理 if (method.returnsVoid() && method.hasResultHandler()) {   executeWithResultHandler(sqlSession, args);   result = null; //执⾏查询，返回列表 } else if (method.returnsMany()) {   result = executeForMany(sqlSession, args); //执⾏查询，返回Map } else if (method.returnsMap()) {   result = executeForMap(sqlSession, args); //执⾏查询，返回Cursor } else if (method.returnsCursor()) {   result = executeForCursor(sqlSession, args); //执⾏查询，返回单个对象 } else {   //转换参数   Object param = method.convertArgsToSqlCommandParam(args);   //查询单条   result = sqlSession.selectOne(command.getName(), param); } break;      case FLUSH: result = sqlSession.flushStatements(); break;      default: throw new BindingException("Unknown execution method for: " + command.getName());    }    //返回结果为null，并且返回类型为基本类型，则抛出BindingException异常    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() + ").");    }    //返回结果    return result;  }}