Java class reloading

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