Interesting!
I played with some source code to see what is happening with and without a conditional breakpoint. Attached below.
Execution in the debugger with the conditional breakpoint:
Duration: 1210623 microseconds
Execution in the debugger without the conditional breakpoint:
Duration: 24 microseconds
IMHO the VM is not stopped because the second thread continues to run side-by-side. Eclipse does have to inject the breakpoint code into the current class. Maybe it does so on every call and maybe it has to recompile the class on every call. Checking the Eclipse sources would reveal what is happening exactly.
My experience running conditional breakpoints in C# and in Visual Studio is even worse: My stomach feeling is that things are a couple of orders of magnitude worse there.
public class BreakPointPlay {
static int breakpointHits;
static volatile int modifiedBySecondThread;
static volatile boolean stopped;
public static void main(String[] args) throws InterruptedException {
Thread secondThread = startSecondThread();
final long LOOPS = 1000;
long counter = 0;
long start = System.nanoTime();
for (long i = 0; i < LOOPS; i++) {
// place breakpoint here and set the condition to the
// #breakPointCondition() method.
counter += i;
}
long stop = System.nanoTime();
long nanos = stop - start;
long micros = nanos / 1000;
System.out.println("\nDuration: " + micros + " microseconds\n");
printInfo();
stopped = true;
secondThread.join();
}
private static Thread startSecondThread() {
Thread thread = new Thread(new Runnable() {
@Override
public void run() {
while(! stopped){
modifiedBySecondThread++;
}
}
});
thread.start();
return thread;
}
private static void printInfo() {
printModifiedBySecondThread();
printThread();
printClassLoader();
printStackTrace();
printModifiedBySecondThread();
}
private static void printStackTrace() {
Exception exception = new Exception();
exception.fillInStackTrace();
exception.printStackTrace(System.out);
}
private static void printModifiedBySecondThread() {
print("modifiedBySecondThread " + modifiedBySecondThread);
}
public static boolean breakPointCondition(){
breakpointHits++;
if(breakpointHits == 100){
printInfo();
}
return false;
}
private static void printClassLoader() {
print("ClassLoader " + new BreakPointPlay().getClass().getClassLoader());
}
private static void printThread() {
print("Thread " + Thread.currentThread());
}
private static void print(String msg){
System.out.println(msg);
}
}