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This article introduces the relevant knowledge of "what are the differences and connections between JVM, JRE and JDK". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

1. The differences and relations among JVM, JRE and JDK.

This is basically an entry-level knowledge cognition into the world of java. First, let's take a look at a picture from the official website of java:

From this picture, we can basically see that "JRE" is an indispensable running environment for running programs written in Java. With JRE, the java program we write can be run and used by users.

The "JDK", commonly known as the java development kit, includes the Java runtime environment JRE (Java Runtime Envirnment) and a bunch of Java tools (javac/java/jdb, etc.) and Java-based class libraries (that is, Java API includes rt.jar).

But both JRE and JDK are based on JVM. It can be said that JVM is the lowest guarantee that java programs can run on a machine.

2. So what is JVM? What is its main function?

JVM is the abbreviation of Java Virtual Machine (Java Virtual Machine). Its purpose is simply that it allows us to write java programs to run on different CPU of different operating systems. The java program we write will use a development tool (such as Intellij idea) to compile it into a .class file, but this class file can not be directly recognized and run by the operating system. We need to use jvm to convert the compiled .class file into a machine language according to the jvm specification, and then submit it to the operating system to cpu for execution.

The main function of evaluating JVM in one sentence is that JVM shields the information related to the specific operating system platform, so that Java programs only need to generate the object code (bytecode) running on the Java virtual machine and can run on a variety of platforms without modification.

3. What are the core functions of such an awesome JVM?

There are three core functions in JVM:

Class loader: loads the required class file into JVM when JVM starts or when the class is running

Execution engine: responsible for executing class files, including allocating runtime data areas (such as program counters, local method stacks and virtual stacks) and finally converting bytecode instructions in class into machine instructions to CPU for execution through the operating system

Garbage collector: manage the heap memory of JVM, reclaim useless resources and release memory space in time

4. The loading mechanism and process of JVM class?

First, let's talk about how the class files compiled by the development tools are loaded by JVM. The so-called class loading mechanism is that the virtual machine (JVM) loads the class file into memory, then verifies its correctness, and checks that through parsing and initialization, the class file is finally turned into a java.lang.Class object that can be used directly in memory.

From loading to destroying a class file, its life cycle can be divided into the following five phases: loading, linking (verification, preparation, and parsing), initialization, use, and unloading.

Load: there are three tasks in the load (Load) phase

(1) to obtain the binary byte stream defined by the fully qualified name of a class, you need to use the class loader (ClassLoader) to complete.

(2) assign an area in the "method area" of the runtime data area to store the information of the class, including the basic information of the class, constants, static variables, and so on.

(3) generate a java.lang.Class object of this class on the "Java heap" memory, which is used to expose the entry that uses the class.

Links: there are also three tasks in the link phase

(1) Verify, validating file formats, metadata, bytecode and symbol references to ensure the accuracy of loaded classes

(2) prepare (Prepare), allocate memory for static variables and initialize them to default values.

(3) Resolve, the parsing phase is the process that the virtual machine replaces the symbolic references in the constant pool with direct references. Parsing actions are mainly aimed at seven types of symbolic references: classes or interfaces, fields, class methods, interface methods, method types, method handles and call qualifiers.

Initialization: what the initialization (Initialize) phase does is initialize assignments or calls to static member variables and static methods of the class.

For example, the value of the static variable age above changes to 10 after initialization.

In steps (2) and (3) of the load phase, you can find the terms such as run-time data area, heap, method area, etc., so what exactly is "run-time data area" and what structure does it have?

5. What is the JVM runtime datazone? And its logical structure

"Runtime data area" is that JVM divides memory into several different areas for the purpose of memory management in the process of executing Java program. These areas have their own uses, and the lifecycle of some areas, like virtual machines, exists with the start of the virtual machine process and dies with the end of the virtual machine process. Some areas are established and destroyed depending on the start and end of the user thread. The details are as follows:

Method area (Method Area):

(1) it is used to store class information, constants, static variables, compiled code and other data that have been loaded by the virtual machine.

(2) the method area is the memory area shared by each thread, which is created when the virtual machine starts, because only one copy of the same class class information needs to be loaded.

(3) the java virtual machine specification describes the method area as a logical part of heap memory, but it has another alias "non-heap" to distinguish it from the java heap. Before JDK8, the method area is called Perm space, in JDK8 and later it is called Metaspace (that is, metadata area).

Heap (Heap): the Java heap is shared by all threads and created when the virtual machine starts. The sole purpose of this memory area is to store object instances. In the Java virtual machine specification, all object instances and arrays are allocated on the heap, but with the development of JIT compiler and the gradual maturity of escape analysis technology, allocation on stack and scalar replacement optimization technology will lead to some subtle changes. All objects are assigned to the heap and become less absolute.

Virtual machine stack (Java Virtual Machine Stacks): the virtual machine stack is thread-private or unique and is created as the thread is created. The running state of a thread (which method is being called) is saved by the virtual machine stack corresponding to that thread.

Each method executed by the thread is a stack frame in the virtual machine stack. When a method is called, a stack frame is pressed into the stack; when a method call is completed, the stack frame is popped out of the stack. The illustration is as follows:

Program counter (The Pc Register): we all know that there are multiple threads executing in a JVM process, and whether the content in the thread can have the right of execution is scheduled according to CPU. If thread An is executing somewhere, suddenly loses the right to execute CPU, switches to thread B, and then when thread A gets the right to execute CPU again, how can it continue to execute? This is the need to maintain a variable in the thread to record the location where the thread executes, which is the program counter.

Local method stack (Native Method Stacks): the role of the local method stack is very similar to that of the virtual machine stack, except that the virtual machine stack performs Java methods (bytecode) services for the virtual machine, while the local method stack serves the native methods used in the virtual machine. That is, if the methods executed by the current thread are of type Native, they will be executed in the local method stack.

In summary, according to the design specification of JVM, the memory of JVM is generally divided into thread private memory area and thread shared memory area from the point of view of usage.

The thread private memory area determines the "program counter" and "virtual stack frame" needed for execution when the class loader compiles a class file, and it will be generated with the generation of the current execution thread, and the execution thread will die out, so the "thread private memory area" does not need to consider the problems of memory management and garbage collection.

The thread sharing memory area is created when the virtual machine starts and is shared by all threads, which is the most concerned and largest piece of memory managed by the Java virtual machine.

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