jinjiankang1980的专栏

文档中有：“java.lang.String:   A value longer than 500 bytes throws a JDOFatalUserException.”。经测试，对String类型属性，最多可将500个汉字，或者500个英文字母保存进Datastore。再多一根稻草，则抛出：java.lang.IllegalArgumentException: 属性名: String ...

The datastore is designed with web applications in mind, with an emphasis on read and query performance.要知道，datastore是为Web应用程序而设计的，其重点关注的是读取和查询的性能。 All queries are pre-indexed for fast results o...

The following features of the JDO interface are not supported by the App Engine implementation:contains() syntax for query filters on Collectionfields. You can test that a multi-valued proper...

You can modify an object after the PersistenceManager has been closedby declaring the class as "detachable." To do this, add the detachable attribute to the @PersistenceCapable annotation:impo...

Embedded classes allow you to model a field value using a class withoutcreating a new datastore entity and forming a relationship.你可以使用嵌套类作为POJO的属性，这样，嵌套类就不会作为单独实体被保存在datastore中，也不和POJO类形成关联关系。 ...

这下可要多读几遍了。

POJO类的某个属性可以是一个Serializable 类的实例，该属性被保存在Datastore中的数据类型是com.google.appengine.api.datastore.Blob。该属性需要使用 @Persistent(serialized=true) 批注。import java.io.Serializable;public class DownloadableF...