How can I set the umask from within java?

I'm new to Java. Where is umask exposed in the api?

You can't fiddle with the umask directly, since Java is an abstraction and the umask is POSIX-implementation specific. But you have the following API:

File f;
f.setExecutable(true);
f.setReadable(false);
f.setWritable(true);

There are some more APIs available, check the docs.

If you must have direct access to the umask, either do it via JNI and the chmod() syscall, or spawn a new process with exec("chmod").

import java.nio.file.Files
import java.nio.file.attribute.PosixFilePermission

File file = new File("/some/path") 
Files.setPosixFilePermissions(file.toPath(), [
                PosixFilePermission.OWNER_READ,
                PosixFilePermission.OWNER_WRITE
            ].toSet())

java.nio.file.attribute.PosixFileAttributes in Java SE 7.

Another approach is to use a 3rd-party Java library that exposes POSIX system calls; e.g.

• Jtux
• The "Posix for Java" library,
• and so on (Google for "java posix library").

The problem with this approach is that it is intrinsically non-portable (won't work on a non-POSIX compliant platform), and requires a platform-specific native library ... and all that that entails.