MP简单的分页查询测试

好久没水后端的东西了，最近在做vue项目写前端的代码，所以cloud也停进度了，吃完饭突然记得我没有在博客里写分页的东西，虽然项目中用到了，但是没有拎出来，这里就拎出来看看。

导入最新的mp依赖是第一步不然太低的版本什么都做不了，3,1以下的好像连分页插件都没有加进去，所以我们用最新的3.5的，保证啥都有：

        <dependency>
            <groupId>com.baomidougroupId>
            <artifactId>mybatis-plus-boot-starterartifactId>
            <version>3.5.2version>
        dependency>

这里我们需要认识两个插件：mp的核心插件MybatisPlusInterceptor与自动分页插件PaginationInnerInterceptor。

MybatisPlusInterceptor的源码（去掉中间的处理代码）：

public class MybatisPlusInterceptor implements Interceptor {
    private List interceptors = new ArrayList();
    public MybatisPlusInterceptor() {}
    public Object intercept(Invocation invocation) throws Throwable {}
    public Object plugin(Object target) {}
    public void addInnerInterceptor(InnerInterceptor innerInterceptor) {}
    public List getInterceptors() {}
    public void setProperties(Properties properties) {}
    public void setInterceptors(final List interceptors) {}
}

我们可以发现它有一个私有的属性列表 List 而这个链表中的元素类型是InnerInterceptor。

InnerInterceptor源码：

public interface InnerInterceptor {
    default boolean willDoQuery(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
        return true;
    }
 
    default void beforeQuery(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    }
 
    default boolean willDoUpdate(Executor executor, MappedStatement ms, Object parameter) throws SQLException {
        return true;
    }
 
    default void beforeUpdate(Executor executor, MappedStatement ms, Object parameter) throws SQLException {
    }
 
    default void beforePrepare(StatementHandler sh, Connection connection, Integer transactionTimeout) {
    }
 
    default void beforeGetBoundSql(StatementHandler sh) {
    }
 
    default void setProperties(Properties properties) {
    }
}

不难发现这个接口的内容大致就是设置默认的属性，从代码的意思上就是提供默认的数据库操作执行时期前后执行的一些逻辑，谁实现它的方法会得到新的功能？

再看看PaginationInnerInterceptor插件的源码：

public class PaginationInnerInterceptor implements InnerInterceptor {
    protected static final List COUNT_SELECT_ITEM = Collections.singletonList((new SelectExpressionItem((new Column()).withColumnName("COUNT(*)"))).withAlias(new Alias("total")));
    protected static final Map countMsCache = new ConcurrentHashMap();
    protected final Log logger = LogFactory.getLog(this.getClass());
    protected boolean overflow;
    protected Long maxLimit;
    private DbType dbType;
    private IDialect dialect;
    protected boolean optimizeJoin = true;
 
    public PaginationInnerInterceptor(DbType dbType) {
        this.dbType = dbType;
    }
 
    public PaginationInnerInterceptor(IDialect dialect) {
        this.dialect = dialect;
    }
 
    public boolean willDoQuery(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
        IPage page = (IPage)ParameterUtils.findPage(parameter).orElse((Object)null);
        if (page != null && page.getSize() >= 0L && page.searchCount()) {
            MappedStatement countMs = this.buildCountMappedStatement(ms, page.countId());
            BoundSql countSql;
            if (countMs != null) {
                countSql = countMs.getBoundSql(parameter);
            } else {
                countMs = this.buildAutoCountMappedStatement(ms);
                String countSqlStr = this.autoCountSql(page, boundSql.getSql());
                MPBoundSql mpBoundSql = PluginUtils.mpBoundSql(boundSql);
                countSql = new BoundSql(countMs.getConfiguration(), countSqlStr, mpBoundSql.parameterMappings(), parameter);
                PluginUtils.setAdditionalParameter(countSql, mpBoundSql.additionalParameters());
            }
 
            CacheKey cacheKey = executor.createCacheKey(countMs, parameter, rowBounds, countSql);
            List