public class DefaultParameterHandler implements ParameterHandler {
    private final MappedStatement mappedStatement;
    private final Object parameterObject;
    private final BoundSql boundSql;
    private final Configuration configuration;
    private final TypeHandlerRegistry typeHandlerRegistry;

    public DefaultParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
        this.mappedStatement = mappedStatement;
        this.parameterObject = parameterObject;
        this.boundSql = boundSql;
        this.configuration = mappedStatement.getConfiguration();
        this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    }
}

@Override
public void setParameters(PreparedStatement ps) {
  ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
  // 獲取SQL中的參數(shù)映射列表（每個(gè)元素對(duì)應(yīng)一個(gè)占位符）
  List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
  if (parameterMappings != null) {
    // 遍歷每個(gè)參數(shù)映射，依次綁定到PreparedStatement
    for (int i = 0; i < parameterMappings.size(); i++) {
      ParameterMapping parameterMapping = parameterMappings.get(i);
      // 忽略輸出參數(shù)（存儲(chǔ)過程場(chǎng)景）
      if (parameterMapping.getMode() != ParameterMode.OUT) {
        Object value;
        String propertyName = parameterMapping.getProperty(); // 參數(shù)名稱（如"id"、"user.name"）
        // 獲取參數(shù)值：從原始參數(shù)對(duì)象中解析propertyName對(duì)應(yīng)的 value
        if (boundSql.hasAdditionalParameter(propertyName)) {
          // 從附加參數(shù)中獲取（<foreach>或<bind>標(biāo)簽可以提供額外參數(shù)）
          value = boundSql.getAdditionalParameter(propertyName);
        } else if (parameterObject == null) {
          value = null; // 參數(shù)為null時(shí)直接設(shè)為null
        } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
          // 參數(shù)本身就是TypeHandler可以處理的類型
          value = parameterObject;
        } else {
          // 參數(shù)是復(fù)雜對(duì)象，通過反射獲取propertyName對(duì)應(yīng)的屬性值
          MetaObject metaObject = configuration.newMetaObject(parameterObject);
          value = metaObject.getValue(propertyName);
        }
        // 獲取參數(shù)對(duì)應(yīng)的TypeHandler（負(fù)責(zé)Java類型→JDBC類型的轉(zhuǎn)換）
        TypeHandler typeHandler = parameterMapping.getTypeHandler();
        JdbcType jdbcType = parameterMapping.getJdbcType();
        if (value == null && jdbcType == null) {
          jdbcType = configuration.getJdbcTypeForNull();
        }
        try {
          // 調(diào)用TypeHandler的方法，將value綁定到PreparedStatement的第i+1個(gè)占位符
          typeHandler.setParameter(ps, i + 1, value, jdbcType);
        } catch (SQLException e) {
          throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
        }
      }
    }
  }
}

public interface TypeHandler<T> {
    void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException;
    
    T getResult(ResultSet rs, String columnName) throws SQLException;
    
    T getResult(ResultSet rs, int columnIndex) throws SQLException;
    
    T getResult(CallableStatement cs, int columnIndex) throws SQLException;
}

<configuration>
    <settings>
        <!-- 設(shè)置null值默認(rèn)的JDBC類型 -->
        <setting name="jdbcTypeForNull" value="NULL"/>
    </settings>
</configuration>