精通SpringBoot--分页查询功能的实现

本文将介绍如何实现分页查询功能，推荐使用github的pagehelper插件实现（事实上大家基本都是这么干的），但本文的实现方式和大多数不同，废话少说，现在就带着大家看看区别在哪里。
先看pom.xml依赖：相当简单的mybatis-spring依赖+mysql+pagehelper+fastjson

 <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.mybatis.spring.boot</groupId>
            <artifactId>mybatis-spring-boot-starter</artifactId>
            <version>1.3.2</version>
        </dependency>
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>fastjson</artifactId>
            <version>1.2.44</version>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
        </dependency>
        <!--pageHelper-->
        <!-- https://mvnrepository.com/artifact/com.github.pagehelper/pagehelper-spring-boot-starter -->
        <dependency>
            <groupId>com.github.pagehelper</groupId>
            <artifactId>pagehelper-spring-boot-starter</artifactId>
            <version>1.2.5</version>
        </dependency>
   
    </dependencies>

application.yml配置

spring:
  datasource:
    username: root
    password: 123456
    url: jdbc:mysql://localhost:3306/javashop
    driver-class-name: com.mysql.jdbc.Driver
  http:
    encoding:
      charset: utf-8
      enabled: true
mybatis:
  type-aliases-package: com.developlee.mybatispagehelper.domain
  mapper-locations: classpath:/mapper/*.xml
pagehelper:
  helper-dialect: mysql
  reasonable: true
  support-methods-arguments: true
  params: count=countSql

撸代码了，先从controller开始，开干！随便写个简单点的

@RestController
public class UserController {

    @Autowired
    private UserServiceImpl userService;

    @PostMapping("/queryList")
    public ResponseEntity queryList(@RequestBody UserEntity userEntity) {
        List<UserEntity> userEntities = userService.findUserList(userEntity);
        return new ResponseEntity( new PageEntity<>(userEntities), HttpStatus.OK);
    }
}

仔细看，是不是发现并_没有PageHelper.startPage(int num, int size)_这句话？当然没有最好，每次分页查询都写这句，是不是很烦。。。还有_PageEntity是个啥_？不要着急，让我们慢慢揭开面纱。
service和dao还有userMapper.xml没什么好讲的，就是为了userService.findList能正常查出个list数组来。代码懒得贴了，占位置，相信大家也都会。
Next，就是重点了， 看看UserEntity实体做了什么。

public class UserEntity extends BaseEntity {
    private Long id;
    private String username;
    private String password;
    private String mobile;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getMobile() {
        return mobile;
    }

    public void setMobile(String mobile) {
        this.mobile = mobile;
    }
}

OMG，继承了一个BaseEntity，那BaseEntity有哪些内容呢，其实相当easy，就是些我们要传的page参数.

public abstract class BaseEntity {

    //分页大小
    private Integer pageSize;
    //分页开始
    private Integer pageNum;

    private Integer total;

    //排序类型DESC  or  AES
    private String sort;

    private String orderBy;

    @JSONField(serialize = false)
    public Integer getPageSize() {
        return pageSize;
    }


    public void setPageSize(Integer pageSize) {
        this.pageSize = pageSize;
    }

    @JSONField(serialize = false)
    public Integer getPageNum() {
        return pageNum;
    }


    public void setPageNum(Integer pageNum) {
        this.pageNum = pageNum;
    }

    @JSONField(serialize = false)
    public Integer getTotal() {
        return total;
    }

    public void setTotal(Integer total) {
        this.total = total;
    }

    @JSONField(serialize = false)
    public String getOrderBy() {
        return orderBy;
    }

    public void setOrderBy(String orderBy) {
        this.orderBy = orderBy;
    }

    @JSONField(serialize = false)
    public String getSort() {
        return sort;
    }

    public void setSort(String sort) {
        this.sort = sort;
    }
}

注意哦，这些个参数返回给前端时都不需要给的哦。所以用了@JSONField(serialize=false) 来搞定。

讲到这，似乎没什么神奇之处，呃呃呃，是的，这些都是为了传参数。看看PageEntity ,这才是今天的主角儿~~~

import com.github.pagehelper.Page;

import java.io.Serializable;
import java.util.Collection;
import java.util.List;

/**
 * 对Page<E>结果进行包装
 * <p/>
 * 新增分页的多项属性
 */
@SuppressWarnings({"rawtypes", "unchecked"})
public class PageEntity<T> implements Serializable {
    private static final long serialVersionUID = 1L;
    //当前页
    private int currentPage;
    //每页的数量
    private int pageSize;
    //总记录数
    private long total;
    //总页数
    private int pages;
    //结果集
    private List<T> list;
    //是否为第一页
    private boolean isFirstPage = false;
    //是否为最后一页
    private boolean isLastPage = false;


    public PageEntity() {
    }

    /**
     * 包装Page对象
     *
     * @param list
     */
    public PageEntity(List<T> list) {
        if (list instanceof Page) {
            Page page = (Page) list;
            this.currentPage = page.getPageNum();
            this.pageSize = page.getPageSize();

            this.pages = page.getPages();
            this.list = page;
            this.total = page.getTotal();
        } else if (list instanceof Collection) {
            this.currentPage = 1;
            this.pageSize = list.size();
            this.pages = 1;
            this.list = list;
            this.total = list.size();
        }
        if (list instanceof Collection) {
            //判断页面边界
            judgePageBoudary();
        }
    }

    /**
     * 判定页面边界
     */
    private void judgePageBoudary() {
        isFirstPage = currentPage == 1;
        isLastPage = currentPage == pages;
    }

    public int getCurrentPage() {
        return currentPage;
    }

    public void setCurrentPage(int currentPage) {
        this.currentPage = currentPage;
    }

    public int getPageSize() {
        return pageSize;
    }

    public void setPageSize(int pageSize) {
        this.pageSize = pageSize;
    }

    public long getTotal() {
        return total;
    }

    public void setTotal(long total) {
        this.total = total;
    }

    public int getPages() {
        return pages;
    }

    public void setPages(int pages) {
        this.pages = pages;
    }

    public List<T> getList() {
        return list;
    }

    public void setList(List<T> List) {
        this.list = List;
    }

    public boolean isIsFirstPage() {
        return isFirstPage;
    }

    public void setIsFirstPage(boolean isFirstPage) {
        this.isFirstPage = isFirstPage;
    }

    public boolean isIsLastPage() {
        return isLastPage;
    }

    public void setIsLastPage(boolean isLastPage) {
        this.isLastPage = isLastPage;
    }

    @Override
    public String toString() {
        final StringBuffer sb = new StringBuffer("PageEntity{");
        sb.append("currentPage=").append(currentPage);
        sb.append(", pageSize=").append(pageSize);
        sb.append(", total=").append(total);
        sb.append(", pages=").append(pages);
        sb.append(", list=").append(list);
        sb.append(", isFirstPage=").append(isFirstPage);
        sb.append(", isLastPage=").append(isLastPage);
        sb.append(", navigatepageNums=");
        sb.append('}');
        return sb.toString();
    }
}

Watch Out!构造函数PageEntiy(List list) 这里头对pageHelper的Page进行了进一步的完善。看看pageHelper的Page类

public class Page<E> extends ArrayList<E> implements Closeable {
    private static final long serialVersionUID = 1L;
    private int pageNum;
    private int pageSize;
    private int startRow;
    private int endRow;
    private long total;
    private int pages;
    private boolean count;
    private Boolean reasonable;
    private Boolean pageSizeZero;
    private String countColumn;
    private String orderBy;
    private boolean orderByOnly;
    //多余不贴了。。。
}

光说不练空把式，postman 工具测试一波看看。

我擦，什么情况，说好的不要这些page参数呢，为什么list里头还是有？嗯哼，SpringBoot默认的消息格式转换，也就是messgeConvert，很明显，我们使用了fastjson ，自然MessageConvert也要改造成用fastjson的。
springboot2.0中实现WebMvcConfiguer接口，然后覆盖该方法

/**
 * @author Lensen
 * @desc
 * @since 2018/8/25 21:45
 */
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
    //消息格式转换
    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        //1.需要定义一个convert转换消息的对象;
        FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter();
        //2.添加fastJson的配置信息，比如：是否要格式化返回的json数据;
        FastJsonConfig fastJsonConfig = new FastJsonConfig();
        fastJsonConfig.setSerializerFeatures(SerializerFeature.PrettyFormat,
                SerializerFeature.WriteMapNullValue,
                SerializerFeature.WriteNullStringAsEmpty,
                SerializerFeature.DisableCircularReferenceDetect,
                SerializerFeature.WriteNullListAsEmpty,
                SerializerFeature.WriteDateUseDateFormat);
        //3处理中文乱码问题
        List<MediaType> fastMediaTypes = new ArrayList<>();
        fastMediaTypes.add(MediaType.APPLICATION_JSON_UTF8);
        //4.在convert中添加配置信息.
        fastJsonHttpMessageConverter.setSupportedMediaTypes(fastMediaTypes);
        fastJsonHttpMessageConverter.setFastJsonConfig(fastJsonConfig);
        //5.将convert添加到converters当中.
        converters.add(fastJsonHttpMessageConverter);
    }
}

@EnableWebMvc别忘了，忘了就不会生效的（我一开始就**忘记了）
在测试，走起

完美~ 代码里我根本不用写PageHelper.startPage(int pageNum, int pageSize)这句话，直接专注自己的逻辑。