Our group of accredited schools, colleges and universities (grouped collectively as The Chubb Institute and Banner Institute) offer online degrees, programs and other distance education courses in the following disciplines:
Take a look inside Java classes Learn to deduce properties of a Java class from inside a Java program Summary The Java language's safety features often make it difficult to get to information about a Java class other than what the Java virtual machine wants you to know. This month I'll take a look at class files with an eye toward understanding what is in them and how you can use that knowledge in your Java applications. (3,300 words) By Chuck McManis Printer-friendly version | Mail this to a friend Page 1 of 3 Advertisement elcome to this month's installment of "Java In Depth." One of the earliest challenges for Java was whether or not it could stand as a capable "systems" language. The root of the question involved Java's safety features that prevent a Java class from knowing other classes that are running alongside it in the virtual machine. This ability to "look inside" the classes is called introspection. In the first public Java release, known as Alpha3, the strict language rules regarding visibility of the internal components of a class could be circumvented though the use of the ObjectScope class. Then, during beta, when ObjectScope was removed from the run time because of security concerns, many people declared Java to be unfit for "serious" development. Why is introspection necessary in order for a language to be considered a "systems" language? One part of the answer is fairly mundane: Getting from "nothing" (that is, an uninitialized VM) to "something" (that is, a running Java class) requires that some part of the system be able to inspect the classes to be run so as to figure out just what to do with them. The canonical example of this problem is simply the following: "How does a program, written in a language that cannot look 'inside' another language component, begin executing the first language component, which is the starting point of execution for all other components?" There are two ways to deal with introspection in Java: class file inspection and the new reflection API that is part of Java 1.1.x. I'll cover both techniques, but in this column I'll focus on the first -- class file inspection. In a future column I will look at how the reflection API solves this problem. (Links to complete source code for this column are available in the Resources section.) Look deeply into my files... In the 1.0.x releases of Java, one of the biggest warts on the Java run time is the way in which the Java executable starts a program. What is the problem? Execution is transiting from the domain of the host operating system (Win 95, SunOS, and so on) into the domain of the Java virtual machine. Typing the line "java MyClass arg1 arg2" sets in motion a series of events that are completely hard-coded by the Java interpreter. As the first event, the operating system command shell loads the Java interpreter and passes it the string "MyClass arg1 arg2" as its argument. The next event occurs when the Java interpreter attempts to locate a class named MyClass in one of the directories identified in the class path. If the class is found, the third event is to locate a method inside the class named main, whose signature has the modifiers "public" and "static" and which takes an array of String objects as its argument. If this method is found, a primordial thread is constructed and the method is invoked. The Java interpreter then converts "arg1 arg2" into an array of strings. Once this method is invoked, everything else is pure Java. This is all well and good except that the main method has to be static because the run time can't invoke it with a Java environment that doesn't exist yet. Further, the first method has to be named main because there isn't any way to tell the interpreter the method's name on the command line. Even if you did tell the interpreter the name of the method, there isn't any general way in which to find out if it was in the class you had named in the first place. Finally, because the main method is static, you can't declare it in an interface, and that means you can't specify an interface like this: public interface Application { public void main(String args[]); } If the above interface was defined, and classes implemented it, then at least you could use the instanceof operator in Java to determine if you had an application or not and thus determine whether or not it was suitable for invoking from the command line. The bottom line is that you can't (define the interface), it wasn't (built into the Java interpreter), and so you can't (determine if a class file is an application easily). So what can you do? Actually, you can do quite a bit if you know what to look for and how to use it. Next page > Page 1 Take a look inside Java classes Page 2 Decompiling class files Page 3 Ok, so now what? Java Classes
The Chubb Institute and Banner Institute (The Chubb Institute and Banner Institute) is fully accredited. When reviewing our offerings, please remember that we are here to serve you, the student. Without you, we could not operate successfully.
Want to start an online program but don't know where to look? Look no further-
this is the place to find a variety of online degree programs. From online MBA,
to Information Technology, to online nursing degree programs, we've compiled
a list of online colleges and universities for you.
We also have extensive campus systems for a more traditional school environment. With easy access to all major forms of transportation, as well as free onsite parking, we can make your educational experience something you will treasure.
Start by choosing an online degree category in Java Classes below. You'll get access to a list
of the online universities and online colleges that offer the degree
programs of your choice:
Sponsors:
Debt Consolidation | Short Term Loans | Designers Network | Wainscot & Beadboard | Debt Reduction
A full array of Master's degrees awaits you as well in Java Classes, including a number of discount programs. Make sure you fill out the simple form as well, to ensure you take advantage of the special membership offerings that are included as part of the use of this site. Programs offered through The Chubb Institute and Banner Institute, Keller School, ITT Technical Institute, Chubb Institute and other fine colleges and universities across the United States and Canada.
Click Here to search for all locations | home | complete city listing Java Classes