MyBatis框架的使用及源码分析(八) MapperMethod

从 <MyBatis框架中Mapper映射配置的使用及原理解析(七) MapperProxy，MapperProxyFactory> 文中，我们知道Mapper，通过MapperProxy代理类执行他的接口方法，当mapper方法被调用的时候对应的MapperProxy会生成相应的MapperMethod并且会缓存起来，这样当多次调用同一个mapper方法时候只会生成一个MapperMethod，提高了时间和内存效率：

//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);
  }

最后2句关键，我们执行所调用Mapper的每一个接口方法，最后返回的是MapperMethod.execute方法。每一个MapperMethod对应了一个mapper文件中配置的一个sql语句或FLUSH配置，对应的sql语句通过mapper对应的class文件名+方法名从Configuration对象中获得。

我们看一下MapperMethod的源码：

package org.apache.ibatis.binding;

import org.apache.ibatis.annotations.MapKey;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.mapping.SqlCommandType;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.session.SqlSession;

import java.lang.reflect.Array;
import java.lang.reflect.Method;
import java.util.*;

/**
 * MapperMethod代理Mapper所有方法
 */
public class MapperMethod {
  //一个内部封 封装了SQL标签的类型 insert update delete select
  private final SqlCommand command;
  //一个内部类 封装了方法的参数信息 返回类型信息等 
  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);
  }

  /**
   * 这个方法是对SqlSession的包装，对应insert、delete、update、select四种操作
   */
public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;//返回结果
　　 //INSERT操作
    if (SqlCommandType.INSERT == command.getType()) {
      //处理参数
      Object param = method.convertArgsToSqlCommandParam(args);
      //调用sqlSession的insert方法 
      result = rowCountResult(sqlSession.insert(command.getName(), param));
    } else if (SqlCommandType.UPDATE == command.getType()) {
      //UPDATE操作 同上
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));
    } else if (SqlCommandType.DELETE == command.getType()) {
      //DELETE操作 同上
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));
    } else if (SqlCommandType.SELECT == command.getType()) {
      //如果返回void 并且参数有resultHandler  ,则调用 void select(String statement, Object parameter, ResultHandler handler);方法  
      if (method.returnsVoid() && method.hasResultHandler()) {
        executeWithResultHandler(sqlSession, args);
        result = null;
      } else if (method.returnsMany()) {
        //如果返回多行结果,executeForMany这个方法调用 <E> List<E> selectList(String statement, Object parameter);   
        result = executeForMany(sqlSession, args);
      } else if (method.returnsMap()) {
        //如果返回类型是MAP 则调用executeForMap方法 
        result = executeForMap(sqlSession, args);
      } else {
        //否则就是查询单个对象
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
    } else {
        //接口方法没有和sql命令绑定
        throw new BindingException("Unknown execution method for: " + command.getName());
    }

    //如果返回值为空 并且方法返回值类型是基础类型 并且不是VOID 则抛出异常  
    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() + ").");
    }
    return result;
  }

  private Object rowCountResult(int rowCount) {
    final Object result;
    if (method.returnsVoid()) {
      result = null;
    } else if (Integer.class.equals(method.getReturnType()) || Integer.TYPE.equals(method.getReturnType())) {
      result = rowCount;
    } else if (Long.class.equals(method.getReturnType()) || Long.TYPE.equals(method.getReturnType())) {
      result = (long) rowCount;
    } else if (Boolean.class.equals(method.getReturnType()) || Boolean.TYPE.equals(method.getReturnType())) {
      result = (rowCount > 0);
    } else {
      throw new BindingException("Mapper method '" + command.getName() + "' has an unsupported return type: " + method.getReturnType());
    }
    return result;
  }

  private void executeWithResultHandler(SqlSession sqlSession, Object[] args) {
    MappedStatement ms = sqlSession.getConfiguration().getMappedStatement(command.getName());
    if (void.class.equals(ms.getResultMaps().get(0).getType())) {
      throw new BindingException("method " + command.getName()
          + " needs either a @ResultMap annotation, a @ResultType annotation,"
          + " or a resultType attribute in XML so a ResultHandler can be used as a parameter.");
    }
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      sqlSession.select(command.getName(), param, rowBounds, method.extractResultHandler(args));
    } else {
      sqlSession.select(command.getName(), param, method.extractResultHandler(args));
    }
  }

  //返回多行结果 调用sqlSession.selectList方法 
  private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
    List<E> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      //如果参数含有rowBounds则调用分页的查询 
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<E>selectList(command.getName(), param, rowBounds);
    } else {
      //没有分页则调用普通查询 
      result = sqlSession.<E>selectList(command.getName(), param);
    }
    // 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;
  }

  private <E> Object convertToDeclaredCollection(Configuration config, List<E> list) {
    Object collection = config.getObjectFactory().create(method.getReturnType());
    MetaObject metaObject = config.newMetaObject(collection);
    metaObject.addAll(list);
    return collection;
  }

  @SuppressWarnings("unchecked")
  private <E> E[] convertToArray(List<E> list) {
    E[] array = (E[]) Array.newInstance(method.getReturnType().getComponentType(), list.size());
    array = list.toArray(array);
    return array;
  }

  private <K, V> Map<K, V> executeForMap(SqlSession sqlSession, Object[] args) {
    Map<K, V> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey(), rowBounds);
    } else {
      result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey());
    }
    return result;
  }

  public static class ParamMap<V> extends HashMap<String, V> {

    private static final long serialVersionUID = -2212268410512043556L;

    @Override
    public V get(Object key) {
      if (!super.containsKey(key)) {
        throw new BindingException("Parameter '" + key + "' not found. Available parameters are " + keySet());
      }
      return super.get(key);
    }

  }

  //封装了具体执行的动作
  public static class SqlCommand {
    //xml标签的id 
    private final String name;
    //insert update delete select的具体类型 
    private final SqlCommandType type;

    public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) throws BindingException {
      //拿到全名 比如 org.mybatis.example.UserMapper.selectByPrimaryKey
      String statementName = mapperInterface.getName() + "." + method.getName();
      //MappedStatement对象,封装一个Mapper接口对应的sql操作 
      MappedStatement ms = null;
      if (configuration.hasStatement(statementName)) {
        //从Configuration对象查找是否有这个方法的全限定名称，如果有则根据方法的全限定名称获取MappedStatement
        ms = configuration.getMappedStatement(statementName);
      } else if (!mapperInterface.equals(method.getDeclaringClass().getName())) { // issue #35
        //如果没有在Configuration对象中找到这个方法，则向上父类中获取全限定方法名
        String parentStatementName = method.getDeclaringClass().getName() + "." + method.getName();
        if (configuration.hasStatement(parentStatementName)) {
          ms = configuration.getMappedStatement(parentStatementName);
        }
      }
      if (ms == null) {
        throw new BindingException("Invalid bound statement (not found): " + statementName);
      }
      //这个ms.getId，其实就是我们在mapper.xml配置文件中配置一条sql语句设置的id属性的值
      name = ms.getId();
      //sql的类型（insert、update、delete、select）
      type = ms.getSqlCommandType();
      if (type == SqlCommandType.UNKNOWN) {
        //判断SQL标签类型 未知就抛异常
        throw new BindingException("Unknown execution method for: " + name);
      }
    }

    public String getName() {
      return name;
    }

    public SqlCommandType getType() {
      return type;
    }
  }

  /**
   * 方法签名,封装了接口当中方法的 参数类型 返回值类型 等信息
   */
  public static class MethodSignature {

    private final boolean returnsMany;//是否返回多条结果 
    private final boolean returnsMap; //返回值是否是MAP 
    private final boolean returnsVoid;//返回值是否是VOID 
    private final Class<?> returnType; //返回值类型 
    private final String mapKey;
    private final Integer resultHandlerIndex;//resultHandler类型参数的位置 
    private final Integer rowBoundsIndex; //rowBound类型参数的位置
    private final SortedMap<Integer, String> params;//用来存放参数信息
    private final boolean hasNamedParameters;  //是否存在命名参数 

    public MethodSignature(Configuration configuration, Method method) throws BindingException {
      this.returnType = method.getReturnType();
      this.returnsVoid = void.class.equals(this.returnType);
      this.returnsMany = (configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray());
      this.mapKey = getMapKey(method);
      this.returnsMap = (this.mapKey != null);
      this.hasNamedParameters = hasNamedParams(method);
      this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
      this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
      this.params = Collections.unmodifiableSortedMap(getParams(method, this.hasNamedParameters));
    }

    /**
     * 创建SqlSession对象需要传递的参数逻辑
     * args是用户mapper所传递的方法参数列表， 如果方法没有参数，则返回null.  
     * 如果方法只包含一个参数并且不包含命名参数， 则返回传递的参数值。
     * 如果包含多个参数或包含命名参数，则返回包含名字和对应值的map对象、
     */
    public Object convertArgsToSqlCommandParam(Object[] args) {
      final int paramCount = params.size();
      if (args == null || paramCount == 0) {
        return null;
      } else if (!hasNamedParameters && paramCount == 1) {
        return args[params.keySet().iterator().next()];
      } else {
        final Map<String, Object> param = new ParamMap<Object>();
        int i = 0;
        for (Map.Entry<Integer, String> entry : params.entrySet()) {
          param.put(entry.getValue(), args[entry.getKey()]);
          // issue #71, add param names as param1, param2...but ensure backward compatibility
          final String genericParamName = "param" + String.valueOf(i + 1);
          if (!param.containsKey(genericParamName)) {
            param.put(genericParamName, args[entry.getKey()]);
          }
          i++;
        }
        return param;
      }
    }

    public boolean hasRowBounds() {
      return (rowBoundsIndex != null);
    }

    public RowBounds extractRowBounds(Object[] args) {
      return (hasRowBounds() ? (RowBounds) args[rowBoundsIndex] : null);
    }

    public boolean hasResultHandler() {
      return (resultHandlerIndex != null);
    }

    public ResultHandler extractResultHandler(Object[] args) {
      return (hasResultHandler() ? (ResultHandler) args[resultHandlerIndex] : null);
    }

    public String getMapKey() {
      return mapKey;
    }

    public Class<?> getReturnType() {
      return returnType;
    }

    public boolean returnsMany() {
      return returnsMany;
    }

    public boolean returnsMap() {
      return returnsMap;
    }

    public boolean returnsVoid() {
      return returnsVoid;
    }

    private Integer getUniqueParamIndex(Method method, Class<?> paramType) {
      Integer index = null;
      final Class<?>[] argTypes = method.getParameterTypes();
      for (int i = 0; i < argTypes.length; i++) {
        if (paramType.isAssignableFrom(argTypes[i])) {
          if (index == null) {
            index = i;
          } else {
            throw new BindingException(method.getName() + " cannot have multiple " + paramType.getSimpleName() + " parameters");
          }
        }
      }
      return index;
    }

    private String getMapKey(Method method) {
      String mapKey = null;
      if (Map.class.isAssignableFrom(method.getReturnType())) {
        final MapKey mapKeyAnnotation = method.getAnnotation(MapKey.class);
        if (mapKeyAnnotation != null) {
          mapKey = mapKeyAnnotation.value();
        }
      }
      return mapKey;
    }

    private SortedMap<Integer, String> getParams(Method method, boolean hasNamedParameters) {
      final SortedMap<Integer, String> params = new TreeMap<Integer, String>();
      final Class<?>[] argTypes = method.getParameterTypes();
      for (int i = 0; i < argTypes.length; i++) {
        if (!RowBounds.class.isAssignableFrom(argTypes[i]) && !ResultHandler.class.isAssignableFrom(argTypes[i])) {
          String paramName = String.valueOf(params.size());
          if (hasNamedParameters) {
            paramName = getParamNameFromAnnotation(method, i, paramName);
          }
          params.put(i, paramName);
        }
      }
      return params;
    }

    private String getParamNameFromAnnotation(Method method, int i, String paramName) {
      final Object[] paramAnnos = method.getParameterAnnotations()[i];
      for (Object paramAnno : paramAnnos) {
        if (paramAnno instanceof Param) {
          paramName = ((Param) paramAnno).value();
        }
      }
      return paramName;
    }

    private boolean hasNamedParams(Method method) {
      boolean hasNamedParams = false;
      final Object[][] paramAnnos = method.getParameterAnnotations();
      for (Object[] paramAnno : paramAnnos) {
        for (Object aParamAnno : paramAnno) {
          if (aParamAnno instanceof Param) {
            hasNamedParams = true;
            break;
          }
        }
      }
      return hasNamedParams;
    }

  }

}

当执行MapperMethod的execute方法的时候，根据当前MapperMethod对应的mapper配置会执行Session的insert, update, delete, select, selectList, selectMap, selectCursor, selectOne或flushStatements方法。
具体执行Session对象的方法对照如下：

Mapper节点	SqlSession方法
insert	insert
update	update
delete	delete
select	select: 方法返回void,并且包含resultHandler配置
select	selectList:方法返回数组或Collection子类
select	selectMap: 存在MapKey注解
select	selectCursor: 方法返回Cursor
select	selectOne 其它
flush	Flush注解