[notes] Inside JVM - Intro to Java architecure

Java's architecture arises out of four distinct but interrelated technologies, each of which is defined by a separate specification from Sun Microsystems:

• the Java programming language
• the Java class file format
• the Java Application Programming Interface
• the Java Virtual Machine

The Java Virtual Machine

At the heart of Javaís network-orientation is the Java Virtual Machine, which supports all three prongs of Javaís network-oriented architecture: platform independence, security, and network-mobility.

A Java Virtual Machineís main job is to load class files and execute the bytecodes they contain. As you can see in Figure 1-3

 

class loader Architecure

 

Primodial class loader

The primordial class loader (there is only one of them) is a part of the Java Virtual Machine implementation. For example, if a Java Virtual Machine is implemented as a C program on top of an existing operating system, then the primordial class loader will be part of that C program. The primordial class loader loads trusted classes, including the classes of the Java API, usually from the local disk.

class loader

class loader objects enable you to dynamically extend a Java application at run-time. As it runs, your application can determine what extra classes it needs and load them through one or more class loader objects. Because you write the class loader in Java, you can load classes in any manner. You can download them across a network, get them out of some kind of database, or even calculate them on the fly.

Java class file

Its role in platform independence is serving as a binary form for Java programs that is expected by the Java Virtual Machine but independent of underlying host platforms.

The Java API

The Java API is set of runtime libraries that give you a standard way to access the system resources of a host computer.

 

The Java Programming Language

• object-orientation
• multi-threading
• structured error-handling
• garbage collection
• dynamic linking
• dynamic extension