The document discusses invokedynamic, a new bytecode introduced in Java 7 that allows for user-definable bytecode behavior. It provides more flexibility compared to the traditional bytecode instructions like invokevirtual. invokedynamic calls a bootstrap method that prepares a CallSite object and MethodHandle to invoke the target method. This allows dynamic dispatch, fast method pointers, and optimizable behavior like normal Java code. The document includes examples of using invokedynamic to implement dynamic language features and define new domain-specific languages.

1. invokedynamic
IN 45 MINUTES!!!
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2. Me
• Charles Oliver Nutter
• headius@headius.com, @headius
• blog.headius.com
• JRuby Guy at Sun, Engine Yard, Red Hat
• JVM enthusiast, educator, contributor
• Earliest adopter of invokedynamic
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3. 27 April, 2011
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4. History
• JVM authors mentioned non-Java languages
• Language authors have targeted JVM
• Hundreds of JVM languages now
• But JVM was a mismatch for many of them
• Usually required tricks that defeated JVM
optimizations
• Or required features JDK could not provide
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5. JVM Languages
Through the Years
• Early impls of Python (JPython), JS (Rhino)
and many others
• No official backing by Sun until 2006
• JRuby team hired
• JSR-292 “invokedynamic” rebooted in 2007
• Java 7 shipped invokedynamic in 2011
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6. What is
invokedynamic
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7. New Bytecode?
Well, yes...but what does that mean?
And is that all?
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8. Wednesday, February 6, 13

9. New form of
invocation?
That’s one use, but there are many others
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10. Wednesday, February 6, 13

11. Only for Dynamic
Languages?
Dynamic dispatch is a common use,
but there are many others
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12. Wednesday, February 6, 13

13. A User-definable
Bytecode
You decide how the JVM implements it
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14. A User-definable
Bytecode
You decide how the JVM implements it
+
Method Pointers
and Adapters
Faster than reflection, with user-defined
argument, flow, and exception handling
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15. Wednesday, February 6, 13

16. invokedynamic
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17. user-def’d bytecode
invokedynamic
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18. user-def’d bytecode method pointers
invokedynamic
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19. bytecode + bootstrap
user-def’d bytecode method pointers
MethodHandles
invokedynamic
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20. https://github.com/headius/indy_deep_dive
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21. MethodHandles
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22. Method Handles
• Function/field/array pointers
• Argument manipulation
• Flow control
• Optimizable by the JVM
• This is very important
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23. java.lang.invoke
• MethodHandle
• An invokable target + adaptations
• MethodType
• Representation of args + return type
• MethodHandles
• Utilities for acquiring, adapting handles
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24. MethodHandles.Lookup
• Method pointers
• findStatic, findVirtual,
findSpecial, findConstructor
• Field pointers
• findGetter, findSetter,
findStaticGetter, findStaticSetter
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25. Why Casting?
value1 = (String)mh1.invoke("java.home");
mh2.invoke((Object)"Hello, world");
• invoke() is “signature polymorphic”
• Call-side types define signature
• At compile time
• Signature must match MethodHandle type
• Or use invokeWithArguments
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26. Adapters
• Methods on j.l.i.MethodHandles
• Argument manipulation, modification
• Flow control and exception handling
• Similar to writing your own command-
pattern utility objects
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27. Argument Juggling
• insert, drop, permute
• filter, fold, cast
• splat (varargs), spread (unbox varargs)
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28. Flow Control
• guardWithTest: boolean branch
• condition, true path, false path
• Combination of three handles
• SwitchPoint: on/off branch
• true and false paths
• Once off, always off
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29. Exception Handling
• catchException
• body, exception type, handler
• throwException
• Throws Throwable in argument 0
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30. bytecode
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31. Goals of JSR 292
• A user-definable bytecode
• Full freedom to define VM behavior
• Fast method pointers + adapters
• Optimizable like normal Java code
• Avoid future modifications
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32. JVM 101
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33. JVM 101
200 opcodes
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34. JVM 101
200 opcodes
Ten (or 16) “endpoints”
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35. JVM 101
200 opcodes
Ten (or 16) “endpoints”
Invocation
invokevirtual
invokeinterface
invokestatic
invokespecial
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36. JVM 101
200 opcodes
Ten (or 16) “endpoints”
Invocation Field Access
invokevirtual getfield
invokeinterface setfield
invokestatic getstatic
invokespecial setstatic
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37. JVM 101
200 opcodes
Ten (or 16) “endpoints”
Invocation Field Access Array Access
invokevirtual getfield *aload
invokeinterface setfield *astore
invokestatic getstatic b,s,c,i,l,d,f,a
invokespecial setstatic
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38. JVM 101
200 opcodes
Ten (or 16) “endpoints”
Invocation Field Access Array Access
invokevirtual getfield *aload
invokeinterface setfield *astore
invokestatic getstatic b,s,c,i,l,d,f,a
invokespecial setstatic
All Java code revolves around these endpoints
Remaining ops are stack, local vars, flow control,
allocation, and math/boolean/bit operations
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39. The Problem
• JVM spec (pre-7) defined 200 opcodes
• All bytecode lives within these 200
• What if your use case does not fit?
• Dynamic language/dispatch
• Lazy initialization
• Non-Java features
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40. // Static
System.currentTimeMillis()
Math.log(1.0)
// Virtual
"hello".toUpperCase()
System.out.println()
// Interface
myList.add("happy happy")
myRunnable.run()
// Special
new ArrayList()
super.equals(other)
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41. // Static
invokestatic java/lang/System.currentTimeMillis:()J
invokestatic java/lang/Math.log:(D)D
// Virtual
invokevirtual java/lang/String.toUpperCase:()Ljava/lang/String;
invokevirtual java/io/PrintStream.println:()V
// Interface
invokeinterface java/util/List.add:(Ljava/lang/Object;)Z
invokeinterface java/lang/Runnable.add:()V
// Special
invokespecial java/util/ArrayList.<init>:()V
invokespecial java/lang/Object.equals:(java/lang/Object)Z
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42. invokestatic
invokevirtual
invokeinterface
invokespecial
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43. invokevirtual invokestatic
1. Confirm object is of correct type 1. Confirm arguments are of correct type
2. Confirm arguments are of correct type 2. Look up method on Java class
3. Look up method on Java class 3. Cache method
4. Cache method 4. Invoke method
5. Invoke method
invokeinterface invokespecial
1. Confirm object’s type implements interface 1. Confirm object is of correct type
2. Confirm arguments are of correct type 2. Confirm arguments are of correct type
3. Look up method on Java class 3. Confirm target method is visible
4. Cache method 4. Look up method on Java class
5. Invoke method 5. Cache method
6. Invoke method
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44. invokevirtual invokestatic
1. Confirm object is of correct type 1. Confirm arguments are of correct type
2. Confirm arguments are of correct type 2. Look up method on Java class
3. Look up method on Java class 3. Cache method
4. Cache method 4. Invoke method
5. Invoke method
invokeinterface invokespecial
1. Confirm object’s type implements interface 1. Confirm object is of correct type
2. Confirm arguments are of correct type 2. Confirm arguments are of correct type
3. Look up method on Java class 3. Confirm target method is visible
4. Cache method 4. Look up method on Java class
5. Invoke method 5. Cache method
6. Invoke method
invokedynamic
1. Call bootstrap handle (your code)
2. Bootstrap prepares CallSite + MethodHandle
3. MethodHandle invoked now and future (until CallSite changes)
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45. Wednesday, February 6, 13

46. invokedynamic bytecode
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47. invokedynamic bytecode
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48. invokedynamic bytecode
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49. invokedynamic bytecode
target method
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50. invokedynamic bytecode
target method
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51. invokedynamic bytecode
target method
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52. All Together Now...
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53. Tools
• ObjectWeb's ASM library
• De facto standard bytecode library
• Jitescript
• DSL/fluent wrapper around ASM
• InvokeBinder
• DSL/fluent API for building MH chains
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54. #1: Trivial Binding
• Simple binding of a method
• j.l.i.ConstantCallSite
• Method returns String
• Print out String
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55. #2: Modifying the Site
• Toggle between two targets each call
• j.l.i.MutableCallSite
• Call site sent into methods
• ...so they can modify it
• Trivial example of late binding
• InvokeBinder usage
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56. #3: Dynamic Dispatch
• Target method not known at compile time
• Dynamically look up after bootstrap
• Rebind call site to proper method
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57. StupidScript
• push <string>: push string on stack
• send <number>: call function with n args
• One arg call: ["print", "Hello, world"]
• def <name> 'n' <op1> [ 'n' <op2> ...] 'n' end
• define function with given ops
• One builtin: print (System.out.println)
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58. StupidScript
• push <string>: push string on stack
• send <number>: call function with n args
• def <name> 'n' <op1> [ 'n' <op2> ...] 'n' end
• One builtin: print (System.out.println)
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59. Implementation
• Simple parser generates AST
• Compiler walks AST, emits .class
• Top level code goes into run() method
• defs create additional methods
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60. Hello, world!
push Hello, world!
push print
send 1
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61. Define Function
def hello
push print
push Hello, world!
send 1
end
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62. Call Function
push hello
send 0
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63. More Information
• My blog: http://blog.headius.com
• Follow me: @headius
• Code on Github
• https://github.com/headius/indy_deep_dive
• Slides posted online
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