Java™ Application Development on Linux - Dator

156Chapter 5The Sun Microsystems Java Software <strong>Development</strong> KitStep completed: "thread=main", FetchURL.main(), line=48 bci=4848 for (i = 0; i < args.length; i++)main[1] step>Step completed: "thread=main", FetchURL.main(), line=53 bci=5753 }main[1] step>The application exited$1. Here we tell the debugger where to break execution to let us run debugger commands.We do so at the start of the FetchURL class’ main() method. If we did not set abreakpoint, the run would have run the program to termination, making it no differentfrom running it with the java command (except perhaps a bit slower).2. The list command shows the source line that is about to be executed, along with somemore lines to either side. It is a handy way to get a little context. The standard “nextline” prompt isn’t enough for most of us to get context (unless, of course, we arelooking at a line-numbered printout of the source or an editor window at the sametime, which we often do).3. The step steps execution one “line” (what a line is can be a bit fuzzy when there’s alot of vertical whitespace in the source, or when multiple method calls occur on oneline). Note the information in the status message. The name of the thread is given (oursample is single-threaded, so it is always “main”), as is the line number in the sourcefile and the bci. Note that there is a very similar command, next, that advances to thenext line in the same stack frame. In other words, it won’t step into method calls, itsteps over them.4, 5. Here we see two uses of the dump command. First, we apply it to this (which is animplicit argument to any nonstatic method call) to dump the currently executing object.The second instance dumps the rc local variable, which is an accumulating stringcontaining the requested Web page. At the moment, it contains only the first few lines.6. Here we set a breakpoint on a specific source line number. We then use the contcommand to resume “full speed” code execution.