mybatisplus PageHelper

拼搏现实的明天。 2022-05-22 05:08 262阅读 0赞

PageHelper用于查询语句分页，让分页更简单、代码更优雅。![发火][angry.gif]如果硬是要纠结效率与资源，那您倒是直接手写BaseDao哇....

集成mybatis-plus，代码中添加分页相关配置

/**
 * mybatis-plus分页插件 
 * 文档：http://mp.baomidou.com 
 */
 @Bean
 public PaginationInterceptor paginationInterceptor() {
 PaginationInterceptor page = new PaginationInterceptor();
 page.setDialectType("mysql");
         // 打开PageHelper localPage 模式
         page.setLocalPage(true);
 return page;
 }

这样就能使用PageHelper来分页了，当然要问了，为什么必须要手动设置打开呢？那就直接看源码咯~

直贴关键代码，具体源码请查阅：**PaginationInterceptor.java**

package com.baomidou.mybatisplus.plugins;
 
 /**
 * 
 * 分页拦截器
 * 
 *
 * @author hubin
 * @Date 2016-01-23
 */
 @Intercepts({@Signature(type = StatementHandler.class, method = "prepare", args = {Connection.class, Integer.class})})
 public class PaginationInterceptor implements Interceptor {
 /* 是否开启 PageHelper localPage 模式 */
 private boolean localPage = false;
 
 /**
 * Physical Pagination Interceptor for all the queries with parameter {@link org.apache.ibatis.session.RowBounds}
 */
 public Object intercept(Invocation invocation) throws Throwable {
 StatementHandler statementHandler = (StatementHandler) PluginUtils.realTarget(invocation.getTarget());
 MetaObject metaStatementHandler = SystemMetaObject.forObject(statementHandler);
 // 先判断是不是SELECT操作
 MappedStatement mappedStatement = (MappedStatement) metaStatementHandler.getValue("delegate.mappedStatement");
 if (!SqlCommandType.SELECT.equals(mappedStatement.getSqlCommandType())) {
 return invocation.proceed();
 }
 RowBounds rowBounds = (RowBounds) metaStatementHandler.getValue("delegate.rowBounds");
 /* 不需要分页的场合 */
 if (rowBounds == null || rowBounds == RowBounds.DEFAULT) {
 // 本地线程分页
 if (localPage) {
 // 采用ThreadLocal变量处理的分页
 rowBounds = PageHelper.getPagination();
 if (rowBounds == null) {
 return invocation.proceed();
 }
 } else {
 // 无需分页
 return invocation.proceed();
 }
 }
 // 针对定义了rowBounds，做为mapper接口方法的参数
 BoundSql boundSql = (BoundSql) metaStatementHandler.getValue("delegate.boundSql");
 String originalSql = boundSql.getSql();
 Connection connection = (Connection) invocation.getArgs()[0];
 DBType dbType = StringUtils.isNotEmpty(dialectType) ? DBType.getDBType(dialectType) : JdbcUtils.getDbType(connection.getMetaData().getURL());
 if (rowBounds instanceof Pagination) {
 Pagination page = (Pagination) rowBounds;
 boolean orderBy = true;
 if (page.isSearchCount()) {
 SqlInfo sqlInfo = SqlUtils.getCountOptimize(sqlParser, originalSql);
 orderBy = sqlInfo.isOrderBy();
 this.queryTotal(overflowCurrent, sqlInfo.getSql(), mappedStatement, boundSql, page, connection);
 if (page.getTotal() <= 0) {
 return invocation.proceed();
 }
 }
 String buildSql = SqlUtils.concatOrderBy(originalSql, page, orderBy);
 originalSql = DialectFactory.buildPaginationSql(page, buildSql, dbType, dialectClazz);
 } else {
 // support physical Pagination for RowBounds
 originalSql = DialectFactory.buildPaginationSql(rowBounds, originalSql, dbType, dialectClazz);
 }
 
 		/*
 * 禁用内存分页 
 * 内存分页会查询所有结果出来处理（这个很吓人的），如果结果变化频繁这个数据还会不准。
 		 */
 metaStatementHandler.setValue("delegate.boundSql.sql", originalSql);
 metaStatementHandler.setValue("delegate.rowBounds.offset", RowBounds.NO_ROW_OFFSET);
 metaStatementHandler.setValue("delegate.rowBounds.limit", RowBounds.NO_ROW_LIMIT);
 return invocation.proceed();
 }
 
 public void setLocalPage(boolean localPage) {
 this.localPage = localPage;
 }
 }

`关键地方已标红，在PaginationInterceptor中，是否开启 PageHelper localPage 模式，默认是关闭的，需要手动打开`

/* 是否开启 PageHelper localPage 模式 */
        private boolean localPage = false;

这货为啥不是默认打开呢？查看PageHelper类可以发现，是基于ThreadLocal实现，了然。

public class PageHelper {
 
 // 分页本地线程变量
 private static final ThreadLocal<Pagination> LOCAL_PAGE = new ThreadLocal<>();
 
 /**
 * 
 * 获取分页
 * 
 *
 * @return
 */
 public static Pagination getPagination() {
 return LOCAL_PAGE.get();
 }
 
 /**
 * 
 * 设置分页
 * 
 *
 * @param page
 */
 public static void setPagination(Pagination page) {
 LOCAL_PAGE.set(page);
 }

好，接下来使用就方便了

// 组装查询条件
 EntityWrapper<User> wrapper = new EntityWrapper<>();
 if (null != status) {
 wrapper.eq("status", status);
 }
 // 设置分页参数
 PageHelper.startPage(pageIndex, pageSize);
 // 执行查询
 List<User> platfProjects = UserMapper.selectList(wrapper);
 // 返回的分页参数
 Pagination pagination = PageHelper.getPagination();

就是这样~

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