University of Washington
Roadmap Memory & data
Integers & floats
Machine code & C
C: Java:
x86 assembly
car *c = malloc(sizeof(car)); Car c = new Car(); Procedures & stacks
c->miles = 100; c.setMiles(100);
Arrays & structs
c->gals = 17; c.setGals(17);
float mpg = get_mpg(c); float mpg =
Memory & caches
free(c); c.getMPG(); Processes
Virtual memory
Assembly get_mpg: Memory allocation
language: pushq %rbp Java vs. C
movq %rsp, %rbp
...
popq %rbp
ret
OS:
Machine 0111010000011000
100011010000010000000010
code: 1000100111000010
110000011111101000011111
Computer
system:
Autumn 2013 Java vs. C 1
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Java vs. C
¢ Reconnecting to Java
§ Back to CSE143!
§ But now you know a lot more about what really happens when we
execute programs
¢ We’ve learned about the following items in C; now we’ll see
what they look like for Java:
§ Representation of data
§ Pointers / references
§ Casting
§ Function / method calls
§ Runtime environment
§ Translation from high-level code to machine code
Autumn 2013 Java vs. C 2
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Meta-point to this lecture
¢ None of the data representations we are going to talk about
are guaranteed by Java
¢ In fact, the language simply provides an abstraction
¢ We can't easily tell how things are really represented
¢ But it is important to understand an implementation of the
lower levels – useful in thinking about your program
§ just like caching, etc.
Autumn 2013 Java vs. C 3
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Data in Java
¢ Integers, floats, doubles, pointers – same as C
§ Yes, Java has pointers – they are called ‘references’ – however, Java
references are much more constrained than C’s general pointers
¢ Null is typically represented as 0
¢ Characters and strings
¢ Arrays
¢ Objects
Autumn 2013 Java vs. C 4
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Data in Java
¢ Arrays
§ Every element initialized to 0 or null
§ Length specified in immutable field at start of array (int – 4 bytes)
§ array.length returns value of this field
§ Since it has this info, what can it do?
int array[5]:
C ?? ?? ?? ?? ??
0 4 20 24
Java 5 00 00 00 00 00
Autumn 2013 Java vs. C 5
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Data in Java
¢ Arrays
§ Every element initialized to 0 or null
§ Length specified in immutable field at start of array (int – 4 bytes)
§array.length returns value of this field
§ Every access triggers a bounds-check
§ Code is added to ensure the index is within bounds
§ Exception if out-of-bounds
Bounds-checking sounds slow, but:
int array[5]: 1. Length is likely in cache.
2. Compiler may store length in register
C ?? ?? ?? ?? ?? for loops.
3. Compiler may prove that some checks
0 4 20 24 are redundant.
Java 5 00 00 00 00 00
Autumn 2013 Java vs. C 6
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Data in Java
¢ Characters and strings
§ Two-byte Unicode instead of ASCII
§ Represents most of the world’s alphabets
§ String not bounded by a ‘\0’ (null character)
§ Bounded by hidden length field at beginning of string
the string ‘CSE351’:
C: ASCII 43 53 45 33 35 31 \0
0 1 4 7 16
Java: Unicode 6 00 43 00 53 00 45 00 33 00 35 00 31
Autumn 2013 Java vs. C 7
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Data structures (objects) in Java
¢ Objects are always stored by reference, never stored inline.
§ Include complex data types (arrays, other objects, etc.) using references
C struct rec { Java class Rec {
int i; int i;
int a[3]; int[] a = new int[3];
struct rec *p; Rec p;
}; …
}
i a p i a p
3 int[3]
0 4 16 20 0 4 8 12
Autumn 2013 Java vs. C 0 4 16 8
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Pointer/reference fields and variables
¢ In C, we have “->” and “.” for field selection depending on
whether we have a pointer to a struct or a struct
§ (*r).a is so common it becomes r->a
¢ In Java, all non-primitive variables are references to objects
§ We always use r.a notation
§ But really follow reference to r with offset to a, just like C’s r->a
struct rec *r = malloc(...); r = new Rec();
struct rec r2; r2 = new Rec();
r->i = val; r.i = val;
r->a[2] = val; r.a[2] = val;
r->p = &r2; r.p = r2;
Autumn 2013 Java vs. C 9
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Pointers/References
¢ Pointers in C can point to any memory address
¢ References in Java can only point to [the starts of] objects
§ And can only be dereferenced to access a field or element of that object
C struct rec { Java class Rec {
int i; int i;
int a[3]; int[] a = new int[3];
struct rec *p; Rec p;
}; }
struct rec* r = malloc(…); Rec r = new Rec();
some_fn(&(r.a[1])) //ptr some_fn(r.a, 1) // ref, index
r
r
x
i a p i a p
3 int[3]
0 4 16 20 0 4 8 12
0 4 16
Autumn 2013 Java vs. C 10
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Casting in C (example from Lab 5)
¢ We can cast any pointer into any other pointer;
just look at the same bits differently
struct BlockInfo {
int sizeAndTags;
struct BlockInfo* next; Cast b into char
struct BlockInfo* prev; pointer so that
}; you can add byte
typedef struct BlockInfo BlockInfo; offset without
scaling
…
int x; Cast back into
BlockInfo *b; BlockInfo pointer
BlockInfo *newBlock; so you can use it
… as BlockInfo struct
newBlock = (BlockInfo *) ( (char *) b + x );
…
s n p s n p
0 4 8 12 x
Autumn 2013 Java vs. C 11
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Type-safe casting in Java
¢ Can only cast compatible object references
class Boat extends Vehicle {
int propellers;
class Object { class Vehicle { }
… int passengers;
} }
class Car extends Vehicle {
int wheels;
}
// Vehicle is a super class of Boat and Car, which are siblings
Vehicle v = new Vehicle();
Car c1 = new Car();
Boat b1 = new Boat();
Vehicle v1 = new Car(); // ok, everything needed for Vehicle
// is also in Car
Vehicle v2 = v1; // ok, v1 is already a Vehicle
Car c2 = new Boat(); // incompatible type – Boat and
// Car are siblings Why are these
Car c3 = new Vehicle(); // wrong direction; elements in Car
// not in Vehicle (wheels) problematic?
Boat b2 = (Boat) v; // run-time error; Vehicle does not contain
// all elements in Boat (propellers)
Car c4 = (Car) v2; // ok, v2 started out as Car
Car c5 = (Car) b1; // incompatible types, b1 is Boat
How is this implemented / enforced?
Autumn 2013 Java vs. C 12
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Java objects
class Point { fields
double x;
double y;
Point() { constructor
x = 0;
y = 0;
}
method
boolean samePlace(Point p) {
return (x == p.x) && (y == p.y);
}
}
… creation
Point p = new Point();
…
Autumn 2013 Java vs. C 13
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Java objects
Point object
p
vtable pointer
header x y
Point class vtable
q Point object code for Point() code for samePlace()
header vtable pointer x y
¢ vtable pointer : points to virtual method table
§ like a jump table for instance (“virtual”) methods plus other class info
§ one table per class
¢ header : GC info, hashing info, lock info, etc.
§ no size – why?
¢ new : allocate space for object; zero/null fields; run constructor
Autumn 2013 § compiler actually resolves constructor like aJava
static
vs. C method 14
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Java Methods
¢ Static methods are just like functions.
¢ Instance methods
§ can refer to this;
§ have an implicit first parameter for this; and
§ can be overridden in subclasses.
¢The code to run when calling an instance method (e.g.,
p.samePlace(q)) is chosen at run-time by lookup in the vtable.
Java: C pseudo-translation:
Point p = new Point(); Point* p = calloc(1,sizeof(Point));
p->header = ...;
p->vtable = &Point_vtable;
p->vtable[0](p);
return p.samePlace(q); return p->vtable[1](p, q);
Autumn 2013 Java vs. C 15
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Method dispatch
Point object
p
vtable pointer
header x y
Point class vtable
q Point object code for Point() code for samePlace()
header vtable pointer x y
Java: C pseudo-translation:
Point p = new Point(); Point* p = calloc(1,sizeof(Point));
p->header = ...;
p->vtable = &Point_vtable;
p->vtable[0](p);
return p.samePlace(q); return p->vtable[1](p, q);
Autumn 2013 Java vs. C 16
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Subclassing
class PtSubClass extends Point{
int aNewField;
boolean samePlace(Point p2) {
return false;
}
void sayHi() {
System.out.println("hello");
}
}
¢ Where does “aNewField” go? At end of fields of Point
§ Point fields are always in the same place, so Point code can run on
PtSubClass objects without modification.
¢ Where does pointer to code for two new methods go?
§ No constructor, so use default Point constructor
§ To override “samePlace”, write over old pointer
§ Add new pointer at end of table for new method “sayHi”
Autumn 2013 Java vs. C 17
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Subclassing
class PtSubClass extends Point{
int aNewField;
boolean samePlace(Point p2) {
return false;
}
void sayHi() {
System.out.println("hello");
}
} aNewField tacked on at end
vtable x y aNewField
constructor samePlace sayHi
vtable for PtSubClass
(not Point)
Pointer to old code for constructor
Pointer to new code for samePlace
Autumn 2013 Java vs. C 18
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Dynamic dispatch
Point object
vtable pointer
header x y
Point vtable
code for Point() code for samePlace()
PtSubclass object
vtable pointer
header x y aNewField
code for sayHi()
PtSubclass vtable
code for samePlace()
Java: C pseudo-translation:
Point p = ???; // works regardless of what p is
return p.samePlace(q); return p->vtable[1](p, q);
Autumn 2013 Java vs. C 19
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Agenda
¢ Inside
¢ HW4 grades/feedback are up
¢ Lab 5 due tonight! Go, go, go!
§ If I’m not in the office today, I might be in the basement labs
¢ Tomorrow: Review Session
§ bring your own questions
¢ Final exam topics/materials
§ See past exams (website)
§ See topic manifest (website: last Friday’s slides)
¢ Today
§ Finish up Java
§ Brief tour of Parallel Processing
§ 351 Conclusions :(
Autumn 2013 Java vs. C 20
University of Washington
Implementing Programming Languages
¢ Many choices in how to implement programming models
¢ We’ve talked about compilation, can also interpret
§ Execute line by line in original source code
§ Simpler/no compiler – less translation
§ More transparent to debug – less translation
§ Easier to run on different architectures – runs in a simulated
environment that exists only inside the interpreter process
§ Slower and harder to optimize
§ All errors at run time
¢ Interpreting languages has a long history
§ Lisp, an early programming language, was interpreted
¢ Interpreters are still in common use:
§ Python, Javascript, Ruby, Matlab, PHP, Perl, …
Autumn 2013 Java vs. C 21
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Interpreted vs. Compiled in practice
¢ Really a continuum, a choice to be made Compiled
§ More or less work done by interpreter/compiler
C
Java
Lisp
Interpreted
¢ Java programs are usually run by a virtual machine
§ JVMs interpret an intermediate language called Java bytecode
§ Many JVMs compile bytecode to native machine code
§ just-in-time (JIT) compilation
§ Java is sometimes compiled ahead of time (AOT) like C
Autumn 2013 Java vs. C 22
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Virtual Machine Model
High-Level Language Program
Bytecode Ahead-of-time
compiler compiler
compile time
Virtual Machine Language
run time
Virtual machine JIT
(interpreter) compiler
Native Machine Language
Autumn 2013 Java vs. C 23
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Java bytecode
¢ like assembly code for JVM, Holds pointer ‘this’
but works on all JVMs: Other arguments to method
hardware-independent Other local variables
¢ typed (unlike ASM)
¢ strong JVM protections 0 1 2 3 4 n
variable table
operand stack
constant
pool
Autumn 2013 Java vs. C 24
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Holds pointer ‘this’
JVM Operand Stack Other arguments to method
Other local variables
machine:
0 1 2 3 4 n
variable table
operand stack
‘i’ stands for integer,
‘a’ for reference,
‘b’ for byte,
‘c’ for char, constant
‘d’ for double, … pool
bytecode: iload 1
iload 2
//
//
push 1st argument from table onto stack
push 2nd argument from table onto stack
iadd // pop top 2 elements from stack, add together, and
// push result back onto stack
istore 3 // pop result and put it into third slot in table
No registers or mov 8(%ebp), %eax
stack locations; mov 12(%ebp), %edx
compiled to x86:
all operations use add %edx, %eax
operand stack. mov %eax, -8(%ebp)
Autumn 2013 Java vs. C 25
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A Simple Java Method
Method java.lang.String getEmployeeName()
0 aload 0 // "this" object is stored at 0 in the var table
1 getfield #5 <Field java.lang.String name> // takes 3 bytes
// pop an element from top of stack, retrieve its
// specified instance field and push it onto
stack.
// "name" field is the fifth field of the object
4 areturn // Returns object at top of stack
0 1 4
aload_0 getfield 00 05 areturn
In the .class file: 2A B4 00 05 B0
http://en.wikipedia.org/wiki/Java_bytecode_instruction_listings
Autumn 2013 Java vs. C 26
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Class File Format
¢ Every class in Java source code is compiled to its own class
file
¢ 10 sections in the Java class file structure:
§ Magic number: 0xCAFEBABE (legible hex from James Gosling – Java’s inventor)
§ Version of class file format: the minor and major versions of the class file
§ Constant pool: set of constant values for the class
§ Access flags: for example whether the class is abstract, static, final, etc.
§ This class: The name of the current class
§ Super class: The name of the super class
§ Interfaces: Any interfaces in the class
§ Fields: Any fields in the class
§ Methods: Any methods in the class
§ Attributes: Any attributes of the class (for example, name of source file, etc.)
¢ A .jar file collects together all of the class files needed for the
program, plus any additional resources (e.g. images)
Autumn 2013 Java vs. C 27
University of Washington
Compiled from Employee.java
Disassembled class Employee extends java.lang.Object {
public Employee(java.lang.String,int);
public java.lang.String getEmployeeName();
Java Bytecode }
public int getEmployeeNumber();
Method Employee(java.lang.String,int)
0 aload_0
1 invokespecial #3 <Method java.lang.Object()>
4 aload_0
5 aload_1
6 putfield #5 <Field java.lang.String name>
9 aload_0
10 iload_2
11 putfield #4 <Field int idNumber>
javac Employee.java 14 aload_0
javap -c Employee 15 aload_1
16 iload_2
17 invokespecial #6 <Method void
storeData(java.lang.String, int)>
20 return
Method java.lang.String getEmployeeName()
0 aload_0
1 getfield #5 <Field java.lang.String name>
4 areturn
Method int getEmployeeNumber()
0 aload_0
1 getfield #4 <Field int idNumber>
4 ireturn
Method void storeData(java.lang.String, int)
…
Autumn 2013 Java vs. C 28
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Other languages for JVMs
¢ JVMs run on so many computers that compilers have been
built to translate many other languages to Java bytecode:
§ AspectJ, an aspect-oriented extension of Java
§ ColdFusion, a scripting language compiled to Java
§ Clojure, a functional Lisp dialect
§ Groovy, a scripting language
§ JavaFX Script, a scripting language for web apps
§ JRuby, an implementation of Ruby
§ Jython, an implementation of Python
§ Rhino, an implementation of JavaScript
§ Scala, an object-oriented and functional programming language
§ And many others, even including C!
Autumn 2013 Java vs. C 29
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Microsoft’s C# and .NET Framework
¢ C# has similar motivations as Java
¢ Virtual machine is called the Common Language Runtime;
Common Intermediate Language is the bytecode for C# and
other languages in the .NET framework
Autumn 2013 Java vs. C 30
