Architecture of Java Systems

Java's goals:

Platform-independence

Security

Mobility

all in the context of an OO language similar to C++.

Architectural components:

Java source language

Java Libraries API

Java Virtual Machine (JVM) specification

Java Class File specification

Many kinds of JVM implementations are possible:

interpreters

compilers

hardware

in many contexts:

in browser (``applets'')

stand-alone (``applications'')

embedded

Important Java Language Features

Essentially a simplified version of C++

Class-based object-oriented language.

Type-safe.

Garbage collected.

No explicit pointers.

Single inheritance of classes (``implementation inheritance'').

Inheritance from (multiple) interfaces (``specification inheritance'').

No templates or generics.

Built-in arrays and strings.

Multi-level break; no goto statement.

Exceptions.

Concurrent threads.

Dynamic linking.

Classes

All Java data and function definitions are organized into classes.

Each class is a subclass of exactly one other class, except for the predefined class Object, which serves as the root of the object hierarchy.

An instance of a class is a record-like value with associated members: fields (variables) and methods (functions). As usual, a class inherits the fields and methods of its parent class and of its interfaces, and can override the methods of its parent. Instance methods take the instance itself as an implicit parameter, which can be referred to via this.

Abstract classes have specifications but no associated code. In addition, classes can implement one or more abstract interfaces (with methods but no mutable fields), effectively allowing multiple inheritance of specifications.

Classes have special constructor methods for creating new instances; as in C++, the class name serves as the constructor name.

More on Classes

Classes can also have static (per-class) variables and methods; this is important, since every function has to be a method within some class or other.

Method and constructor names can be overloaded, so long as they can be distinguished by the number and types of their parameters (their signatures).

Class and member definitions have a variety of modifiers controlling things like access. For example, a class can be declared final to indicate that it cannot be sub-classed; a member can be declared private to indicate that it can only be accessed by method code within its own class.

Example:

Packages

At the very top level, class definitions are organized into packages, each with a separate internal name space.

Packages consist of class and interface definitions, and possibly subpackages.

In particular, the standard API is presented in the following packages (which always exist):

java.lang
java.util
java.io

Simple applications can just define clases in the ``unnamed'' package.

Package names are prepended to class or interface names to produce fully qualified names which are what appear in class files.

Objects = Class Instances

Class instances are essentially like records. They are always heap-allocated and manipulated via an (implicit) pointer.

Java doesn't have explicit pointers.

Instances are created by calling a constructor. Instances are removed by an automatic garbage collector at some point after they cease to be referenced by the program. There is no explicit deallocation.

The Object class has a finalize method which is guaranteed to be called before an object instance is garbage-collected; this can be overridden

Types

Each class declaration essentially defines a type (called a reference type, because class instances are handled by reference).

There are also built-in primitive types (boolean, char, byte, short, int, long, float, double) whose values are not class instances; there are standard library wrapper classes for these types.

Variables (class or instance fields, parameters, or local variables within methods) and expressions are always typed with a primitive, class or interface name. The basic typing rule is that it is valid to assign an expression of type C to a variable of type D if and only if C inherits from D.

It is also possible to explicitly cast an expression of type D to some subtype C; this is only legal if the expression actually evaluates at runtime to an object of type C, which requires a runtime check.

Types Example

Any reference variable can take on the special value null. Attempts to dereference a null value are also checked at runtime. If a runtime check fails, an exception is raised.

Arrays and Strings

Java arrays are a special sort of object (belonging to the root class Object), which are created dynamically.

Arrays can contain either primitive or reference types.

Arrays are single-dimensional; multi-dimensional arrays can be generated as arrays of arrays.

All references to arrays are subject to runtime bounds checking. There is no way to point directly into an array.

Java strings are not just arrays of characters; String is a special built-in class. Strings are immutable; their contents never change.

Java has special support for:

defining string literals using the ordinary "" mechanism;

appending strings, using the built-in + operator;

converting arbitrary types to string form.

There is also a class of mutable StringBuffers.

Strings are encoded in Unicode.

Multi-level break

Java has no general-purpose goto statement.

But it provides a restricted goto by allowing labels on loops, which can be referenced by break or continue statements.

Example:

Exceptions

Exceptions indicate abnormal conditions in code.

Java exceptions can be thrown by the internal runtime system (e.g., by memory access violations, division by zero, etc.) or explicitly by the user program using the throw statement.

The exception itself is represented as an object belong to (a subclass of) class Throwable.

Exceptions can be caught by enclosing the statements whose execution led to the exception in a try...catch construct. Exception handlers can be restricted to handle only certain subclasses of exceptions. When an exception is raised, control passes to the nearest dynamically enclosing handler.

Exceptions Example

Finally Clauses

The finally construct can be used to clean up at the end of a block that might be exited abnormally.

Example:

This is essentially equivalent to:

But note the duplicated code in this version.

Threads

Java has integral support for multi-threaded programming using per-object locks. Locks can be handled using monitor protocols (executing an object method requires holding the object's lock) or explicitly.

Threads are initiated by invoking the start method of an object inheriting from class Thread; this in turn invokes the object's run method.

Monitor-like semantics for a method are specified by using the modifier synchronized on the method definition.

Locks can be obtained explicitly by using the synchronized statement.

The wait and notify statements are used to to coordinate access to shared objects among threads.

To obtain sensible semantics for non-synchronized access to shared variables, those variables may be declared volatile.

Dynamic linking

The language specification includes detailed rules for how inter-class linking occurs. These are described with reference to the Java Virtual Machine (JVM) and to a set of standard functions in the java.lang API.

In particular, the specification insists that binaries generated by Java compilers use symbolic names for all references from the code of one class to the members of other classes and interfaces. These names are not resolved until the external member is actually needed at runtime. (More precisely, errors arising from attempts to resolve symbolic names must not be reflected until then.)

Loading and resolution are provided by a runtime system class called ClassLoader. The default, built-in class loader obtains binaries from the local host system (in some implementation-dependent way). User code may provide subclasses of ClassLoader that obtain binaries in some other way. For example, net browsers might get binaries over the network using a specialized ClassLoader.

Once a class has been (explicitly) loaded with a specialized instance of ClassLoader, any classes references are automatically loaded using the same ClassLoader instance. Thus, ClassLoader instances effectively provide a top-level partitioning of the symbolic name space.

More Dynamic linking

Specialized ClassLoader instances can be used to load new code into a running Java program and jump into it. However, it is apparently not possible to replace existing class definitions in a running program.

Also, Java 1.1 contains reflection facilities for examining class definitions at runtime.

Despite these facilities, Java provides only imprecise control over versioning of components, because symbolic names don't contain any global identification information, and the precise time when class loading occurs is still rather flexible.

Java API

The language specification insists on the existence of a basic set of library facilities, which must be present (with the specified semantics) in every Java runtime environment.

Many of these may be implemented in Java, but some require access to underlying facilities of the host system (e.g., for IO) or of the java execution engine itself (e.g., class loading).

java.lang contains: Object - the root of the object hierarchy
wrapper classes for primitive types
String and StringBuffer
ClassLoader
Class - represents class and interface defns at runtime
Process,Runtime,System - basic RTS stuff
SecurityManager
Thread and ThreadGroup
Throwable and its subclasses

java.util contains useful ADTs, from Date to Dictionary.

java.io is the basic I/O library.