1. JVM Overview
Java Virtual Machine (JVM) is an abstract computing machine. Java Runtime Environment (JRE) is an implementation of the JVM. Java Development Kit (JDK) contains JRE along with various development tools like Java libraries, Java source compilers, Java debuggers, bundling and deployment tools.
JVM becomes an instance of JRE at runtime of a java program. It is widely known as a runtime interpreter. The Java virtual machine (JVM) is the cornerstone on top of which the Java technology is built upon. It is the component of the Java technology responsible for its hardware and platform independence. JVM largely helps in the abstraction of inner implementation from the programmers who make use of libraries for their programmes from JDK
Like a real computing machine, JVM has an instruction set and manipulates various memory areas at run time. Thus for different hardware platforms one has corresponding implementation of JVM available as vendor supplied JREs. It is common to implement a programming language using a virtual machine. Historicaly the best-known virtual machine may be the P-Code machine of UCSD Pascal.
A Java virtual machine instruction consists of an opcode specifying the operation to be performed, followed by zero or more operands embodying values to be operated upon. From the point of view of a compiler, the Java Virtual Machine (JVM)is just another processor with an instruction set, Java byte code, for which code can be generated. Life cycle is as follows, source code to byte code to be interpreted by the JRE and gets converted to the platform specific executable ones.
The Java Virtual Machine (JVM) executes the Java programs bytecode (.class file). The bytecode is generated after the compilation of the program by the Java compiler. The Java Virtual Machine is the software program and data structures that is on the top of the hardware. The Java virtual machine is called "virtual" because it is an abstract computer (that runs compiled programs) defined by a specification. The Java Virtual Machine is the abstraction between the compiled Java program and used hardware and operating system. The JVM is part of both the JDK and the JRE. It is the 'engine' that executes java bytecodes, performs garbage collection and does just-in-time compilation. The JVM for different systems is not same. JVM forms the Java Runtime Environment (JRE) for the program execution. Each operating system and CPU architecture requires a different JRE. The JRE comprises a set of base classes, which are an implementation of the base Java API, as well as a JVM.
The JDK (Java Development Kit) is used for developing java applications. The JDK includes JRE, set of API classes, Java compiler, Webstart and additional files needed to write Java applications. The JDK (Java Development Kit) contains software development tools which are used to compile and run the Java program. Both JDK and JRE contains the JVM.
The above picture helps you to understand the difference between JDK, JRE, JVM
OpenJDK is the open-source implementation of the Java SE 7 JSR (JSR 336). Now there is almost no difference between the Oracle JDK and the OpenJDK. Last year, Oracle took this decision : Moving to OpenJDK as the official Java SE 7 Reference Implementation
Java Virtual Machine (JVM) is an abstract computing machine. Java Runtime Environment (JRE) is an implementation of the JVM. Java Development Kit (JDK) contains JRE along with various development tools like Java libraries, Java source compilers, Java debuggers, bundling and deployment tools.
JVM becomes an instance of JRE at runtime of a java program. It is widely known as a runtime interpreter. The Java virtual machine (JVM) is the cornerstone on top of which the Java technology is built upon. It is the component of the Java technology responsible for its hardware and platform independence. JVM largely helps in the abstraction of inner implementation from the programmers who make use of libraries for their programmes from JDK
Like a real computing machine, JVM has an instruction set and manipulates various memory areas at run time. Thus for different hardware platforms one has corresponding implementation of JVM available as vendor supplied JREs. It is common to implement a programming language using a virtual machine. Historicaly the best-known virtual machine may be the P-Code machine of UCSD Pascal.
A Java virtual machine instruction consists of an opcode specifying the operation to be performed, followed by zero or more operands embodying values to be operated upon. From the point of view of a compiler, the Java Virtual Machine (JVM)is just another processor with an instruction set, Java byte code, for which code can be generated. Life cycle is as follows, source code to byte code to be interpreted by the JRE and gets converted to the platform specific executable ones.
The Java Virtual Machine (JVM) executes the Java programs bytecode (.class file). The bytecode is generated after the compilation of the program by the Java compiler. The Java Virtual Machine is the software program and data structures that is on the top of the hardware. The Java virtual machine is called "virtual" because it is an abstract computer (that runs compiled programs) defined by a specification. The Java Virtual Machine is the abstraction between the compiled Java program and used hardware and operating system. The JVM is part of both the JDK and the JRE. It is the 'engine' that executes java bytecodes, performs garbage collection and does just-in-time compilation. The JVM for different systems is not same. JVM forms the Java Runtime Environment (JRE) for the program execution. Each operating system and CPU architecture requires a different JRE. The JRE comprises a set of base classes, which are an implementation of the base Java API, as well as a JVM.
Sun’s JVM
Sun’s implementations of the Java virtual machine (JVM) is itself called as JRE. Sun’s JRE is available as a separate application and also available as part of JDK. Sun’s Java Development Tool Kit (JDK) comes with utility tools for byte code compilation “javac”. Then execution of the byte codes through java programs using “java” and many more utilities found in the binary directory of JDK. ‘java’ tools forks the JRE. Implementation of JVMs are also actively released by other companies besides Sun Micro Systems.JVM for other languages
A JVM can also be used to implement programming languages other than Java. For example, Ada source code can be compiled to Java bytecode, which may then be executed by a Java virtual machine (JVM). That is, any language with functionality that can be expressed in terms of a valid class file can be hosted by the Java virtual machine (JVM). Attracted by a generally available, machine-independent platform, implementers of other languages are turning to the Java virtual machine (JVM) as a delivery vehicle for their languages. PHP with Quercus is such an example.Just-in-time Compiler (JIT)
JIT is the part of the Java Virtual Machine (JVM) that is used to speed up the execution time. JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.The JDK (Java Development Kit) is used for developing java applications. The JDK includes JRE, set of API classes, Java compiler, Webstart and additional files needed to write Java applications. The JDK (Java Development Kit) contains software development tools which are used to compile and run the Java program. Both JDK and JRE contains the JVM.
The above picture helps you to understand the difference between JDK, JRE, JVM
OpenJDK is the open-source implementation of the Java SE 7 JSR (JSR 336). Now there is almost no difference between the Oracle JDK and the OpenJDK. Last year, Oracle took this decision : Moving to OpenJDK as the official Java SE 7 Reference Implementation
Java Virtual Machine :
JVM Memory Area
Heap memory
The heap memory is the runtime data area from which the Java VM allocates memory for all class instances and arrays. The heap may be of a fixed or variable size. The garbage collector is an automatic memory management system that reclaims heap memory for objects.- Eden Space: The pool from which memory is initially allocated for most objects.
- Survivor Space: The pool containing objects that have survived the garbage collection of the Eden space.
- Tenured Generation: The pool containing objects that have existed for some time in the survivor space.
Non-heap memory
Non-heap memory includes a method area shared among all threads and memory required for the internal processing or optimization for the Java VM. It stores per-class structures such as a runtime constant pool, field and method data, and the code for methods and constructors. The method area is logically part of the heap but, depending on the implementation, a Java VM may not garbage collect or compact it. Like the heap memory, the method area may be of a fixed or variable size. The memory for the method area does not need to be contiguous.- Permanent Generation: The pool containing all the reflective data of the virtual machine itself, such as class and method objects. With Java VMs that use class data sharing, this generation is divided into read-only and read-write areas.
- Code Cache: The HotSpot Java VM also includes a code cache, containing memory that is used for compilation and storage of native code.
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