1. JVM : JVM stands for Java Virtual Machine. It is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are compiled to Java bytecode. The JVM provides a runtime environment for executing Java applications, translating the compiled Java bytecode into machine code specific to the underlying hardware. This allows Java programs to be platform-independent, as they can run on any device with a compatible JVM. The JVM plays a crucial role in the portability and cross-platform compatibility of Java applications.
2. JRE : JRE stands for Java Runtime Environment. It is a set of software tools that provide the necessary runtime support for Java applications. The JRE includes the Java Virtual Machine (JVM), class libraries, and other components needed to run Java applications and applets. While developers use the Java Development Kit (JDK) to create and compile Java programs, end-users typically need only the JRE to run Java applications on their machines. The JRE is responsible for interpreting Java bytecode and executing the program on the user's system, providing the essential runtime environment for Java applications.
3. Java Run time System :
Class Loader:
The Class Loader is responsible for loading Java classes into the Java Virtual Machine (JVM) dynamically at runtime.
It locates and reads class files, which are typically compiled Java source code, and then creates a binary representation of these classes in the JVM.
Class Area (Method Area):
The Class Area is a part of the memory where class structures are stored.
It contains metadata about classes, such as method information, field information, and other runtime constants.
This area is shared among all threads and is where the JVM stores the representation of each loaded class.
Heap:
The Heap is the runtime data area in which objects are allocated.
All Java objects are stored in the heap, and it is managed by the garbage collector, which automatically reclaims memory that is no longer in use by the program.
Stack:
Each thread in a Java application has its own stack.
The stack is used for the execution of methods, and it stores local variables, method call information, and partial results.
It follows the Last In, First Out (LIFO) principle.
Program Counter (PC) Register:
The PC Register is a small, dedicated area of memory within the CPU.
It keeps track of the current instruction being executed by a thread.
For each thread, the PC Register points to the next instruction to be executed.
Native Method Stack:
The Native Method Stack is used for native methods, which are methods written in languages other than Java, like C or C++.
It is distinct from the regular stack and is used for executing native methods.
Native Method Interface (JNI):
JNI is a framework that enables Java code running in a JVM to call and be called by native applications and libraries written in other languages.
It provides a way for Java code to interact with applications and libraries written in languages such as C and C++.
Java Native Library:
The Java Native Library is a collection of functions and procedures that are written in languages like C and C++.
These libraries provide an interface for Java applications to interact with the underlying operating system and perform tasks that require platform-specific functionality.
In summary, the Java runtime system is a complex environment that includes various components working together to execute Java programs efficiently and provide a bridge to interact with native code when needed.
The Bootstrap Class Loader is responsible for loading essential Java classes as part of the Java Virtual Machine (JVM) initialization.
It is written in native code and is not represented by a specific Java class.
Examples of classes loaded by the Bootstrap Class Loader include those in the core Java libraries.
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// Example: Loading a class using the Bootstrap Class Loader
// This is typically handled automatically by the JVM during initialization.
Extension Class Loader:
The Extension Class Loader is an extension of the standard class loader and is responsible for loading classes from the extension directories.
It is parented by the Bootstrap Class Loader.
This loader loads classes that extend the standard functionality of the Java platform.
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// Example: Loading a class using the Extension Class Loader
// Classes in the extension directories, such as JAR files in the lib/ext directory, are loaded by this loader.
System Class Loader (Application Class Loader):
The System Class Loader is responsible for loading classes from the application classpath.
It is the default class loader used by most Java applications.
Classes in the classpath are loaded by this loader.
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// Example: Loading a class using the System Class Loader
// This is the default loader for classes in the application classpath.
Execution Engine:
The Execution Engine is responsible for executing Java bytecode.
It includes the Just-In-Time (JIT) compiler, which translates bytecode into native machine code for improved performance.
The Execution Engine works closely with the Java Virtual Machine to execute Java programs efficiently.
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// Example: Execution of Java bytecode by the Execution Engine
// The bytecode is translated into native machine code for execution.
Native Method Interface (JNI):
The Native Method Interface (JNI) is a framework that allows Java code to call and be called by native applications and libraries.
It provides a way for Java programs to interact with native code written in languages like C and C++.
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// Example: Using JNI to call native methods from Java
// This involves writing Java native methods and corresponding native code in another language.
In a proper sequence, the definitions would be:
Bootstrap Class Loader
Extension Class Loader
System Class Loader
Execution Engine
Native Method Interface (JNI)
These components collectively form the Java runtime system, ensuring the loading, execution, and interaction of Java programs in a platform-independent manner.
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