Esper：EPL 查询中的链接属性访问和方法调用
我目前正在努力让一些 Esper EPL 查询正常工作。查询如下所示：
select a.center.distance(b.center) as delta
from pattern [
every-distinct(a.id, b.id) (
a=org.example.PositionEvent -> b=org.example.PositionEvent
)
]
当我尝试通过 EPAdministrator.createEPL() 将其编译为 EPLStatement 时，它会抛出以下异常：
com.espertech.esper.client.EPStatementException：启动语句时出错：无法按名称“a.center”加载类，请检查导入
如果我修改事件类和查询以像这样读取，
select a.distance(b) as delta
from pattern [
every-distinct(a.id, b.id) (
a=org.example.PositionEvent -> b=org.example.PositionEvent
)
]
它编译得很好。 Esper 似乎将 a.center.distance(...) 解释为类名，后跟静态方法调用，而它解释 a.distance(...)作为对对象 a 的方法调用。
如何让 Esper 按预期解释我的原始查询（即作为属性访问，然后是方法调用）？
I am currently struggling to get some Esper EPL queries to work. The queries are looking like this:
select a.center.distance(b.center) as delta
from pattern [
every-distinct(a.id, b.id) (
a=org.example.PositionEvent -> b=org.example.PositionEvent
)
]
When I try to compile this into an EPLStatement via EPAdministrator.createEPL() it throws the following exception:
com.espertech.esper.client.EPStatementException: Error starting statement: Could not load class by name 'a.center', please check imports
If I modify the event classes and the query to read like this
select a.distance(b) as delta
from pattern [
every-distinct(a.id, b.id) (
a=org.example.PositionEvent -> b=org.example.PositionEvent
)
]
it compiles just fine. Esper seems to interpret a.center.distance(...) as a class name followed by a static method invocation, while it interprets a.distance(...) as a method call on the object a.
How can I make Esper interpret my original query as intended (i.e. as a property access followed by a method invocation)?
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解决方案实际上简单明了（但有点难看）：使用括号，就像在其他地方当某些内容可能不明确时所做的那样。因此，为了使第一个查询工作，请像这样编写：
在这种情况下，通过添加括号，它实际上看起来更具可读性。
The solution is actually simple and straightforward (yet a little bit ugly): Use parentheses, just like you do everywhere else when something could be ambiguous. So to make that first query work, write it like this:
In this case, it actually looks even slightly more readable with the added parentheses.