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?  
   

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2. Simon  Maple  -­‐  @sjmaple  
   @sjmaple
   Simon Maple
   

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

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7. state
   external
   serializable
   temporary
   derivative
   

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8. How can I reload a single
   class preserving the state?
   

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9. ClassLoader API
   

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11. How to reload web application?
    (and preserve state)
    

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

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

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

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16. Leaking ClassLoaders
    

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17. Hello, HotSwap!
    Hotswapping methods since 2002
    

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18. 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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19. The  Heap  &  Garbage  CollecBon  
    

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20. Hello, JRebel!
    Improving HotSwap since 2007
    

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21. Demo Time!
    

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22. 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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23. How does it work???
    

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24. The
    engine
    

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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
    

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26. 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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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");
    }
    }
    

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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");
    }
    }
    

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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();
    }
    //...
    }
    

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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;
    }
    //...
    }
    

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31. JRebel Reloading
    

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

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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
    

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36. “package” visibility
    package -> public -> package
    

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

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38. serialization
    Serializable Externalizable
    transient
    

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39. reflection
    Class, Method, Field,
    Constructor, Proxy
    

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41. The
    Ecosystem
    

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42. Javassist: binary patching
    for fun and profit
    

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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
    

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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();
    }
    }
    

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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();
    }
    }
    

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46. JRebel Plugins
    

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

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48. Remote?
    

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49. THE IMPACT
    

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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.”
    

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51. Questions?
    @sjmaple
    

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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
    

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