HotSwap, DCEVM, class loaders, Spring Boot Devtools, JRebel

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

10. HotSwap

11. HotSwap
Class loaders

12. HotSwap
Class loaders
Java agents & instrumentation

13. HotSwap
Class loaders
Java agents & instrumentation

14. hotswapEST. 2001

24. HOW ABOUT…
refactoring?

28. JDB

29. 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

30. 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

31. 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

32. 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

33. What if…
hotswap++

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

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

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

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

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

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

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

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

43. Classloaders

44. 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;
}

45. 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;
}

46. 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;
}

47. 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;
}

48. 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;
}

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

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

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

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

53. 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);
}

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

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

61. Java agents

62. 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
});
}
}

63. 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

64. 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

65. 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

66. 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

69. HotSwap

70. HotSwap
Class loaders

71. HotSwap
Class loaders
Java agents & instrumentation

72. anton@zeroturnaround.com
@antonarhipov