This document discusses techniques for reloading Java classes at runtime, known as hot reloading or hot swapping. It begins with an introduction and overview of class loaders, Java agents, and instrumentation which can be used to dynamically modify classes. Specific techniques are then presented for reloading classes, fields, methods and code using custom class loaders and bytecode manipulation. The goal is to allow reloading parts of an application without restarting the Java Virtual Machine.

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

2. whoami
Anton Arhipov
@antonarhipov

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

6. 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

7. 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

8. HotSwap
Class loaders
Java agents & instrumentation
Challenges

9. hotswapEST. 2001

17. HOW ABOUT…
refactoring?

20. 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

21. What if…
hotswap++

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

24. Classloaders

25. 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;
}

26. 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;
}

27. 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;
}

28. 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;
}

29. 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;
}

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

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

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

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

34. 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);
}

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

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

43. 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

44. 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

45. public class C extends X {
int y = 5;
int method1(int x) {
return x + y;
}
void method2(String s) {
System.out.println(s);
}
}

46. public class C extends X {
int y = 5;
int method1(int x) {
Object[] o = new Object[1];
o[0] = x;
return Runtime.redirect(this, o, "C", "method1", "(I)I");
}
void method2(String s) {
Object[] o = new Object[1];
o[0] = s;
return Runtime.redirect(
this, o, "C", "method2", "(Ljava/lang/String;)V");
}
}

47. public class C extends X {
int y = 5;
int method1(int x) {
Object[] o = new Object[1];
o[0] = x;
return Runtime.redirect(this, o, "C", "method1", "(I)I");
}
void method2(String s) {
Object[] o = new Object[1];
o[0] = s;
return Runtime.redirect(
this, o, "C", "method2", "(Ljava/lang/String;)V");
}
}

48. public class C extends X {
int y = 5;
int method1(int x) {
Object[] o = new Object[1];
o[0] = x;
return Runtime.redirect(this, o, "C", "method1", "(I)I");
}
void method2(String s) {
Object[] o = new Object[1];
o[0] = s;
return Runtime.redirect(
this, o, "C", "method2", "(Ljava/lang/String;)V");
}
}

49. public abstract class C0 {
public static int method1(C c, int x) {
int tmp1 = Runtime.getFieldValue(c, "C", "y", "I");
return x + tmp1;
}
public static void method2(C c, String s) {
PrintStream tmp1 = Runtime.getFieldValue(
null, "java/lang/System", "out", "Ljava/io/PrintStream;");
Object[] o = new Object[1];
o[0] = s;
Runtime.redirect(
tmp1, o, "java/io/PrintStream;", "println",
"(Ljava/lang/String;)V");
}
}
int method1(int x) {
return x + y;
}

50. public abstract class C0 {
public static int method1(C c, int x) {
int tmp1 = Runtime.getFieldValue(c, "C", "y", "I");
return x + tmp1;
}
public static void method2(C c, String s) {
PrintStream tmp1 = Runtime.getFieldValue(
null, "java/lang/System", "out", "Ljava/io/PrintStream;");
Object[] o = new Object[1];
o[0] = s;
Runtime.redirect(
tmp1, o, "java/io/PrintStream;", "println",
"(Ljava/lang/String;)V");
}
}

51. public class C extends X {
int y = 5;
int method1(int x) {
return x + y;
}
//...
}
public class C extends X {
int y = 5;
int z() {
return 10;
}
int method1(int x) {
return x + y + z();
}
//...
}

52. public class C1 {
public static int z(C c) {
return 10;
}
public static int method1(C c, int x) {
int tmp1 = Runtime.getFieldValue(c, "C", "y", "I");
int tmp2 = Runtime.redirect(c, null, "C", "z", "(V)I");
return x + tmp1 + tmp2;
}
//...
}

53. WHAT ABOUT…
configuration?

54. DEMO

55. HotSwap
Class loaders
Java agents & instrumentation

56. anton@zeroturnaround.com
@antonarhipov