Question

So I’ve got this code that compiles a class from a string and then runs it. It works fine from the command line but not in eclipse…

import java.lang.reflect.Method;
import java.util.Arrays;
import javax.tools.DiagnosticCollector;
import javax.tools.JavaCompiler;
import javax.tools.JavaCompiler.CompilationTask;
import javax.tools.JavaFileObject;
import javax.tools.ToolProvider;

public class Another2 {

    public static void main(String args[]) throws Exception {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject>();
        String temp = "public class ByeWorld {\n" + "  public static void main(String args[]) {"
                + " System.out.println(\"First of our Compiled Class\");}}";
        JavaFileObject file = new JavaSourceFromString("ByeWorld", temp);
        Iterable<? extends JavaFileObject> compilationUnits = Arrays.asList(file);
        CompilationTask task = compiler.getTask(null, null, diagnostics, null, null, compilationUnits);
        task.call();
        ClassLoader classLoader = Another2.class.getClassLoader();
        Class<?> myclass = classLoader.loadClass("ByeWorld");
        Method mymethod = myclass.getDeclaredMethod("main", new Class[] { String[].class });
        mymethod.invoke(null, new Object[] { null });
    }
}

In the command line I get:

$ javac Another2.java 
$ java Another2
First of our Compiled Class

But in eclipse I get:

Exception in thread "main" java.lang.ClassNotFoundException: ByeWorld
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
    at Another2.main(Another2.java:21)

Is something permission-ally going on? is it permanent? And what’s my approach for fixing it?

Answer 1

I would bet the directory the file goes in and the directory it’s looking to load classes from are different. So essentially it’s a working-directory/classpath issue. Remember that Eclipse is trying to make things easier, and it wouldn’t expect to look in the current working directory for a .class file.

Try hard-coding where the files should go and be loaded from and see if that changes things. I suspect it will, but can’t be sure.