JRebel Under the Covers

This talk presents a number of conceptual and technical challenges that we discovered while building JRebel. JRebel performs class reloading and framework integrations which the JVM doesn't provide out the box. This session demystifies the magic.
This talk presents a number of conceptual and technical challenges that we discovered while building JRebel. At first, the JVM wasn't designed for live updates, so we will talk about the engine that mitigates the problem. Secondly, the diversity of Java ecosystem, created by the variety of application servers, the frameworks and tools, makes it challenging in creating a generic solution that would fit the majority of developers. We will see, how Java platform itself allows us to develop a solution by applying bytecode instrumentation mechanism.
JRebel does live code reloading to ensure that the developer can keep instantly alternating between the developing environment and the web browser, to save wasted time and increase the productivity flow.

Transcript

1. JRebel  Under  the  Covers   How  is  it  even  possible?  
2. Simon  Maple  -­‐  @sjmaple   @sjmaple Simon Maple
5. Make a change Compile Build Deploy Observe the result Take some coffee Avg: 3 min
7. state external serializable temporary derivative
8. How can I reload a single class preserving the state?
9. ClassLoader API
11. How to reload web application? (and preserve state)
12. Classes Libraries OldClassLoader NewClassLoader Sevlet New Classes New Libraries Sevlet Session Session init() App State App State Serialize/deserialize
13. OldClassLoader NewClassLoader Sevlet New Classes New Libraries Sevlet Session Session App State App State
14. Class loaders are good for isolation, but suck at reloading the code … (and perfect for memory leaks)
16. Leaking ClassLoaders
17. Hello, HotSwap! Hotswapping methods since 2002
18. M. Dmitriev. Safe class and data evolution in large and long-lived java[tm] applications. Technical report, Mountain View, CA, 2001.
19. The  Heap  &  Garbage  CollecBon  
20. Hello, JRebel! Improving HotSwap since 2007
21. Demo Time!
22. HotSwap   JRebel   Changing  method  bodies   Adding/removing  methods   Adding/removing  constructors   Adding/removing  fields   Adding/removing  annotaBons   Changing  superclass   Adding/removing  interfaces  
23. How does it work???
24. The engine
25. ClassA +field1 +field2 +field3 +method1(n:int) +method2(s:String) +method3(z:double) ClassA +field1 +field2 +field3 +proxy methods ClassA0 +method1(n:int) +method2(s:String) +method3(z:double) Transformer
26. public class C extends X { int y = 5; int method1(int x) { return x + y; } void method2(String s) { System.out.println(s); } }
27. 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"); } }
28. 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"); } }
29. 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(); } //... }
30. 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; } //... }
31. JRebel Reloading
34. static { } Adding new static fields Removing static fields Changing values of static fields Factories etc
35. package a; public class A { int say() {return 1;} } package a; class Test { public static void main(String args[]) { a.A a = new b.B(); System.out.println(a.say()); } } package b; public class B extends a.A { int say() {return 2;} } An Exercise
36. “package” visibility package -> public -> package
37. concurrency synchronized volatile Object.wait() Object.notify()
38. serialization Serializable Externalizable transient
39. reflection Class, Method, Field, Constructor, Proxy
41. The Ecosystem
42. Javassist: binary patching for fun and profit
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 { }); } } java  –javaagent:agent.jar  …   META-INF/MANIFEST.MF Premain-Class: Agent
44. new ClassFileTransformer() { public byte[] transform(ClassLoader loader, String className, Class<?>classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer){ ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass(new ByteArrayInputStream(classfileBuffer)); transformClass(ct, cp); return ct.toBytecode(); } }
45. new ClassFileTransformer() { public byte[] transform(ClassLoader loader, String className, Class<?>classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer){ ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass(new ByteArrayInputStream(classfileBuffer)); transformClass(ct, cp); return ct.toBytecode(); } }
46. JRebel Plugins
47. @Path(“/”) public String foo() { return “Foo”; } @Path(“/foobar”) public String foo() { return “FooBar”; } JRebel   core   JRebel   Spring   plugin  
48. Remote?
49. THE IMPACT
50. http://zeroturnaround.com/company/case-studies/ “Team velocity was increased by 40.6%, and according to t-test it was a statistically significant difference.” “Netty and Spring together with JRebel is real pleasure to work with. The value of time saved is over 50% of my development time.”
51. Questions? @sjmaple
54. Credits https://www.flickr.com/photos/smemon/4556099850/ https://www.flickr.com/photos/gforsythe/10173857405 https://www.flickr.com/photos/dkshots/6967173295 https://www.flickr.com/photos/reckless_sniper/4545673070 https://www.flickr.com/photos/omcoc/8350510425 https://www.flickr.com/photos/bufivla/3619927485 https://www.flickr.com/photos/39747297@N05/5229733647 http://leadershipelements.files.wordpress.com/2014/01/square-peg-round-hole.jpg