Java™ Application Development on Linux - Dator

104Chapter 4Where Am I? Execution Contextor even a whole package of classes that write log messages to System.out(e.g., System.out.println("some message")). Now you realize that you’dlike the output to go somewhere else.You could redirect standard out, as in:$ java SomeClass > logbut that requires the user to remember to redirect the output every time theprogram is invoked. That’s fine for testing, or if the output is intended to goto a different place each time it is invoked. But, in this example, we always wantthe output to go to the same location.Without changing any of the System.out.println() statements, all themessages can be sent to a new location by reassigning the System.out printstream. The System class has a setter for out—that is, a method which will letyou set a new value for out. In your Java program, open the new destinationfile and give this to the System class:PrintStream ps = new PrintStream("pathname");System.setOut(ps);It will be used from that point forward in the execution of this program as itsout output stream.CAUTIONChanging standard out (or in, or err) will make the change for all classesfrom here on in this invocation of the Java runtime—they are static fields ofthe one System class. Since this is so serious a move, the Java SecurityManager (see Section 5.8.4.2) provides a check for setIO to see if the Javaprogram is allowed to make such changes. If such a security manager is inplace and you are not allowed to make such changes, an exception(SecurityException) will be thrown. Note also that the permission appliesto setting any of the fields; it doesn’t divide the permission into setting one(e.g., out) but not another (e.g., in).4.3.2 Environment VariablesWhen Linux programs are run they have the open file descriptors describedabove. They also carry with them a list of “name=value” pairs called theirenvironment. These environment variables allow for context to be shared among