Java Virtual Machine (JVM):
Definition: JVM is an abstract computing machine that provides the runtime environment in which Java bytecode can be executed. It abstracts the underlying hardware and operating system, allowing Java programs to run on any platform that has a compatible JVM implementation.
Key Components:
Class Loader: Loads Java class files into memory as they are referenced by a running Java program.
Runtime Data Areas:
- Method Area: Stores class structures such as runtime constants, field and method data, and method code.
- Heap: Memory area where objects are allocated. It's shared among all threads and used for dynamic memory allocation.
- Java Stack: Stores method invocation frames, including local variables, operand stacks, and frame data.
- Program Counter (PC) Register: Keeps track of the currently executing instruction.
Execution Engine: Interprets Java bytecode and executes the instructions. It can use various techniques such as Just-In-Time (JIT) compilation to improve performance.
Native Method Interface (JNI): Provides a bridge between Java code and native code written in languages like C or C++. It allows Java programs to call native methods and vice versa.
Functionality:
- Platform Independence: JVM abstracts the hardware and operating system, providing a consistent runtime environment for Java programs across different platforms.
- Memory Management: JVM manages memory allocation and garbage collection, automatically deallocating memory for objects that are no longer in use.
- Security: JVM enforces various security features such as bytecode verification to ensure that Java programs cannot violate the runtime environment's integrity.
Java Runtime Environment (JRE):
Definition: JRE is a set of software components that provides the runtime environment for executing Java programs. It includes JVM, libraries, and other components required to run Java applications but does not include development tools such as compilers and debuggers.
Key Components:
JVM: The core component responsible for executing Java bytecode.
Java Class Library: A collection of precompiled Java classes and packages providing essential functionality such as data structures, input/output operations, networking, and more.
Java Deployment Technologies: Tools and components for deploying Java applications, including Java Web Start and Java Plugin.
Functionality:
- Running Java Applications: JRE provides the necessary runtime environment to execute Java applications, allowing users to run Java programs on their systems.
- Application Deployment: JRE includes deployment technologies for distributing and launching Java applications through web browsers or standalone applications.
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