拼多多二面：Mybatis是如何执行一条SQL命令的？-技术圈

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阅读本文大概需要 3 分钟。

Mybatis中的Sql命令，在枚举类SqlCommandType中定义的。

public enum SqlCommandType {  UNKNOWN, INSERT, UPDATE, DELETE, SELECT, FLUSH;}

下面，我们以Mapper接口中的一个方法作为例子，看看Sql命令的执行完整流程。

public interface StudentMapper {  List findAllStudents(Map<String, Object> map, RowBounds rowBounds, ResultSetHandler rh);  }

参数RowBounds和ResultSetHandler是可选参数，表示分页对象和自定义结果集处理器，一般不需要。

一个完整的Sql命令，其执行的完整流程图如下：

（Made In Edrawmax）

对于上面的流程图，如果看过前面的文章的话，大部分对象我们都比较熟悉了。一个图，就完整展示了其执行流程。

MapperProxy的功能：

1. 因为Mapper接口不能直接实例化，MapperProxy的作用，就是使用JDK动态代理功能，间接实例化Mapper的proxy对象。可参看系列的第二篇。

2. 缓存MapperMethod对象。

  private final Map 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);      }    }    // 投鞭断流    final MapperMethod mapperMethod = cachedMapperMethod(method);    return mapperMethod.execute(sqlSession, args);  }
  // 缓存MapperMethod  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;  }

MapperMethod的功能：

1. 解析Mapper接口的方法，并封装成MapperMethod对象。

2. 将Sql命令，正确路由到恰当的SqlSession的方法上。

public class MapperMethod {
  // 保存了Sql命令的类型和键id  private final SqlCommand command;  // 保存了Mapper接口方法的解析信息  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);  }
  // 根据解析结果，路由到恰当的SqlSession方法上  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()) {        result = executeForMany(sqlSession, args);      } 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;  }  // ...

org.apache.ibatis.binding.MapperMethod.SqlCommand。

public static class SqlCommand {    // full id, 通过它可以找到MappedStatement    private final String name;    private final SqlCommandType type;// ...

org.apache.ibatis.binding.MapperMethod.MethodSignature。


  public static class MethodSignature {    private final boolean returnsMany;    private final boolean returnsMap;    private final boolean returnsVoid;    private final Class returnType;    private final String mapKey;    private final Integer resultHandlerIndex;    private final Integer rowBoundsIndex;    private final SortedMap params;    private final boolean hasNamedParameters;
    public MethodSignature(Configuration configuration, Method method) {      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);      // 分页参数      this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);      // 自定义ResultHandler      this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);      this.params = Collections.unmodifiableSortedMap(getParams(method, this.hasNamedParameters));    }