使用reify创建的一次性实例会导致NPE

Question

我正在尝试围绕Java API编写一个Clojure层，如下所示：

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，允许将fn作为ExecutorJob的execute方法的主体执行。这是我想出来的：

(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，考虑到这个调用：

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--

我尝试将一些有意义的execute-as调用放在Java参数中，我可以看到，使用调试器可以很好地执行这些调用。

这个宏有什么问题？

Answer 1

不要紧，我明白了:我误用了宏参数，并试图实际调用表单f的执行结果。而且它的收益率为零，因此产生了NPE。

更正后的版本：

(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)))