Shulou(Shulou.com)06/02 Report--

We need to have a perceptual understanding of the structure of the JVM virtual machine. After all, we are not programmers, and our awareness is not that deep. The bounden duty of a runtime Java virtual machine instance is to run a java program. When you start a Java program, a virtual machine instance is born. When the program closes and exits, the virtual machine instance dies. If you run three Java programs on the same computer at the same time, you will get three Java virtual machine instances. Each Java program runs in its own Java virtual machine instance.

1. Class loading subsystem

It is responsible for loading Class information from the file system or network, and the loaded class information is stored in a memory space called the method area. In addition to class information, run-time pool information, including literal strings and numbers, may also be stored in the method area.

2. Java heap

Established when the virtual machine starts, it is the main memory working area of the Java program. Almost all Java object instances are placed in the Java heap. Heap space is shared by all threads, which is a memory interval that is closely related to Java applications.

3. NIO library of Java (direct memory)

Allow Java programs to use direct memory. Direct memory is the memory range outside the Java heap and requested directly from the system. Access to direct memory is usually faster than the Java heap. Therefore, for performance reasons, direct memory may be considered in situations where reads and writes are frequent. Because it is directly stored outside the Java heap, its size is not limited to the maximum heap size specified by Xmx. However, the system memory is limited, and the sum of Java heap and direct memory is still limited by the maximum memory that the operating system can give.

4. Garbage collection system

The garbage collection system is an important part of the Java virtual machine. The garbage collector can collect the normal area, Java heap and direct memory.

5. Java stack

Each Java virtual machine thread has a private Java stack. A thread's Java stack is created when the thread is created. Java stores frame information, local variables and method parameters are stored in Java stack, and it is closely related to the call and return of Java method.

6. Local method

Very similar to the Java stack, the biggest difference is that the Java stack is used for Java method calls, while the local method stack is used for local method calls. As an important extension of the Java virtual machine, the Java virtual machine runs the Java program to directly call local methods (usually written in C).

7. PC register

Each thread has private space, and the Java virtual machine creates PC registers for each Java thread. At any given time, a Java thread is always executing a method, and the method being executed is called the current method. If the current method is not a local method, the PC register points to the instruction that is currently being executed. If the current method is a local method, then the value that PC registers is undefined.

8. Execution engine

Is one of the core components of the Java virtual machine, which is responsible for executing the bytecode of the virtual machine. In order to improve the execution efficiency, the modern virtual machine. The method is compiled into machine code using just-in-time compilation techniques before execution.

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