java - 使用 reify 创建的一次性实例会导致 NPE

Question

我正在尝试围绕如下所示的 Java API 编写 Clojure 层:

public class Executor {

  public interface ExecutorJob<Result> {
    public Result execute () throws Exception;
  }

  public static <R> R executeAsUser(RunAsWork<R> executorJob, String uid) {
    try {
      //...
      R result = executorJob.execute();
      return result;
    }
    finally {
      //...
    }
  }

}

我的目标是创建一个 Clojure API，允许执行 fn 作为 ExecutorJob 的 execute 方法的主体。这是我想出的:

(defmacro execute-as
  "Runs the form f while impersonating the given user"
  [user f]
  `(let [work# (reify Executor$ExecutorJob
                     (~'execute [~'this]
                               (~f)))]
     (Executor/executeAsUser work# ~user)))

不幸的是，给出这个调用:

user> (macroexpand '(run-as "admin" (.println System/out "test")))
(let* [work__2928__auto__ (clojure.core/reify package.to.Executor$ExecutorJob (execute [this] ((.println System/out "test"))))] (package.to.Executor/executeAsUser work__2928__auto__ "admin"))

它会导致 NPE:

user> (execute-as "admin" (.println System/out "test"))

No message.
  [Thrown class java.lang.NullPointerException]

Restarts:
 0: [QUIT] Quit to the SLIME top level

Backtrace:
  0: user$eval2936$reify__2937.doWork(NO_SOURCE_FILE:1)
  1: package.to.Executor.executeAsUser(Executor.java:508)
  2: user$eval2936.invoke(NO_SOURCE_FILE:1)
  3: clojure.lang.Compiler.eval(Compiler.java:5424)
  4: clojure.lang.Compiler.eval(Compiler.java:5391)
  5: clojure.core$eval.invoke(core.clj:2382)
 --more--

我尝试在 execute-as 第二个参数中放置一些有意义的 Java 调用，我可以看到使用调试器执行得很好。

那个宏有什么问题？

Answer 1

没关系，我明白了:我误用了宏参数并试图实际调用 f 形式的执行结果。它没有产生任何 yield ，因此是 NPE。

更正版本:

(defmacro execute-as
  "Runs the form f while impersonating the given user"
  [user f]
  `(let [work# (reify Executor$ExecutorJob
                     (~'execute [~'this]
                               ~f))]
     (Executor/executeAsUser work# ~user)))