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Before finishing up this chapter, I have a couple of exercises for you that will give you some experience using the MOVE block along with the previous blocks you ve learned about (WAIT and DISPLAY). Try to create the programs yourself; if you find you need help, I ll put two possible solutions at the end of the chapter.

// Demonstrate throw class ThrowDemo { static void demoproc() { try { throw new NullPointerException("demo"); } catch(NullPointerException e) { Systemoutprintln("Caught inside demoproc"); throw e; // rethrow the exception } } public static void main(String args[]) { try { demoproc(); } catch(NullPointerException e) { Systemoutprintln("Recaught: " + e); } } }

To better control the resources available to processes started by the shell, the ulimit facility can be used System-wide defaults can be configured using the /etc/security/ limitsconf file ulimit options can be used to control such things as the number of files that may open, how much memory they may use, CPU time they may use, how many processes they may open, etc The settings are read by the PAM (Pluggable Authentication Module) libraries when a user starts up The key to choosing ulimit values is to consider the purpose of the system For example, in the case of an application server, if the application is going to require a lot of processes to run, then the system administrator needs to ensure that ulimit caps don t cripple the functionality of the system Other types of servers, such as a Domain Name System (DNS) server for example, should not need more than a small handful of processes It should be noted that there is a caveat here: PAM has to have a chance to run to set the settings before the user does something If the application starts as root and then drops permissions, PAM is not likely to run From a practical point of view, this means that having individual per-user settings is not likely to do you a lot of good in most server environments What will work are global settings that apply to both root and normal users This detail turns out to be a good thing in the end; having root under control helps keep the system from spiraling away both from attacks and from broken applications

This program gets two chances to deal with the same error First, main( ) sets up an exception context and then calls demoproc( ) The demoproc( ) method then sets up another exceptionhandling context and immediately throws a new instance of NullPointerException, which is caught on the next line The exception is then rethrown Here is the resulting output: Caught inside demoproc Recaught: javalangNullPointerException: demo The program also illustrates how to create one of Java s standard exception objects Pay close attention to this line:

throw new NullPointerException("demo");

A common trick that students still play on other students is to log into their workstations and run a fork bomb This is a program that simply creates so many processes that it overwhelms the system and brings it to a grinding halt For a student, this is annoying For a production server, this is fatal A simple shell-based fork bomb using Bourne Again Shell (BASH) is

Here, new is used to construct an instance of NullPointerException Many of Java s builtin run-time exceptions have at least two constructors: one with no parameter and one that takes a string parameter When the second form is used, the argument specifies a string that describes the exception This string is displayed when the object is used as an argument to print( ) or println( ) It can also be obtained by a call to getMessage( ), which is defined by Throwable

If a method is capable of causing an exception that it does not handle, it must specify this behavior so that callers of the method can guard themselves against that exception You do this by including a throws clause in the method s declaration A throws clause lists the types of exceptions that a method might throw This is necessary for all exceptions, except those of