Google Datastore for Java 文档摘录（一）

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 over very large data sets.

为了在大量数据集中快速查询到结果，在datastore中所做的所有查询都必须事先建立索引。与在关系型数据库系统中建立索引的方式不同，在Google App Engine for Java应用中，整个应用程序所使用到的索引被保存在一个XML文件中。你可以手工编写，也可以由Google Plugin for Eclipse自动生成。

 

With the App Engine datastore, every attempt to create, update or delete an entity happens in a transaction .

对App Engine datastore而言，每次创建、更新或删除一个实体的行为都在一个事务中完成。

 

All entities fetched, created, updated or deleted in a transaction must be in the same entity group.

在一个事务中，对多个实体进行增、删、改、查操作时，这些实体必须属于同一“实体组”。在这里，对“实体组”的概念不好理解，下面以一组代码段来解释：

public class Employee {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Long id;

    @Persistent
    private String firstName;

    @Persistent
    private String lastName;

    @Persistent
    private Date hireDate;

    ...
}

 

public class ContactInfo {
	@PrimaryKey
	@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
	private Key key;

	@Persistent
	private String streetAddress;

	@Persistent
	private String city;

	@Persistent
	private String stateOrProvince;

	@Persistent
	private String zipCode;
      
        ...
}

 

		Employee em = new Employee("Jiankang", "Jin", new Date());
		ContactInfo info = new ContactInfo("ZhiChun Road", "Beijing",
				"Beijing", "010");

		PersistenceManager pm = PMF.get().getPersistenceManager();
		Transaction tx = pm.currentTransaction();

		try {
			tx.begin();
			pm.makePersistent(em);
			pm.makePersistent(info);
			tx.commit();







		} catch (Exception e) {
			System.out.println(e);
		} finally {
			if (tx.isActive()) {
				tx.rollback();
			}
			pm.close();
		}

运行Servlet，在控制台看到：

javax.jdo.JDOFatalUserException: Illegal argument
NestedThrowables:
java.lang.IllegalArgumentException: can't operate on multiple entity groups in a single transaction. 

修改后：

public class Employee {
    ...
    @Persistent
    private ContactInfo contactInfo;
    ...
}

 

		ContactInfo info = new ContactInfo("ZhiChun Road", "Beijing",
				"Beijing", "010");
		Employee em = new Employee("Jiankang", "Jin", new Date(), info);

		PersistenceManager pm = PMF.get().getPersistenceManager();
		Transaction tx = pm.currentTransaction();

		try {
			tx.begin();
			pm.makePersistent(em);
			pm.makePersistent(info);
			tx.commit();
		} catch (Exception e) {
			System.out.println(e);
		} finally {
			if (tx.isActive()) {
				tx.rollback();
			}
			pm.close();
		}

 再次运行Servlet，无异常。查看http://127.0.0.1:8080/_ah/admin，Kind Employee有一条记录，Kind ContactInfo有一条记录（注意：不是两条，代码pm.makePersistent(info)是不必要的，即使写了，也只保存一条ContactInfo记录）。而且，The key of the ContactInfo entity has the key of the Employee entity as its entity group parent. 这句话可能引起歧义，我原以为前者的key包含后者的key，但结果是：

the key of Employee is           'agByDgsSCEVtcGxveWVlGAEM',

and the key of ContactInfo is 'agByHwsSCEVtcGxveWVlGAEMCxILQ29udGFjdEluZm8YAgw'.

在Google Groups上请教了该问题，上述现象是正常的，两者的key确实不是简单的字符串包含关系。但可以肯定的是：

KeyFactory.stringToKey(employeeKeyString).equals(KeyFactory.stringToKey(con

tactInfoKeyString).getParent())

参见：http://groups.google.com/group/google-appengine-java/browse_thread/thread/76ca595de10a5a5a/95509d1364acc9d4?lnk=gst&q=jinjiankang1980#95509d1364acc9d4

 

Entity groups are defined by a hierarchy of relationships between entities.

如果多个实体间有关联关系，则它们组成一个实体组。据我我理解，这里的“a hierarchy of relationships”就是指UML中的组合或聚合。

 

（如何使用附件源代码注：使用Google Eclipse插件建立工程，工程名GAE_Test，包名com.gaetest）

（所使用的GAE JDK：appengine-java-sdk-1.2.2）