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This article mainly introduces "what are the basic principles and related tools of the Java virtual machine". In the daily operation, I believe that many people have doubts about the basic principles of the Java virtual machine and related tools. The editor consulted all kinds of materials and sorted out simple and easy-to-use methods of operation. I hope it will be helpful to answer the doubts about "what are the basic principles and related tools of the Java virtual machine?" Next, please follow the editor to study!

The question I'm going to ask you today is, tell me about your understanding of the Java platform? is it true that "Java is interpretation and execution"?

Typical answer

Java itself is an object-oriented language, and the most prominent features are in two aspects:

One is the so-called "compile once, execute everywhere" (Compile once, run anywhere), which can easily acquire cross-platform capabilities.

In addition, there is garbage collection (GC, Garbage Collection). Java allocates memory through garbage collector (Garbage Collector). In most cases, programmers do not have to worry about memory allocation and recycling.

We come into contact with JRE (Java Runtime Environment) or JDK (Java Development Kit) every day. JRE, the Java runtime environment, includes JVM and Java class libraries, as well as some modules. JDK can be seen as a superset of JRE, providing more tools, such as compilers, various diagnostic tools, and so on.

For the sentence "Java is interpretation and execution", this is not very accurate.

The source code of the Java we developed is first compiled into bytecode (bytecode) through Javac, and then, at run time, the bytecode is converted into the final machine code through the interpreter embedded in the Java virtual machine (JVM). But common JVM, such as the Hospot JVM provided by Oracle JDK, which we use in most cases, provides a JIT (Just-In-Time) compiler, commonly known as a dynamic compiler. JIT can compile hot code into machine code at run time, in which case some hot code is compiled and executed rather than interpreted.

Test site analysis

In fact, this question is a bit general. The topic itself is very open, often examining many aspects, such as whether the understanding of the basic knowledge is very clear, whether to master the main modules and operating principles of the Java platform, and so on. Many interviewers will suffer from this kind of question, a little nervous, do not know where to start, give a very brief answer.

For such general questions, you need to try to show that your thinking is deep and systematic, and you have a comprehensive understanding of Java, so be sure to avoid giving the interviewer the impression that you are a person who "knows but doesn't know why". After all, understanding the basic composition and mechanism is the basis of many things such as problem diagnosis or performance tuning in daily work. I believe that no recruiter will not like the interviewer who "loves learning and thinking".

Even if you feel that your answer is not very perfect, don't worry. Personally, I think it is normal for such general questions to be answered a little one-sidedly. Most experienced interviewers will not easily draw conclusions to the interviewer just because of a single question. Usually try to guide the interviewer to show his true level, this kind of question is to do an opening warm-up, the interviewer will often expand the relevant questions according to your answer.

Knowledge expansion

To get back to the point, the understanding of the Java platform can be briefly discussed in many aspects, such as Java language features, including generics, Lambda and other language features, and basic class libraries, including collection, IO/NIO, network, concurrency, security and other basic class libraries. For the class libraries that are widely used in our daily work, we can systematically summarize them before the interview, which is helpful to play on the spot.

Or talk about some basic concepts and mechanisms of JVM, such as Java's class loading mechanism, Class-Loader embedded in common versions of JDK (such as JDK 8), such as Bootstrap, Application and Extension Class-loader; class loading process: loading, verification, linking, initialization (here refer to Zhou Zhiming's "in-depth understanding of the Java Virtual Machine", great JVM hands-on books); custom Class-Loader, etc. There are also the basic principles of garbage collection, and the most common garbage collectors, such as SerialGC, Parallel GC, CMS, G1, etc., have a good idea of what kind of workload they are suitable for. These are areas that can be expanded, which I will introduce more systematically in a later column.

Of course, there are tools included in JDK or other tools in the Java domain, such as compilers, runtime environments, security tools, diagnostic and monitoring tools, and so on. These basic tools are the guarantee of daily work efficiency, and they are also helpful for us to work on other language platforms, many of which are analogous.

The following picture is a relatively broad blueprint I have summarized for your reference.

Instead of extending it, go back to the questions asked earlier about interpreting and compiling execution. Some interviewers like to "get to the bottom of" on specific issues, because this is an effective way to learn more about the interviewer's mastery of knowledge.

As we all know, we usually divide Java into compile time and run time. There are different meanings between the compilation of Java and the compilation of Java source code. There are actually bytecode in the ".class" file, rather than machine code that can be executed directly. Java shields the details of the operating system and hardware through the cross-platform abstraction of bytecode and Java virtual machine (JVM), which is the basis of "compile once, execute everywhere".

At run time, JVM loads the bytecode through the class loader (Class-Loader), interprets it, or compiles it. As I mentioned earlier, mainstream Java versions, such as JDK 8, are actually a mixed mode of interpretation and compilation, the so-called mixed mode (- Xmixed). JVM, which usually runs in server mode, makes tens of thousands of calls to gather enough information for efficient compilation, while the threshold for client mode is 1500. Oracle Hotspot JVM has built-in two different JIT compiler,C1 corresponding to the above-mentioned client mode, which is suitable for startup speed-sensitive applications, such as ordinary Java desktop applications; C2 corresponds to server mode, which is optimized for long-running server-side applications. The default is the so-called hierarchical compilation (TieredCompilation). I won't expand any more details on JIT here, so there's no need to drill into it all at once. I'll talk about hierarchical compilation later.

When the Java virtual machine starts, you can specify different parameters to select the running mode. For example, specifying "- Xint" tells JVM to only interpret execution and not compile the code, a mode that abandons the performance advantages that JIT may bring. After all, the interpreter (interpreter) is read in one by one and run one by one. In contrast, there is a "- Xcomp" parameter that tells JVM to turn off the interpreter and not to perform interpretation execution, or the level of optimization. Then you may ask whether this model is effective? To put it simply, not necessarily. "- Xcomp" can cause JVM startup to slow down a lot, and some JIT compiler optimizations, such as branch prediction, often fail to optimize effectively without profiling.

In addition to our most common daily Java usage pattern, there is actually a new compilation method, the so-called AOT (Ahead-of-Time Compilation), which compiles the bytecode directly into machine code, thus avoiding the overhead of JIT preheating and other aspects. For example, Oracle JDK 9 introduces experimental AOT features and adds new jaotc tools. Use the following command to compile a class or module into an AOT library.

Jaotc-output libHelloWorld.so HelloWorld.class jaotc-output libjava.base.so-module java.base

Then, you can specify it directly at startup.

Java-XX:AOTLibrary=./libHelloWorld.so,./libjava.base.so HelloWorld

Moreover, Oracle JDK supports hierarchical compilation and AOT collaboration, which is not a choice between the two.

In addition, JVM as a powerful platform, not only Java language can run on JVM, essentially compliant bytecode can be run, Java language itself also provides convenience for this, we can see a large number of JVM languages such as Clojure, Scala, Groovy, JRuby, Jython and so on, active in different scenarios.

At this point, the study of "what are the basic principles and related tools of the Java virtual machine" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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