聊聊Mybatis的动态Sql之SqlSource

小咪咪 2024-04-08 10:46 83阅读 0赞

### 聊聊Mybatis的动态Sql之SqlSource ###

#### 构建SqlSource对象 ####

当Mapper.xml的各个标签被解析后SqlNode，然后SqlSourceBuilder进一步处理，

public SqlSource parse(String originalSql, Class<?> parameterType, Map<String, Object> additionalParameters) {
        ParameterMappingTokenHandler handler = new ParameterMappingTokenHandler(configuration, parameterType, additionalParameters);
        GenericTokenParser parser = new GenericTokenParser("#{", "}", handler);
        String sql;
        if (configuration.isShrinkWhitespacesInSql()) {
          sql = parser.parse(removeExtraWhitespaces(originalSql));
        } else {
          sql = parser.parse(originalSql);
        }
        return new StaticSqlSource(configuration, sql, handler.getParameterMappings());
      }

1.  ParameterMappingTokenHandler是静态内部类，它用来保存每个占位符参数解析后的结果
2.  创建识别\#\{\}占位符的解析器GenericTokenParser，解析sql，最终形成StaticSqlSource对象

#### SqlSource接口 ####

SqlSource接口是用来创建被数据库执行的sql,它只有一个getBoundSql()方法

public interface SqlSource {
    
      BoundSql getBoundSql(Object parameterObject);
    
    }

实现类有DynamicSqlSource、StaticSqlSource、RawSqlSource、ProviderSqlSource、VelocitySqlSource，这里重点说一下前三个实现类

#### 解析动态Sql类 ####

DynamicSqlSource是解析动态sql的类

public class DynamicSqlSource implements SqlSource {
    
      private final Configuration configuration;
      private final SqlNode rootSqlNode;
    
      public DynamicSqlSource(Configuration configuration, SqlNode rootSqlNode) {
        this.configuration = configuration;
        this.rootSqlNode = rootSqlNode;
      }
    
      @Override
      public BoundSql getBoundSql(Object parameterObject) {
        DynamicContext context = new DynamicContext(configuration, parameterObject);
        rootSqlNode.apply(context);
        SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
        Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
        SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());
        BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
        context.getBindings().forEach(boundSql::setAdditionalParameter);
        return boundSql;
      }
    
    }

它的getBoundSql()方法中：

1.  创建DynamicContext对象
2.  调用SqlNode的apply()方法完成对sql片段的解析
3.  创建SqlSourceBuilder对象，调用parse()方法进行解析\#\{\}，替换成？占位符，返回StaticSqlSource对象，
4.  调用StaticSqlSource的getBoundSql()方法，返回BoundSql对象，这里存储着sql语句的相关信息
5.  返回BoundSql对象

#### 解析静态SQL类 ####

RawSqlSource是解析静态sql文件的，在程序启动的时候就解析了

public class RawSqlSource implements SqlSource {
    
      private final SqlSource sqlSource;
    
      public RawSqlSource(Configuration configuration, SqlNode rootSqlNode, Class<?> parameterType) {
        this(configuration, getSql(configuration, rootSqlNode), parameterType);
      }
    
      public RawSqlSource(Configuration configuration, String sql, Class<?> parameterType) {
        SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
        Class<?> clazz = parameterType == null ? Object.class : parameterType;
        sqlSource = sqlSourceParser.parse(sql, clazz, new HashMap<>());
      }
    
      private static String getSql(Configuration configuration, SqlNode rootSqlNode) {
        DynamicContext context = new DynamicContext(configuration, null);
        rootSqlNode.apply(context);
        return context.getSql();
      }
    
      @Override
      public BoundSql getBoundSql(Object parameterObject) {
        return sqlSource.getBoundSql(parameterObject);
      }
    
    }

它的构造方法调用了getSql()方法，这个方法里调用SqlSource的apply()方法组装成完整sql，然后通过SqlSourceBuilder调用parse()方法处理\#\{\}占位符，返回StaticSqlSource对象

#### StaticSqlSource ####

StaticSqlSource类：

public class StaticSqlSource implements SqlSource {
      @Override
      public BoundSql getBoundSql(Object parameterObject) {
        return new BoundSql(configuration, sql, parameterMappings, parameterObject);
      }
    
    }

它的getBoundSql()方法就是创建BoundSql对象

#### 总结 ####

这篇文章讲了SqlSource的接口和它的几个实现类，其中DynamicSqlSource类和RawSqlSource类最最终生成StaticSqlSource类，由StaticSqlSource类调用getBoundSql()方法来创建BoundSql类，DynamicSqlSource是解析动态sql的类，RawSqlSource是解析静态sql的类，在程序启动的时候就生成sql了