关于System.out.println()中的out=null不报空指针问题

在System类的最上面有个静态块，在第一次使用System类的时候加载这个静态块。如下：

   /* register the natives via the static initializer.
      *
      * VM will invoke the initializeSystemClass method to complete
      * the initialization for this class separated from clinit.
      * Note that to use properties set by the VM, see the constraints
      * described in the initializeSystemClass method.
      */
     private static native void registerNatives();
     static {
         registerNatives();
     }

这部分代码调用了本地方法registerNatives()来初始化in,out,err。
Java代码前面加了final只是让用户无法赋值而已。

■参考这个解释
https://stackoverflow.com/questions/31743760/system-out-is-declared-as-static-final-and-initialized-with-null
■registerNatives()方法参考这个解释
https://stackoverflow.com/questions/1010645/what-does-the-registernatives-method-do

■如果想重新制定System.out的输出方向，参考这个
https://stackoverflow.com/questions/5339499/resetting-standard-output-stream