Java’s Magic: The Bytecode

The key that allows Java to solve both the security and the portability problems just
described is that the output of a Java compiler is not executable code. Rather, it is
bytecode. Bytecode is a highly optimized set of instructions designed to be executed
by the Java run-time system, which is called the Java Virtual Machine (JVM). That is,
in its standard form, the JVM is an interpreter for bytecode. This may come as a bit of
a surprise. As you know, C++ is compiled to executable code. In fact, most modern
languages are designed to be compiled, not interpreted—mostly because of
performance concerns. However, the fact that a Java program is executed by the
JVM helps solve the major problems associated with downloading programs over
the Internet. Here is why.
Translating a Java program into bytecode helps makes it much easier to run a
program in a wide variety of environments. The reason is straightforward: only the
JVM needs to be implemented for each platform. Once the run-time package exists
for a given system, any Java program can run on it. Remember, although the details
of the JVM will differ from platform to platform, all interpret the same Java bytecode.
If a Java program were compiled to native code, then different versions of the same
program would have to exist for each type of CPU connected to the Internet. This is,
of course, not a feasible solution. Thus, the interpretation of bytecode is the easiest way
to create truly portable programs.
The fact that a Java program is interpreted also helps to make it secure. Because the
execution of every Java program is under the control of the JVM, the JVM can contain
the program and prevent it from generating side effects outside of the system. As you
will see, safety is also enhanced by certain restrictions that exist in the Java language.
When a program is interpreted, it generally runs substantially slower than it would
run if compiled to executable code. However, with Java, the differential between the
two is not so great. The use of bytecode enables the Java run-time system to execute
programs much faster than you might expect.

Although Java was designed for interpretation, there is technically nothing about
Java that prevents on-the-fly compilation of bytecode into native code. Along these
lines, Sun supplies its Just In Time (JIT) compiler for bytecode, which is included in
the Java 2 release. When the JIT compiler is part of the JVM, it compiles bytecode into
executable code in real time, on a piece-by-piece, demand basis. It is important to
understand that it is not possible to compile an entire Java program into executable
code all at once, because Java performs various run-time checks that can be done only
at run time. Instead, the JIT compiles code as it is needed, during execution. However,
the just-in-time approach still yields a significant performance boost. Even when
dynamic compilation is applied to bytecode, the portability and safety features still
apply, because the run-time system (which performs the compilation) still is in charge
of the execution environment. Whether your Java program is actually interpreted in the
traditional way or compiled on-the-fly, its functionality is the same.