Java IO.pdf - Nguyen Dang Binh

Java I/O
in Apple's Macintosh Runtime for Java, shown in Figure 1.5, provides a simple pop-up menu
interface for selecting a file, /dev/null, or a message window as the target of System.out or
source for System.in.
Figure 1.5. Redirecting stdout and stdin from JBindery
It's sometimes convenient to be able to redirect System.out, System.in, and System.err
from inside the running program. The following three static methods in the
java.lang.System class do exactly that:
public static void setIn(InputStream in)
public static void setOut(PrintStream out)
public static void setErr(PrintStream err)
For example, to specify that data written on System.out is sent to the file yankees99.out and
data read from System.in comes from yankees99.tab, you could write:
System.setIn(new FileInputStream("yankees99.tab"));
System.setOut(new PrintStream(new FileOutputStream("yankees99.out")));
These methods are especially useful when making a quick and dirty port of a program that
makes heavy use of System.out, System.in, or System.err from an application to an
applet. However, there is no absolute guarantee that console redirection will be allowed in all
web browsers. Internet Explorer 4.0b2 allowed it, but the released version does not. HotJava
1.1 allows it with the security settings turned down, but not with security at the default level.
Netscape Navigator 4.0 and 4.5 and HotJava 1.0 do not allow console redirection.
The SecurityManager class does not have a specific method to test whether or not
redirecting System.out or System.err is allowed. However, in Java 1.1 Sun's JDK checks
whether this is permitted by calling checkExec("setIO"). (The source code contains a
comment to the effect that there should be a separate method for this sort of check in future
versions of Java.) checkExec() determines whether the security manager allows a subprocess
called setio to be spawned. The AppletSecurity security manager used by appletviewer in
JDK 1.1 always disallows this call.
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