Java IO.pdf - Nguyen Dang Binh

Security Issues
Java I/O
I don't know if there's one most frequently asked question about Java Network Programming,
but there's definitely a most frequent answer, and it applies to this book too. My mistake in
Java Network Programming was hiding that answer in the back of a chapter most people
didn't read. Since that very same answer should answer an equal number of questions from
readers of this book, I want to get it out of the way right up front:
Java's security manager prevents almost all the examples and methods discussed in this book
from working in an applet.
This book focuses very much on applications. There is very little that can be done with I/O
from an untrusted applet without running afoul of the security manager. The problem may not
always be obvious—not all web browsers properly report security exceptions—but it is there.
There are some exceptions. Byte array streams and piped streams work without limitation in
applets. Network connections can be made back to the host from whence the applet came (and
only to that host). System.in and System.out may be accessible from some, though not all,
web browsers. And in Java 2 and later, there are ways to relax the restrictions on applets so
they get limited access to the filesystem or unlimited access to the network. However, these
are exceptions, not the rule.
If you can make an applet work when run as a standalone application and you cannot get it to
work inside a web browser, the problem is almost certainly a conflict with the browser's
security manager.
Conventions Used in This Book
Italic is used for:
• Filenames (readme.txt )
• Host and domain names (http://www.oreilly.com/)
• URLs (http://metalab.unc.edu/javafaq/)
Constant width is used for:
• Code examples and fragments
• Class, variable, and method names, and Java keywords used within the text
Significant code fragments and complete programs are generally placed in a separate
paragraph like this:
InputStream in = new FileInputStream("/etc/mailcap");
When code is presented as fragments rather than complete programs, the existence of the
appropriate import statements should be inferred. For example, in the previous code fragment
you may assume that java.io.InputStream and java.io.FileInputStream were
imported.
9