That First Gulp of Java
Most modern languages are “fully compiled.” The compiler generates the “native code” of a specific target platform, i.e. the machine language appropriate to a particular operating system running in a particular processor. Once a program is installed on a user's machine, the operating system executes its instructions directly—an arrangement that achieves efficiency at the expense of portability. For example, if you are running Linux on a Pentium-based PC and creating a C program with GNU's gcc compiler, the resulting executable will run just fine on your machine and ones like it, but not on a Pentium running OS/2, and not on a DEC Alpha running Linux.
If you want to distribute your program widely, you will need to recompile it for a dismaying number of platforms, probably using a number of different development tools. Oh, and you want to keep supporting your software after sale, as well? Nice of you—start hiring. Experience has shown that the long-term effort of maintaining software products on multiple platforms far exceeds the effort of developing them in the first place. And the costs are proportional to the effort—better hunt up some heavy financing to pay all those people.
Java eliminates the complexity of cross-platform development and support through its reliance on a “virtual machine.”
As the word “virtual” implies, a Java compiler's target is a machine that does not actually exist. Instead of generating the native code of a particular platform, it produces “bytecode”, a sequence of 8-bit codes that no actual machine can execute directly. Your program will run, however, and not only on your Linux box, but on any platform that supports Java—and these days that's the same as saying “on any popular platform.”
To execute a Java program, a machine must have a Java Run-time System (JRTS), an implementation of the JVM for that platform—but that is all it needs to run any program written in Java. The JRTS executes the bytecode much as an operating system executes native machine code. Because the run-time system handles all those nasty machine-specific issues for every program, the program itself does not have to.
It is a common mistake to confuse the run-time system with the virtual machine. Even people who should know better sometimes refer to “a program running on [a particular computer's] virtual machine”—and thereby conceal a crucial distinction. Part of Java's unique character is that only one piece of software, the JRTS, knows anything about the particular platform. Programs themselves remain blissfully ignorant of hardware dependencies—and so do programmers. They write their code for a machine that does not exist, serenely confident that doing so makes it portable to any popular platform.
A JRTS loads compiled classes as needed, performs security checks, and dynamically binds calls to methods. At that point many run-time systems begin executing the bytecode, interpreting each one as it is encountered. This continuous interpretation limits execution speed, and is the source of many early complaints about poor performance. An increasing number of Java implementations solve this problem by performing a second compilation step, “just in time.”
Native-code compilers produce fast executables at the expense of portability. Java compilers that produce bytecode achieve portability at the expense of speed—if the JRTS interprets each instruction every time it is encountered.
Many run-time systems don't. In place of the interpreter they include a just-in-time (JIT) compiler. The first time the JRTS loads a portion of bytecode, the JIT compiler translates it into native code. Thereafter, the run-time system executes the native code instead of interpreting the bytecode; execution speed improves dramatically.
It is worth stressing that users get the speed of fully compiled programs without sacrificing portability. The JIT compiler is part of the JRTS, not the Java source-code compiler, so all platform-specific knowledge resides only in the run-time system, on the user's machine, where it belongs. Software developers continue to compile and distribute the same portable, architecture-neutral bytecode files.
A second compilation step is not as expensive as it might sound. JIT compilation is actually quite fast in practice, because the most time-consuming tasks are completed in the first translation, from original Java source code to bytecode. JIT-compiled code is currently running 20 to 30 times faster than interpreted bytecode; this level of performance compares favorably with that of object-oriented code written in C++. Future improvements could boost this ratio to 50 or more, which would put Java executables on a par with optimized C code.
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