第1步：引入缓存上下文

import com.zhangziwa.practisesvr.model.Student;
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import static org.apache.commons.lang3.ObjectUtils.anyNull;
public class StudentContextHolder {
    private static final ThreadLocal> studentContextHolder = ThreadLocal.withInitial(HashMap::new);
    public static Student getStudent(Integer id) {
        if (Objects.isNull(id)) {
            return null;
        }
        return studentContextHolder.get().get(id);
    }
    public static void setStudent(Integer id, Student student) {
        if (anyNull(id, student)) {
            return;
        }
        if (getStudent(id) != null) {
            throw new UnsupportedOperationException("Student with id " + id + " already exists.");
        }
        studentContextHolder.get().put(id, student);
    }
    public static void clear() {
        studentContextHolder.remove();
    }
}

第2步：查询先查缓存，查询到值先存缓存

public Student queryById(Integer id) {
    if (Objects.isNull(id)) {
        return null;
    }
    
    // 线程缓存里去
    Student student = StudentContextHolder.getStudent(id);
    if (nonNull(student)) {
        return student;
    }
    
    student = studentsMapper.queryById(id);
    
    // 查询数据库值先存缓存
    StudentContextHolder.setStudent(id, student);
    
    return student;
}

第3步：清理缓存上下文

public class ResponsePostInterceptor implements HandlerInterceptor {
    //在Controller执行之前调用，如果返回false，controller不执行
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        System.err.println("***ResponsePostInterceptor.preHandle***");
        return true;
    }
    //controller执行之后，且页面渲染之前调用
    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
        System.err.println("***ResponsePostInterceptor.postHandle***");
    }
    //页面渲染之后调用，一般用于资源清理操作
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        System.err.println("***ResponsePostInterceptor.afterCompletion***");
        StudentContextHolder.clear(); //  清除student上下文
    }
}

第4步：验证使用

@GetMapping("{id}")
public ResponseEntity queryById(@PathVariable("id") Integer id) {
    System.out.println("***StudentController.queryById***");
    
    Student body = studentService.queryById(id);
    System.out.println(body);
    
    System.out.println(studentService.queryById(id));
    
    return ResponseEntity.ok(body);
}

执行日志

[2024-01-26 01:16:16.068_068] [WARN ] [http-nio-8080-exec-2] [LogFilter.java:21] → [LogFilter.doFilter: Start processing request at 2024-01-25T17:16:16.068132700Z - /students/8]
***LogFilter.doFilter.start***
***RequestHeaderCheckFilter.doFilter.start***
***ResponsePostInterceptor.preHandle***
***LogInterceptor.preHandle***
[2024-01-26 01:16:16.094_094] [WARN ] [http-nio-8080-exec-2] [LogInterceptor.java:37] → [LogInterceptor.postHandle: Start processing request at 2024-01-25T17:16:16.094950300Z - /students/8]
***StudentController.queryById***
Creating a new SqlSession
SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@3bfba234] was not registered for synchronization because synchronization is not active
[2024-01-26 01:16:16.128_128] [INFO ] [http-nio-8080-exec-2] [HikariDataSource.java:110] → [practisedb - Starting...]
[2024-01-26 01:16:16.248_248] [INFO ] [http-nio-8080-exec-2] [HikariPool.java:565] → [practisedb - Added connection com.mysql.cj.jdbc.ConnectionImpl@12e4ef1]
[2024-01-26 01:16:16.252_252] [INFO ] [http-nio-8080-exec-2] [HikariDataSource.java:123] → [practisedb - Start completed.]
JDBC Connection [HikariProxyConnection@688850212 wrapping com.mysql.cj.jdbc.ConnectionImpl@12e4ef1] will not be managed by Spring
==>  Preparing: select id, username, password, age, height, gender, class_id, is_delete from students where id = ?
***SqlExecuteInterceptor.intercept***
***SqlReadRowInterceptor.intercept***
==> Parameters: 8(Integer)
<==    Columns: id, username, password, age, height, gender, class_id, is_delete
<==        Row: 8, 汪子韬, lq2fks1eg5, 24, 161.84, 女, 5, 0
<==      Total: 1
Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@3bfba234]
Student(id=8, username=汪子韬, password=lq2fks1eg5, age=24, height=161, gender=女, classId=5, isDelete=false)
Student(id=8, username=汪子韬, password=lq2fks1eg5, age=24, height=161, gender=女, classId=5, isDelete=false)
***ResponsePostAdvice.supports***
***ResponsePostAdvice.beforeBodyWrite***
***LogInterceptor.postHandle***
***ResponsePostInterceptor.postHandle***
***LogInterceptor.afterCompletion***
[2024-01-26 01:16:16.394_394] [WARN ] [http-nio-8080-exec-2] [LogInterceptor.java:57] → [LogInterceptor.postHandle: Finished processing request at 2024-01-25T17:16:16.393566400Z - /students/8 in 299 ms. Status code: 200]
[2024-01-26 01:16:16.413_413] [INFO ] [http-nio-8080-exec-2] [logUtils.java:70] → [{"traceId":"9287f21b215b49e19ed7fd94c9aed4e6","endDate":"2024-01-26T01:16:16.3972818+08:00[Asia/Shanghai]","cost":299,"remoteHost":"0:0:0:0:0:0:0:1","remoteAddr":"0:0:0:0:0:0:0:1","remotePort":12742,"method":"GET","requestURI":"/students/8","status":200,"requestContentLength":-1,"sqlCount":1,"sqlCost":31,"sqlSearchedRowCount":1,"currentThreadTime":109,"currentThreadUserTime":78,"currentThreadAllocatedBytes":20845224}]
[2024-01-26 01:16:16.419_419] [WARN ] [http-nio-8080-exec-2] [LogFilter.java:30] → [LogFilter.doFilter: Finished processing request at 2024-01-25T17:16:16.419516700Z - /students/8 in 351 ms. Status code: 200]
***ResponsePostInterceptor.afterCompletion***
***RequestHeaderCheckFilter.doFilter.end***
***LogFilter.doFilter.end***