为什么System.console（）为命令行应用程序返回null？ [英] Why does System.console() return null for a command line app?

问题描述

我正在使用一个依赖于用户命令行输入的旧版应用程序：

  String key = System.console .readLine（请输入许可证密钥：）;但是，我得到一个 NullPointerException 因为 System.console（）给我一个null。
 
 
 
为什么 System.console ）为命令行应用程序返回null？ 
解决方案
如果从终端窗口启动java， 
 
 
 
如果你直接从终端运行一个简单的测试，它工作？
  echo'public class Test {public static void main（String [] args）{System.console printf（hello world％n）;}}'> Test.java&&& javac Test.java&& java Test 
  
预期输出：
  hello world 
  
如果不起作用， 
 
 
 
但是，如果它可以正常工作，而你的程序没有，那么你的程序是如何启动的。
 
 
 
检查java二进制文件是如何开始的？它是从shell脚本启动的吗？检查stdin / stdout没有被重定向或管道输入到一些东西，并且可能还没有在后台启动。
 
 
 
 ex：这可能会使System.console ）return null。
  tee> app.log 
  
和此：
  java Test> /tmp/test.log 
  
这似乎工作在我的机器（linux）
  java测试& 
  
似乎 System.setOut ， System.setErr 或 System.setIn 会影响控制台，即使经过几个gc和finalizer。 
 
 
 
 
 
 
 不过，
 
 
 
关闭（原始） System.out 或 System.in 将禁用控制台。 
  echo'public class Test {public static void main（String [] args）{System.out.close .console（）。printf（hello world％n）;}}'> Test.java&& javac Test.java&& java Test 
  
预期输出：
 线程main中的异常java.lang.NullPointerException 
 at Test.main（Test.java:1）
  
 So;扫描您的代码以查找关闭流的位置，或将System.out传递到可能关闭的位置。
 
I am working on a legacy app which depends on user command line input:
String key = System.console().readLine("Please enter the license key: ");
However, I am getting a NullPointerException because System.console() gives me a null.

Why does System.console() return null for a command line app?  It happens when running it out of the terminal as well as IDE.
解决方案
If you start java from a terminal window, then it really should work, even though I haven't tried on OSX.

If you run a simple test using java directly from the terminal, does it work?
echo 'public class Test { public static void main(String[] args) {System.console().printf("hello world%n");}}' >Test.java && javac Test.java && java Test 
Expected output:
hello world
If it doesn't work, then sorry, no console support on your platform.

However, if it works, and your program doesn't then there is a problem with how your program is started.

Check how the java binary started? Is it started from a shell script? Check that stdin/stdout have not been  redirected or piped into something, and possibly also that it's not started in the background.

ex: This will probably make System.console() return null.
java Test | tee >app.log    
and this:
java Test >/tmp/test.log
This seems to work on my machine (linux)
java Test &
Neither does it seem as if System.setOut, System.setErr or System.setIn affects the console, even after a couple of gc's and finalizers. 



However:

Closing (the original) System.out or System.in will disable the console too. 
echo 'public class Test { public static void main(String[] args) {System.out.close();System.console().printf("hello world%n");}}' >Test.java && javac Test.java && java Test 
Expected output:
Exception in thread "main" java.lang.NullPointerException
at Test.main(Test.java:1)
So; scan your code for places where it closes streams, or passes System.out somewhere it might get closed.

                        
这篇关于为什么System.console（）为命令行应用程序返回null？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！