A little bit about Javassist

1. Having fun with Javassist

2. whoami
Anton Arhipov
@antonarhipov

4. @Entity
@Table(name = "owners")
public class Owner extends Person {
@Column(name = "address")
@NotEmpty
private String address;
@Column(name = "city")
@NotEmpty
private String city;
@Column(name = "telephone")
@NotEmpty
@Digits(fraction = 0, integer = 10)
private String telephone;
@OneToMany(cascade = CascadeType.ALL,
mappedBy = "owner")
private Set<Pet> pets;

5. public class JavassistLazyInitializer
extends BasicLazyInitializer
implements MethodHandler {
final JavassistLazyInitializer instance
= new JavassistLazyInitializer(…);
ProxyFactory factory = new ProxyFactory();
factory.setSuperclass(interfaces.length == 1?persistentClass:null);
factory.setInterfaces(interfaces);
factory.setFilter(FINALIZE_FILTER);
Class cl = factory.createClass();
final HibernateProxy proxy = (HibernateProxy) cl.newInstance();
((ProxyObject)proxy).setHandler(instance);
instance.constructed = true;
return proxy;

6. public class JavassistLazyInitializer
extends BasicLazyInitializer
implements MethodHandler {
final JavassistLazyInitializer instance
= new JavassistLazyInitializer(…);
ProxyFactory factory = new ProxyFactory();
factory.setSuperclass(interfaces.length == 1?persistentClass:null);
factory.setInterfaces(interfaces);
factory.setFilter(FINALIZE_FILTER);
Class cl = factory.createClass();
final HibernateProxy proxy = (HibernateProxy) cl.newInstance();
((ProxyObject)proxy).setHandler(instance);
instance.constructed = true;
return proxy;
Generates proxy!

7. The main use case for
bytecode generation
in Java framewoks
is to generate proxies

9. Agenda
Javassist
-javaagent
basics
… and little bit about о JRebel

10. Javassist
www.javassist.org

11. CtPool
CtClass
CtClass
CtClass
CtClass
CtField
CtMethod
CtConst
CtMethod
insertBefore
insertAfter
instrument
Almost like Java Reflection API :)

12. https://github.com/antonarhipov/jpoint
Ex1, Ex2, Ex3

13. Java Agent

14. Java Agent
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.Instrumentation;
public class Agent {
public static void premain(String args, Instrumentation inst)
throws Exception {
inst.addTransformer(new ClassFileTransformer {
// here be code
});
}
}
META-INF/MANIFEST.MF
Premain-Class: Agent
$> java –javaagent:agent.jar application.Main

15. ClassFileTransformer
new ClassFileTransformer() {
public byte[] transform(ClassLoader loader,
String className,
Class<?> classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer){
ClassPool cp = ClassPool.getDefault();
CtClass ct = cp.makeClass(new
ByteArrayInputStream(classfileBuffer));
// here we can do all the things to ‘ct’
return ct.toBytecode();
}
}

16. ClassFileTransformer
new ClassFileTransformer() {
public byte[] transform(ClassLoader loader,
String className,
Class<?> classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer){
ClassPool cp = ClassPool.getDefault();
CtClass ct = cp.makeClass(new
ByteArrayInputStream(classfileBuffer));
// here we can do all the things to ‘ct’
return ct.toBytecode();
}
}

17. ClassFileTransformer
new ClassFileTransformer() {
public byte[] transform(ClassLoader loader,
String className,
Class<?> classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer){
ClassPool cp = ClassPool.getDefault();
CtClass ct = cp.makeClass(new
ByteArrayInputStream(classfileBuffer));
// here we can do all the things to ‘ct’
return ct.toBytecode();
}
}

18. ClassFileTransformer
new ClassFileTransformer() {
public byte[] transform(ClassLoader loader,
String className,
Class<?> classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer){
ClassPool cp = ClassPool.getDefault();
CtClass ct = cp.makeClass(new
ByteArrayInputStream(classfileBuffer));
// here we can do all the things to ‘ct’
return ct.toBytecode();
}
}

24. https://github.com/zeroturnaround/callspy

25. Javassist in

26. JRebel
core
Spring
plugin
Hibernate
plugin
EJB
plugin

27. JRebel
core
Spring
plugin
Hibernate
plugin
EJB
plugin
Reloads
classes

28. JRebel
core
Spring
plugin
Hibernate
plugin
EJB
pluginNotifies
plugins

29. JRebel
core
Spring
plugin
Hibernate
plugin
EJB
plugin
Refresh
configurations

30. JRebel
core
Spring
plugin
Hibernate
plugin
EJB
plugin
Javassist lives here

31. JRebel
core
Spring
plugin
Hibernate
plugin
EJB
plugin
Spring
Hibernate
OpenEJB

32. https://github.com/antonarhipov/jpoint
HowItWorks

33. Your task
Javassist

34. @antonarhipov
anton@zeroturnaround.com
https://speakerdeck.com/antonarhipov
http://www.slideshare.net/arhan