Welcome to Java Developer

Java is the new language developed by Sun
Microsystems that significantly raises the level of technology on the
Web. Java allows individuals, Web designers, and organizations large
and small to build highly interactive and safe applications. The
technology is based on

• true executable content embedded in an extensible language;
  
• automatic download of applications ("applets") in a safe manner;
  
• portability by standardizing on a "neutral" architecture;
  
• client/server functionality on the Internet or "intranet;"
  
• built-in security that is extensible.
  

Running some Java applets will demonstrate the above features. For
those of you with a mathematical or financial background run the derivative calculator applet
at http://zeitgeist.com. For those of you with a consumer preference,
there is the multimedia application at http://www.vectorman.com. (If you do not have HotJava and you click
on the link, your browser should just ignore the applet.)
For those of you using HotJava you have just downloaded a
program that can be run on your system. This same program runs on
Solaris for SPARC, Solaris for Intel, Windows 95, and NT without
modification.

HotJava Alpha 3 is the current release of the browser. Until now,
releases contained a browser and interpreter, with its corresponding
API. The JDK (Java Development Kit) is the next iteration of the API.
The JDK does not contain a browser but does contain an
appletviewer, which allows you to view applets for debugging
and development, and deployments that use no HTML other than a tag to
call an applet. More
information on obtaining the JDK is available.

Alpha 3 applets do not work with the JDK or Netscape. Small
modifications are required. The procedure for converting an Alpha 3
applet is described in Converting Alpha 3
applets (http://java.sun.com/JDK-prebeta1/converting/). The HTML interface language was changed slightly,
requiring some minor edits to applets. To get an Alpha 3
applet working with the JDK and Netscape you need to convert the applet
and modify the HTML file that calls the applet.

At the time of this writing a new Netscape version of a browser that
supports Java Applets was not released. To avoid confusion, we will
discuss Netscape's Java abilities in the next issue and focus on
HotJava this month.

Let the games begin

The HotJava browser reads standard HTML just like any other Web
browser. It also has the ability to download Java applets. To
accomplish this, a small addition was made to HTML. An additional tag
was added (<APPLET>) that allows the specification of a Java applet class to be
loaded. When a user clicks on an application that requires an applet, the HotJava browser calls a thread that loads the
applet from the network or local filesystem. The code that is loaded is
the architecture-neutral bytecodes produced by the Java compiler. These
bytecodes are verified and then decoded into machine instructions. It
is hoped future versions will compile these bytecodes to native machine
instructions for improved performance.

Many languages share some of the features of Java. The Java designers
built a practical language for distributed computation without the
complexity associated with such languages. Why another language? It
can be argued that there are systems with enough ubiquity (consider
the proliferation of DOS/Windows 3.x) that there is no need for
Java's benefits. Many years ago, Japan launched a project
called TRON. The purpose of this project was to provide one OS and one
hardware platform for the entire country of Japan. This project
ultimately failed. Java, unlike TRON, is based on standards, proven
network infrastructure, and is architecture independent at runtime.

The salient features of the Java Language are:

• Object-oriented. Like Smalltalk, everything in
  Java is an object. Objects are instances of Java classes. One example
  of a Java class is the Applet class. Classes are created from Java
  source files with the Java compiler.
  
  
  
  
• Total Pointer Control. In C and C++, pointers are
  sometimes too powerful for our own good. Not only are they a common
  source of programming errors (so common that the VAX used to store at
  address 0 so that de-referencing a NULL pointer would not
  be invalid), pointers are also a common source of security violations.
  In C and C++ for example you can subvert the type-checking system at
  runtime by explicitly casting the address of a memory area and then
  getting to a private member causing an undetected access violation.
  Consider the following example from C++:

class A { 
	public	int	amount;
	private int	total; 
} 

p = new amount 
p = p + 4; 	// Find location of private member
*p = 10000; 

In Java this is not possible. All accesses to memory areas must be
done using object instance variables. This feature provides additional
security protection against internal security threats -- programmers
leaving a Trojan horse that accesses private data. It also provides
protection against inadvertent pointer over-run or under-run errors, as
well as incorrect offset calculations.



A typical programming error is introduced by a casting operation. Consider
the structure foo defined below. This structure has a couple of
useful members -- a name field, and an id field:

struct foo {
	char	name[10];
	int	id;
}

If this structure was known to your program, you could access
any piece of memory as a structure of type foo by simply
using this fragment of code:

struct	foo *a;
a = (struct foo *)(0x1234);
printf("user %s, id = %d\n",a->name,a->id);

However if the memory so addressed was not a structure of type foo
your results would be unpredictable at best. In Java this is not
possible. All accesses to memory areas must be done using object
references. When used, these references are checked for type
compliance. Loosely typed data structures defined by
struct or typedef in C or C++ are replaced
with a strongly typed data structure defined by class in Java.

record = (struct foo *)malloc(sizeof (struct foo)); 
... some code
record = next_record

In C and C++ we would be left with a memory block in the heap without
a corresponding pointer. In Java, once this allocated memory was no
longer referenced, it would automatically be returned to the heap. In
C and C++ you have to perform frees over and over again and keep track
of a pointer that is separate from the object in some cases.



Another advantage is the number of errors that are prevented when one
portion of the C code calls free() to return allocated
memory to the heap but another portion is still holding pointer to it. If
the memory gets re-used the program will crash when the stale pointer
is used, sometimes much later in the execution. Problems of this type
are hard to debug and often complicated by timing differences and code
paths. In Java, a section of memory is not reclaimed until there are
no more references to it in the active code.

public boolean synchronized enqueue(Item i){
	... Implementation
}

public boolean synchronized enqueue(Item i){
	... Implementation
}

Alternatively one can code a lock only around the section of code that
needs to be locked. This would allow other callers to execute more
code before blocking. For very small functions this overhead may not
be worth it:

public class SomeClass {
	Lock myLock = new Lock();
	void someMethod() {
		myLock.lock();
		try {
			StartOperation();
			ContinueOperation();
			EndOperation();
		}
		finally {
			myLock.unlock();
		}
	}
}

Security -- downloading code over the Internet can be safe

There are several layers to the Java security model. These layers
build on each other to provide built-in security that can be extended
and or modified.

• Ground rules from the language.
  The Java Language, which was designed to be a safe language, and the
  Java compiler, which ensures that source code doesn't violate the
  safety rules.
  
  
  
  
• Bytecode verification. Bytecodes imported into
  the runtime engine are put through a verification process to ensure
  that they obey the language's safety rules. This layer guards against
  an altered compiler producing code that violates the safety rules. Once
  the bytecodes pass the verifier we know the following:

• The code causes no operand stack overflows or underflows; since
  all objects are known the exact allocation required is known.
  
• The types of parameters to all opcodes are known to always be
  correct; an opcode will not be given the wrong type of argument.
  
• No illegal data conversions are done, such as converting integers
  to pointers. This is detected by the bytecode verifier.
  
• Object field accesses are known to be legal.
  

• No access -- disallows any applets from loading any documents via
  URLs, no matter where they come from. Turns HotJava into a simple
  passive browser.
  
• Applet host -- allows an applet to load documents only from the same
  place it came from.
  
• Firewall -- lets you specify a firewall (a scope of hosts that you
  are willing to trust). Once you configure this firewall, applets that
  are inside the firewall can access any documents via URLs, but applets
  outside the firewall can access only those URLs that are also outside
  the firewall.
  
• Unrestricted -- allows any applet to load any URL.
  

• HOTJAVA_READ_PATH -- used to determine where an applet
  has permission to read a file. Defaults to
  <hotjava-install-dir>:$HOME/public_html, where
  <hotjava-install-dir> is where HotJava was installed.
  
• HOTJAVA_WRITE_PATH -- used to determine where an applet has
  permission to write a file. Defaults to NULL.
  

• Local filesystem -- same applet in different hierarchies will have a
  different name.
  
• Network source -- an applet or class on another system accessed via
  HTTP will have a unique name that identifies the network source.
  

A common technique for compromising a system is to replace a good
program with a doctored copy. Often this cannot be done directly. An
indirect approach looks to replace an applet or class in the search
path. HotJava always searches for built-in functions in explicit
areas. This design limits spoofing since built-ins are always checked
first

• File access control lists. Dialog boxes pop up if there is an access
  violation. The policy is conservative.
  
• Network Security Check. When a class is requested that is located at a
  machine that is outside the firewall, additional security rules can be
  applied to it. Some users may not want any class to be loaded that is
  outside the firewall. Other users may only want to allow certain sites
  and others may want to allow all classes to load inside or outside the
  firewall. In future articles we will discuss this in greater detail as
  well as techniques to prevent applet theft and how you can be sure that
  the applet you are running is the one the trusted programmer intended you
  to run.
  

Late-breaking news

While this article was being written the beta API was released. This
API is referred to as the JDK -- Java development Kit. The JDK offers
many new improvements and new functionality. Two of our favorite
additions are the Java Debugger and the Java Applet Viewer. The imaging
model has also been changed significantly and offers an excellent
graphics foundation that understands various color models as well as
the performance issues with loading images over slow lines. We'll
discuss these and other parts of the JDK in future columns. In the
mean time, additional information on several topics is available
below:

• The source for HotJava/Java -- http://java.sun.com
  
• Netscape and Java -- http://www.netscape.com/comprod/products/navigator/version_2.0/java_appl...
  
• About JDK -- http://java.sun.com/JDK-prebeta1/filesinkit/README
  
• Converting applets -- http://java.sun.com/JDK-prebeta1/converting
  
• Online Java course -- http://java.sun.com/progGuide/index.html
  
• Sample applets -- http://java.sun.com/applets/
  

Uses for Java

Java will play a key role in defining the infrastructure of the
emerging electronic world. We can actually think of several hundred
uses for Java. Here are some of the more common:

• Corporate MIS departments begin replacing mission-critical systems
  for internal use with a Web-based architecture. The corporate
  backbones evolve into "intranets".
  
• Legacy applications on mainframes and other computers get Web-based
  interfaces so printouts are no longer shipped around via
  snail-mail or e-mail.
  
• Web-based Java applets interface information from several
  different legacy systems into understandable Web pages. For example
  accessing the 3270 to find a database that tells you where a file is.
  This can become one mouse-click preserving investment and allowing
  workers to be more productive.
  
• Replacement of expense reports, time sheets, and other personnel
  functions with Java applets.
  
• The automation of factories with thousands of applets with nice
  readable graphics interfaces tied into a 10,000-node network.
  
• Access to decision-support systems from a Web interface
  
• Web access while you watch TV -- the start of interactive TV.
  

Future columns

In future columns we will discuss:

• Security issues authentication, authorization and how to build it into
  your applications.
  
• Good Java programming techniques and style.
  
• CELLB video example
  
• Java and CORBA
  
• GUI programming examples
  
• Java development environments
  
• Embedding Java
  

Next month, we discuss Netscape 2.0, including example applets
demonstrating the use of various methods.