Byte Code In Java, What Is Byte Code, Why We Use Byte Code In Java Explain
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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). In essence, the original JVM was designed as
an interpreter for bytecode . This may come as a bit of a surprise since many modern languages
are designed to be compiled into executable code because of performance concerns. However,
the fact that a Java program is executed by the JVM helps solve the major problems associated
with web-based programs. Here is why.
Translating a Java program into bytecode makes it much easier to run a program in
a wide variety of environments because 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
understand 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 execution of bytecode by
the JVM is the easiest way to create truly portable programs.
The fact that a Java program is executed by the JVM also helps to make it secure.
Because the JVM is in control, it 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.
In general, when a program is compiled to an intermediate form and then interpreted
by a virtual machine, it runs slower than it would run if compiled to executable code.
However, with Java, the differential between the two is not so great. Because bytecode has
been highly optimized, the use of bytecode enables the JVM to execute programs much
faster than you might expect.
Although Java was designed as an interpreted language, there is nothing about Java that
prevents on-the-fly compilation of bytecode into native code in order to boost performance.
For this reason, Sun began supplying its HotSpot technology not long after Java’s initial
release. HotSpot provides a Just-In-Time (JIT) compiler for bytecode. When a JIT compiler
is part of the JVM, selected portions of bytecode are compiled into executable code in real
time, on a piece-by-piece, demand basis. It is important to understand that it is not practical
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, a JIT compiler compiles
code as it is needed, during execution. Furthermore, not all sequences of bytecode are
compiled—only those that will benefit from compilation. The remaining code is simply
interpreted. 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 JVM is still in charge of the execution environment.