After decompiling many undocumented classes, I finally discovered
a way to invoke Javac from within the JVM without using exec, running
the compiler inside the same JVM. This is MUCH faster than using JavaC
if you want to do repeat compiles. This is how ANT does it.

// compiling from within a JVM
// without spawning javac.exe or a separate JVM
// com.sun.tools.javac.Main lives in tools.jar
// Make sure it is on the classpath. It won't be by default.
import com.sun.tools.javac.Main;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;

// ...

// simulate an arbitrarily long command line.
// No wildcards since no command line interpreter to expand them.
String[] optionsAndSources = { "-g", "-source", "1.5", "-target",
"1.5", "Apple.java", "Banana.java", "Cantaloup.java"};

// where Javac output goes
PrintWriter out = new PrintWriter( new FileWriter( "C:/temp/out.txt" )
);

// Compile all three sources at once
int status =  Main.compile( optionsAndSources, out );
System.out.println( "status: " + status );

For future reference, this is documented in the Java glossary under
javac.exe  at
http://mindprod.com/jgloss/javacexe.html