Java Reference
In-Depth Information
Note
The files that store compiled Java code are known as
classfiles
because they
often store the runtime representation ofJava classes, alanguage feature discussed in
The Java language was designed with portability in mind. Ideally, Java developers
writeaJavaprogram'ssourcecodeonce,compilethissourcecodeintobytecodeonce,
and run the bytecode on any platform (e.g., Windows, Linux, and Mac OS X) where
Javaissupported,withouteverhavingtochangethesourcecodeandrecompile. Port-
ability is achieved in part by ensuring that primitive types have the same sizes across
platforms. For example, the size of Java's integer type is always 32 bits.
TheJavalanguagewasalsodesignedwithrobustnessinmind.Javaprogramsshould
belessvulnerabletocrashesthantheirC/C++counterparts.Javaachievesrobustnessin
part by not implementing certain C/C++ features that can make programs less robust.
Forexample,
pointers
(variablesthatstoretheaddressesofothervariables)increasethe
likelihood of program crashes, which is why Java doesn't support this C/C++ feature.
Java Is a Platform
Java
is a platform that executes Java-based programs. Unlike platforms with physical
processors(e.g.,anIntelprocessor)andoperatingsystems(e.g.,Windows7),theJava
platform consists of a virtual machine and execution environment.
A
virtual machine
isasoftware-basedprocessorwithitsownsetofinstructions.The
JavaVirtualMachine(JVM)'sassociated
execution environment
consistsofahugelib-
raryofprebuiltfunctionality,commonlyknownasthe
standard class library
,thatJava
programscanusetoperformroutinetasks(e.g.,openafileandreaditscontents).The
executionenvironmentalsoconsistsof“glue”codethatconnectstheJVMtotheunder-
lying operating system.
Note
The “glue” code consists of platform-specific libraries for accessing the op-
erating system's windowing, networking, and other subsystems. It also consists of
code that uses the Java Native Interface (JNI) to bridge between Java and the oper-
ating system. I discuss the JNI in Appendix C. You might also want to check out
Wikipedia's“JavaNativeInterface”entry(
http://en.wikipedia.org/wiki/
Java_Native_Interface
)
to learn about the JNI.
WhenaJavaprogramlauncherstartstheJavaplatform,theJVMislaunchedandtold
to load a Java program's starting classfile into memory, via a component known as a
classloader
. After the classfile has loaded, the following tasks are performed:
