Where compiled code will be stored if not in class file

Question

Recently I came across a query while reading THE STRUCTURE OF THE JAVA VIRTUAL MACHINE

On page no-5 ch. 2,

Compiled code to be executed by the Java Virtual Machine is represented using a hardware- and operating system-independent binary format, typically (but not necessarily) stored in a file, known as the class file format.

That bracket But not necessarily is the reason.

Question is,

In which case compiled code will not be stored in class file ? and if it will not be stored in class file, then where and how ?

Edit : Please note that question is not about ClassLoader.

Answer 1

You seem more interest in how to generate a java class file in memory so here goes:

public class CompileSourceInMemory {
    public static void main(String args[]) throws IOException {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        DiagnosticCollector<JavaFileObject> diagnostics = new    DiagnosticCollector<JavaFileObject>();

        StringWriter writer = new StringWriter();
        PrintWriter out = new PrintWriter(writer);
        out.println("public class HelloWorld {");
        out.println("  public static void main(String args[]) {");
        out.println("    System.out.println(\"This is in another java file\");");    
        out.println("  }");
        out.println("}");
        out.close();
        JavaFileObject file = new JavaSourceFromString("HelloWorld", writer.toString());

        Iterable<? extends JavaFileObject> compilationUnits = Arrays.asList(file);
        CompilationTask task = compiler.getTask(null, null, diagnostics, null, null, compilationUnits);

        boolean success = task.call();

        if (success) {
            try {
                Class.forName("HelloWorld").getDeclaredMethod("main", new Class[] { String[].class }).invoke(null, new Object[] { null });
            } catch (ClassNotFoundException e) {
                System.err.println("Class not found: " + e);
            } catch (NoSuchMethodException e) {
                System.err.println("No such method: " + e);
            } catch (IllegalAccessException e) {
                System.err.println("Illegal access: " + e);
            } catch (InvocationTargetException e) {
                System.err.println("Invocation target: " + e);
            }
        }
    }
}

class JavaSourceFromString extends SimpleJavaFileObject {
    final String code;

    JavaSourceFromString(String name, String code) {
        super(URI.create("string:///" + name.replace('.','/') + Kind.SOURCE.extension),Kind.SOURCE);
        this.code = code;
    }

    @Override
    public CharSequence getCharContent(boolean ignoreEncodingErrors) {
        return code;
    }
}

Source: In memory compilation

This generates a class in memory without any external representation of the java source or compiled class.