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，允许执行fn作为ExecutorJob的execute方法的主体。这就是我想出的：

(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，给了这个电话：

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--

我试图在execute-as秒参数中放入一些有意义的Java调用，我可以看到使用调试器执行得很好。

该宏出了什么问题？

Answer 1

没关系，我明白了：我误用了宏参数，试图实际调用执行表格f的结果。它产生零，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)))

使用reify创建的一次性实例会导致NPE

1 个答案: