A Java virtual machine is a process virtual machine that can execute Java bytecode.
It is the code execution component of the Java platform.
Sun Microsystems has stated that there are over 5.5 billion JVM-enabled devices.
Overview A Java virtual machine interprets compiled
Java binary code for a computer's processor so that it can perform a Java program's instructions.
James Gosling designed Java to allow programmers to write code that could run on any platform
without the need for rewriting or recompilation for each separate platform.
A Java virtual machine makes this possible because of its awareness of the specific instruction
lengths and other particularities of the platform.
As of 2014 most JVMs use JIT compiling, not interpreting, to achieve greater speed.
JVM languages
Although the JVM was primarily aimed at running compiled Java programs, many other languages
can now run on top of it.
The JVM has currently no built-in support for dynamically typed languages: the existing
JVM instruction set is statically typed, although the JVM can be used to implement interpreters
for dynamic languages.
Java 7 supports dynamically typed languages which started as Sun's "Da Vinci Machine"
project.
Bytecode verifier A basic philosophy of Java is that it is inherently
safe from the standpoint that no user program can crash the host machine or otherwise interfere
inappropriately with other operations on the host machine, and that it is possible to protect
certain methods and data structures belonging to trusted code from access or corruption
by untrusted code executing within the same JVM.
Furthermore, common programmer errors that often lead to data corruption or unpredictable
behavior such as accessing off the end of an array or using an uninitialized pointer
are not allowed to occur.
Several features of Java combine to provide this safety, including the class model, the
garbage-collected heap, and the verifier.
The JVM verifies all bytecode before it is executed.
This verification consists primarily of three types of checks:
Branches are always to valid locations Data is always initialized and references
are always type-safe Access to private or package private data
and methods is rigidly controlled The first two of these checks take place primarily
during the verification step that occurs when a class is loaded and made eligible for use.
The third is primarily performed dynamically, when data items or methods of a class are
first accessed by another class.
The verifier permits only some bytecode sequences in valid programs, e.g. a jump instruction
can only target an instruction within the same method.
Furthermore, the verifier ensures that any given instruction operates on a fixed stack
location, allowing the JIT compiler to transform stack accesses into fixed register accesses.
Because of this, that the JVM is a stack architecture does not imply a speed penalty for emulation
on register-based architectures when using a JIT compiler.
In the face of the code-verified JVM architecture, it makes no difference to a JIT compiler whether
it gets named imaginary registers or imaginary stack positions that must be allocated to
the target architecture's registers.
In fact, code verification makes the JVM different from a classic stack architecture, of which
efficient emulation with a JIT compiler is more complicated and typically carried out
by a slower interpreter.
Code verification also ensures that arbitrary bit patterns cannot get used as an address.
Memory protection is achieved without the need for a memory management unit.
Thus, JVM is an efficient way to get memory protection on simple architectures that lack
an MMU.
This is analogous to managed code in Microsoft's .NET Common Language Runtime, and conceptually
similar to capability architectures such as the Plessey 250, and IBM System/38.
The original specification for the bytecode verifier used natural language that was incomplete
or incorrect in some respects.
A number of attempts have been made to specify the JVM as a formal system.
By doing this, the security of current JVM implementations can more thoroughly be analyzed,
and potential security exploits prevented.
It will also be possible to optimize the JVM by skipping unnecessary safety checks, if
the application being run is proved to be safe.
Bytecode instructions
The JVM has instructions for the following groups of tasks:
Load and store Arithmetic
Type conversion Object creation and manipulation
Operand stack management Control transfer
Method invocation and return Throwing exceptions
Monitor-based concurrency The aim is binary compatibility.
Each particular host operating system needs its own implementation of the JVM and runtime.
These JVMs interpret the bytecode semantically the same way, but the actual implementation
may be different.
More complex than just emulating bytecode is compatibly and efficiently implementing
the Java core API that must be mapped to each host operating system.
Heap The Java virtual machine heap is the area
of memory used by the JVM, specifically HotSpot, for dynamic memory allocation.
The heap is divided into generations: The young generation stores short-lived objects
that are created and immediately garbage collected.
Objects that persist longer are moved to the old generation.
The permanent generation was used for class definitions and associated metadata prior
to Java 8.
Permanent generation was not part of the heap.
The permanent generation was removed from Java 8.
Originally there was no permanent generation, and objects and classes were stored together
in the same area.
But as class unloading occurs much more rarely than objects are collected, moving class structures
to a specific area allowed significant performance improvements.
Secure execution of remote code A virtual machine architecture allows very
fine-grained control over the actions that code within the machine is permitted to take.
This is designed to allow safe execution of untrusted code from remote sources, a model
used by Java applets.
Applets run within a VM incorporated into a user's browser, executing code downloaded
from a remote HTTP server.
The remote code runs in a restricted sandbox, which is designed to protect the user from
misbehaving or malicious code.
Publishers can purchase a certificate with which to digitally sign applets as safe, giving
them permission to ask the user to break out of the sandbox and access the local file system,
clipboard, execute external pieces of software, or network.
C to bytecode compilers From the viewpoint of a compiler, the Java
virtual machine is just another processor with an instruction set, Java bytecode, for
which code can be generated.
The JVM was originally designed to execute programs written in the Java language.
However, the JVM provides an execution environment in the form of a bytecode instruction set
and a runtime system that is general enough that it can be used as the target for compilers
of other languages.
Because of its close association with the Java language, the JVM performs the strict
runtime checks mandated by the Java specification.
That requires C to bytecode compilers to provide their own lax machine abstraction, for instance
producing compiled code that uses a Java array to represent main memory, and linking the
C library to a centralized Java class that emulates system calls.
Most or all of the compilers listed below use a similar approach.
Several C to bytecode compilers exist: NestedVM translates C to MIPS machine language
first before converting to Java bytecode.
Cibyl works similarly to NestedVM but targets J2ME devices.
LLJVM compiles C to LLVM IR, which is then translated to JVM bytecode.
C2J is also GCC-based, but it produces intermediary Java source code before generating bytecode.
Supports the full ANSI C runtime.
Available as a Win32 binary or as a Java executable.
Java Backend for GCC, possibly the oldest project of its kind, was developed at The
University of Queensland in 1999.
Javum is an attempt to port the full GNU environment to the JVM, and includes one of the above
compilers packaged with additional utilities.
Compilers targeting Java bytecode have been written for other programming languages, including
Ada and COBOL.
Licensing Starting with Java Platform, Standard Edition
5.0, changes to the JVM specification have been developed under the Java Community Process
as JSR 924.
As of 2006, changes to specification to support changes proposed to the class file format
are being done as a maintenance release of JSR 924.
The specification for the JVM is published in book form, known as blue book.
The preface states:
We intend that this specification should sufficiently document the Java Virtual Machine to make
possible compatible clean-room implementations.
Oracle provides tests that verify the proper operation of implementations of the Java Virtual
Machine.
One of Oracle's JVMs is named HotSpot.
Clean-room Java implementations include Kaffe and IBM J9.
Oracle retains control over the Java trademark, which it uses to certify implementation suites
as fully compatible with Oracle's specification.
See also
List of Java virtual machines Comparison of Java virtual machines
Comparison of application virtual machines Automated exception handling
Java performance List of JVM languages
Java processor Common Language Runtime
Notes
References
External links The Java Virtual Machine Specification
Java implementations at DMOZ Java Virtual Machine Download Link
JVM implementation in pure Java