Basic concepts of Java virtual machine

1. Class loading subsystem: Responsible for loading Class information from the file system or the network. The loaded information is stored in a memory space called the method area.

2. Method area: It stores class information, constant information, constant pool information, including stringsliterals and numeric constants, etc. The method area is the block permanent area of ​​the auxiliary stack, which is a prerequisite for solving the generation of stack information.

3. Java heap: The Java heap is established when the java virtual machine is started. It is the main memory working area of ​​the java program. Almost all object instances are stored in the Java heap. , the heap space is shared by all threads. The heap solves the problem of data storage, that is, how and where to put data.

4. Direct memory: Java's NIO library allows Java programs to use direct memory, thus improving performance. Usually direct memory speed is better than Java heap. You may consider using it in situations where reading and writing are frequent.

5. Each virtual machine thread has a private stack. The Java stack of a thread is created when the thread is created. The Java stack stores local variables, method parameters, At the same time, Java method calls, return values, etc. The stack solves the running problem of the program, that is, how the program executes, or how to process data.

6. Local method stack: It is very similar to the Java stack. The biggest difference is that the local method stack is used for local method calls. The Java virtual machine allows Java to directly call local methods.

7. Garbage collection system: It is the core of Java and indispensable. Java has its own mechanism for garbage cleaning, so developers do not need to clean it manually.

8. PC register: a private space for each thread. The Java virtual machine creates a PC register for each thread. At any time, a Java thread is always executing a method. This method is called the current method. , if the current method is not a local method, the PC register will execute the instruction currently being executed. If it is a local method, the PC register value is undefined. The register stores such as the current execution environment pointer, program counter, operation stack pointer, and calculated variables. Pointers and other information.

9. The core component of the virtual machine is the execution engine, which is responsible for executing the bytecode of the virtual machine. Generally, users first compile it into machine code and then execute it.

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