Clojure：使用 LazySeq 的 NullPointerException

Question

第一次尝试访问 LazySeq 时出现 NullPointerException， line-items - 这需要一些时间来评估，然后在下一行计算 total-exc:

时出现 NullPointerException

(defn orders->invoice [db billing-account-id dt orders resource-name]
    (let [line-items (mapcat (partial order->line-items db dt resource-name) orders) ;;lazy
          total-exc  (reduce + (map :invoice-item/total-exclusive line-items))       ;; NullPointerException
          total-inc  (reduce + (map :invoice-item/total-inclusive line-items))]

     {:invoice/items           line-items
      :invoice/total-exclusive total-exc
      :invoice/total-inclusive total-inc}))

没有数据丢失，就好像我把行放在 try catch 块中的中断处吞下它并在 catch 块中尝试相同它会起作用：

(try 
    (reduce + (map :invoice-item/total-exclusive line-items)) 
    (catch Exception e
      (clojure.pprint/pprint (.getStackTrace e))
      (reduce + (map :invoice-item/total-exclusive line-items))))

堆栈跟踪：

[[java.util.concurrent.FutureTask report "FutureTask.java" 122],
 [java.util.concurrent.FutureTask get "FutureTask.java" 192],
 [clojure.core$deref_future invokeStatic "core.clj" 2300],
 [clojure.core$future_call$reify__8439 deref "core.clj" 6974],
 [clojure.core$deref invokeStatic "core.clj" 2320],
 [clojure.core$deref invoke "core.clj" 2306],
 [clojure.core$map$fn__5851 invoke "core.clj" 2755],
 [clojure.lang.LazySeq sval "LazySeq.java" 42],
 [clojure.lang.LazySeq seq "LazySeq.java" 58],
 [clojure.lang.RT seq "RT.java" 531],
 [clojure.core$seq__5387 invokeStatic "core.clj" 137],
 [clojure.core$filter$fn__5878 invoke "core.clj" 2809],
 [clojure.lang.LazySeq sval "LazySeq.java" 42],
 [clojure.lang.LazySeq seq "LazySeq.java" 51],
 [clojure.lang.RT seq "RT.java" 531],
 [clojure.core$seq__5387 invokeStatic "core.clj" 137],
 [clojure.core$map$fn__5851 invoke "core.clj" 2746],
 [clojure.lang.LazySeq sval "LazySeq.java" 42],
 [clojure.lang.LazySeq seq "LazySeq.java" 51],
 [clojure.lang.RT seq "RT.java" 531],
 [clojure.core$seq__5387 invokeStatic "core.clj" 137],
 [clojure.core$concat$cat__5480$fn__5481 invoke "core.clj" 734],
 [clojure.lang.LazySeq sval "LazySeq.java" 42],
 [clojure.lang.LazySeq seq "LazySeq.java" 58],
 [clojure.lang.RT seq "RT.java" 531],
 [clojure.core$seq__5387 invokeStatic "core.clj" 137],
 [clojure.core$map$fn__5851 invoke "core.clj" 2746],
 [clojure.lang.LazySeq sval "LazySeq.java" 42],
 [clojure.lang.LazySeq seq "LazySeq.java" 51],
 [clojure.lang.Cons next "Cons.java" 39],
 [clojure.lang.RT next "RT.java" 709],
 [clojure.core$next__5371 invokeStatic "core.clj" 64],
 [clojure.core.protocols$fn__8144 invokeStatic "protocols.clj" 169],
 [clojure.core.protocols$fn__8144 invoke "protocols.clj" 124],
 [clojure.core.protocols$fn__8099$G__8094__8108 invoke "protocols.clj" 19],
 [clojure.core.protocols$seq_reduce invokeStatic "protocols.clj" 27],
 [clojure.core.protocols$fn__8131 invokeStatic "protocols.clj" 75],
 [clojure.core.protocols$fn__8131 invoke "protocols.clj" 75],
 [clojure.core.protocols$fn__8073$G__8068__8086 invoke "protocols.clj" 13],
 [clojure.core$reduce invokeStatic "core.clj" 6824],
 [clojure.core$reduce invoke "core.clj" 6810],
 [user$eval106182 invokeStatic "form-init3664079349963806059.clj" 15],
 [user$eval106182 invoke "form-init3664079349963806059.clj" 14],
 [clojure.lang.Compiler eval "Compiler.java" 7176],
 [clojure.lang.Compiler eval "Compiler.java" 7131],
 [clojure.core$eval invokeStatic "core.clj" 3214],
 [clojure.core$eval invoke "core.clj" 3210],
 [clojure.main$repl$read_eval_print__9068$fn__9071 invoke "main.clj" 414],
 [clojure.main$repl$read_eval_print__9068 invoke "main.clj" 414],
 [clojure.main$repl$fn__9077 invoke "main.clj" 435],
 [clojure.main$repl invokeStatic "main.clj" 435],
 [clojure.main$repl doInvoke "main.clj" 345],
 [clojure.lang.RestFn applyTo "RestFn.java" 137],
 [clojure.core$apply invokeStatic "core.clj" 665],
 [clojure.core$apply invoke "core.clj" 660],
 [refactor_nrepl.ns.slam.hound.regrow$wrap_clojure_repl$fn__62865 doInvoke "regrow.clj" 18],
 [clojure.lang.RestFn invoke "RestFn.java" 1523],
 [clojure.tools.nrepl.middleware.interruptible_eval$evaluate$fn__53116 invoke "interruptible_eval.clj" 87],
 [clojure.lang.AFn applyToHelper "AFn.java" 152],
 [clojure.lang.AFn applyTo "AFn.java" 144],
 [clojure.core$apply invokeStatic "core.clj" 665],
 [clojure.core$with_bindings_STAR_ invokeStatic "core.clj" 1973],
 [clojure.core$with_bindings_STAR_ doInvoke "core.clj" 1973],
 [clojure.lang.RestFn invoke "RestFn.java" 425],
 [clojure.tools.nrepl.middleware.interruptible_eval$evaluate invokeStatic "interruptible_eval.clj" 85],
 [clojure.tools.nrepl.middleware.interruptible_eval$evaluate invoke "interruptible_eval.clj" 55][clojure.tools.nrepl.middleware.interruptible_eval$interruptible_eval$fn__53161$fn__53164 invoke "interruptible_eval.clj" 222],
 [clojure.tools.nrepl.middleware.interruptible_eval$run_next$fn__53156 invoke "interruptible_eval.clj" 190],
 [clojure.lang.AFn run "AFn.java" 22],
 [java.util.concurrent.ThreadPoolExecutor runWorker "ThreadPoolExecutor.java" 1149],
 [java.util.concurrent.ThreadPoolExecutor$Worker run "ThreadPoolExecutor.java" 624],
 [java.lang.Thread run "Thread.java" 748]]

返回值：

176805

这已经工作了很多年，唯一的区别是有更多的数据要处理，所以它需要更长的时间。

我试过用 doall 实现 LazySeq - 同样的事情，但只是第一次。

我以前从未见过这样的行为，因此非常感谢您的帮助。

Answer 1

你的 line-items 有一个例外，直到你试图实现它以求和才浮出水面。您将从这样的值中看到相同的行为：

user=> (def xs (take 10 (map #(/ 2 %) (iterate dec 3))))
#'user/xs
user=> (apply + xs)
Execution error (ArithmeticException) at user/fn (REPL:1).
Divide by zero
user=> (apply + xs)
11/3 ;; (which is 2/3 + 2/2 + 2/1)

第二次尝试获得有效结果并不意味着您的输入有效。该异常只是导致 xs 丢弃了最后一个有效项目之后的所有内容。

因此，与其查找您发布的代码中的错误，不如弄清楚您最终是如何生成无效的 line-items。

Clojure：使用 LazySeq 的 NullPointerException

Clojure: NullPointerException using LazySeq

clojure

nullpointerexception

lazy-sequences