How to correctly use @ResultType annotation in MyBatis?

Correct use of @ResultType annotation in MyBatis

This article discusses the usage of @ResultType annotation in MyBatis and clarifies its relationship with ResultHandler . Source code analysis shows that the @ResultType annotation only takes effect when the method return type is void .

Misconceptions and clarifications:

Many developers mistakenly believe that @ResultType directly specifies the return value type. In fact, when the method returns void , @ResultType tells MyBatis how to map the query result to the specified type ( Student.class ), which is usually used with ResultHandler . If the method returns a non- void type, @ResultType will be ignored and MyBatis will map according to the method return type.

Correct usage method:

The following example shows the correct combination of @ResultType and ResultHandler :

 @Select("select * from student")
@ResultType(Student.class)
void queryStudent(StudentResultHandler resultHandler);

public class StudentResultHandler implements ResultHandler<student> {
    private final List<student> students = new ArrayList();

    @Override
    public void handleResult(ResultContext extends Student> context) {
        Student student = context.getResultObject();
        students.add(student);
    }

    public List<student> getStudents() {
        return students;
    }
}</student></student></student>

In this example:

1. queryStudent method returns void .
2. @ResultType(Student.class) Tell MyBatis to map the query results to Student objects.
3. StudentResultHandler implements the ResultHandler interface, processes query results row by row, and adds each Student object to the students list.
4. The caller needs to get the result list through resultHandler.getStudents() .

Other situations:

• @ResultMap annotation: If the XML mapping file is used<resultmap></resultmap> Defining the result map, you should use the @ResultMap annotation instead of @ResultType .

• Non- void return type: if method returns List<student></student> or Student , without @ResultType , MyBatis will automatically map according to the return type.

Summarize:

@ResultType annotation is mainly used for void return type methods, and it implements custom result processing logic in conjunction with ResultHandler . For non- void return types, MyBatis will automatically map based on the return type without @ResultType . Selecting @ResultType or @ResultMap depends on whether XML result mapping is used. Only by understanding these differences can you use MyBatis annotations correctly and effectively.

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