Interviews are often asked how to tune the JVM? How to answer this question? What if there is no actual tuning experience?
Generally, I ask about JVM tuning during an interview. The main reason is that this technology is not naturally understood by understanding Java. It is necessary to understand some underlying principles and have some depth. So it is more suitable for checking whether the interviewer is "advanced" enough.
So during the interview, how to answer the JVM tuning? You mainly need to be able to answer two questions:
- How to observe the operating parameters of the JVM. For example, you find that Java programs run slowly, freeze regularly, and OOM hangs. In order to find the reason, you must learn how to see how the JVM runs. At this time, you need to understand some of the tool commands that come with the JVM, such as jps, jstack, jhat, and the relatively new JMC graphical interface tool that comes with Java. Through them, you can know how many threads a Java process has, what state each thread has, whether it is waiting for a lock: how much CPU and memory the process takes up; what is the GC state, how frequently Full GC is in frequency, etc.; in memory Is there a leak? Where might it be leaked?
- How to give a plan. You need to understand the working principle of JVM and solve your problems according to the operating parameters. For example, if you find that the program starts slowly, you may guess that the code may not be well written, that is, it is running slowly; maybe the load resource is too large at the beginning, and the heap is not large, causing repeated Full GC. You must verify your conjecture by running parameters, and then solve those problems in a targeted manner. For this, you need to have a general understanding of the working principle of multithreading, Java memory management, and the working principle of GC (serial, CMS, and G1).
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JVM advanced features and best practices
Detailed catalog
Friends who need help to forward the one-click triple connection, see the picture below and add the assistant VX (gyhycx7980) to receive it for free!
Content overview
The first part approaches Java
The first part of this book has established a good foundation for the subsequent research and explanation. 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 introduces the past, present situation and future development trend of the Java technology system, and introduces 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. However, for this reason, once memory leaks and overflow problems occur, if you don’t understand virtual How does the computer use memory, that troubleshooting will become an extremely difficult task.
Chapter 2 introduces 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 explains the common reasons for the memory overflow exception in each area.
Chapter 3 introduces the garbage collection algorithm and the characteristics and operating principles of several garbage collectors provided in the HotSpot virtual machine. The main rules of automatic memory allocation and recovery in the Java virtual machine are verified through code examples.
Chapter 4 introduces how to use the basic command line tools and visual troubleshooting tools released with the JDK.
Chapter 5 shares a few more representative practical cases, and also prepares an exercise that all developers can "hand-in-hand". I hope that readers can gain experience in troubleshooting and tuning through practice.
Friends who need help to forward the one-click triple connection, see the picture below and add the assistant VX (gyhycx7980) to receive it for free!
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 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 Class data is stored and accessed in a practical way.
Chapter 7 introduces 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 effect on the virtual machine. The meaning of the machine.
Chapter 8 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 when executing the code.
Chapter 9 introduces some ideas worth appreciating and learning from using class loaders and processing bytecode through several cases of class loading and execution subsystems, and deepens readers' understanding of the previous theoretical knowledge through a practical exercise.
The fourth part 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 analyzes 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 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. It also selects several common compiler optimization techniques. explain.
Part 5 Efficient Concurrency
Java language and virtual machine provide native and complete multi-threading support, making it inherently suitable for developing multi-threaded concurrent applications. But we can't 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 explains the structure and operation of the Java memory model of the virtual machine, 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 threads in the Java language How to achieve it; also introduced in advance the relevant content of the Java coroutine that is still in the laboratory state.
Chapter 13 introduces the concepts and classifications involved in thread safety, synchronization implementation methods, and the underlying operating principles of virtual machines, and introduces a series of lock optimization measures for virtual machines to achieve efficient concurrency.
This time, I will share this JVM note that helped me get the Ali offer for free . Friends who need help forward it with one-click three consecutive times, see the picture below and add the assistant VX (gyhycx7980) to get it for free!
At last
Not long ago, a friend interviewed Ali. The interviewer asked: Can the Dalvik virtual machine execute class files?
His answer at the time was: No, but it is executing the dex file of class conversion.
When the interviewer continued to ask: Why can't the class file be executed?
His answer was only: the reason for the optimization inside the Dalvik virtual machine, and he failed to give a correct answer to the specific reason.
In fact, the note shared above has an answer: Dakvik is not a Java virtual machine. It does not follow the Java virtual machine specification and cannot execute Java class files. It uses the register architecture instead of the common stack architecture in JVM, but it It is inextricably related to Java. The dex file it executes can be transformed from the class file