聊聊Mybatis的动态Sql之SqlSource

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 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