This document discusses different techniques for reloading Java classes at runtime: - HotSwap allows reloading individual statements but is limited. Class loaders allow reloading entire classes but each class loader isolates its classes. - More advanced techniques like Java agents and bytecode instrumentation allow redefining classes in-place and reloading methods, fields, and class hierarchies in a binary-compatible way. - Demonstrations show reloading an entire "region" of related classes like a service class and its dependencies, allowing a live application thread to see the changes immediately without restarts. Java agents in particular provide a powerful but complex way to implement class reloading.

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

2. whoami
Anton Arhipov
@antonarhipov

3. whoami
Anton Arhipov
@antonarhipov

6. https://zeroturnaround.com/rebellabs/java-ee-productivity-report-2011/
10%
20%
30%
0.5 1 2 3 4 5 6 7 8 10+ minutes

7. https://zeroturnaround.com/rebellabs/java-ee-productivity-report-2011/
10%
20%
30%
0.5 1 2 3 4 5 6 7 8 10+
The only effective developers??
minutes

8. https://zeroturnaround.com/rebellabs/java-ee-productivity-report-2011/
10%
20%
30%
0.5 1 2 3 4 5 6 7 8 10+
How about those?
minutes

9. The problem:
where did the time go?

10. The problem:
where did the time go?
1-30
sec
Container startup time

11. The problem:
where did the time go?
1-30
sec
Container startup time
~3 min
30 min1 s
Application deployment

12. The problem:
where did the time go?
1-30
sec
Container startup time
~3 min
30 min1 s
Application deployment
~1
min
Navigation

13. Technical solutions

14. Technical solutions
HotSwap

15. Technical solutions
HotSwap
Class loaders

16. Technical solutions
HotSwap
Class loaders
Java agents & instrumentation

17. Technical solutions
HotSwap
Class loaders
Java agents & instrumentation

18. hotswapEST. 2001

28. HOW ABOUT…
refactoring?

32. JDB

33. 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

34. 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

35. 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

36. 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

37. 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
Limited to individual statements

38. What if…
hotswap++

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

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

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

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

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

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

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

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

48. Classloaders

49. 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;
}

50. 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;
}

51. 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;
}

52. 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;
}

53. 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;
}

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

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

56. while(true) {
Class<?> uc = new DynamicClassLoader().load("com.zt.User");
ReflectUtil.invokeStatic("getHobby", uc);
}
public class User {
public Hobby getHobby() {
return Basketball();
}
}
Assume there is a trigger / event

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

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

59. 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);
}

60. DynamicClassLoader
Context class
HobbyService class
User class
Context object
HobbyService object
User object
Reloadable
“region”
Live thread

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

67. Java agents

68. 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 dragons
});
}
}

69. 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 dragons
});
}
}
META-INF/MANIFEST.MF
Premain-Class: Agent

70. 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 dragons
});
}
}
$> java –javaagent:agent.jar application.Main
META-INF/MANIFEST.MF
Premain-Class: Agent

71. 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

72. 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

74. What did we learn today?

75. What did we learn today?
HotSwap - a feature of your JVM, not IDE!

76. What did we learn today?
HotSwap - a feature of your JVM, not IDE!
Class loaders - easy to implement, but limited

77. What did we learn today?
HotSwap - a feature of your JVM, not IDE!
Class loaders - easy to implement, but limited
Java agents & instrumentation - very tricky!

78. anton@zeroturnaround.com
@antonarhipov
slideshare.net/arhan