JRebel Under the Covers

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JRebel Under the Covers How is it even possible?

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Simon Maple -‐ @sjmaple @sjmaple Simon Maple

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Make a change Compile Build Deploy Observe the result Take some coffee Avg: 3 min

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state external serializable temporary derivative

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How can I reload a single class preserving the state?

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ClassLoader API

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How to reload web application? (and preserve state)

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Classes Libraries OldClassLoader NewClassLoader Sevlet New Classes New Libraries Sevlet Session Session init() App State App State Serialize/deserialize

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OldClassLoader NewClassLoader Sevlet New Classes New Libraries Sevlet Session Session App State App State

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Class loaders are good for isolation, but suck at reloading the code … (and perfect for memory leaks)

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Leaking ClassLoaders

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Hello, HotSwap! Hotswapping methods since 2002

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M. Dmitriev. Safe class and data evolution in large and long-lived java[tm] applications. Technical report, Mountain View, CA, 2001.

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The Heap & Garbage CollecBon

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Hello, JRebel! Improving HotSwap since 2007

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Demo Time!

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HotSwap JRebel Changing method bodies Adding/removing methods Adding/removing constructors Adding/removing ﬁelds Adding/removing annotaBons Changing superclass Adding/removing interfaces

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How does it work???

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The engine

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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

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

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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"); } }

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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"); } }

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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(); } //... }

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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; } //... }

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JRebel Reloading

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static { } Adding new static fields Removing static fields Changing values of static fields Factories etc

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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

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“package” visibility package -> public -> package

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concurrency synchronized volatile Object.wait() Object.notify()

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serialization Serializable Externalizable transient

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reflection Class, Method, Field, Constructor, Proxy

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The Ecosystem

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Javassist: binary patching for fun and profit

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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

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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(); } }

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JRebel Plugins

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@Path(“/”) public String foo() { return “Foo”; } @Path(“/foobar”) public String foo() { return “FooBar”; } JRebel core JRebel Spring plugin

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Remote?

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THE IMPACT

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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.”

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Questions? @sjmaple

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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