With the development of the Internet, high concurrency, high availability, and fast response have become necessary for software, and JVM is closely related to these. Many of the previous JVM series were due to STW affecting fast response problems, and suddenly the website was slow (under jitter) and other problems. Let’s talk about the high concurrency and high availability problems that have been checked through JVM recently. (In the case of high availability and high concurrency, there may be many reasons for the problems, such as abnormally high CPU, high disk IO, SWAP space, etc., many problems may be comprehensive problems).
And while enjoying the benefits of the Java virtual machine, we should also understand and think about "how these technical features are implemented" to understand the underlying principles. Only if you are familiar with JVM, you will not be helpless when encountering abnormalities such as OutOfMemory, and you will not be confused on the Internet to find a solution. In the end, even if you change a few startup parameters to solve the problem, you will still be in the cloud.
In fact, the question of "why learn Java virtual machine" is the same as "why learn data structure and algorithm": if you want to do well, you must first sharpen your tools. It can be said that the Java virtual machine is a weapon for every Java engineer to advance their salary. You want to go up, you want to go deep into the technology, and you don’t want to stay in simple development, or when you are doing Java performance analysis and tuning work, then , Java virtual machine is definitely a powerful sword.
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The following is divided into five parts:
- Approaching Java
- Automatic memory management
- Virtual machine execution subsystem
- Program compilation
- Code optimization, efficient concurrency
first part
The first part establishes a good foundation for the research and explanation in the following text. Although understanding the ins and outs of Java technology and compiling your own OpenJDK is not necessary for readers to understand the Java virtual machine, these preparation processes can provide a good guide for approaching Java technology and the Java virtual machine.
Chapter 1 Approaching Java
Introduced the past, present situation and future development trend of the Java technology system, and introduced how to compile an OpenJDK 12 by yourself in practice.
The second part of automatic memory management
Because programmers give the power to control memory to the Java virtual machine, they can enjoy the many advantages of automatic memory management when coding, but for this reason, once memory leaks and overflow problems occur, if you don’t understand virtual How does the computer use memory, and troubleshooting errors will become an extremely difficult task.
Chapter 2 Java memory area and memory overflow exception
Introduced how the memory in the virtual machine is divided, which part of the area, what kind of code and operation may cause the memory overflow exception, and explained the common causes of the memory overflow exception in each area.
Chapter 3 Garbage Collector and Memory Allocation Strategy
The algorithm of garbage collection and the characteristics and operating principles of several garbage collectors provided in the HotSpot virtual machine are introduced. The main rules of automatic memory allocation and recovery in the Java virtual machine are verified through code examples.
Chapter 4 Virtual Machine Performance Monitoring and Fault Handling Tools
Introduced how to use the basic command line tools and visual troubleshooting tools released with the JDK.
Chapter 5 Tuning Case Analysis and Actual Combat
I shared a few more representative practical cases, and prepared a "hands-on" exercise for all developers. I hope that readers can gain experience in troubleshooting and tuning through practice.
The third part of the virtual machine execution subsystem
The execution subsystem is an indispensable part of the virtual machine. If you understand how the virtual machine executes the program, you can better understand how to write excellent code.
Chapter 6 Class File Structure
Explains the various components in the Class file structure, as well as the definition, data structure and usage of each part, and demonstrates how the data of the Class is stored and accessed in a practical way.
Chapter 7 Virtual Machine Class Loading Mechanism
Introduced what actions the virtual machine performs in the five stages of "loading", "verification", "preparation", "analysis" and "initialization" of the class loading process. It also introduces the working principle of the class loader and its significance to the virtual machine. .
Chapter 8 Virtual Machine Bytecode Execution Engine
It analyzes how to find the correct method when the virtual machine executes the code, how to execute the bytecode in the method, and the memory structure involved in the execution of the code.
Chapter 9 Cases and Actual Combats of Class Loading and Execution Subsystem
Through several cases of class loading and execution subsystems, some ideas worth appreciating and learning from using class loader and processor decoding are introduced, and a practical exercise is used to deepen readers' understanding of the previous theoretical knowledge.
The fourth part of the program compilation and code optimization
The two processes of compiling a Java program from source code into bytecode, and then compiling from bytecode to native machine code, are actually equivalent to the compilation front-end and back-end processes performed by a traditional compiler.
Chapter 10 Front End Compilation and Optimization
Analyzed the causes and consequences of various syntactic sugars in the Java language, such as generics, active boxing and unboxing, and conditional compilation, and practiced how to use the plug-in annotation processor to complete a compiler plug-in that checks program naming conventions.
Chapter 11 Backend Compilation and Optimization
It explains the hot spot detection method of the virtual machine, HotSpot's just-in-time compiler, compilation trigger conditions, and how to observe and analyze the data and results of the real-time compilation from the outside of the virtual machine. Several common compiler optimization techniques are also selected for explanation.
Part 5 Efficient Concurrency
The Java language and virtual machine provide native and complete multi-threading support, making it inherently suitable for developing multi-threaded concurrent applications. However, we cannot expect the system to complete all the processing related to concurrency. Understanding the inside story of concurrency is also an indispensable course for a senior programmer.
Chapter 12 Java Memory Model and Threads
Explains the structure and operation of the virtual machine Java memory model, as well as the embodiment of atomicity, visibility and order in the Java memory model; introduces the rules and use of the principle of first occurrence, and how threads are implemented in the Java language的; Also introduced in advance the relevant content of the Java coroutine that is still in the laboratory state.
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Chapter 13 Thread Safety and Lock Optimization
It introduces the concepts and classifications involved in thread safety, the synchronization implementation method and the underlying operating principle of the virtual machine, and introduces a series of lock optimization measures made by the virtual machine to achieve efficient concurrency.
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