The most common source of unintentional object retention is the use of a Map to associate metadata with transient objects.

无意识对象保留最常见的原因是使用Map将元数据与临时对象（transient object）相关联。

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It also makes it possible to retain metadata at source code level in class files, and at runtime controlled by a retention policy.

它还使得在类文件中以源代码级别保留元数据、并在运行时由保持(retention)策略控制成为可能。

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With the growing increase of the retention of the vehicles in the city, the gas emitted from vehicles has become the most important source of the urban air pollution.

随着城市机动车保有量的不断增长，机动车尾气污染问题已经成为城市空气污染最重要的来源，控制机动车尾气污染也已经成为改善城市大气环境质量的当务之急。

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With the growing increase of the retention of the vehicles in the city, the gas emitted from vehicles has become the most important source of the urban air pollution.

随着城市机动车保有量的不断增长，机动车尾气污染问题已经成为城市空气污染最重要的来源，控制机动车尾气污染也已经成为改善城市大气环境质量的当务之急。

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