1 The Java Virtual Machine Yearly Programming Project

2 Introduction An abstract computing machine. An instruction set Various memory areas Knows: –Nothing of the Java programming language –Only a “class” file format: Java Virtual Machine instructions (bytecodes) a symbol table other ancillary information. Secure!

3 Data Types Two kinds of types: primitive and reference. –Primitive: byte, short, int, long, char, float, double, returnAddress Two kinds of values: primitive and reference. All type checking is done at compile time: –data is not tagged –data is not inspectable to determine types. Security: the JVM checks prior to loading that the instructions are type correct

4 Objects Explicit support for objects: instance or array. All objects: JVM type reference. –Can be thought of as pointers to objects. –More than one reference may exist to an object. – Objects are only operated on via values of type reference. Reference Types –Kinds: class, interface, array

5 Stack Each thread has a private stack –No direct operations –Can be implemented as a heap –Either fixed or varying size A stack stores frames. Each frame has: local variables partial results operand stack data for method invocation and return. There is always a current frame for each thread Frames use only words. Two words for long and double.

6 Other Areas of Memory Heap –Shared among all threads –Pool for all classes and arrays. –Reclaimed by an automatic storage management system Method area –Similar to text segment –Shared among all threads –Subject to memory management! Constant Pool Native methods stack

7 Operand Stack –A stack machine: No registers! –Computation: operations on operand stack –IADD instruction: pop first value pop second value push their sum –Type checked: Illegal to push two ints and pop a long

8 Local Variables Serve as registers Indexed by numbers: 0, 1, 2, 3,... Operation (also for constant pool) –Loaded (pushed) into the stack –Stored from a stack Similar operations for the constant pool

9 Instruction Set Stack manipulation: Load/Store/Pop/Dup Arithmetic, type conversion Control: goto, if, call, exception, syncornization Object creation: new, newarray Object manipulation: –field access (static and instance) –array access –typing: check cast and object type, get array length

10 The class File Contains one Java type, either a class or an interface. A stream of 8-bit bytes Described using C like structures –Uses types u1, u2, and u4 for represent an unsigned one-, two-, or four-byte quantity. –No alignment or padding –Can be read using readUnsignedByte, readUnsignedShort, and readInt –Uses pseudo-array notation, even for varying size records

11 The Class File Structure ClassFile { u4 magic; u2 minor_version; u2 major_version; u2 constant_pool_count; cp_info constant_pool[constant_pool_count-1]; u2 access_flags; // is-abstract, private, and other flags u2 this_class; // index to constant pool u2 super_class; // index to constant pool u2 interfaces_count; u2 interfaces[interfaces_count]; // indices of constant pool u2 fields_count; field_info fields[fields_count]; u2 methods_count; method_info methods[methods_count]; u2 attributes_count; attribute_info attributes[attributes_count]; // Currently only source file information }

12 The Constant Pool cp_info { u1 tag; u1 info[]; } Tags for: class name, field and method references, strings of two kinds, various numerical constants, etc. The info field varies accordingly, may also include other constant pool references.

13 Methods Include both regular and construction methods method_info { u2 access_flags; // is-private, is-final, etc. u2 name_index; // index into the constant pool u2 descriptor_index; // index into the constant pool, giving method signature u2 attributes_count; attribute_info attributes[attributes_count]; // currently, only Code and Exceptions are required // optional: line number information }

14 The Code Attribute Code_attribute { u2 attribute_name_index; // Always the word “Code” u4 attribute_length; // Size of this structure u2 max_stack; // The maximal # words in the operand stack u2 max_locals; // Max # of registers used u4 code_length; u1 code[code_length]; // The byte codes, finally! u2 exception_table_length; { u2 start_pc; u2 end_pc; u2 handler_pc; u2 catch_type; } exception_table[exception_table_length]; u2 attributes_count; attribute_info attributes[attributes_count]; }

15 Verification of Class Files Compiler: generate only valid class file JVM: verify that the class file is valid –Security –Version skew Verification: –Static: basic structure –Static or Dynamic: type checking Static implementation by Sun’s JVM

16 The Verification Process Pass 1: ensures basic format integrity: magic number, lengths, etc. Pass 2: All additional verification that can be done without looking at the code array of the Code attribute. –Final class is not subclassed. –Has a super class. –References to constant pool. Pass 3: Data-flow analysis on each method.

17 Data Flow Analysis: At any given point in the program, no matter what code path is taken to reach that point: –The operand stack is always the same size and contains the same types of objects. –No local variable is accessed unless it is known to contain a value of an appropriate type. –Methods are invoked with the appropriate arguments. –Fields are assigned only using values of appropriate types. –All opcodes have appropriate type arguments on the operand stack and in the local variables.

18 Pass 4 Only for efficiency reasons Example: another method that returns an instance of class A. –Instance is only assigned to a field of the same type- No need to check that class A exists. –If instance is assigned to type B, the definitions of both A and B must be loaded in to ensure that A is a subclass of B. Implemented dynamically, at the first time a method is invoked or a field is accessed.