DevoxxPL: JRebel Under The Covers
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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.
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DevoxxPL: JRebel Under The Covers
- 1. JRebel Under the Covers How is it even possible?
- 2. Simon Maple -‐ @sjmaple @sjmapleSimon Maple
- 5. Make a change Compile Build Deploy Observe the result Take some coffee Avg: 3 min
- 8. How can I reload a single class preserving the state?
- 11. How to reload web application? (and preserve state)
- 14. Class loaders are good for isolation, but suck at reloading the code … (and perfect for memory leaks)
- 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
- 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
- 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.”