In sandboxed programming platforms like .Net and Java, the developer is insulated from the details of the underlying machine upon which the code is running. At times, however, it is often useful to have more knowledge about the environment. One way to provide such information without introducing any machine-dependent code into the sandbox is via system properties. They can be implemented in a number of ways, but the most common is a set of name/value pairs that can be accessed via a core class in the language runtime. In java, one can get a list of all the system properties using System.getProperties().
The properties are not limited to those set by the language virtual machine; they can be extended to include any property the developer wants to set. Most languages provide a way to do this at virtual machine initialization or programmatically within the application code. Having system properties is a powerful tool and can enable code to handle certain conditions based on the capabilities of the underlying machine in an elegant way. With power, though, comes responsibility.
Some developers have been abusing this construct and using it as if were a container for global variables: instead of passing parameters into methods, a method will simply look up a value from the system properties. This is especially egregious when the property is not universally true from the system perspective. One such example is specifying a HTTP proxy. In Java, the default HttpConneciton class uses system properties to specify a proxy. In most cases, this is fine, but in some enterprise environments, the proxy server one must use depends on the address being accessed. If your program needs to access two addresses that use different proxies, then changing proxies involves updating the values in the system properties. Again, this is usually not a big problem, but if you have a multi-threaded program, this could cause a race condition unless one enforces the atomicity of the property update with the HTTP request. Depending on the application requirements, this may or may not be an acceptable answer.
While there is not much one can do about the proxy issue (outside of using some other library to make HTTP requests), developers can prevent imposing similar limitations on their own code. Since it is impossible to anticipate every way in which code may be used and it is equally impossible to know every possible system configuration, the use of system properties that do not actually describe the system upon which the code is executing should be avoided.
