简介
利用 MyBatis Plugin 插件技术实现分页功能。
分页插件实现思路如下:
- 业务代码在 ThreadLocal 中保存分页信息;
- MyBatis Interceptor 拦截查询请求,获取分页信息,实现分页操作,封装分页列表数据返回;
测试类:com.yjw.demo.PageTest
插件开发过程
确定需要拦截的签名
MyBatis 插件可以拦截四大对象中的任意一个,从 Plugin 源码中可以看到它需要注册签名才能够运行插件,签名需要确定一些要素。
确定需要拦截的对象
- Executor 是执行 SQL 的全过程,包括组装参数,组装结果集返回和执行 SQL 过程,都可以拦截。
- StatementHandler 是执行 SQL 的过程,我们可以重写执行 SQL 的过程。
- ParameterHandler 是拦截执行 SQL 的参数组装,我们可以重写组装参数规则。
- ResultSetHandler 用于拦截执行结果的组装,我们可以重写组装结果的规则。
拦截方法和参数
当确定了需要拦截什么对象,接下来就要确定需要拦截什么方法和方法的参数。比如分页插件需要拦截 Executor 的 query 方法,我们先看看 Executor 接口的定义,代码清单如下:
public interface Executor { ResultHandler NO_RESULT_HANDLER = null; int update(MappedStatement ms, Object parameter) throws SQLException; <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException; <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException; <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException; List<BatchResult> flushStatements() throws SQLException; void commit(boolean required) throws SQLException; void rollback(boolean required) throws SQLException; CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql); boolean isCached(MappedStatement ms, CacheKey key); void clearLocalCache(); void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType); Transaction getTransaction(); void close(boolean forceRollback); boolean isClosed(); void setExecutorWrapper(Executor executor); }
以上的任何方法都可以拦截,从接口定义而言,query 方法有两个,我们可以按照代码清单来定义签名。
@Intercepts({ @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}), @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class})})
其中,@Intercepts 说明它是一个拦截器。@Signature 是注册拦截器签名的地方,type 是四大对象中的一个,method 是需要拦截的方法,args 是方法的参数。
插件接口定义
在 MyBatis 中开发插件,需要实现 Interceptor 接口,接口的定义如下:
public interface Interceptor { Object intercept(Invocation invocation) throws Throwable; Object plugin(Object target); void setProperties(Properties properties); }
- intercept 方法:它将直接覆盖你所拦截对象原有的方法,因此它是插件的核心方法。通过 invocation 参数可以反射调度原来对象的方法。
- plugin 方法:target 是被拦截对象,它的作用是给被拦截对象生成一个代理对象,并返回它。为了方便 MyBatis 使用 org.apache.ibatis.plugin.Plugin 中的 wrap 静态方法提供生成代理对象。
- setProperties 方法:允许在 plugin 元素中配置所需参数,方法在插件初始化的时候就被调用了一次,然后把插件对象存入到配置中,以便后面再取出。
实现类
根据分页插件的实现思路,定义了三个类。
Page 类
Page 类继承了 ArrayList 类,用来封装分页信息和列表数据。
/** * 分页返回对象 * * @author yinjianwei * @date 2018/11/05 */ public class Page<E> extends ArrayList<E> { private static final long serialVersionUID = 1L; /** * 页码,从1开始 */ private int pageNum; /** * 页面大小 */ private int pageSize; /** * 起始行 */ private int startRow; /** * 末行 */ private int endRow; /** * 总数 */ private long total; /** * 总页数 */ private int pages; public int getPageNum() { return pageNum; } public void setPageNum(int pageNum) { this.pageNum = pageNum; } public int getPageSize() { return pageSize; } public void setPageSize(int pageSize) { this.pageSize = pageSize; } public int getStartRow() { return startRow; } public void setStartRow(int startRow) { this.startRow = startRow; } public int getEndRow() { return endRow; } public void setEndRow(int endRow) { this.endRow = endRow; } public long getTotal() { return total; } public void setTotal(long total) { this.total = total; this.pages = (int)(total / pageSize + (total % pageSize == 0 ? 0 : 1)); if (pageNum > pages) { pageNum = pages; } this.startRow = this.pageNum > 0 ? (this.pageNum - 1) * this.pageSize : 0; this.endRow = this.startRow + this.pageSize * (this.pageNum > 0 ? 1 : 0); } public int getPages() { return pages; } public void setPages(int pages) { this.pages = pages; } /** * 返回当前对象 * * @return */ public List<E> getResult() { return this; } }
PageHelper 类
PageHelper 类是分页的帮助类,主要利用 ThreadLocal 线程变量存储分页信息。代码清单如下:
/** * 分页帮助类 * * @author yinjianwei * @date 2018/11/05 */ @SuppressWarnings("rawtypes") public class PageHelper { private static final ThreadLocal<Page> PAGE_THREADLOCAT = new ThreadLocal<Page>(); /** * 设置线程局部变量分页信息 * * @param page */ public static void setPageThreadLocal(Page page) { PAGE_THREADLOCAT.set(page); } /** * 获取线程局部变量分页信息 * * @return */ public static Page getPageThreadLocal() { return PAGE_THREADLOCAT.get(); } /** * 清空线程局部变量分页信息 */ public static void pageThreadLocalClear() { PAGE_THREADLOCAT.remove(); } /** * 设置分页参数 * * @param pageNum * @param pageSize */ public static void startPage(Integer pageNum, Integer pageSize) { Page page = new Page(); page.setPageNum(pageNum); page.setPageSize(pageSize); setPageThreadLocal(page); } }
PageInterceptor 类
PageInterceptor 类实现了 Interceptor 接口,是分页插件的核心类。代码清单如下:
/** * 分页拦截器 * * @author yinjianwei * @date 2018/11/05 */ @Intercepts({ @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}), @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class})}) public class PageInterceptor implements Interceptor { private Field additionalParametersField; @SuppressWarnings({"rawtypes", "unchecked"}) @Override public Object intercept(Invocation invocation) throws Throwable { Executor executor = (Executor)invocation.getTarget(); Object[] args = invocation.getArgs(); MappedStatement ms = (MappedStatement)args[0]; Object parameter = args[1]; RowBounds rowBounds = (RowBounds)args[2]; ResultHandler resultHandler = (ResultHandler)args[3]; CacheKey cacheKey; BoundSql boundSql; // 4个参数 if (args.length == 4) { boundSql = ms.getBoundSql(parameter); cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql); } // 6个参数 else { cacheKey = (CacheKey)args[4]; boundSql = (BoundSql)args[5]; } // 判断是否需要分页 Page page = PageHelper.getPageThreadLocal(); // 不执行分页 if (page.getPageNum() <= 0) { return executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql); } // count查询 MappedStatement countMs = newCountMappedStatement(ms); String sql = boundSql.getSql(); String countSql = "select count(1) from (" + sql + ") _count"; BoundSql countBoundSql = new BoundSql(ms.getConfiguration(), countSql, boundSql.getParameterMappings(), parameter); Map<String, Object> additionalParameters = (Map<String, Object>)additionalParametersField.get(boundSql); for (Entry<String, Object> additionalParameter : additionalParameters.entrySet()) { countBoundSql.setAdditionalParameter(additionalParameter.getKey(), additionalParameter.getValue()); } CacheKey countCacheKey = executor.createCacheKey(countMs, parameter, rowBounds, countBoundSql); Object countResult = executor.query(countMs, parameter, RowBounds.DEFAULT, resultHandler, countCacheKey, countBoundSql); Long count = (Long)((List)countResult).get(0); page.setTotal(count); // 分页查询 String pageSql = sql + " limit " + page.getStartRow() + "," + page.getPageSize(); BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter); for (Entry<String, Object> additionalParameter : additionalParameters.entrySet()) { pageBoundSql.setAdditionalParameter(additionalParameter.getKey(), additionalParameter.getValue()); } CacheKey pageCacheKey = executor.createCacheKey(ms, parameter, rowBounds, pageBoundSql); List listResult = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageCacheKey, pageBoundSql); page.addAll(listResult); // 清空线程局部变量分页信息 PageHelper.pageThreadLocalClear(); return page; } @Override public Object plugin(Object target) { return Plugin.wrap(target, this); } @Override public void setProperties(Properties properties) { try { additionalParametersField = BoundSql.class.getDeclaredField("additionalParameters"); additionalParametersField.setAccessible(true); } catch (NoSuchFieldException | SecurityException e) { e.printStackTrace(); } } /** * 创建count的MappedStatement * * @param ms * @return */ private MappedStatement newCountMappedStatement(MappedStatement ms) { MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(), ms.getId() + "_count", ms.getSqlSource(), ms.getSqlCommandType()); builder.resource(ms.getResource()); builder.fetchSize(ms.getFetchSize()); builder.statementType(ms.getStatementType()); builder.keyGenerator(ms.getKeyGenerator()); if (ms.getKeyProperties() != null && ms.getKeyProperties().length != 0) { StringBuilder keyProperties = new StringBuilder(); for (String keyProperty : ms.getKeyProperties()) { keyProperties.append(keyProperty).append(","); } keyProperties.delete(keyProperties.length() - 1, keyProperties.length()); builder.keyProperty(keyProperties.toString()); } builder.timeout(ms.getTimeout()); builder.parameterMap(ms.getParameterMap()); // count查询返回值int List<ResultMap> resultMaps = new ArrayList<ResultMap>(); ResultMap resultMap = new ResultMap.Builder(ms.getConfiguration(), ms.getId() + "_count", Long.class, new ArrayList<ResultMapping>(0)).build(); resultMaps.add(resultMap); builder.resultMaps(resultMaps); builder.resultSetType(ms.getResultSetType()); builder.cache(ms.getCache()); builder.flushCacheRequired(ms.isFlushCacheRequired()); builder.useCache(ms.isUseCache()); return builder.build(); } }
配置
MyBatis 配置文件增加 plugin 配置项。
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"> <configuration> <settings> <setting name="lazyLoadingEnabled" value="true"/> <setting name="aggressiveLazyLoading" value="false"/> </settings> <typeHandlers> <typeHandler javaType="com.yjw.demo.mybatis.common.constant.Sex" jdbcType="TINYINT" handler="com.yjw.demo.mybatis.common.type.SexEnumTypeHandler"/> </typeHandlers> <plugins> <plugin interceptor="com.yjw.demo.mybatis.common.page.PageInterceptor"> </plugin> </plugins> </configuration>
MyBatis 实用篇