MyBatis SQL映射

深入理解MyBatis的SQL映射机制，掌握SQL语句的解析和执行流程，理解参数映射、结果映射等核心功能。

📋 SQL映射概览

SQL映射是MyBatis的核心功能，负责将Java方法调用转换为SQL语句执行。

🎯 一、SQL解析

1.1 XMLMapperBuilder

XMLMapperBuilder：

public class XMLMapperBuilder extends BaseBuilder {
    
    public void parse() {
        if (!configuration.isResourceLoaded(resource)) {
            configurationElement(parser.evalNode("/mapper"));
            configuration.addLoadedResource(resource);
            bindMapperForNamespace();
        }
    }
}

1.2 MappedStatement

MappedStatement：

public final class MappedStatement {
    private String resource;
    private Configuration configuration;
    private String id;
    private SqlSource sqlSource;
    private SqlCommandType sqlCommandType;
}

🎯 二、参数映射

2.1 参数处理

ParameterHandler：

public interface ParameterHandler {
    Object getParameterObject();
    void setParameters(PreparedStatement ps) throws SQLException;
}

2.2 参数绑定

DefaultParameterHandler：

@Override
public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters")
            .object(mappedStatement.getParameterMap().getId());
    List<ParameterMapping> parameterMappings = 
            boundSql.getParameterMappings();
    if (parameterMappings != null) {
        for (int i = 0; i < parameterMappings.size(); i++) {
            ParameterMapping parameterMapping = parameterMappings.get(i);
            if (parameterMapping.getMode() != ParameterMode.OUT) {
                Object value;
                String propertyName = parameterMapping.getProperty();
                if (boundSql.hasAdditionalParameter(propertyName)) {
                    value = boundSql.getAdditionalParameter(propertyName);
                } else if (parameterObject == null) {
                    value = null;
                } else if (typeHandlerRegistry.hasTypeHandler(
                        parameterObject.getClass())) {
                    value = parameterObject;
                } else {
                    MetaObject metaObject = configuration.newMetaObject(
                            parameterObject);
                    value = metaObject.getValue(propertyName);
                }
                TypeHandler typeHandler = parameterMapping.getTypeHandler();
                JdbcType jdbcType = parameterMapping.getJdbcType();
                if (value == null && jdbcType == null) {
                    jdbcType = configuration.getJdbcTypeForNull();
                }
                try {
                    typeHandler.setParameter(ps, i + 1, value, jdbcType);
                } catch (TypeException | SQLException e) {
                    throw new TypeException(...);
                }
            }
        }
    }
}

🎯 三、SQL执行

3.1 Executor

Executor接口：

public interface Executor {
    <E> List<E> query(MappedStatement ms, Object parameter, 
            RowBounds rowBounds, ResultHandler resultHandler) 
            throws SQLException;
    int update(MappedStatement ms, Object parameter) throws SQLException;
}

3.2 SimpleExecutor

SimpleExecutor.query：

@Override
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();
        StatementHandler handler = configuration.newStatementHandler(
                wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
        stmt = prepareStatement(handler, ms.getStatementLog());
        return handler.query(stmt, resultHandler);
    } finally {
        closeStatement(stmt);
    }
}

📖 学习资源

视频教程

• MyBatis SQL映射源码解析

书籍推荐

• 《MyBatis技术内幕》 - 徐郡明

网站资源

• MyBatis官方文档

✅ 学习检查清单

• [ ] 已理解SQL解析机制
• [ ] 已理解参数映射
• [ ] 已理解SQL执行流程
• [ ] 已总结核心知识点

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