MyBatis参数为0时跟空字符串''判断相等的分析以及解决办法

• 起因
  之前在一个Mapper文件中写了这样一个if标签，结果当参数为0时不论如何if标签的判断都是false，打印sql也发现当参数为0时这个条件会不起作用，因此查看了一下MyBatis中相关的源码，if标签如下：

        <if test="backCarType != null and backCarType != ''">
			and m.back_car = #{backCarType,jdbcType=BIT}
		</if>

• MyBatis 创建BoundSql的过程
  上文提到了MappedStatement这个类，这个类中有一个getBoundSql()方法，现在来跟踪一下这个方法：

  - MappedStatement.getBoundSql(Object parameterObject) 
  public BoundSql getBoundSql(Object parameterObject) {
   
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings == null || parameterMappings.isEmpty()) {
   
      boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject);
    }

    // check for nested result maps in parameter mappings (issue #30)
    for (ParameterMapping pm : boundSql.getParameterMappings()) {
   
      String rmId = pm.getResultMapId();
      if (rmId != null) {
   
        ResultMap rm = configuration.getResultMap(rmId);
        if (rm != null) {
   
          hasNestedResultMaps |= rm.hasNestedResultMaps();
        }
      }
    }

    return boundSql;
  }

  - DynamicSqlSource.getBoundSql(Object parameterObject)
  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;
  }

重点关注这个rootSqlNode，因为它就是解析我们写在mapper文件中的各种标签，比如if、choose、foreach等，接下来看一下是怎么解析if标签的：

public class IfSqlNode implements SqlNode {
   
  private final ExpressionEvaluator evaluator;
  private final String test;
  private final SqlNode contents;

  public IfSqlNode(SqlNode contents, String test) {
   
    this.test = test;
    this.contents = contents;
    this.evaluator = new ExpressionEvaluator();
  }

  @Override
  public boolean apply(DynamicContext context) {
   
    if (evaluator.evaluateBoolean