Lecture 14 Java Virtual Machine

1. Lecture 14Java Virtual Machine Instructors: Fu-Chiung Cheng (鄭福炯) Associate Professor Computer Science & Engineering Tatung Institute of Technology 1

2. Java Program class SumI { public static void main (String[] args) { int count=10; int sum =0; for (int index=1;index<count;index++) sum=sum+index; } // method main } // class SumI 2

3. Java ByteCode Method void main(java.lang.String[]) 0 bipush 10 // byte push 10 to stack (0x10) 2 istore_1 // load 10 (top of stack) to count 3 iconst_0 // push 0 to stack 4 istore_2 // load 0 (top of stack to sum 5 iconst_1 // push 1 to stack 6 istore_3 // load 1 (top of stack) to index 7 goto 17 // go to 17 3

4. Java ByteCode 10 iload_2 // load sum to stack 11 iload_3 // load index to stack 12 iadd // add 13 istore_2 // store “top of stack” to sum 14 iinc 3 1 // index ++ 17 iload_3 // load index to stack 18 iload_1 // load count to stack 19 if_icmplt 10 // if index < count goto 10 22 return 3

5. Internal Architecture of JVM Class loader subsystem class files method area heap Java stacks pc registers native method stacks Runtime data area Native Method Libraries Native Method Interface Execution engine

6. Internal Architecture of JVM • Class loader subsystem: a mechanism for loading • classes or interfaces. • Execution engine: a mechanism for executing the • instructions contained in the methods of loaded classes. • Runtime data area: a memory to store • bytecodes (method area), objects (heap), • parameters, return values, local variables, (stack) • results of intermediate computations (stack). • JVM spec. is abstract ==> designers are free to • implement JVM by their own structure.

7. JVM Execution • JVM execution: • A. class loading, • B. Linking: Verification, Preparation, and Resolution • C. Initialization • D. Executing. • Each instance of JVM has one method area and one heap • Method area and heap are shared by all threads. • JVM parses class and store methods info in method area. • Objects are put onto heap.

8. Runtime Data Area Shared among threads object class data object class data object class data object object object class data class data object object class data object Method area heap

9. Threads • Java supports multi-thread applications. • Multi-thread: Processing can be broken into several • separate threads of control which execute at the same time • A thread is one sequential flow of execution that occurs • at the same time another thread is executing the statement • of the same program • Each thread has its own PC register (program counter) and • Java Stack. • PC register: pointer to next instruction to be executed. • Java Stack: parameters, return values, local variables, • results of intermediate computations.

10. Thread’s Runtime Data Area thread 2 thread 3 thread 1 thread 1 stack frame stack frame stack frame thread 3 thread 2 stack frame stack frame stack frame thread 3 stack frame stack frame stack frame native method stacks pc registers java stacks

11. Thread’s Runtime Data Area • Java Stacks: state of Java method invocation. • Native method stacks: state of native method invocation. • Java Stack is composed of stack frames. • Each stack frame corresponds to one method invocation. • In the example: • A. Thread 1 and 2 are executing Java methods: • PC registers of Thread 1and 2 are pointed to the next • instruction to be executed • B. Thread 3 is executing a native method: • PC register of Thread 3 is undefined.

12. Class Loader Subsystem • Class loader subsystem: • A. Loading: find and import the binary data for a type. • B. Linking: • 1. Verification: ensure the correctness of imported type. • A. verify .class file is well-formed with a proper • symbol table. • B. verify that bytecode obeys the semantic • requirements of the Java Virtual Machine. • C. Example: VM checks • 1. every instruction has a valid operation code • 2. Every branch instruction branches to the start • (not middle) of some other instruction.

13. Class Loader Subsystem B. Linking: 2. Preparation: A. allocating memory for class variables and initializing the memory to default values. B. allocating data structures that are used internally by the virtual machine: 1. method tables. 2. data structure that allows any method to be invoked on instances of a class without requiring a search of superclasses at invocation time.

14. Class Loader Subsystem B. Linking: 3. Resolution: A. transforming symbolic references (e.g. class.field) into direct references. B. symbolic reference is replaced with a direct reference that can be more efficiently processed if the reference is used repeatedly. (Quick instructions) C. Implementation choice: static linkage vs. laziest resolution

15. Class Loader Subsystem C. Initialization: invoke java code that initializes class variables to their proper staring values. A. execution of any class variable initializers B. Before a class can be initialized, its direct superclass must be initialized, as well as the direct superclass of its direct superclass, and so on, recursively. C. Initialization may cause loading, linking, and initialization errors D. Because Java is multithreaded, initialization of a class or interface requires careful synchronization.

16. Class Loader Subsystem • JVM contains two kinds of class loader: • A. Primordial class loader: load trusted class. • It looks in the each directory, in the order the • directories appear in the CLASSPATH, until a file • with appropriate name (filename.class) is found. • B. Class loader objects: • 1. Class loader objects(java.lang.ClassLoader) are • part of the Java APIs. • 2. Three methods in ClassLoader (defineClass • findSystemClass, resolveClass) are the gateway • into JVM.

17. Class Loader Subsystem • DefineClass converts an array of bytes into an instance • of class Class. • Instances of this newly defined class can be created • using the newInstance method in class Class. • findSystemClass

18. Method Area • Inside a Java Virtual Machine Instance, information of • about loaded types(classes and interface) are loaded • into a logical area of memory called method area. • A. Class loader reads in the class file (a linear stream of • bytes) and pass it to VM. • B. VM extracts information about the type from the • binary data and stores the information in method area. • PS: Class (static) variables are stored in method area. • All threads share the Method area. (Thus, access to • the data area’s data structure must be thread-safe.)

19. Type Information stored in Method Area • Fully qualified name of the type. • Fully qualified name of its superclass • class or interface • type’s modifier (public, abstract, final) • List of interface • Constant pool: • literals, symbolic links to types, fields, methods. • Field information: field name, type, modifiers • Method information: name, return type, parameters, • modifiers, method’s bytecodes, size of operand stack • size of local variables, exception table

20. Type Information stored in Method Area • Static variables: class variables are shared by all • instances of a class. (must allocate space in method • area before anyone uses it.) • A reference to class ClassLoader: • for dynamic linking • A reference to class Class

21. Method Table • The type information stored in method area must be • organized to be quickly accessible. • A method table of a class is an array of direct references • to all its methods and the method inherited from its • superclass.

22. Heap • New objects are allocated by JVM in heap. • A heap exists in every JVM instance. Thus, two • different applications can not trample on each other’s • heap. • Heap are shared by all threads. Thus, synchronization • between threads is needed. • Garbage Collector is responsible for freeing Objects. • Note objects (in heap) and class (in method area) may • be freed by GC.

23. Object Representation • JVM spec does not specify how object should be • represented on the heap. • The data structure of object representation and its GC • may greatly influence performance. • Three possible object representations: • heap contains two parts: handle pool and object pool

24. the object pool the handle pool instance data ptr to object pool instance data ptr to class data instance data an object reference instance data ptr to handle pool the heap class data the method area • Splitting an object across a handle pool and object pool.

25. ptr to class data instance data instance data an object reference instance data ptr to heap instance data the heap class data The method area Keeping object data all in one place.

26. prt to class data prt to class data length=(2) length=(2) ar[0][0](int) int [ ] [ ] ar= new int [2][2]; ar[0] (ref) ar[0][1] (int) ar[1] (ref) prt to class data ar (an array ref) length=(2) ar[1][0] (int) ar[1][1] (int) the heap class data for [[I class data for [I the method area One possible heap representation for arrays.

27. prt to special structure instance data ptr into heap instance data the heap entry point into all data for the class prt to full class data prt to method data prt to method data method table prt to method data ● ● ● method data method data method data the method area Keeping the method table close at hand.

28. Object Representation • Each object (instance) of a class has a lock (mutex). • Only one thread at a time can own the object’s lock. • All threads that attempt to access to a locked object, • are put into a wait set. • (for wait(), notify() and notifyAll())

29. Program Counter • Each thread of a running program has its own • pc register (program counter). • Program counter is created when the thread is started. • Pc register can point to a native code or bytecode.

30. Java Stack • When a new method is launched, JVM creates a new • Java stack for the thread. • A Java stack contains stack frames for method • invocation. • Java Stack frame: • A. method’s local variables. B. operand stack. • C. frame data (constant pool resolution, return values, • return address, exception dispatch). • A method can complete itself in either of two ways: • normal and abrupt completion (exception). Either Way • the stack frame is popped and discarded.

31. Class Example3a { // local variable in stack frame public static int runClassMethod(int i, long l, float f, double d, Object o, byte b) { return 0;} public int runInstanceMethod(int i, double d, short s, boolean b) { return 0;} } runInstanceMethod() runClassMethod() index type index type parameter parameter 0 int i hidden this type 0 reference char c long 1 int 1 long l double double d 3 2 float f float double 4 short s int 4 double d 5 boolean b int Object o 6 reference 7 int byte b

32. iload_0 // push local variable 0 (an int) iload_1 // push local variable 1 (an int) iadd // pop two ints, add them and push result istore_2 // pop int, store into local variable 2 after iload_0 before starting after iload_1 after iadd after istore_2 0 0 0 0 0 100 100 100 100 100 local variables 1 1 1 1 1 98 98 98 98 98 2 2 2 2 2 198 operand stack 100 100 198 98

33. Java Stack Example Class Example3c { public static void addAndPrint() { double result = addTwoTypes(1, 88.88); System.out.println(result); } public static double addTwoTypes (int i, double d) { return i + d; } }

34. before invoke addTwoTypes() After invoke addTwoTypes() addTwoTypes() returns 0 0 0 1 1 1 1 89.88 88.88 local variables 0 1 frames for addAndPrint( ) 1 88.88 frame data frame for addTwoTypes( ) operand stack

35. After invoke addTwoTypes() before invoke addTwoTypes() addTwoTypes() returns 0 0 1 1 frames for addAndPrint( ) 0 1 1 1 88.88 88.88 89.88 frame for addTwoTypes( )

36. This C function invokes another C function this Java method invokes a native method. stack frame stack frame This C function invokes a Java method stack frame the current frame stack frame a native method stack Java stacks