Java Cookbook

Debugging Printouts

Problem

You want to have debugging statements left in your code to be enabled at runtime.

Solution

Use my Debug class.

Discussion

Instead of using the conditional compilation mechanism of Section 1.11, you may want to leave your debugging statements in the code, but enable them only at runtime, when a problem surfaces. This is a good technique for all but the most compute-intensive applications because the overhead of a simple if statement is not all that great. Let’s combine the flexibility of runtime checking with the simple if statement to debug a hypothetical fetch( ) method (part of Fetch.java):

String name = "poem";
if (System.getProperty("debug.fetch") != null) {
    System.err.println("Fetching " + name);
}
value = fetch(name);

Then, we can compile and run this normally and the debugging statement will be omitted. But if we run it with a -D argument to enable debug.fetch, the printout will occur:

> java Fetch          # See? No output
> java -Ddebug.fetch Fetch
Fetching poem
>

Of course this kind of if statement is tedious to write in large quantities, so I have encapsulated it into a Debug class, which is part of my com.darwinsys.util package. Debug.java appears in full at the end of this chapter, in Section 1.19. My Debug class also provides the string “debug”. as part of the System.getProperty( ) , so we can simplify the previous Fetch example as follows (code in FetchDebug.java):

String name = "poem", value;
Fetch f = new Fetch(  );
Debug.println("fetch", "Fetching " + name);
value = f.fetch(name);

Running it behaves identically to the original Fetch:

> java FetchDebug     # again, no output
> java -Ddebug.fetch FetchDebug
Fetching poem
>