深入浅出Mybatis-改造Cache

为了方便修改BUG，我在github上创建了一个仓库，地址：https://github.com/hupanfeng/hdd。欢迎大家在留言里提交问题，我会尽快修复，并将修复的代码提交至github上。

在前面的文章里，我开发了两个插件：根据注解实现的sql自动生成插件和分页插件。这两个插件在没有开启cache的情况下可以很好的使用，但开启cache后却出现了一些问题，为了解决这些问题，我编写了拦截cache的插件，通过这个拦截器修正了这些问题。

问题

什么问题

最容易出现的问题是开启cache后，分页查询时无论查询哪一页都返回第一页的数据。另外，使用sql自动生成插件生成get方法的sql时，传入的参数不起作用，无论传入的参数是多少，都返回第一个参数的查询结果。

 

为什么出现这些问题

在之前讲解Mybatis的执行流程的时候提到，在开启cache的前提下，Mybatis的executor会先从缓存里读取数据，读取不到才去数据库查询。问题就出在这里，sql自动生成插件和分页插件执行的时机是在statementhandler里，而statementhandler是在executor之后执行的，无论sql自动生成插件和分页插件都是通过改写sql来实现的，executor在生成读取cache的key（key由sql以及对应的参数值构成）时使用都是原始的sql，这样当然就出问题了。

解决问题

找到问题的原因后，解决起来就方便了。只要通过拦截器改写executor里生成key的方法，在生成可以时使用自动生成的sql（对应sql自动生成插件）或加入分页信息（对应分页插件）就可以了。

拦截器签名

@Intercepts({@Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})
public class CacheInterceptor implements Interceptor {
...
}

@Intercepts({@Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})
public class CacheInterceptor implements Interceptor {
...
}

从签名里可以看出，要拦截的目标类型是Executor（注意：type只能配置成接口类型），拦截的方法是名称为query的方法。

intercept的实现

public Object intercept(Invocation invocation) throws Throwable {
        Executor executorProxy = (Executor) invocation.getTarget();
        MetaObject metaExecutor = MetaObject.forObject(executorProxy, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
        // 分离代理对象链
        while (metaExecutor.hasGetter("h")) {
            Object object = metaExecutor.getValue("h");
            metaExecutor = MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
        }
        // 分离最后一个代理对象的目标类
        while (metaExecutor.hasGetter("target")) {
            Object object = metaExecutor.getValue("target");
            metaExecutor = MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
        }
        Object[] args = invocation.getArgs();
        return this.query(metaExecutor, args);
    }

    public <E> List<E> query(MetaObject metaExecutor, Object[] args) throws SQLException {
        MappedStatement ms = (MappedStatement) args[0];
        Object parameterObject = args[1];
        RowBounds rowBounds = (RowBounds) args[2];
        ResultHandler resultHandler = (ResultHandler) args[3];
        BoundSql boundSql = ms.getBoundSql(parameterObject);
        // 改写key的生成
        CacheKey cacheKey = createCacheKey(ms, parameterObject, rowBounds, boundSql);
        Executor executor = (Executor) metaExecutor.getOriginalObject();
        return executor.query(ms, parameterObject, rowBounds, resultHandler, cacheKey, boundSql);
    }

    private CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
        Configuration configuration = ms.getConfiguration();
        pageSqlId = configuration.getVariables().getProperty("pageSqlId");
        if (null == pageSqlId || "".equals(pageSqlId)) {
            logger.warn("Property pageSqlId is not setted,use default '.*Page$' ");
            pageSqlId = defaultPageSqlId;
        }
        CacheKey cacheKey = new CacheKey();
        cacheKey.update(ms.getId());
        cacheKey.update(rowBounds.getOffset());
        cacheKey.update(rowBounds.getLimit());
        List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
        // 解决自动生成SQL，SQL语句为空导致key生成错误的bug
        if (null == boundSql.getSql() || "".equals(boundSql.getSql())) {
            String id = ms.getId();
            id = id.substring(id.lastIndexOf(".") + 1);
            String newSql = null;
            try {
                if ("select".equals(id)) {
                    newSql = SqlBuilder.buildSelectSql(parameterObject);
                }
                SqlSource sqlSource = buildSqlSource(configuration, newSql, parameterObject.getClass());
                parameterMappings = sqlSource.getBoundSql(parameterObject).getParameterMappings();
                cacheKey.update(newSql);
            } catch (Exception e) {
                logger.error("Update cacheKey error.", e);
            }
        } else {
            cacheKey.update(boundSql.getSql());
        }

        MetaObject metaObject = MetaObject.forObject(parameterObject, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);

        if (parameterMappings.size() > 0 && parameterObject != null) {
            TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
            if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
                cacheKey.update(parameterObject);
            } else {
                for (ParameterMapping parameterMapping : parameterMappings) {
                    String propertyName = parameterMapping.getProperty();
                    if (metaObject.hasGetter(propertyName)) {
                        cacheKey.update(metaObject.getValue(propertyName));
                    } else if (boundSql.hasAdditionalParameter(propertyName)) {
                        cacheKey.update(boundSql.getAdditionalParameter(propertyName));
                    }
                }
            }
        }
        // 当需要分页查询时，将page参数里的当前页和每页数加到cachekey里
        if (ms.getId().matches(pageSqlId) && metaObject.hasGetter("page")) {
            PageParameter page = (PageParameter) metaObject.getValue("page");
            if (null != page) {
                cacheKey.update(page.getCurrentPage());
                cacheKey.update(page.getPageSize());
            }
        }
        return cacheKey;
}

public Object intercept(Invocation invocation) throws Throwable {
        Executor executorProxy = (Executor) invocation.getTarget();
        MetaObject metaExecutor = MetaObject.forObject(executorProxy, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
        // 分离代理对象链
        while (metaExecutor.hasGetter("h")) {
            Object object = metaExecutor.getValue("h");
            metaExecutor = MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
        }
        // 分离最后一个代理对象的目标类
        while (metaExecutor.hasGetter("target")) {
            Object object = metaExecutor.getValue("target");
            metaExecutor = MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
        }
        Object[] args = invocation.getArgs();
        return this.query(metaExecutor, args);
    }

    public <E> List<E> query(MetaObject metaExecutor, Object[] args) throws SQLException {
        MappedStatement ms = (MappedStatement) args[0];
        Object parameterObject = args[1];
        RowBounds rowBounds = (RowBounds) args[2];
        ResultHandler resultHandler = (ResultHandler) args[3];
        BoundSql boundSql = ms.getBoundSql(parameterObject);
        // 改写key的生成
        CacheKey cacheKey = createCacheKey(ms, parameterObject, rowBounds, boundSql);
        Executor executor = (Executor) metaExecutor.getOriginalObject();
        return executor.query(ms, parameterObject, rowBounds, resultHandler, cacheKey, boundSql);
    }

    private CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
        Configuration configuration = ms.getConfiguration();
        pageSqlId = configuration.getVariables().getProperty("pageSqlId");
        if (null == pageSqlId || "".equals(pageSqlId)) {
            logger.warn("Property pageSqlId is not setted,use default '.*Page$' ");
            pageSqlId = defaultPageSqlId;
        }
        CacheKey cacheKey = new CacheKey();
        cacheKey.update(ms.getId());
        cacheKey.update(rowBounds.getOffset());
        cacheKey.update(rowBounds.getLimit());
        List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
        // 解决自动生成SQL，SQL语句为空导致key生成错误的bug
        if (null == boundSql.getSql() || "".equals(boundSql.getSql())) {
            String id = ms.getId();
            id = id.substring(id.lastIndexOf(".") + 1);
            String newSql = null;
            try {
                if ("select".equals(id)) {
                    newSql = SqlBuilder.buildSelectSql(parameterObject);
                }
                SqlSource sqlSource = buildSqlSource(configuration, newSql, parameterObject.getClass());
                parameterMappings = sqlSource.getBoundSql(parameterObject).getParameterMappings();
                cacheKey.update(newSql);
            } catch (Exception e) {
                logger.error("Update cacheKey error.", e);
            }
        } else {
            cacheKey.update(boundSql.getSql());
        }

        MetaObject metaObject = MetaObject.forObject(parameterObject, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);

        if (parameterMappings.size() > 0 && parameterObject != null) {
            TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
            if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
                cacheKey.update(parameterObject);
            } else {
                for (ParameterMapping parameterMapping : parameterMappings) {
                    String propertyName = parameterMapping.getProperty();
                    if (metaObject.hasGetter(propertyName)) {
                        cacheKey.update(metaObject.getValue(propertyName));
                    } else if (boundSql.hasAdditionalParameter(propertyName)) {
                        cacheKey.update(boundSql.getAdditionalParameter(propertyName));
                    }
                }
            }
        }
        // 当需要分页查询时，将page参数里的当前页和每页数加到cachekey里
        if (ms.getId().matches(pageSqlId) && metaObject.hasGetter("page")) {
            PageParameter page = (PageParameter) metaObject.getValue("page");
            if (null != page) {
                cacheKey.update(page.getCurrentPage());
                cacheKey.update(page.getPageSize());
            }
        }
        return cacheKey;
} 

plugin的实现

public Object plugin(Object target) {
    // 当目标类是CachingExecutor类型时，才包装目标类，否者直接返回目标本身,减少目标被代理的
    // 次数
    if (target instanceof CachingExecutor) {
        return Plugin.wrap(target, this);
    } else {
        return target;
    }
}

    public Object plugin(Object target) {
        // 当目标类是CachingExecutor类型时，才包装目标类，否者直接返回目标本身,减少目标被代理的
        // 次数
        if (target instanceof CachingExecutor) {
            return Plugin.wrap(target, this);
        } else {
            return target;
        }
    }

源码

下载地址：http://download.csdn.net/detail/hupanfeng/6609263