[Java知识库]MyBatis分页插件Pagehelper原理

分页插件

<!-- 分页插件 -->
<dependency>
    <groupId>com.github.pagehelper</groupId>
    <artifactId>pagehelper-spring-boot-starter</artifactId>
    <version>1.2.3</version>
</dependency>

利用springboot自动装配

pagehelper-spring-boot-autoconfigure-1.2.3.jar?

?spring.factories

SpringFactories是spring提供的SPI机制,它读取META-INF/spring.factories文件中配置的接口实现类名称,然后在程序中读取这些配置文件并实例化

# Auto Configure org.springframework.boot.autoconfigure.EnableAutoConfiguration=\ com.github.pagehelper.autoconfigure.PageHelperAutoConfiguration?

?自动实例化?PageHelperAutoConfiguration

com.github.pagehelper.autoconfigure.PageHelperAutoConfiguration?

利用addPageInterceptor() 方法完成属性初始化和增加mybatis拦截器

/**
 * 自定注入分页插件
 *
 * @author liuzh
 */
@Configuration
@ConditionalOnBean(SqlSessionFactory.class)
@EnableConfigurationProperties(PageHelperProperties.class)
@AutoConfigureAfter(MybatisAutoConfiguration.class)
public class PageHelperAutoConfiguration {

    @Autowired
    private List<SqlSessionFactory> sqlSessionFactoryList;

    @Autowired
    private PageHelperProperties properties;

    /**
     * 接受分页插件额外的属性
     *
     * @return
     */
    @Bean
    @ConfigurationProperties(prefix = PageHelperProperties.PAGEHELPER_PREFIX)
    public Properties pageHelperProperties() {
        return new Properties();
    }

    @PostConstruct
    public void addPageInterceptor() {
        PageInterceptor interceptor = new PageInterceptor();
        Properties properties = new Properties();
        //先把一般方式配置的属性放进去
        properties.putAll(pageHelperProperties());
        //在把特殊配置放进去，由于close-conn 利用上面方式时，属性名就是 close-conn 而不是 closeConn，所以需要额外的一步
        properties.putAll(this.properties.getProperties());
        interceptor.setProperties(properties);
        for (SqlSessionFactory sqlSessionFactory : sqlSessionFactoryList) {
            sqlSessionFactory.getConfiguration().addInterceptor(interceptor);
        }
    }

}

?mybatis拦截器

com.github.pagehelper.PageInterceptor?

 @Override
    public Object intercept(Invocation invocation) throws Throwable {
        try {
            Object[] args = invocation.getArgs();
            MappedStatement ms = (MappedStatement) args[0];
            Object parameter = args[1];
            RowBounds rowBounds = (RowBounds) args[2];
            ResultHandler resultHandler = (ResultHandler) args[3];
            Executor executor = (Executor) invocation.getTarget();
            CacheKey cacheKey;
            BoundSql boundSql;
            //由于逻辑关系，只会进入一次
            if(args.length == 4){
                //4 个参数时
                boundSql = ms.getBoundSql(parameter);
                cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
            } else {
                //6 个参数时
                cacheKey = (CacheKey) args[4];
                boundSql = (BoundSql) args[5];
            }
            List resultList;
            //调用方法判断是否需要进行分页，如果不需要，直接返回结果
            if (!dialect.skip(ms, parameter, rowBounds)) {
                //反射获取动态参数
                String msId = ms.getId();
                Configuration configuration = ms.getConfiguration();
                Map<String, Object> additionalParameters = (Map<String, Object>) additionalParametersField.get(boundSql);
                //判断是否需要进行 count 查询
                if (dialect.beforeCount(ms, parameter, rowBounds)) {
                    String countMsId = msId + countSuffix;
                    Long count;
                    //先判断是否存在手写的 count 查询
                    MappedStatement countMs = getExistedMappedStatement(configuration, countMsId);
                    if(countMs != null){
                        count = executeManualCount(executor, countMs, parameter, boundSql, resultHandler);
                    } else {
                        countMs = msCountMap.get(countMsId);
                        //自动创建
                        if (countMs == null) {
                            //根据当前的 ms 创建一个返回值为 Long 类型的 ms
                            countMs = MSUtils.newCountMappedStatement(ms, countMsId);
                            msCountMap.put(countMsId, countMs);
                        }
                        count = executeAutoCount(executor, countMs, parameter, boundSql, rowBounds, resultHandler);
                    }
                    //处理查询总数
                    //返回 true 时继续分页查询，false 时直接返回
                    if (!dialect.afterCount(count, parameter, rowBounds)) {
                        //当查询总数为 0 时，直接返回空的结果
                        return dialect.afterPage(new ArrayList(), parameter, rowBounds);
                    }
                }
                //判断是否需要进行分页查询
                if (dialect.beforePage(ms, parameter, rowBounds)) {
                    //生成分页的缓存 key
                    CacheKey pageKey = cacheKey;
                    //处理参数对象
                    parameter = dialect.processParameterObject(ms, parameter, boundSql, pageKey);
                    //调用方言获取分页 sql
                    String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);
                    BoundSql pageBoundSql = new BoundSql(configuration, pageSql, boundSql.getParameterMappings(), parameter);
                    //设置动态参数
                    for (String key : additionalParameters.keySet()) {
                        pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
                    }
                    //执行分页查询
                    resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);
                } else {
                    //不执行分页的情况下，也不执行内存分页
                    resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);
                }
            } else {
                //rowBounds用参数值，不使用分页插件处理时，仍然支持默认的内存分页
                resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
            }
            return dialect.afterPage(resultList, parameter, rowBounds);
        } finally {
            dialect.afterAll();
        }
    }

获取count?

count = executeAutoCount(executor, countMs, parameter, boundSql, rowBounds, resultHandler);

?生成分页语句

//调用方言获取分页 sql
String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);