数据库迁移活动的目的是确保相同的应用程序层能够与另一个RDBMS交互。

Database migration activity is done to ensure that the same application layer is able to talk to a different RDBMS.

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类包装背后的理念是：目标类可以包装在一层插装代码中，后者具有与被包装的类相同的外部行为。

The idea behind class wrapping is that a target class can be wrapped in a layer of instrumenting code that retains the same external behavior as the class that's wrapped.

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您还可以使用与清单4内相同的方法向地图添加道路层，只不过需要用适当的值代替layers和map选项。

You add the roads layer to the map using the same method as in Listing 4, substituting the layers and map options with appropriate values.

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