JavaOne 2017 - The hitchhiker’s guide to Java class reloading
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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.
JavaOne 2017 - The hitchhiker’s guide to Java class reloading
- 1. The hitchhiker’s guide to Java class reloading @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
- 16. Technical solutions HotSwap Class loaders Java agents & instrumentation
- 17. Technical solutions HotSwap Class loaders Java agents & instrumentation
- 18. hotswapEST. 2001
- 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
- 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
- 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!