在前面的文章里,介绍了两个插件: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,编写了拦截cache的插件,通过这个拦截器修正了这些问题。
1 问题
1.1 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。
1.2 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。
1.3 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。
2 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 {
...
}
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。
3 intercept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);
while (metaExecutor.hasGetter("h")) {
Object object = metaExecutor.getValue("h");
metaExecutor = MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
}
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);
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();
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));
}
}
}
}
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;
}
4 plugin实现
public Object plugin(Object target) {
if (target instanceof CachingExecutor) {
return Plugin.wrap(target, this);
} else {
return target;
}
}