How to use @ResultType annotation correctly in MyBatis?

When studying the mybatis framework, developers often encounter various problems with annotation, one of the common problems is how to use @resulttype annotations correctly. This article will discuss in detail the usage scenarios of @resulttype annotation and answer how to correctly apply the annotation in actual code.

Mybatis annotation @resulttype usage scenario

When looking at the source code of mybatis, we will find that the @resulttype annotation will only take effect if the return type of the method is void. This code is excerpted from org.apache.ibatis.builder.annotation.mapperannotationbuilder#getreturntype:

 if (void.class.equals(returntype)) {
  resulttype rt = method.getannotation(resulttype.class);
  if (rt != null) {
      returntype = rt.value();
  }
}

The above code shows that if the return type of the method is void, mybatis will try to get the actual return type from the @resulttype annotation.

Application code examples and problem analysis

Suppose we define the querystudent method in the code like this:

 @select("select * from student")
@resulttype(student.class)
void querystudent();

In this example, we use the @select annotation to specify the SQL query and use @resulttype(student.class) to specify the return type as student. But since the return type of the method is void, this means that the student object cannot be returned directly.

How does querystudent return student object? What is the meaning of @resulttype?

@resulttype annotation is mainly used with resultandler. resulthandler is a callback interface for processing query results, through which you can customize the way to process query results. When a method's return type is void and resultsandler is used, mybatis needs to know the type of each result row, and the @resulttype annotation comes in handy.

In order for the querystudent method to return the student object correctly, we need to define a resultandler. Here is a modified code example:

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

public class StudentResultHandler implements ResultHandler {
  private final List<student> students;

  public StudentResultHandler() {
    students = new ArrayList();
  }

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

In this example, the studentresulthander class implements the resultandler interface and processes each query result in the handleresult method, adding it to a list. In this way, although the return type of querystudent method is void, studentresulthander can effectively collect and process student objects through studentresulthander.

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