Wojtek Erbetowski

WJUG, Warsjawa 2012

http://[blog.]erbetowski.pl

@wojtassj

Groovy, Java, Scala, Python, JS developer

Code + docs at Polidea/RoboSpock

Instances of the class Class represent classes and interfaces in a running Java application.

public static Class<?> forName(String name, boolean initialize,
        ClassLoader loader) throws ClassNotFoundException{
    if (loader == null) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            ClassLoader ccl = ClassLoader.getCallerClassLoader();
            if (ccl != null) {
                sm.checkPermission(
                    SecurityConstants.GET_CLASSLOADER_PERMISSION);
            }
        }
    }
    return forName0(name, initialize, loader);
}

private static native Class forName0(...)

A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class.

Given the binary name of a class, a class loader should attempt to locate or generate data that constitutes a definition for the class.

A typical strategy is to transform the name into a file name and then read a "class file" of that name from a file system.

protected final Class<?> defineClass(
        String name, byte[] b, int off, int len)

The very low level libraries

ObjectWeb ASM

Javassist

• PowerMock
• Play!Framework
• Robolectric
• Spring 3.2?

• CGLib
• Terracota
• Cobertura
• Groovy
• Clojure
• Byteman

• Hibernate
• Spring
• Guice
• jMock
• Mockito
• Tumbler

Cache

Mock/Stub/Spy

Transaction

Contracts

Logging

Performance (optimization, profiling)

ORM

Security

ASM Hello World!

ClassWriter cw = new ClassWriter(0);
FieldVisitor fv;
MethodVisitor mv;
AnnotationVisitor av0;

cw.visit(49,
    ACC_PUBLIC + ACC_SUPER,
    "Hello",
    null,
    "java/lang/Object",
    null);

cw.visitSource("Hello.java", null);

{
    mv = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
    mv.visitVarInsn(ALOAD, 0);
    mv.visitMethodInsn(INVOKESPECIAL,
        "java/lang/Object",
        "<init>",
        "()V");

    mv.visitInsn(RETURN);
    mv.visitMaxs(1, 1);
    mv.visitEnd();
}
{
    mv = cw.visitMethod(ACC_PUBLIC + ACC_STATIC,
        "main",
        "([Ljava/lang/String;)V",
        null,
        null);
    mv.visitFieldInsn(GETSTATIC,
        "java/lang/System",
        "out",
        "Ljava/io/PrintStream;");
    mv.visitLdcInsn("hello");
    mv.visitMethodInsn(INVOKEVIRTUAL,
        "java/io/PrintStream",
        "println",
        "(Ljava/lang/String;)V");
    mv.visitInsn(RETURN);
    mv.visitMaxs(2, 1);
    mv.visitEnd();
}
cw.visitEnd();

return cw.toByteArray();

public class WeekendBlocker implements MethodInterceptor {
    public Object invoke(MethodInvocation invocation)
            throws Throwable {

        Calendar today = new GregorianCalendar();
        if (today.getDisplayName(DAY_OF_WEEK, LONG, ENGLISH)
                .startsWith("S")) {

            throw new IllegalStateException(
                invocation.getMethod().getName()
                    + " not allowed on weekends!");
        }

        return invocation.proceed();
    }
}

public class NotOnWeekendsModule extends AbstractModule {
    protected void configure() {
        bindInterceptor(
            Matchers.any(),
            Matchers.annotatedWith(NotOnWeekends.class),
            new WeekendBlocker());
    }
}

Metaprogramming is the writing of computer programs that write or manipulate other programs (or themselves) as their data, or that do part of the work at compile time that would otherwise be done at runtime.

AOP (Aspect Oriented Programming)

see api