PageInterceptor是Mybatis的插件,用于拦截Executor的query方法,增强这个方法用于分页查询。
@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 { //缓存count查询的ms protected Cache<String, MappedStatement> msCountMap = null; private Dialect dialect; private String default_dialect_class = "com.github.pagehelper.PageHelper"; private Field additionalParametersField; private String countSuffix = "_COUNT";
对于Mybatis配置了插件后,实例化插件后就会调用Interceper的setProperties方法。
public void setProperties(Properties properties) { //缓存 count ms msCountMap = CacheFactory.createCache(properties.getProperty("msCountCache"), "ms", properties); String dialectClass = properties.getProperty("dialect"); if (StringUtil.isEmpty(dialectClass)) { dialectClass = default_dialect_class; } try { Class<?> aClass = Class.forName(dialectClass); dialect = (Dialect) aClass.newInstance(); } catch (Exception e) { throw new PageException(e); } dialect.setProperties(properties); String countSuffix = properties.getProperty("countSuffix"); if (StringUtil.isNotEmpty(countSuffix)) { this.countSuffix = countSuffix; } try { //反射获取 BoundSql 中的 additionalParameters 属性 additionalParametersField = BoundSql.class.getDeclaredField("additionalParameters"); additionalParametersField.setAccessible(true); } catch (NoSuchFieldException e) { throw new PageException(e); } }
这个方法中首先会实例化一个Cache实例,用于缓存用于查询数据数量的MappedStatement,如果没有配置dialect就会使用PageHelper,然后调用dialect的setProperties()方法。
public void setProperties(Properties properties) { setStaticProperties(properties); pageParams = new PageParams(); autoDialect = new PageAutoDialect(); pageParams.setProperties(properties); autoDialect.setProperties(properties); }
PageHelper中有一个PageParams对象和一个AutoDialect对象,都会调用自身的setProperties()方法。看一下autoDialect的:
public void setProperties(Properties properties) { //多数据源时,获取 jdbcurl 后是否关闭数据源 String closeConn = properties.getProperty("closeConn"); if (StringUtil.isNotEmpty(closeConn)) { this.closeConn = Boolean.parseBoolean(closeConn); } String dialectAlias = properties.getProperty("dialectAlias"); if (StringUtil.isNotEmpty(dialectAlias)) { String[] alias = dialectAlias.split(";"); for (int i = 0; i < alias.length; i++) { String[] kv = alias[i].split("="); if(kv.length != 2){ throw new IllegalArgumentException("dialectAlias 参数配置错误," + "请按照 alias1=xx.dialectClass;alias2=dialectClass2 的形式进行配置!"); } for (int j = 0; j < kv.length; j++) { try { Class<? extends Dialect> diallectClass = (Class<? extends Dialect>) Class.forName(kv[1]); //允许覆盖已有的实现 registerDialectAlias(kv[0], diallectClass); } catch (ClassNotFoundException e) { throw new IllegalArgumentException("请确保 dialectAlias 配置的 Dialect 实现类存在!", e); } } } } //指定的 Helper 数据库方言,和 不同 String dialect = properties.getProperty("helperDialect"); //运行时获取数据源 String runtimeDialect = properties.getProperty("autoRuntimeDialect"); //1.动态多数据源 if (StringUtil.isNotEmpty(runtimeDialect) && "TRUE".equalsIgnoreCase(runtimeDialect)) { this.autoDialect = false; this.properties = properties; } //2.动态获取方言 else if (StringUtil.isEmpty(dialect)) { autoDialect = true; this.properties = properties; } //3.指定方言 else { autoDialect = false; this.delegate = initDialect(dialect, properties); } }
默认情况下的结果为autoDialect = true。此类还有个方法initDelegateDialect()会在PageHelper的skip方法中调用(马上会说到)。
initDelegateDialect方法会根据jdbcurl自动选取合适的Dialect实例赋值给一个委托变量delegate,PageHelper是一个委派模式,此后对PageHelper对象方法的调用都会委托delegate对象。
接下来就是看Intecepter的核心方法intercept()方法。
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(); } }
dialect.skip()方法用于判断当前查询是否跳过分页,PageHelper的skip()方法。
public boolean skip(MappedStatement ms, Object parameterObject, RowBounds rowBounds) { if(ms.getId().endsWith(MSUtils.COUNT)){ throw new RuntimeException("在系统中发现了多个分页插件,请检查系统配置!"); } Page page = pageParams.getPage(parameterObject, rowBounds); if (page == null) { return true; } else { //设置默认的 count 列 if(StringUtil.isEmpty(page.getCountColumn())){ page.setCountColumn(pageParams.getCountColumn()); } autoDialect.initDelegateDialect(ms);//注意这里哦 return false; } }
PageParams的getPage方法如果返回一个Page对象则不可以跳过分页,说明我们使用了查询分页。
public Page getPage(Object parameterObject, RowBounds rowBounds) { Page page = PageHelper.getLocalPage(); if (page == null) { if (rowBounds != RowBounds.DEFAULT) { if (offsetAsPageNum) { page = new Page(rowBounds.getOffset(), rowBounds.getLimit(), rowBoundsWithCount); } else { page = new Page(new int[]{rowBounds.getOffset(), rowBounds.getLimit()}, rowBoundsWithCount); //offsetAsPageNum=false的时候,由于PageNum问题,不能使用reasonable,这里会强制为false page.setReasonable(false); } if(rowBounds instanceof PageRowBounds){ PageRowBounds pageRowBounds = (PageRowBounds)rowBounds; page.setCount(pageRowBounds.getCount() == null || pageRowBounds.getCount()); } } else if(supportMethodsArguments){ try { page = PageObjectUtil.getPageFromObject(parameterObject, false); } catch (Exception e) { return null; } } if(page == null){ return null; } PageHelper.setLocalPage(page); } //分页合理化 if (page.getReasonable() == null) { page.setReasonable(reasonable); } //当设置为true的时候,如果pagesize设置为0(或RowBounds的limit=0),就不执行分页,返回全部结果 if (page.getPageSizeZero() == null) { page.setPageSizeZero(pageSizeZero); } return page; }
1.PageHelper的getLocalPage()方法是调用ThreadLocal的get()方法,如果返回一个Page对象,说明我们在调用查询接口之前使用了PageHelper的startPage等可以调用 ThreadLocal的set()方法。
2.如果page==null,在判断是否满足rowBounds != RowBounds.DEFAULT,因为该分页插件也支持RowBounds分页的这也是比较建议的一种方式,因为侵入性小。在判断该rowBounds instanceof PageRowBounds,因为PageRowBounds继承RowBounds,使用它可以支持对该分页查询的count查询,先做个标记page.setCount(pageRowBounds.getCount() == null || pageRowBounds.getCount());
3.如果上述条件不满足在判断是否supportMethodsArguments==true,这种条件下会使用参数对象的属性作为分页的关键字。这种用的方式较少暂不分析,默认值如下:
PARAMS.put("pageNum", "pageNum"); PARAMS.put("pageSize", "pageSize"); PARAMS.put("count", "countSql"); PARAMS.put("orderBy", "orderBy"); PARAMS.put("reasonable", "reasonable"); PARAMS.put("pageSizeZero", "pageSizeZero");
情况2和3是插件为我们自动创建的Page对象,调用PageHelper.setLocalPage(page)保存到当前线程。
下面就说一下满足分页查询的情况。
1、dialect.beforeCount()方法判断是否需要count()查询。
beforeCount()方法定义在AbstractHelperDialect,就是查看ThreadLocal中Page对象的count属性。
public boolean beforeCount(MappedStatement ms, Object parameterObject, RowBounds rowBounds) { Page page = getLocalPage(); return !page.isOrderByOnly() && page.isCount(); }
需要count查询,接下来会根据当前查询的MappedStatement构建一个count查询的MappedStatement。除了id与返回类型不同其余都一样。
public static MappedStatement newCountMappedStatement(MappedStatement ms, String newMsId) { MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(), newMsId, 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(), Long.class, EMPTY_RESULTMAPPING).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(); }
之后执行方法executeAutoCount(executor, countMs, parameter, boundSql, rowBounds, resultHandler)获取count插叙的结果。
private Long executeAutoCount(Executor executor, MappedStatement countMs, Object parameter, BoundSql boundSql, RowBounds rowBounds, ResultHandler resultHandler) throws IllegalAccessException, SQLException { Map<String, Object> additionalParameters = (Map<String, Object>) additionalParametersField.get(boundSql); //创建 count 查询的缓存 key CacheKey countKey = executor.createCacheKey(countMs, parameter, RowBounds.DEFAULT, boundSql); //调用方言获取 count sql String countSql = dialect.getCountSql(countMs, boundSql, parameter, rowBounds, countKey); //countKey.update(countSql); BoundSql countBoundSql = new BoundSql(countMs.getConfiguration(), countSql, boundSql.getParameterMappings(), parameter); //当使用动态 SQL 时,可能会产生临时的参数,这些参数需要手动设置到新的 BoundSql 中 for (String key : additionalParameters.keySet()) { countBoundSql.setAdditionalParameter(key, additionalParameters.get(key)); } //执行 count 查询 Object countResultList = executor.query(countMs, parameter, RowBounds.DEFAULT, resultHandler, countKey, countBoundSql); Long count = (Long) ((List) countResultList).get(0); return count; }
dialect.getCountSql根据当前查询sql语句返回一个select count(0)。。。形式的sql语句,因为AbstractHelperDialect内的countSqlParser提供了这样的能力(有兴趣的自己看下具体实现,这里不再展开)。构建好了新的BoundSql后传入executor的query方法返回数量。
2.dialect.beforePage(ms, parameter, rowBounds)) 判断是否需要进行分页查询
public boolean beforePage(MappedStatement ms, Object parameterObject, RowBounds rowBounds) { Page page = getLocalPage(); if (page.isOrderByOnly() || page.getPageSize() > 0) { return true; } return false; }
String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);这句是分页的核心方法,用于加入分页语句,对于mysql就是limit xx,yy。
//执行分页查询
resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);
最后调用dialect.afterPage(resultList, parameter, rowBounds);填充resultList和count到ThreadLocal的Page中。
在intercept方法中最后的finally代码块中会调用 dialect.afterAll();内部会清除ThreadLocal的变量,所以每次调用分页方法都要重新设置Page到ThreadLocal中。