Mybatis执行SQL命令的流程分析

目录

• MapperProxy的功能：
• MapperMethod的功能：

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<Student> 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：

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<Method, MapperMethod> 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 = exe编程客栈cuteForMany(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;

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org.apache.ibatis.binding.MapperMethod.MethodSignature：

public static class MethodSignature {
    private f开发者_JAV培训inal 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<Integer, String> params;
    private final boolean hasNamedParameters;
 
    public MENzurbBeethodSignature(Configuration configuration, Method method) {
      this.returnType = method.getReturnType();
      this.returnjssVoid = void.class.equals(this.returnType);
      this.returnsMany = (configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray());
      this.mapKey = getMapKey(method);
      this.returnsMap = (this.mapKey != null);
      thipythons.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));
    }

以上是对MapperMethod的补充说明。 

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