Transcript Java Virtual Machine

Java Virtual Machine
Student Name: Wei Liu
Student ID: 104076
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Java Virtual Machine
 JVM is part of Java programming language.
 JVM is a software, staying on top of Operating
System, such as UNIX, Windows NT.
 It help Java create high level of portability by
hiding the difference between the Operating System
implementations.
 It creates an environment that Java language lives.
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Why JVM?
 An ordinary language can not create a system
independent program.
 Java’s goal is “Write-Once-Run-Anywhere”. Java
programs are not computer, operating system
dependent.
 Need to create an “abstract computer” of its own
and runs on it, a kind of virtual machine which
hiding the different OS implementations.
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Java Runtime Environment.
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How JVM works?
 Java programs are compiled into byte code.
 JVM interprets and converts Java byte code
into machine code in order to execute on a
CPU.
 Most web browser has an integrated JVM to
run applets.
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How JVM works?
 Other JVM tasks include:
 Object creations of Java programs.
 Garbage collection.
 Security responsibility.
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JVM Fundamental Parts.
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A byte code instruction set
A set of registers
A stack
A garbage-collected heap
An area for storing methods
All five parts are necessary, may be implemented by a
compiler, an interpreter or a hardware chip.
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Byte Code Instruction Set
 JVM keeps a compact set of Byte Code Instructions
in order to interpret byte code into native binary code.
 Java compilers do not translate programs directly into
native binary code, which is system dependent.
Instead, programs are translated into byte code, just in
its mid-way to a runnable.
 JVM interprets these half-cooked byte code into
executable machine code on different computer
systems and platforms.
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Registers.
 The registers of the Java virtual machine are just like
the registers inside a “real” computer. (32 bit wide)
 PC: program counter
 OPTOP: Pointer to operation stack.
 FRAME: Pointer to execution environment of current
method.
 VARS: Pointer to the first local variable of current
method.
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Stacks
 JVM is stack based.
 The stack is used to supply parameters to byte
codes and methods, and to receive results
back from them.
 Each stack frame contains three (possibly
empty) sets of data: the local variables for the
method call, its execution environment, and
its operand stack.
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Heaps.
 The heap is that part of memory from which newly
created instances (objects) are allocated.
 The heap is often assigned a large, fixed size when
the Java run-time system is started, but on systems
that support virtual memory, it can grow as needed,
in a nearly unbounded fashion.
 Objects in heap are automatically garbage-collected
when they are not needed.
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The Method Area.
 The method area stores the Java byte codes that
implement almost every method in the Java system.
 The method area also stores the symbol tables
needed for dynamic linking, and any other
additional information debuggers or development
environments might want to associate with each
method’s implementation.
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Drawbacks of JVM.
 JVM is a layer on the top of your operating
system that consumes additional memory.
 JVM is additional layer between compiler
and machine. (Comparing Java program and
fast C program!)
 Byte code is compiled for system
independence so it does not take advantage
of any particular operating system.
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JIT Compiler.
 JIT stands for “Just In Time”.
 10 years ago, a smart idea was discovered by Peter
Deutsch while trying to make Smalltalk run faster. He
called it “dynamic translation” during interpretation.
 Every time JIT compiler interprets byte codes, it will
keep the binary code in log and optimize it. Next time,
when the same method is running, the optimized code
will run. Experiments show Java programs using JIT
could be as fast as a compiled C program.
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JIT Example.
 ( Loop with 1000 times )
for(int i=0;i<1000;i++){
do_action( );
}
Without JIT, JVM will interpret do_action() method
1000 times. (A waste of time!)
With JIT, JVM interprets do_action() method only
once and keeps it in log, and the binary native code
will execute for the rest 999 loops.
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JIT Compiler.
Java Virtual
Machine
Java Compiler
Optimize
d
& Kept in
Log
Compiled
Byte
Code
JIT Compiler
Native Machine
Code
Machine
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JVM Security Capability.
 JVM has many capabilities to keep the security of
the computer system.
 “Sandbox”: prohibit a Java applet:
 Reading or writing to the local disk
 Making a network connection to any host, except the
host from which the applet came
 Creating a new process
 Loading a new dynamic library and directly calling a
native method
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Security Holes of
JVM
 Netscape 4.X consists of JVM that has flaws.
 A hostile applet could turn the client browser
into an http server that allows almost anyone in
the world to read/modify/delete files residing on
the client (turned server) machine.
 Ways to deal with “malicious” applets.
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Current JVM.
 Microsoft Java Virtual Machine.
 Netscape Java Virtual Machine.
 Sun Java Virtual Machine.
 All these Java Virtual Machines implement
Java core class packages and their own
specific class packages.
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References.
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http://java.sun.com
http://www.cs.princeton.edu
http://www.research.IBM.com
http://www.javaworld.com
http://www.sans.org
http://www.javacoffeebreak.com
http://www.computerworld.com
http://www.zdnet.com
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Questions?
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