In Java, a typical workflow involves restarting the application (almost) with every class change. For some applications it is not a problem at all, for some – it is a disaster. From HotSwap to agent-based reloading. In this session, we are going to take a look at the options available for Java class reloading. There is plenty of tools that you can use for this task: rely on standard JVM HotSwap, redesign your application to rely on dynamic class loaders, to comprehend the Zen of OSGi, or to integrate a reloading agent. Every option has its own drawbacks and benefits and we’re going to take a deep dive on the subject. Finally, there are also the conceptual challenges in reloading Java classes. What to do with the state? What should happen with the static initialisers? What if super class changes? Join this session to gain a better understanding of class reloading technologies and become more productive Java developer.

1. The hitchhiker’s guide to
Java class reloading
JEEConf 2017
@antonarhipov

2. whoami
Anton Arhipov
@antonarhipov

3. whoami
Anton Arhipov
@antonarhipov

6. HotSwap

7. HotSwap
Class loaders

8. HotSwap
Class loaders
Java agents & instrumentation

9. hotswapEST. 2001

19. HOW ABOUT…
refactoring?

23. JDB

24. instanceKlass constantPoolOop
constants()
pool_holder()
klassVTable
Embedded
klassITable
Embedded
Embedded
statics
M. Dmitriev. Safe class and data evolution in large and long-lived Java (тм) applications. Technical report, Mountain View. 2001

25. instanceKlass constantPoolOop
constants()
constantPoolCacheOop
cache()
pool_holder()
klassVTable
Embedded
klassITable
Embedded
Embedded
statics
M. Dmitriev. Safe class and data evolution in large and long-lived Java (тм) applications. Technical report, Mountain View. 2001

26. instanceKlass constantPoolOop
constants()
constantPoolCacheOop
cache()
pool_holder()
klassVTable
Embedded
klassITable
Embedded
Embedded
statics
objArrayOop
methodOop
methods()
M. Dmitriev. Safe class and data evolution in large and long-lived Java (тм) applications. Technical report, Mountain View. 2001

27. instanceKlass constantPoolOop
constants()
constantPoolCacheOop
cache()
pool_holder()
klassVTable
Embedded
klassITable
Embedded
Embedded
statics
nmethod
code()
method()
constants()
objArrayOop
methodOop
methods()
M. Dmitriev. Safe class and data evolution in large and long-lived Java (тм) applications. Technical report, Mountain View. 2001

28. What if…
hotswap++

29. Dynamic Code Evolution for Java
T. Würthinger, C. Wimmer, L. Stadler. 2010

30. Dynamic Code Evolution for Java
T. Würthinger, C. Wimmer, L. Stadler. 2010
Statements

31. Dynamic Code Evolution for Java
T. Würthinger, C. Wimmer, L. Stadler. 2010
Statements
Methods

32. Dynamic Code Evolution for Java
T. Würthinger, C. Wimmer, L. Stadler. 2010
Statements
Methods Fields

33. Dynamic Code Evolution for Java
T. Würthinger, C. Wimmer, L. Stadler. 2010
Statements
Methods Fields Hierarchy

34. Dynamic Code Evolution for Java
T. Würthinger, C. Wimmer, L. Stadler. 2010
Statements
Methods Fields Hierarchy

35. Dynamic Code Evolution for Java
T. Würthinger, C. Wimmer, L. Stadler. 2010
Statements
Methods Fields Hierarchy
+ + +
Binary-compatible

36. Dynamic Code Evolution for Java
T. Würthinger, C. Wimmer, L. Stadler. 2010
Statements
Methods Fields Hierarchy
+ + +
x x x
Binary-compatible
Binary-incompatible

38. Classloaders

39. Class<?> uc1 = User.class;
Class<?> uc2 = new DynamicClassLoader().load("com.zt.User");
out.println(uc1.getName()); // com.zt.User
out.println(uc2.getName()); // com.zt.User
out.println(uc1.getClassLoader()); // sun.misc.Launcher$AppClassLoader@18b4aac2
out.println(uc2.getClassLoader()); // com.zt.DynamicClassLoader@22b4bba7
User.age = 11;
out.println((int) ReflectUtil.getStaticFieldValue("age", uc1)); // 11
out.println((int) ReflectUtil.getStaticFieldValue("age", uc2)); // 10
public class User {
public static int age = 10;
}

40. Class<?> uc1 = User.class;
Class<?> uc2 = new DynamicClassLoader().load("com.zt.User");
out.println(uc1.getName()); // com.zt.User
out.println(uc2.getName()); // com.zt.User
out.println(uc1.getClassLoader()); // sun.misc.Launcher$AppClassLoader@18b4aac2
out.println(uc2.getClassLoader()); // com.zt.DynamicClassLoader@22b4bba7
User.age = 11;
out.println((int) ReflectUtil.getStaticFieldValue("age", uc1)); // 11
out.println((int) ReflectUtil.getStaticFieldValue("age", uc2)); // 10
public class User {
public static int age = 10;
}

41. Class<?> uc1 = User.class;
Class<?> uc2 = new DynamicClassLoader().load("com.zt.User");
out.println(uc1.getName()); // com.zt.User
out.println(uc2.getName()); // com.zt.User
out.println(uc1.getClassLoader()); // sun.misc.Launcher$AppClassLoader@18b4aac2
out.println(uc2.getClassLoader()); // com.zt.DynamicClassLoader@22b4bba7
User.age = 11;
out.println((int) ReflectUtil.getStaticFieldValue("age", uc1)); // 11
out.println((int) ReflectUtil.getStaticFieldValue("age", uc2)); // 10
public class User {
public static int age = 10;
}

42. Class<?> uc1 = User.class;
Class<?> uc2 = new DynamicClassLoader().load("com.zt.User");
out.println(uc1.getName()); // com.zt.User
out.println(uc2.getName()); // com.zt.User
out.println(uc1.getClassLoader()); // sun.misc.Launcher$AppClassLoader@18b4aac2
out.println(uc2.getClassLoader()); // com.zt.DynamicClassLoader@22b4bba7
User.age = 11;
out.println((int) ReflectUtil.getStaticFieldValue("age", uc1)); // 11
out.println((int) ReflectUtil.getStaticFieldValue("age", uc2)); // 10
public class User {
public static int age = 10;
}

43. Class<?> uc1 = User.class;
Class<?> uc2 = new DynamicClassLoader().load("com.zt.User");
out.println(uc1.getName()); // com.zt.User
out.println(uc2.getName()); // com.zt.User
out.println(uc1.getClassLoader()); // sun.misc.Launcher$AppClassLoader@18b4aac2
out.println(uc2.getClassLoader()); // com.zt.DynamicClassLoader@22b4bba7
User.age = 11;
out.println((int) ReflectUtil.getStaticFieldValue("age", uc1)); // 11
out.println((int) ReflectUtil.getStaticFieldValue("age", uc2)); // 10
public class User {
public static int age = 10;
}

44. while(true) {
Class<?> uc = new DynamicClassLoader().load("com.zt.User");
ReflectUtil.invokeStatic("getHobby", uc);
}
public class User {
public Hobby getHobby() {
return Basketball();
}
}

45. while(true) {
Class<?> uc = new DynamicClassLoader().load("com.zt.User");
ReflectUtil.invokeStatic("getHobby", uc);
}
public class User {
public Hobby getHobby() {
return Basketball();
}
}

46. public static class Context {
public HobbyService hobbyService = new HobbyService();
public void init() {
hobbyService.user = new User();
anyService.initialize()
}
}

47. public static class Context {
public HobbyService hobbyService = new HobbyService();
public AnyService anyService = new AnyService();
public void init() {
hobbyService.user = new User();
anyService.initialize()
}
}

48. public static class Context {
public HobbyService hobbyService = new HobbyService();
public AnyService anyService = new AnyService();
public void init() {
hobbyService.user = new User();
anyService.initialize()
}
}
while(true) {
Class<?> c = new DynamicClassLoader().load("com.zt.Context");
Object context = c.newInstance();
ReflectUtil.invokeMethod("init", context);
invokeService(context);
}

49. DynamicClassLoader
Context class
HobbyService class
User class
Context object
HobbyService object
User object
Live thread

52. <dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-devtools</artifactId>
<scope>runtime</scope>
</dependency>
DEMO

56. Java agents

58. ClassFileTransformer
ClassA
ClassA
ClassA0
+field1
+field2
+field3
+method1
+method2
+method3
+method1
+method2
+method3
+field1
+field2
+field3
+proxy_methods
A thousand years of productivity: the JRebel story
E. Kabanov, V. Vene, 2012

59. ClassFileTransformer
ClassA
ClassA
ClassA0
+field1
+field2
+field3
+method1
+method2
+method3
+method1
+method2
+method3
+field1
+field2
+field3
+proxy_methods
A thousand years of productivity: the JRebel story
E. Kabanov, V. Vene, 2012

60. DEMO

62. HotSwap

63. HotSwap
Class loaders

64. HotSwap
Class loaders
Java agents & instrumentation

65. anton@zeroturnaround.com
@antonarhipov