@SelectProvider中mybatis参数替换是如何工作的

发布于 2024-12-27 13:50:51 字数 1091 浏览 1 评论 0原文

我继承了一些我试图理解的代码，并且在 @SelectProvider 上进行的任何搜索都一无所获。

Java DAO

@SelectProvider(type = CategoryDaoSelectProvider.class, method = "findByParentIdAndName")
Category findByParentIdAndName(@Param("parentId") Long parentId, @Param("name") String name);

选择提供程序

public class CategoryDaoSelectProvider {
    public static String findByParentIdAndName(Map<String, Object> params) {
        Long parentId = (Long)params.get("parentId");  // WHY IS THIS HERE???

        StringBuffer buffer = new StringBuffer();
        buffer.append("SELECT COUNT(id) FROM Category ");

        if (parentId == null) {
            buffer.append("WHERE parentId IS NULL ");
        } else {
            buffer.append("WHERE parentId = #{parentId} ");
        }

        buffer.append("AND LOWER(name) = LOWER(#{name}) ");

        return buffer.toString();
    }
}

这段代码中的参数parentId 有何用途？据我所知，它实际上从未执行任何操作，除非以某种方式神奇地将 #{parentId} 替换为该值。在这种情况下是否不使用此参数？ mybatis 实际上在哪里进行查询注入？

原文

I've in inherited some code that I'm trying to understand and any searching I do to find something on @SelectProvider turns up a whole lot of nothing.

Java DAO

@SelectProvider(type = CategoryDaoSelectProvider.class, method = "findByParentIdAndName")
Category findByParentIdAndName(@Param("parentId") Long parentId, @Param("name") String name);

Select Provider

public class CategoryDaoSelectProvider {
    public static String findByParentIdAndName(Map<String, Object> params) {
        Long parentId = (Long)params.get("parentId");  // WHY IS THIS HERE???

        StringBuffer buffer = new StringBuffer();
        buffer.append("SELECT COUNT(id) FROM Category ");

        if (parentId == null) {
            buffer.append("WHERE parentId IS NULL ");
        } else {
            buffer.append("WHERE parentId = #{parentId} ");
        }

        buffer.append("AND LOWER(name) = LOWER(#{name}) ");

        return buffer.toString();
    }
}

What purpose does the param parentId serve in this code? As far as I can tell it never actually does anything unless somehow magically the #{parentId} is replaced with the value. Is this param just not used in this situation? Where does mybatis actually do the injections into the query?

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温柔一刀 2025-01-03 13:50:51

SelectProviders 通过两种方式接收参数：作为 params Map 参数中的项目和作为 #{parentId} （在您的示例中）。您的代码显示在 select 语句中使用parentId 之前先对其进行检查。需要parentId变量，因为您无法查询#{parentId}。

顺便说一句，这不是 SelectProviders 的最佳实现，您应该在最后使用 SELECT()、WHERE() 和 SQL() 来返回编译后的语句。我想你的例子也有效。

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才能让你更想念 2025-01-03 13:50:51

您可以将这段代码重写如下，这也许更清晰一些？ parentId 的实际值确实不是必需的，但需要该参数是否存在的信息。

 // ...
 boolean hasIdParam = params.containsKey("parentId");

 StringBuffer buffer = new StringBuffer();
 buffer.append("SELECT COUNT(id) FROM Category ");

 if (!hasIdParam) {
    buffer.append("WHERE parentId IS NULL ");
 } else {
    buffer.append("WHERE parentId = #{parentId} ");
 }
 // ...

You could rewrite that piece of code as follows, which is perhaps a bit clearer? The actual value of parentId is indeed not necessary, but the info on whether or not the parameter is present is required.

 // ...
 boolean hasIdParam = params.containsKey("parentId");

 StringBuffer buffer = new StringBuffer();
 buffer.append("SELECT COUNT(id) FROM Category ");

 if (!hasIdParam) {
    buffer.append("WHERE parentId IS NULL ");
 } else {
    buffer.append("WHERE parentId = #{parentId} ");
 }
 // ...

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