详解MyBatis Mapper 代理实现数据库调用原理

public class MapperProxy<T> implements InvocationHandler, Serializable {
 private final SqlSession sqlSession;
 private final Class<T> mapperInterface;
 private final Map<Method, MapperMethod> methodCache;

 public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> 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())) {
   try {
    return method.invoke(this, args);
   } catch (Throwable t) {
    throw ExceptionUtil.unwrapThrowable(t);
   }
  }
  // 接口里声明的方法，转换为 MapperMethod 来调用
  final MapperMethod mapperMethod = cachedMapperMethod(method);

  // 与 Spring 集成时此处的 sqlSession 仍然 SqlSessionTemplate
  return mapperMethod.execute(sqlSession, args);
 }

 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;
 }
}

public Object execute(SqlSession sqlSession, Object[] args) {
 Object result;
 if (SqlCommandType.INSERT == command.getType()) {
  // 把参数转换为 SqlSession 能处理的格式
  Object param = method.convertArgsToSqlCommandParam(args);

  // 在 sqlSession 上执行并处理结果
  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));
 ...省略

public class DefaultSqlSession implements SqlSession {
 private Configuration configuration;
 private Executor executor;

 private boolean autoCommit;
 private boolean dirty;
 private List<Cursor<?>> cursorList;

 public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
  this.configuration = configuration;
  this.executor = executor;
  this.dirty = false;
  this.autoCommit = autoCommit;
 }

 public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
  try {
   MappedStatement ms = configuration.getMappedStatement(statement);
   return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
  } catch (Exception e) {
   throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
  } finally {
   ErrorContext.instance().reset();
  }
 }

// SimpleExecutor
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
 Statement stmt = null;
 try {
  Configuration configuration = ms.getConfiguration();

  // 创建 handler 来负责具体的执行
  StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);

  // 创建数据库语句
  stmt = prepareStatement(handler, ms.getStatementLog());

  // 执行数据库操作
  return handler.<E>query(stmt, resultHandler);
 } finally {
  closeStatement(stmt);
 }
}

// Configuration
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
 StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
 statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
 return statementHandler;
}

// RoutingStatementHandler
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
 // 根据SQL语句的执行方式创建对应的 handler 实例
 switch (ms.getStatementType()) {
  case STATEMENT:
  delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
  break;
  case PREPARED:
  delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
  break;
  case CALLABLE:
  delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
  break;
  default:
  throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
 }
}

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
 // 创建数据库连接
 Connection connection = getConnection(statementLog);

 // 创建数据库语句
 Statement stmt = handler.prepare(connection, transaction.getTimeout());

 // 参数化设置
 handler.parameterize(stmt);
 return stmt;
}

protected Connection getConnection(Log statementLog) throws SQLException {
 Connection connection = transaction.getConnection();
 if (statementLog.isDebugEnabled()) {
  return ConnectionLogger.newInstance(connection, statementLog, queryStack);
 } else {
  return connection;
 }
}

// BaseStatementHandler
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
 ErrorContext.instance().sql(boundSql.getSql());
 Statement statement = null;
 try {
  // 由具体的子类来创建对应的 Statement 实例
  statement = instantiateStatement(connection);

  // 通用参数设置
  setStatementTimeout(statement, transactionTimeout);
  setFetchSize(statement);
  return statement;
 } catch (SQLException e) {
  closeStatement(statement);
  throw e;
 } catch (Exception e) {
  closeStatement(statement);
  throw new ExecutorException("Error preparing statement. Cause: " + e, e);
 }
}

// PreparedStatementHandler
protected Statement instantiateStatement(Connection connection) throws SQLException {
 String sql = boundSql.getSql();
 if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
  String[] keyColumnNames = mappedStatement.getKeyColumns();
  if (keyColumnNames == null) {
   return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
  } else {
   return connection.prepareStatement(sql, keyColumnNames);
  }
 } else if (mappedStatement.getResultSetType() != null) {
  return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
 } else {
  return connection.prepareStatement(sql);
 }
}

// PreparedStatementHandler
public void parameterize(Statement statement) throws SQLException {
 parameterHandler.setParameters((PreparedStatement) statement);
}